AbstractPhotodynamic therapy (PDT) is a noninvasive protocol for the treatment of various cancers and nonmalignant diseases. Light, oxygen, and photosensitizer (PS) are the essential three elements in a typical PDT process. Currently, there are two major barriers limiting the further development of PDT. One issue is limited tissue penetration, and the other is the lack of high-performance PSs. Therefore, the newly emerging two-photon excited PDT (2PE-PDT) has attracted considerable attention in recent years due to its advantages such as a higher spatial resolution and a greater penetration depth. In this review, focus is on (i) the principle of 2PE-PDT, (ii) the progression of PSs for 2PE-PDT, and (iii) the potential indications and future directions in this field.

•A liquid optical limiting material (LBDBP) with high optical/thermal stability was designed and synthesized.•LBDBP exhibited an obvious optical limiting behavior under an 800 nm fs pulsed laser.•The intensity fluctuation of an ultraintense and ultrashort laser facility (SILEX-1) was reduced significantly by LBDBP.A new liquid malononitrile derivative (LBDBP) has been synthesized by incorporating four tetraethylene glycol groups into the prototype scaffold of 2-[Bis-(4′-diethylamino-biphenyl-4-yl)-methylene]-malononitrile (BDBP). The linear photophysical properties, optical/thermal stabilities and optical limiting behaviors of LBDBP and BDBP have been investigated. The results show that LBDBP has equivalent optical/thermal stability but much better solubility compared with BDBP. Its saturation concentration in DMF is increased to 0.075 M, while the corresponding datum for BDBP is only 0.01 M. The improved solubility of LBDBP insures a very significant optical limiting behavior. The saturated DMF solution of LBDBP can significantly reduce the intensity fluctuation of laser pulses in an 800 nm laser setup.

•Novel liquid optical limiting materials with high solubilities were synthesized.•Remarkable optical limiting behaviors were shown under 800 nm fs pulsed laser.•The compound with cyano substituted on the vinylene is superior for application.•Solid optical limiting devices with high linear transmittances were fabricated.Two liquid cyano substituted distyrylbenzene derivatives (P2 and P4) have been synthesized by incorporating two tetraethylene glycol groups into the prototype scaffolds of 2,5-bis(4-(diethylamino)styryl)terephthalonitrile (P1) and 2,2'-(1,4-phenylene)bis(3-(4-(diethylamino)phenyl)acrylonitrile) (P3), respectively, for increasing their solubilities in liquid and solid substrates. The linear photophysical properties, optical/thermal stabilities and optical limiting behaviors of P1–P4 have been investigated. Results show that both P2 and P4 have significant optical limiting behaviors on 800 nm laser pulses and high thermal stabilities. However, only P4 whose cyano groups substituted on the vinylene bond exhibits good optical stability, while P2 whose cyano groups substituted on the central phenylene ring presents gradual photobleaching under illumination. Through doping P4 into a solid epoxy resin, effective optical limiting devices to 800 nm pulsed laser are fabricated.

A series of bis(arylidene)cycloalkanone photosensitizers modified by polyethylene glycol (PEG) have been studied for two-photon excited photodynamic therapy (2PE-PDT). As compared with their prototype compounds, these PEGylated photosensitizers show enhanced water solubilities while their photophysical and photochemical properties, including linear absorption, two-photon absorption, fluorescence, and singlet oxygen quantum yield, remain unaltered. In vitro behaviors (cellular uptake, subcellular localization, photocytotoxicity in both PDT and 2PE-PDT) of these photosensitizers reveal that an optimized lipid–water partition coefficient can be obtained by adjusting the length and position of the PEG chains. Among them, the photosensitizer modified asymmetrically by two tetraethylene glycol chains presents the best performance as a 2PE-PDT candidate. Selective blood-vessel closure and obvious therapeutic effect in inhibiting the growth of tumors are confirmed by in vivo 2PE-PDT after intravenous injection of this photosensitiezer. The survival periods of treated tumor-bearing mice are significantly prolonged. This study demonstrates the feasibility of using a simple molecule to construct a potential candidate for 2PE-PDT.

To inactivate both standard strains as well as antibiotic-resistant strains of bacteria with minimum damage to host cells, three new pyridyl cationic-modified benzylidene cyclopentanone photosensitizers (PSs), P1 (with one cationic group), P2 (with two cationic groups arranged bilaterally), and P3 (with two cationic groups arranged unilaterally) were synthesized and characterized. Their selective uptakes by bacteria over HepG2 cells and their photodynamic inactivation efficiencies against Staphylococcus aureus (ATCC 6538), Escherichia coli (ATCC 25922), and the drug-resistant Escherichia coli (CA-31) were studied using methylene blue (MB) and hematoporphyrin monomethyl ether (HMME) as references. The results showed that the uptake amounts of P1, P2, and P3 by all strains were at least 2, 20, and 18 times more than those by HepG2 cells, respectively. All PSs exhibited good antimicrobial photodynamic therapy (aPDT) effects towards all three strains at low concentrations of ≤8.0 μM, while MB was invalid towards all three strains and HMME was invalid toward Escherichia coli (CA-31) at concentrations up to 32.0 μM. In particular, the minimum inhibitory concentrations (MICs) of P3 against these strains were all ≤2.0 μM, under which about 94% HepG2 cells were still alive, indicating P3 had high aPDT selectivity for bacterial cells over mammalian cells. The relationships between structure and antimicrobial properties of these cationic PSs were discussed to reveal their high photodynamic inactivation selectivity for bacterial cells.

Triethylene glycol functionalized coumarin derivatives 1–5 were synthesized and investigated as photosensitizers for one- and two-photon excited photodynamic therapy (PDT). Their absorption, fluorescence, triplet-state quantum yields, and singlet oxygen quantum yields were found to be significantly related to the substituent at the 7-position of the coumarin ring. In vitro photocytotoxicity of these derivatives toward HepG2 cells was examined and compared with a clinical drug. In vitro generation of reactive oxygen species (ROS), cellular uptake, and intracellular distribution of these derivatives were also characterized to fully reveal their structure–property relationships. The results showed that derivative 5, with a two-photon absorption cross section value of 1556 GM, could be a powerful PDT agent for both superficial diseases and solid tumors.

A series of bis(arylidene)cycloalkanone compounds based on cyclobutanone, cyclopentanone, cyclohexanone and cycloheptanone, C4–C7, respectively, with a D-π-A-π-D structure containing the same donor and acceptor but different alicyclic rings was prepared. The effects of alicyclic ring size on the photophysical, photochemical and electrochemical properties of these compounds were investigated systematically. We found that an increase of the number of carbons in the central alicyclic ring leads to changes in geometry, which has significant effects on the conjugation, and photophysical and photochemical properties. These effects include decreases in the fluorescence quantum yield, transient lifetimes, peak extinction coefficients, and the singlet oxygen quantum yield with the increase of the ring size. The one-photon absorption spectra, the two-photon absorption (2PA) spectra, and the fluorescence spectra all show a hypsochromic shift with increasing ring size. The results of this study provide guidance for the design of new cycloketone-based D-π-A-π-D 2PA compounds for photopolymerization and photodynamic therapy applications.

Benzylidene cyclopentanone dyes have high photosensitization activity and large two-photon absorption (TPA) cross-section in near infrared region. To explore their application potentials in two-photon excited photodynamic therapy (TPE-PDT), a series of carboxylate modified benzylidene cyclopentanone dyes were synthesized by varying number of carboxylate groups. Their linear and nonlinear photophysical properties, reactive oxygen yields and in vitro PDT activities were investigated systematically. The results showed that the water solubility of these dyes was significantly enhanced after introducing carboxylate groups. All dyes exhibited large TPA cross-section around 1000–1400 GM at 840 nm. In addition, they could generate both superoxide anion radical and singlet oxygen species under illumination. However, only the dye Y1 modified by one carboxylate group presented strong PDT activity to human rectal cancer 1116 cells, which indicated that moderate water–lipid amphipathy was a very important factor for PDT dyes. Finally, Y1 was proved to have large potential in TPE-PDT by in vitro experiments.Highlights► Four water soluble benzylidene cyclopentanone dyes were synthesized by introducing carboxylate group. ► All dyes could effectively generate reactive oxygen species through both Type I and Type II mechanisms. ► Their maximum TPA cross-sections were all above 1000 GM at 840 nm. ► The dye modified by only one carboxylate group presented strong one- and two-photon excited PDT activities to human rectal cancer 1116 cells. ► The water–lipid amphipathy is a very important factor to maintain the PDT activity of a dye.

A series of multibranched benzylidene cyclopentanone dyes with non-conjugated central moiety were synthesized. Their one- and two-photon photophysical properties were systematically studied in comparison with corresponding triphenylamine derivatives. Electronic coupling and vibronic coupling contributions to cooperative enhancement of two-photon absorption in multibranched structure were revealed. The results showed that the crucial contribution for this enhancement was from electronic coupling effect among branches. In addition, no contribution of vibronic coupling was observed. Furthermore, multibranched effect on photosensitizing efficiency of these compounds was investigated by one-photon polymerization and the result indicated the electronic coupling effect was also beneficial to their photosensitizing efficiency.

A novel water-soluble benzylidene cyclopentanone dye was synthesized by introducing four sodium carboxylate groups into 2,5-Bis-[4-(diethylamino)-benzylidene]-cyclopentanone (BDEA). Its two-photon absorption (TPA) cross-section in water solution was determined by nonlinear transmission method with Ti:sapphire femtosecond laser as exciting light source. 287 GM at 800 nm was obtained, which was quite larger than TPA cross-section of water-soluble xanthene dyes. Using this new dye as initiator, two-photon polymerization (TPP) of water-soluble acrylate could be induced directly. Low threshold energy of 0.51 mW and a high resolution of 400 nm were achieved, indicating this dye would have extensive application prospects in TPP under aqueous environments.

A series of ketocoumarin derivatives were synthesized using Aldol condensation and Wittig reactions and their one- and two-photon photophysical properties were investigated. Two-photon absorption cross-sections (δ) of the derivatives revealed that most of the new compounds possessed δ values that were some one to two orders of magnitudes larger than those of commercial coumarin dyes. The largest δ value within measured range was obtained as 1570 GM, which is comparable to that of reported strong two-photon absorption compounds. These novel derivatives containing the functional ketocoumarin group should open new opportunities for two-photon polymerization.

Two malononitrile derivatives, 2-[2,5-bis-(4-dimethylamino-benzylidene)-cyclopentylidene]-malononitrile (1) and 2-{2,6-bis-[2-(4-dimethylamino-phenyl)-vinyl]-pyran-4-ylidene}-malononitrile (2), were synthesized and their photophysical characteristics were investigated using UV–vis spectra, fluorescence spectra, nonlinear transmission and two-photon excited spectra. The results showed that both compounds exhibited strong intramolecular charge transfer absorption bands within 400–600 nm, but only compound 2 presented a strong fluorescence emission. By nonlinear transmission method, the two-photon absorption cross-section (σ) values of two compounds were determined as 250 GM and 283 GM, respectively, with 800 nm femtosecond laser pulses. By two-photon excited fluorescence method, the two-photon excited spectra of compound 2 within 720–880 nm was measured and its σmax was obtained as 773 GM at 770 nm.

A series of stilbene modified asymmetric ketocoumarin derivatives (compounds 1–4) with varied electron-donating terminal groups used on two-photon photopolymerization were reported. Solvatochromism analysis showed that the compounds with strong electron-donating terminal groups exhibited larger dipole moment change than those with weak electron-donating groups upon excitation. In addition, these compounds with strong electron-donating groups presented excellent two-photon absorption properties. Their two-photon absorption cross-sections are two magnitudes larger than those of common UV–vis sensitive photosensitizers. The results of photobleaching and one-photon polymerization experiments also showed that these compounds had high photoactivity on photosensitized photolysis of o-chloro hexaarylbiimidazoles (o-Cl-HABI) and 4,4′-dimethyldiphenyliodinium hexafluorophosphate (DPI) to generate radicals. As an example, the combination of compound 4 with o-Cl-HABI, an efficient two-photon photopolymerization initiation system, has been preliminarily demonstrated for fabrication of 3D micro-structures.

Multibranched benzylidene cyclopentanone dyes with a triphenylamine core were synthesized. Their two-photon optical properties were characterized using a Ti:sapphire femtosecond laser. The results showed that the two-photon absorption cross-sections (δ) of these compounds were increased with increasing numbers of branches. The largest δ value of the three-branched compound 3 was 3298 GM which was almost six times larger than that of a model compound, 2,5-bis-[4-(dimethylamino)benzylidene]cyclopentanone (BDMA). The sensitizing efficiencies of three new dyes in photopolymerization systems were all higher than that of BDMA matching with the commercial initiator 4,4′-dimethyldiphenyliodinium hexafluorophosphate (Omnicat 820). The two-photon polymerization initiated by the bimolecular system composed of compound 3 and Omnicat 820 was investigated under femtosecond laser pulses.

Novel multi-branched two-photon absorbing dyes containing highly efficient UV–vis curing initiator, ketocoumarin (3-acetyl-7-diethylaminocoumarin), were synthesized. There linear and non-linear optical properties were studied and the cooperatively enhanced two-photon absorption of two- and three-branched dyes were confirmed by femtosecond laser pulses. The largest two-photon absorption cross-section was obtained as 1117 GM. The results of photobleaching experiments showed that all dyes had very fast electron transferring speed with the commercial coinitiator o-Cl-hexaarylbisimidazoles (HABI). The two-photon polymerization initiated by a bimolecular system composed of the two-branched dye and HABI was investigated. This photopolymer system presented high photoinitiating efficiency. The single-shot two-photon exposure of the resin film was achieved with a threshold as 1 TW/cm2 at 800 nm.

•Three water-soluble dyes T1-T3 with high initiation efficiency and relatively large 2PA cross-section were synthesized.•Much lower threshold energies were obtained in the formulations containing T1-T3 as PIs compared with commercial ones.•A favorable biocompatibility of T1-T3 indicated their application prospects in 3D fabrication of biomaterials.A series of water-soluble benzylidene cyclanone dyes T1-T3 were synthesized. Their photo-physical properties were investigated using UV–Vis spectra, fluorescence spectra and two-photon absorption spectra. The maximum two-photon absorption cross-sections (σ2) of T1-T3 in deionized water were determined as 567 GM, 808 GM and 231 GM. Using T1-T3 as photoinitiators (PIs) directly, 2D and 3D nano patterns based on two-photon crosslinking or polymerization of bovine serum albumin (BSA), water-soluble acrylic ester monomer (SR610) and hyaluronic acid derivative (HAGM) were successfully fabricated, respectively. Much lower threshold energies and wider fabrication windows were obtained in the formulations containing these PIs compared with commercial ones. Furthermore, the cytotoxicity study showed a favorable biocompatibility of these PIs, which indicated T1-T3 would have application prospects in 3D fabrication of biomaterials.

A series of bis(arylidene)cycloalkanone compounds based on cyclobutanone, cyclopentanone, cyclohexanone and cycloheptanone, C4–C7, respectively, with a D-π-A-π-D structure containing the same donor and acceptor but different alicyclic rings was prepared. The effects of alicyclic ring size on the photophysical, photochemical and electrochemical properties of these compounds were investigated systematically. We found that an increase of the number of carbons in the central alicyclic ring leads to changes in geometry, which has significant effects on the conjugation, and photophysical and photochemical properties. These effects include decreases in the fluorescence quantum yield, transient lifetimes, peak extinction coefficients, and the singlet oxygen quantum yield with the increase of the ring size. The one-photon absorption spectra, the two-photon absorption (2PA) spectra, and the fluorescence spectra all show a hypsochromic shift with increasing ring size. The results of this study provide guidance for the design of new cycloketone-based D-π-A-π-D 2PA compounds for photopolymerization and photodynamic therapy applications.