Chuan-kui Wang

Controlling and optimizing the performance of molecular electronic devices is a great challenge in molecular electronics. By using a first-principles method, here we show that rectifying direction and rectification performance of molecular diodes, consisting of a single bipyridyl-embedded alkanethiolate molecule sandwiched between two parallel Ag(111) electrodes, can be precisely controlled by placing the bipyridyl group at different locations of the alkanethiolate backbone. The analysis reveals that the monotonic shift of energy levels of frontier molecular orbitals induced by the electrostatic effect of external bias voltage on the strongly localized wave functions is responsible for the features of rectification. The spatial distributions of frontier molecular orbitals are highly dependent on the location of the bipyridyl group. Hence, varying the bipyridyl location in the alkanethiolate backbone essentially changes the evolving behavior of the frontier molecular orbitals under external bias voltages. This work is helpful for the rational design of molecular diodes.

Theoretical insights on the electroluminescent mechanism of thermally activated delayed fluorescence emitters

Co-reporter:Lili Lin, Zhongjie Wang, Jianzhong Fan, Chuankui Wang

Organic Electronics 2017 Volume 41() pp:17-25

Publication Date(Web):February 2017

DOI:10.1016/j.orgel.2016.11.035

•The S1 states for TADF molecules have both CT and LE character.•There are two stable geometric structures for 4CzIPN molecule.•A “hybrid” method is proposed to study the excited states of D-A type molecules.•Phenomenological rate equations are used to describe the dynamics of excitons.Thermally activated delayed fluorescence (TADF) emitters, with internal quantum efficiency (IQE) in organic light-emitting diodes (OLED) approaching 100%, have attracted great attention recently. However, theoretical investigation on the electroluminescent mechanism of TADF emitters is quite rare. In this paper, the time-dependent density functional theory is used to study the property of excited states of the TADF emitters, and it is found that both the geometry and the electronic structure are quite dependent on the functionals. By comparing with the experimental results, a ‘hybrid’ method is adopted to study the photophysical properties of the TADF emitters. Based on the energy structure of the states, the lowest three states are found to have close relation to the electroluminescent process. The dynamics of two lowest excited states are investigated and the rate equation is used to analyze the evolution of the three states involved. A visual picture of the exciton evolution process is obtained, and one can get a better understanding of the up-conversion mechanism of TADF emitters. The analysis of the electron distribution of the transition orbitals indicates that the first singlet excited state of the molecule possesses both the charge transfer and local excitation components, which is a necessary character for a TADF emitter. The comparison of the property of two isomers indicates that the appropriate arrangement of donor groups and acceptor groups is important for a high-efficient TADF emitter.

Theoretical perspective for internal quantum efficiency of thermally activated delayed fluorescence emitter in solid phase: A QM/MM study

Co-reporter:Lili Lin, Jianzhong Fan, Chuan-Kui Wang

Organic Electronics 2017 Volume 51(Volume 51) pp:

Publication Date(Web):1 December 2017

DOI:10.1016/j.orgel.2017.09.021

•Excited state dynamics of a TADF emitter is theoretically investigated by QM/MM method.•Hole and electron transfer rates are studied by Marcus theory.•Charge mobility is obtained by Monte Carlo simulation.•Factors that influence internal quantum efficiency of TADF-OLEDs are analyzed.Excited state dynamics and charge transfer property of an orange-red thermally activated delayed fluorescence (TADF) emitter are theoretically investigated by a quantum mechanics/molecular mechanics (QM/MM) method and kinetic Monte Carlo simulation. The factors that influence internal quantum efficiency of the organic light-emitting diode (OLED) based on an asymmetric donor–acceptor (D–A) type molecule 10-(7-fluoro-2,3-diphenylquinoxalin-6-yl)-10H-phenoxazine (FDQPXZ) are analyzed. The results show that the intramolecular rotation of donor unit is restricted because of the enhanced intermolecular interaction in solid phase, which hinders the non-radiative consumption of the excited state energy. The decreased reorganization energy in solid phase is mainly contributed by dihedral angle in low-frequency (<500 cm−1) region. Moreover, the non-radiative decay rate from the first singlet excited state (S1) to the ground state (S0) in solid phase is shown to be smaller than that in gas phase. In order to explore the charge transfer process in the film of FDQPXZ, Marcus theory is used to study the hole and electron transfer rates, and the charge mobility is thus obtained by Monte Carlo simulation. The theoretical calculation indicates that the FDQPXZ film is a p-type organic molecular material under the hopping mechanism. Intermolecular interaction for theoretical simulation of the working principle of OLEDs is highlighted.Investigation about the excited state dynamics of TADF emitter through a combined quantum mechanics/molecular mechanics (QM/MM) method.Download high-res image (170KB)Download full-size image

Is there a specific correlation between conductance and molecular aromaticity in single-molecule junctions?

Co-reporter:Guang-Ping Zhang, Zhen Xie, Yang Song, Ming-Zhi Wei, Gui-Chao Hu, Chuan-Kui Wang

Organic Electronics 2017 Volume 48(Volume 48) pp:

Publication Date(Web):1 September 2017

DOI:10.1016/j.orgel.2017.05.032

•Aromaticity does not necessarily decrease the conductance in single-molecule junctions.•The relationship between conductance and aromaticity depends on the anchoring group.•The contact configuration also mediates the relationship between conductance and molecular aromaticity.By using the first-principles method, conductance of molecular junctions containing one of five-atom cyclic unit cyclopentadiene, furan, and thiophene connected to two gold electrodes via different kinds of anchoring groups is theoretically investigated. The numerical results reveal that the experimental finding of negative relationship between conductance and molecular aromaticity (NRCA rule) does not always hold, and it is affected by the molecular anchoring group and contact manner between the molecule and electrodes. When the thiol or methylene anchoring group is used, NRCA rule is proved valid. However, this rule is not applicable for the case where isocyanide end group is used. Moreover, for methylene as the anchoring groups, it is found that the NRCA rule becomes invalid when the contact manner between the molecule and electrodes is changed from a cross style to a parallel one. The results are understood in terms of the molecular projected self-consistent Hamiltonian states as well as transmission spectra. This work facilitates the deep understanding of experimental observation from the theoretical point of view and is helpful for rational design of functional molecular devices.Download high-res image (213KB)Download full-size image

Mechanisms of the odd-even effect and its reversal in rectifying performance of ferrocenyl-n-alkanethiolate molecular diodes

Co-reporter:Shan Wang, Ming-Zhi Wei, Gui-Chao Hu, Chuan-Kui Wang, Guang-Ping Zhang

Organic Electronics 2017 Volume 49(Volume 49) pp:

Publication Date(Web):1 October 2017

DOI:10.1016/j.orgel.2017.06.025

•High rectifying performance and an odd-even effect in rectification ratios are observed in ferrocenyl-n-alkanethiolates.•The monotonic evolutions for the strongly localized frontier molecular orbitals result in the high rectifying performance.•The odd-even effect is attributed to different alignments between frontier molecular orbitals and the EF of electrodes.•Reversed odd-even effects are observed on Au and Ag electrodes due to different bond angles of Au-S-C and Ag-S-C.Using nonequilibrium Green's function method combined with density functional theory, here we show that the rectification performance of molecular diodes, consisting of a single n-alkanethiolate molecule with a ferrocenyl head group sandwiched between two metal (Au or Ag) electrodes, can be modulated by tuning the number n of methylene units in the ferrocenyl-n-alkanethiolate backbone. Specifically, there is an odd-even effect in the rectifying performance as the methylene unit number n in ferrocenyl-n-alkanethiolates is varied. More interestingly, a reversal of the odd-even effect is observed when the Au electrodes are replaced by Ag electrodes. Further analysis reveals that the rectification is governed by the monotonic evolution of the strongly localized frontier molecular orbitals under external bias voltages. Meanwhile, the odd-even effect in the rectification ratios is attributed to different alignments between frontier molecular orbitals and Fermi energy of electrodes for odd- and even-numbered ferrocenyl-n-alkanethiolates, which is originated from the odd-even dependent coupling strength between the ferrocenyl head group and the adjacent electrode. And the odd-even dependence of coupling strength is completely reversed on Au and Ag electrodes due to different Au-S-C and Ag-S-C bond angles that contributes to the reversal of the odd-even effect in rectification ratios. This work is helpful for future rational design and performance control of molecular diodes.Download high-res image (332KB)Download full-size image

Theoretical perspective of the excited state intramolecular proton transfer for a compound with aggregation induced emission in the solid phase

Co-reporter:Lili Lin;Jianzhong Fan;Lei Cai

RSC Advances (2011-Present) 2017 vol. 7(Issue 70) pp:44089-44096

Publication Date(Web):2017/09/11

DOI:10.1039/C7RA06934K

In this study, we have investigated the excited state intramolecular proton transfer (ESIPT) for the diphenylethylene-modified 2-(2-hydroxyphenyl)benzothiazole derivative (HBT-d-Ph) with aggregation induced emission (AIE) in the solid phase through a combined quantum mechanics and molecular mechanics (QM/MM) approach and thermal vibration correlation formalism for non-radiative decay rate. In comparison with the molecule in the solid phase, we find that the ESIPT for HBT-d-Ph is prone to occur in toluene due to the intramolecular hydrogen bonding (H-bond). In addition, the rotation of benzene units involved in the low frequency vibration modes (<500 cm−1) is effectively impeded due to the intermolecular interaction in the solid state, thus the energy consumption pathway through the rotation of benzene units could be blocked. Further, a dual fluorescence with blue and green emission in the solid phase could be obtained. Our study could provide some useful information for designing highly efficient emitting materials with ESIPT and AIE features.

Excited state dynamics for hybridized local and charge transfer state fluorescent emitters with aggregation-induced emission in the solid phase: a QM/MM study

Co-reporter:Jianzhong Fan;Lei Cai;Lili Lin

Physical Chemistry Chemical Physics 2017 vol. 19(Issue 44) pp:29872-29879

Publication Date(Web):2017/11/15

DOI:10.1039/C7CP05009G

Highly efficient organic light-emitting diodes (OLEDs) based on fluorescent emitters with a hybridized local and charge transfer (HLCT) state have attracted significant attention. Recently, a near-infrared fluorescent compound, 2,3-bis(4′-(diphenylamino)-[1,1′-biphenyl]-4-yl)fumaronitrile (TPATCN), with an HLCT state has been synthetized, and the features of OLEDs based on this compound have been explored. In this study, excited state dynamics of TPATCN in the solid phase has been theoretically studied through a combined quantum mechanics and molecular mechanics (QM/MM) method. By analyzing the changes in geometry, the Huang–Rhys factor, and reorganization energy, non-radiative consumption ways through the torsional motions of diphenylamino and central fumaronitrile in low frequency regions (<200 cm−1) are effectively hindered by the restricted intramolecular rotation (RIR) effect in the solid phase. The fluorescence efficiency of the OLED has been quantitatively calculated. The results show that the fluorescence efficiency is greatly enhanced from 0.16% in the gas phase to 52.1% in the solid phase; this demonstrates the aggregation-induced emission (AIE) mechanism for the OLED. Furthermore, by combining the dynamics of the excited states and the adiabatic energy structures calculated in the solid phase, the so-called hot-exciton process from higher triplet states to a singlet state has been illustrated. Our investigation elucidates the experimental measurement and helps understand the AIE mechanism for HLCT compounds, which is beneficial for developing highly efficient emitters.

Excited state properties of non-doped thermally activated delayed fluorescence emitters with aggregation-induced emission: a QM/MM study

Co-reporter:Jianzhong Fan;Lili Lin

Journal of Materials Chemistry C 2017 vol. 5(Issue 33) pp:8390-8399

Publication Date(Web):2017/08/24

DOI:10.1039/C7TC02541F

The excited state properties of dibenzothiophene-benzoyl-9,9-dimethyl-9,10-dihydroacridine (DBT-BZ-DMAC) in the solid phase are theoretically studied through a combined quantum mechanics and molecular mechanics (QM/MM) method. The results indicate that the non-radiative decay rate of the molecule in the solid phase is significantly decreased due to the suppression of the rotation of the DMAC and DBT units in the molecule, while the radiative rate is greatly increased owing to the enhancement of the transition dipole moment. Moreover, the fluorescence efficiency in the solid phase (20.5%) is shown to be much larger than that in the gas phase (0.01‰), which confirms that DBT-BZ-DMAC is a typical aggregation-induced emission (AIE) system. The results further display that both the intersystem crossing (ISC) and reverse intersystem crossing (RISC) processes take place between the first singlet excited state (S1) and the lowest degenerate triplet excited states (T1 and T2). In addition, the charge transfer rate is studied using the Marcus theory and the intrinsic charge mobility is calculated by performing the kinetic Monte Carlo method. The results show that the DBT-BZ-DMAC crystal is a p-type semiconductor with a hole mobility of 0.14 cm2 V−1 s−1 at room temperature. Our investigation elucidates the experimental measurements and helps one to understand the AIE mechanisms of the DBT-BZ-DMAC fluorescence emitter, which is beneficial for developing new TADF emitters.

Molecular stacking effect on photoluminescence quantum yield and charge mobility of organic semiconductors

Co-reporter:Jianzhong Fan;Lili Lin

Physical Chemistry Chemical Physics 2017 vol. 19(Issue 44) pp:30147-30156

Publication Date(Web):2017/11/15

DOI:10.1039/C7CP05451C

The photoluminescence quantum yield (PLQY) and charge transfer property of the high mobility emissive organic semiconductors, 2,6-diphenylanthracene (DPA) and 2,6-diphenyl-9,10-bis(phenylethynyl)anthracene (DP-BPEA), are theoretically investigated. A quantum mechanics/molecular mechanics (QM/MM) method is adopted to investigate the photophysical properties, the Marcus equation is used to describe hole and electron transfer rates, and kinetic Monte Carlo simulation is performed to obtain charge mobility. In both solution and solid phase, the PLQY and the charge mobility of DPA and DP-BPEA are calculated, and the molecular stacking effect is analyzed. For DPA, the rotation motions of the terminal phenyl ring are obviously restricted in the solid phase, which results in the decrease of the Huang–Rhys factor and reorganization energy. This restricted intramolecular rotation (RIR) effect suppresses dissipation pathways of the excited state energy. As a result, the aggregation induced enhancement emission (AIEE) is thus revealed for this emitter from the tetrahydrofuran (THF) solution to the solid phase. However for DP-BPEA, the geometrical variations between the ground and excited states are almost unchanged from the THF solution to the solid phase, and only a restricted intramolecular vibration (RIV) of bond length (e.g. –CC–) is displayed. As RIV is not obvious as RIR, a similar fluorescence efficiency is obtained for DP-BPEA in THF solution and the solid phase. In addition, DPA and DP-BPEA crystals are proven to be p-type semiconductors. The calculated mobility of the hole for DPA (3.39 cm2 V−1 s−1) is larger than that of DP-BPEA (1.62 cm2 V−1 s−1) because the number of effective transition pathways of DPA is more than that of DP-BPEA. Our study demonstrates that the different molecular stacking of organic semiconductors has an important effect on their photophysical and charge transfer properties.

Obvious modulation of rectifying performance by conjugation breaking of the bridging fragment in donor–bridge–acceptor molecular diodes

Co-reporter:Wen-Xia Su;Xi Zuo;Zhen Xie;Guang-Ping Zhang

RSC Advances (2011-Present) 2017 vol. 7(Issue 23) pp:14200-14205

Publication Date(Web):2017/02/28

DOI:10.1039/C7RA00254H

The rectifying properties of three donor–bridge–acceptor (D–B–A) type thiolated arylethynylene diodes designed by replacing one of the thiol anchoring groups to cyano, where the central bridging fragment is varied from cross-conjugated 9,10-anthraquinone (AQ), linearly-conjugated anthracene (AC) to broken-conjugated 9,10-dihydroanthracene (AH), are theoretically investigated using a non-equilibrium Green's function (NEGF) method combined with density functional theory (DFT). The numerical results reveal that both the rectifying performance and rectification direction of the D–B–A diodes are closely related with the conjugation type of the central bridging segment. The rectification performance is improved and the rectifying direction is inverted when the central bridge is changed from cross-conjugated AQ to linearly-conjugated AC. Moreover, when conjugation of the bridge part is broken, i.e., AH, the rectification performance is further enhanced remarkably. Further analysis reveals that the asymmetric evolution of strongly localized frontier molecular orbitals under positive and negative bias voltages, induced by conjugation breaking, is responsible for the great enhancement in rectification ratio for AH. This work is helpful for rational design of molecular diodes with diverse rectification performance.

Towards Rectifying Performance at the Molecular Scale

Co-reporter:Guang-Ping Zhang;Zhen Xie;Yang Song;Gui-Chao Hu

Topics in Current Chemistry 2017 Volume 375( Issue 6) pp:85

Publication Date(Web):24 October 2017

DOI:10.1007/s41061-017-0170-3

Molecular diode, proposed by Mark Ratner and Arieh Aviram in 1974, is the first single-molecule device investigated in molecular electronics. As a fundamental device in an electric circuit, molecular diode has attracted an enduring and extensive focus during the past decades. In this review, the theoretical and experimental progresses of both charge-based and spin-based molecular diodes are summarized. For the charge-based molecular diodes, the rectifying properties originated from asymmetric molecules including D–σ–A, D–π–A, D–A, and σ–π type compounds, asymmetric electrodes, asymmetric nanoribbons, and their combination are analyzed. Correspondingly, the rectification mechanisms are discussed in detail. Furthermore, a series of strategies for modulating rectification performance is figured out. Discussion on concept of molecular spin diode is also involved based on a magnetic co-oligomer. At the same time, the intrinsic mechanism as well as the modulation of the spin-current rectification performance is introduced. Finally, several crucial issues that need to be addressed in the future are given.

Decreasing the singlet–triplet gap for thermally activated delayed fluorescence molecules by structural modification on the donor fragment: First-principles study

Co-reporter:Jian-zhong Fan, Li-li Lin, Chuan-kui Wang

Chemical Physics Letters 2016 Volume 652() pp:16-21

Publication Date(Web):16 May 2016

DOI:10.1016/j.cplett.2016.04.027

•90 deg D–A dihedral angle is advantageous to decrease S–T energy gap.•The planarity of the donor groups is a key factor for regulating the S–T energy gap.•S–T gap and orbital overlap decrease as electron donating ability is increased.The small energy gap between singlet excitons (S) and triplet excitons (T) of organic molecules is a dominant condition for high efficient thermally activated delayed fluorescence (TADF). In this study, influence of modification in donor groups of a series of molecules on their geometries, S–T energy gaps, and photophysical properties, is investigated based on first-principles calculations. Investigation shows that, as the electron donating ability is increased, both S–T energy gap and overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are decreased. This work provides strategy for designing high efficient and multi-color TADF devices.Graphical abstract

Rectifying enhancement induced by conjugation breaking in thiolated arylethynylene single-molecular diodes

Co-reporter:Xi Zuo, Lei Chu, Guang-Ping Zhang, Chuan-Kui Wang

Chemical Physics Letters 2016 Volume 663() pp:74-78

Publication Date(Web):16 October 2016

DOI:10.1016/j.cplett.2016.09.075

Highlights

•: Rectifying properties of three arylethynylene thiolate molecules are investigated.
•: A negative relationship between rectifying performance and conjugation is revealed.
•: Conjugation breaking in the molecule contributes to the rectifying enhancement.

Responsive mechanism of a newly synthesized fluorescent probe for sensing H2O2, NO and H2O2/NO

Co-reporter:Yu-Jin Zhang, Xin Wang, Yong Zhou, Ke Zhao, Chuan-Kui Wang

Chemical Physics Letters 2016 Volume 662() pp:107-113

Publication Date(Web):1 October 2016

DOI:10.1016/j.cplett.2016.04.026

Highlights

•: Responsive mechanism of the fluorescent probe is theoretically analyzed.
•: Possibility of the probe as a two-photon fluorescent sensor is theoretically studied.
•: The works provide theoretical explanation of the experimental results.

Influence of Donor on the Sensing Performance of a Series of Through-Bond Energy Transfer-Based Two-photon Fluorescent Cu2+ Probes

Co-reporter:Yu-Jin Zhang;Xin Wang;Yong Zhou

Photochemistry and Photobiology 2016 Volume 92( Issue 4) pp:528-536

Publication Date(Web):

DOI:10.1111/php.12597

Abstract

Optical properties of a series of molecular two-photon fluorescent Cu²⁺ probes containing the same acceptor (rhodamine group) are analyzed using time-dependent density functional theory in combination with analytical response theory. Special emphasis is placed on evolution of the probes' optical properties in the presence of Cu²⁺. In this study, the compound with naphthalene as the donor is shown to be excellent ratiometric fluorescent chemosensor, whereas the compound with quinoline derivative as the donor shows off/on-typed colorimetric fluorescent response. For the compound with naphthalimide derivative as the donor, changing the connection between the donor and acceptor can efficiently prevent the fluorescent quenching of the probe both in the absence and presence of Cu²⁺. The donor moiety and the connection between donor and acceptor are thus found to play dominant roles on sensing performance of these probes. Moreover, distributions of molecular orbitals involved in the excitation and emission of the probes are analyzed to explore responsive mechanism of the probes. The through-bond energy transfer process is theoretically demonstrated. Our results are used to elucidate the available experimental measurements. This work is helpful to understand the relationships of structure with optical properties for the studied probes.

Influence of terminal group on sensing performance of a series of N-monoalkylated diketopyrrolopyrrole-based two-photon fluorescent fluoride anion probes

Co-reporter:Yu-Jin Zhang, Xin Wang, Yong Zhou, Chuan-Kui Wang

Chemical Physics Letters 2016 Volume 658() pp:125-129

Publication Date(Web):1 August 2016

DOI:10.1016/j.cplett.2016.06.026

•Responsive mechanism of the fluorescent probe is theoretically analyzed.•Influence of terminal group on sensing performance of the fluorescent probe is theoretically studied.•The works provide theoretical explanation of the experimental results.Optical properties of newly synthesized molecular two-photon fluorescent probes containing diketopyrrolopyrrole are investigated using density functional theory. Drastic changes in photoabsorption and photoemission of the probes are demonstrated when the probes react with fluoride anion. Responsive mechanism of the probes is analyzed by comparing milliken population of the molecules in the absence and presence of fluoride anion, which is attributed to intramolecular charge transfer. In addition, terminal group is found to be crucial to the sensing performance of the probes. Our results can elucidate the available experimental measurements. Additionally, valuable knowledge for designing efficient fluorescent chemosensors is proposed.

Detecting CO, NO and NO2 gases by Boron-doped graphene nanoribbon molecular devices

Co-reporter:Zhen Xie, Xi Zuo, Guang-Ping Zhang, Zong-Liang Li, Chuan-Kui Wang

Chemical Physics Letters 2016 Volume 657() pp:18-25

Publication Date(Web):16 July 2016

DOI:10.1016/j.cplett.2016.05.052

Highlights

•: Graphene nanoribbons based molecular devices are used to detect toxic gas molecules.
•: Toxic gas molecules CO, NO and NO₂ have been detected.
•: The I–V profile or magnitude of current depends on the adsorbed gas molecule.

Understanding the light-emitting mechanism of an X-shape organic thermally activated delayed fluorescence molecule: First-principles study

Co-reporter:Jianzhong Fan, Lei Cai, Lili Lin, Chuankui Wang

Chemical Physics Letters 2016 Volume 664() pp:33-38

Publication Date(Web):1 November 2016

DOI:10.1016/j.cplett.2016.10.009

Highlights

•: There are two stable conformers for the ACRSA molecule, and conformer A has lower energy than conformer B.
•: Conformer B is responsible for the light-emitting of the ACRSA molecule, since conformer A is non-radiative.
•: Up-conversion happens between the S1 and T1 states.
•: Out-of-plane vibration modes contribute most to the reorganization energy of S1.

Effect of fluorophore on sensing NO for newly synthesized PET-based two-photon fluorescent probes

Co-reporter:Yu-Jin Zhang, Wen-Jing Yang, Chuan-Kui Wang

Chemical Physics 2016 Volume 468() pp:37-43

Publication Date(Web):1 April 2016

DOI:10.1016/j.chemphys.2016.01.004

Abstract

Density functional theory in combination with polarizable continuum model is used to investigate one- and two-photon absorption (TPA) as well as emission properties of newly synthesized photoinduced electron transfer (PET)-based two-photon fluorescent probes QNO and LNO in the absence and presence of nitric oxide (NO). Both fluorescent intensity and TPA are enhanced when QNO (LNO) reacts with NO, which theoretically proves that the compounds QNO and LNO are promising two-photon fluorescent probes for NO. Moreover, QNO is demonstrated to be preferable because of its superior fluorophore. Analysis of molecular orbitals is presented to explore responsive mechanism of QNO (LNO) for NO.

Dynamics of Excited States for Fluorescent Emitters with Hybridized Local and Charge-Transfer Excited State in Solid Phase: A QM/MM Study

Co-reporter:Jianzhong Fan, Lei Cai, Lili Lin, and Chuan-Kui Wang

The Journal of Physical Chemistry A 2016 Volume 120(Issue 47) pp:9422-9430

Publication Date(Web):November 8, 2016

DOI:10.1021/acs.jpca.6b09852

The highly efficient organic light-emitting diodes (OLEDS) based on fluorescent emitters with hybridized local and charge-transfer (HLCT) excited state have attracted great attention recently. The excited-state dynamics of the fluorescent molecule with consideration of molecular interaction are studied using the hybrid quantum mechanics/molecular mechanics method. The results show that, in solid state, the internal conversion rate (KIC) between the first singlet excited state (S1) and the ground state (S0) is smaller than the fluorescent rate (Kr), while in gas phase KIC is much larger than Kr. By analyzing the Huang–Rhys (HR) factor and reorganization energy (λ), we find that these two parameters in solid state are much smaller than those in gas phase due to the suppression of the vibration modes in low-frequency regions (<200 cm–1) related with dihedral angles between donor and acceptor groups. This is further demonstrated by the geometrical analysis that variation of the dihedral angle between geometries of S1 and S0 is smaller in solid state than that in gas phase. Moreover, combining the dynamics of the excited states and the adiabatic energy structures calculated in solid state, we illustrate the suggested “hot-exciton” mechanism of the HLCT emitters in OLEDs. Our work presents a rational explanation for the experimental results and demonstrates the importance of molecular interaction for theoretical simulation of the working principle of OLEDs.

Rectifying and conductance switch behaviors of molecular devices modulated by intramolecular hydrogen bonding

Co-reporter:Yang Song, Zhen Xie, Ming-Zhi Wei, Jian-Cai Leng, Zong-Liang Li, Chuan-Kui Wang

Computational Materials Science 2015 Volume 108(Part A) pp:8-13

Publication Date(Web):October 2015

DOI:10.1016/j.commatsci.2015.06.009

•The influence of intramolecular hydrogen bonding on rectifying behaviors of the diode is explored.•The rectifying direction shows inversion when asymmetric electrode–molecule arrangements are adopted.•The intramolecular hydrogen bonding induced conductance switch behavior is presented.•The inside view of observed results is given.By applying nonequilibrium Green’s function method in combination with density functional theory, we theoretically investigate the effect of intramolecular hydrogen bonding on transport properties of a new synthesized α-hydroxyphenyl pyridine molecular diode (Wang and Yu, 2011). Numerical results show that intramolecular hydrogen bonding presents obvious influence on rectifying behaviors of the diode, as the hydrogen bonding could rotate the adjacent aromatic rings into coplanar structure. Appropriate contact configurations between the molecule and electrodes are shown to be important for obtaining the experimental rectifying characteristics. Furthermore, bistable state configurations related to the intramolecular hydrogen bonding are obtained and conductance switch behavior is presented. Accordingly, a molecular conductance switch with high On–Off ratios is proposed. The analysis of molecular projected self-consistent Hamiltonian and the spatial distribution of frontier molecular orbitals as well as the transmission coefficients reveals the mechanism of these consequences.

More aromatic molecular junction has lower conductance

Co-reporter:Zhen Xie, Xiao-Li Ji, Yang Song, Ming-Zhi Wei, Chuan-Kui Wang

Chemical Physics Letters 2015 Volume 639() pp:131-134

Publication Date(Web):16 October 2015

DOI:10.1016/j.cplett.2015.09.017

Highlights

•: The relationship between aromaticity of molecules and their conductance is explored.
•: Less aromatic device shows higher currents.
•: The correlation between the aromaticity and conductance is robust on the molecule–electrode contact configuration.
•: The inside view of observed results is given.

Molecular-length induced inversion of rectification in diblock pyrimidinyl–phenyl molecular junctions

Co-reporter:Guang-Ping Zhang, Zhen Xie, Yang Song, Gui-Chao Hu, Chuan-Kui Wang

Chemical Physics Letters 2014 Volume 591() pp:296-300

Publication Date(Web):20 January 2014

DOI:10.1016/j.cplett.2013.11.057

Highlights

•: Rectifying direction in diblock pyrimidinyl–phenyl molecular diodes can be modulated.
•: There are two competitive mechanisms in determining the rectifying direction.
•: The dominant rectification mechanism can be exchanged by changing molecular length.

Protonation-induced modulation of one- and two-photon absorption properties for quadripolar dyes

Co-reporter:Hong-Juan Ding, Jie Sun, Yu-Jin Zhang, Chuan-Kui Wang

Chemical Physics Letters 2014 Volume 591() pp:142-148

Publication Date(Web):20 January 2014

DOI:10.1016/j.cplett.2013.11.015

Highlights

•: A numerical method is suggested to deal with (de)protonation effects on optical properties.
•: Optical properties show obvious changes upon passing from neutral to doubly (de)protonated state.
•: The numerical results are consistent with the experimental observations.

Giant Rectification Ratios of Azulene-like Dipole Molecular Junctions Induced by Chemical Doping in Armchair-Edged Graphene Nanoribbon Electrodes

Co-reporter:Yang Song ; Zhen Xie ; Yong Ma ; Zong-liang Li

The Journal of Physical Chemistry C 2014 Volume 118(Issue 32) pp:18713-18720

Publication Date(Web):July 25, 2014

DOI:10.1021/jp504448n

Electron transport properties of an azulene-like dipole molecule anchored with carbon atomic chains sandwiched between two graphene nanoribbon (GNR) electrodes are theoretically investigated at the ab initio level. The molecular junctions are constructed with a strategy of modulating symmetry of Bloch wave functions. The chemical doping in an armchair-edged GNR is shown to play a significant role in determining the conductance behavior and rectifying performance of the molecular junctions. Giant rectification ratios up to 104 at low bias voltages are obtained for the molecular junctions with asymmetric arrangement of undoped zGNR and doped aGNR electrodes. The boron (aluminum) dopants in the aGNR electrode induce a better rectifying performance for the molecular junctions than the respective nitrogen (phosphorus) dopants. Moreover, the boron or nitrogen doping is more advantageous than the respective aluminum or phosphorus doping in view of improving rectifying behaviors of the molecular junctions. Taking double doping in the aGNR electrode, we just demonstrate that the double boron-doping displays an improvement of rectifying features in comparison with the single case. The observed results are understood in terms of the transmission spectrum and the molecular projected self-consistent Hamiltonian as well as band structures of the electrodes with applied bias combined with symmetry analyses of Bloch wave functions of the corresponding subbands.

Protonation and deprotonation effects on charge transports of butane-based molecular junctions

Co-reporter:Yang Song, Dong-Qing Zou, Zhen Xie, Guang-Ping Zhang, Zong-Liang Li, Chuan-Kui Wang

Chemical Physics Letters 2013 Volume 588() pp:155-159

Publication Date(Web):19 November 2013

DOI:10.1016/j.cplett.2013.10.006

Highlights

•: A computational method is suggested to deal with the protonation and deprotonation processes for the molecular junctions.
•: The current of the molecular junction has a significant change after (de)protonating.
•: The coupling energy between molecules and electrodes is manipulated by pH.
•: The numerical results are consistent with the experimental observations.

Bias Dependence of Rectifying Direction in a Diblock Co-oligomer Molecule with Graphene Nanoribbon Electrodes

Co-reporter:Yang Song ; Zhen Xie ; Guang-Ping Zhang ; Yong Ma

The Journal of Physical Chemistry C 2013 Volume 117(Issue 40) pp:20951-20957

Publication Date(Web):September 17, 2013

DOI:10.1021/jp406746n

By applying nonequilibrium Green’s function method in combination with density functional theory, we study the rectifying properties of dipyrimidinyl-diphenyl co-oligomer molecules embedded in a carbon atomic chain sandwiched between two graphene nanoribbon (GNR) electrodes. Both the length of the carbon atomic chains and the edge geometry of the graphene nanoribbon electrodes are shown to play a significant role in determining the conductance behavior and rectifying performance of the molecular devices. As for GNRs with zigzag edges, the parallel (perpendicular) conformation between the principal plane of the molecule and the zigzag-edged GNR electrode is observed to be dependent on the odd (even) number of carbon atoms in the carbon chain, whereas for armchair-edged GNRs the parallel (perpendicular) case corresponds to an even (odd) number of carbon atoms. Taking an asymmetric arrangement of armchair and zigzag GNR electrodes, we demonstrate a molecular device having very interesting rectifying behaviors with marked rectification ratios at low bias voltages and inversion of rectifying direction when the bias voltage is large. Analysis of the transmission coefficients and molecular projected self-consistent Hamiltonian as well as band structures of the electrodes under various external bias voltages reveals an underlying mechanism of the observed results.

Modulation of Rectification in Diblock Co-oligomer Diodes by Adjusting Anchoring Groups for Both Symmetric and Asymmetric Electrodes

Co-reporter:Guang-Ping Zhang, Gui-Chao Hu, Yang Song, Zong-Liang Li, and Chuan-Kui Wang

The Journal of Physical Chemistry C 2012 Volume 116(Issue 41) pp:22009-22014

Publication Date(Web):September 21, 2012

DOI:10.1021/jp304890p

The rectifying properties in dipyrimidinyl-diphenyl co-oligomer diodes with asymmetric anchoring groups were investigated using density functional theory combined with the nonequilibrium Green’s function method. Effects of asymmetric interfaces caused by both the anchoring groups and/or contact geometries of electrodes have been investigated. Our results showed that the rectifying behavior of the co-oligomer diode could be reversed or largely enhanced by adjusting asymmetric anchoring groups. Whether the asymmetric contact geometries play a positive or negative role in improving the rectifying behavior is closely related to each molecular diode. The mechanism of modulation was analyzed in terms of molecular projected self-consistent Hamiltonian states and transmission spectra. The theoretical simulations are helpful for understanding recent experimental results [Lee et al. Langmuir2009, 25, 1495 and Hihath et al. ACS Nano2011, 5, 8331]. Moreover, the mechanism of the rectification only due to the electrode asymmetry was explained, and a single-molecule diode with significant rectifying behaviors has been theoretically designed.

Theoretical Studies on Protonation-Induced Inversion of the Rectifying Direction in Dipyrimidinyl–Diphenyl Diblock Molecular Junctions

Co-reporter:Guang-Ping Zhang ; Gui-Chao Hu ; Zong-Liang Li

The Journal of Physical Chemistry C 2012 Volume 116(Issue 5) pp:3773-3778

Publication Date(Web):January 9, 2012

DOI:10.1021/jp211021t

The protonation effects on electron transport through a conjugated dipyrimidinyl–diphenyl diblock oligomer sandwiched between two gold electrodes were theoretically investigated using the fully self-consistent nonequilibrium Green’s function method combined with density functional theory. All configurations of protonation on the dipyrimidinyl group of the molecule were considered. The numerical results show that the protonation prefers to improve electron transport ability through the molecular junction. The transmission spectra around the Fermi energy are tuned by the number and locations of protons residing in the molecular junction. It is found that the protonation in the outer pyrimidinyl is favorable for enhancing the rectification ratio, while the protonation in the inner pyrimidinyl plays a dominative role for inverting the rectifying direction. This theoretical work for the first time presents a mechanism for the experimental findings of inversion of the rectifying effect in diblock molecular diodes caused by protonation [J. Am. Chem. Soc.2005, 127, 10456].

Determination of the Configuration of a Single Molecule Junction by Inelastic Electron Tunneling Spectroscopy

Co-reporter:Li−Li Lin, Xiu-Neng Song, Jian-Cai Leng, Zong-Liang Li, Yi Luo and Chuan-Kui Wang

The Journal of Physical Chemistry C 2010 Volume 114(Issue 11) pp:5199-5202

Publication Date(Web):March 2, 2010

DOI:10.1021/jp101428d

First-principles calculations for inelastic electron tunneling spectroscopy (IETS) of a single 1,3-propanedithiol molecule covalently bound to gold electrodes are presented. Inelastic electron tunneling spectra of the single molecule junction with different contact geometries and molecular orientations at the interface are simulated. It is demonstrated that the delicate variation in the configuration of the single molecule junction caused by separating the two electrodes can result in significant changes in the inelastic electron tunneling spectral profile of the junction. The two most probable configurations of the molecular junction formed in the experiment (Nano Lett. 2008, 8, 1673) are theoretically identified, and the experimental IET spectra are correctly assigned.

Orientation of Decanethiol Molecules in Self-Assembled Monolayers Determined by Inelastic Electron Tunneling Spectroscopy

Co-reporter:Jian-Cai Leng, Li−Li Lin, Xiu-Neng Song, Zong-Liang Li and Chuan-Kui Wang

The Journal of Physical Chemistry C 2009 Volume 113(Issue 42) pp:18353-18357

Publication Date(Web):September 28, 2009

DOI:10.1021/jp9052264

We present first-principles calculations for the inelastic electron tunneling spectroscopy (IETS) of decanethiolate molecules sandwiched between Au(111) surface and scanning tunneling microscope (STM) tip, as reported by Hallbäck et al. (Nano Lett. 2004, 4, 2393). It is demonstrated here that the IET spectra are very sensitive to the molecule−metal contact structure, orientation of the molecule adsorbed on the surface, and the distance between the carbon of the terminal methyl group and the STM tip. With correct assignation of the experimental spectral features, the orientation of the molecule and then the probable configuration of the molecular junction are determined.

Effect of Aromatic Coupling on Electronic Transport in Bimolecular Junctions

Co-reporter:Li-Li Lin, Jian-Cai Leng, Xiu-Neng Song, Zong-Liang Li, Yi Luo and Chuan-Kui Wang

The Journal of Physical Chemistry C 2009 Volume 113(Issue 32) pp:14474-14477

Publication Date(Web):July 17, 2009

DOI:10.1021/jp900908w

We have performed a systematic first-principles study on conductance−voltage characteristics of bioligo(phenylene ethynylene)−monothiol molecular junctions as recently reported by Wu et al.[Nature Nanotech. 2008, 3, 569]. It is found that the molecular conductance is very sensitive to the vertical distance between two molecules as well as the titled angle between two molecular planes. By comparing with experimental results, key structure parameters for bimolecular junction are determined, indicating that in the experimental devices, the vertical distance between two molecules is around 0.30 nm, the two planar molecules have a cofacial arrangement, and the length of the molecular bridge is about 2.88 nm. The underlying mechanism for electron transport in these aromatically coupled bimolecular junctions has also been discussed.

The theoretical simulation for two-photon absorption properties of novel A-π-D-π-A two-photon carbazole compounds

Co-reporter:Jing Li, Xiu-Neng Song, Yu-Ping Sun, Chuan-Kui Wang

Journal of Molecular Structure: THEOCHEM 2008 Volume 867(1–3) pp:53-58

Publication Date(Web):30 October 2008

DOI:10.1016/j.theochem.2008.07.019

The optical properties of three newly synthesized A-π-D-π-A compounds have been studied by use of analytical response theory at the density functional theory level. The theoretical results show that there exist three charge-transfer states for each compound in the low energy regime. The trends for both oscillator strengths and excitation energies among the three compounds as predicted by the numerical simulations are highly consistent with those given by measurement. Large two-photon absorption cross sections are predicted for the three compounds. Furthermore, the solvent effect on the optical properties of the compounds is investigated. It is found that the excitation energies in solvent DMF show a red-shift compared to those in gas case, and the two-photon absorption cross sections are highly enhanced. The optical properties of the compounds with experimental geometric parameters are also investigated. The oscillator strengths are found to be smaller due to the bent molecular structures. The calculated results predict that much higher two-photon absorption cross sections would be observed when a laser with a shorter wavelength is applied.

Theoretical studies on one-photon and two-photon absorption properties of a series of dibenzothiophene derivatives

Co-reporter:Xiao-Juan Xing, Jing Li, Yu-Ping Sun, Chuan-Kui Wang

Journal of Molecular Structure: THEOCHEM 2008 Volume 849(1–3) pp:116-121

Publication Date(Web):30 January 2008

DOI:10.1016/j.theochem.2007.10.006

The one-photon and two-photon absorption properties of a series of dibenzothiophene derivatives are investigated by use of the analytic response theory at DFT level. The numerical results show that these molecules have relatively strong two-photon absorption cross-sections. In the visible light region, the maximal one-photon absorption strengths of the molecules occur in the first excited state. (E,E)-1,4-Bis[3-vinyl-dibenzothiophene]-2,5-dimethoxybenzene molecule has the largest values for both one-photon absorption intensity and two-photon absorption cross-section among the studied molecules. Furthermore, it is shown that the maximal one-photon absorption intensity is monotonically increased as the electron-donating ability of donor group is enhanced. However, the two-photon absorption cross-section displays a non-monotonic increment. The charge-transfer process is analyzed when the molecule is excited from the ground state to charge-transfer state. A general agreement with experimental measurement is obtained.

Effects of field-induced geometry relaxation on the electron transport properties of 4,4′-biphenyldithiol molecular junction

Co-reporter:Bin Zou, Zong-Liang Li, Xiu-Neng Song, Yi Luo, Chuan-Kui Wang

Chemical Physics Letters 2007 Volume 447(1–3) pp:69-73

Publication Date(Web):15 October 2007

DOI:10.1016/j.cplett.2007.08.076

The non-linear charge transport properties of 4,4′-biphenyldithiol molecular junction have been studied using the generalized Green’s function theory. It is shown that the torsion angle between two phenyls is slightly decreased as increase of the external voltage while the whole molecule moves slightly along the reversed direction of the electric field. Calculations indicate that the inclusion of molecular geometry relaxation can avoid a false prediction of negative differential resistance behavior. The charge redistribution under the external bias results in resistive dipoles inside the molecule. The calculated I–V curves of 4,4′-biphenyldithiol molecular junction is consistent with experimental observations in some ways.The non-linear charge transport properties of 4,4′-biphenyldithiol molecular junction have been studied using the generalized Green’s function theory. It is shown that the torsion angle between two phenyls is slightly decreased as increase of the external voltage while the whole molecule moves slightly along the reversed direction of the electric field. Calculations indicate that the inclusion of molecular geometry relaxation can avoid a false prediction of negative differential resistance behavior. The charge redistribution under the external bias results in resistive dipoles inside the molecule. The calculated I–V curves of 4,4′-biphenyldithiol molecular junction is consistent with experimental observations in some ways. The figure shows the conductance curve of 4,4′-biphenyldithiol molecular junction. The oscillating peaks of conductance are observed and the asymmetry of conductance curve related to the direction of applied bias is displayed. When the molecular geometry relaxation is included (dot line), the negative differential resistance behavior is not demonstrated.

Solvent effects on two-photon absorption cross sections of a newly synthesized polymerization initiator

Co-reporter:Yu-Zhi Song, Dong-Mei Li, Xiu-Neng Song, Xiao-Ming Huang, Chuan-Kui Wang

Journal of Molecular Structure: THEOCHEM 2006 Volume 772(1–3) pp:75-79

Publication Date(Web):23 October 2006

DOI:10.1016/j.theochem.2006.06.024

The solvent effects on the geometrical and electronic structures, as well as one- and two-photon absorption process of a newly synthesized asymmetrical photon polymerization initiator 4-vinylpyridine-4′-(N,N-diphenylamino)benene (VPDB) are studied, using the time-dependent hybrid density functional theory (DFT/B3LYP) in combination with polarized continuum model (PCM). The two-photon absorption cross sections of the compound in gas and solvents are calculated by employing the generalized three-state model. The numerical results show that the solvatochromic shift for charge transfer states takes red-shift and exhibits nonmonotonic behavior with respect to the polarity of solvents. The two-photon cross sections are enhanced in related to the solvent and the enhancement shows nonmonotonic trend according to the polarity of solvents. The compound has large TPA cross section and can be used as an excellent candidate for the two-photon photopolymerization initiators.

Theoretical studies on nonlinear optical properties of two newly synthesized compounds: PVPHC and DPVPA

Co-reporter:Yuan-Hong Sun, Ke Zhao, Chuan-Kui Wang, Yi Luo, Yunxing Yan, Xu-Tang Tao, Min-Hua Jiang

Chemical Physics Letters 2004 Volume 394(1–3) pp:176-181

Publication Date(Web):11 August 2004

DOI:10.1016/j.cplett.2004.07.003