A combination of spectroscopic methods and density functional theory (DFT) computations was used to study the excited state proton transfer (ESPT) processes of (8-bromo-7-hydroxyquinolin-2-yl)methyl-protected phenol (BHQ-OPh). Characterization of the prototropic forms of BHQ-OPh in different solvent environments revealed that the neutral form predominates in acetonitrile and in 1 : 1 acetonitrile/water (pH 5.0), whereas the anionic form predominates in 1 : 1 acetonitrile/PBS (pH 7.4). Both the neutral and anionic forms were significantly populated in 1 : 1 acetonitrile/water. Upon irradiation in acetonitrile the triplet neutral form was observed, whereas the triplet anionic form was detected in 1 : 1 acetonitrile/PBS (pH 7.4). The existence of the triplet tautomeric form of BHQ-OPh in both 1 : 1 acetonitrile/water and 1 : 1 acetonitrile/water (pH 5.0), and the ESPT processes from the neutral to the anionic to the tautomeric forms in the excited state were observed using time-resolved spectroscopy. A reaction mechanism in 1 : 1 acetonitrile/water and 1 : 1 acetonitrile/water (pH 5.0) was proposed based on the spectroscopic and DFT computational results. A comparison of the results for BHQ-OPh with those of BHQ-OAc reveals that the initial prototropic states and photochemical processes are similar. The understanding gained of the initial photo-induced processes of BHQ-based photoremovable protecting groups (PPGs) is useful for the design of new quinolinyl-based PPGs for specialized applications.

The excited nπ* and ππ* triplets of two benzophenone (BP) and two anthraquinone (AQ) derivatives have been observed in acetonitrile, isopropanol, and mixed aqueous solutions using time-resolved resonance Raman spectroscopic and nanosecond transient absorption experiments. These experimental results, combined with results from density functional theory calculations, reveal the effects of solvent and substituents on the properties, relative energies, and chemical reactivities of the nπ* and ππ* triplets. The triplet nπ* configuration was found to act as the reactive species for a subsequent hydrogen atom transfer reaction to produce a ketyl radical intermediate in the isopropanol solvent, while the triplet ππ* undergoes a proton-coupled electron transfer (PCET) in aqueous solutions to produce a ketyl radical intermediate. This PCET reaction, which occurs via a concerted proton transfer (to the excited carbonyl group) and electron transfer (to the excited phenyl ring), can account for the experimental observation by several different research groups over the past 40 years of the formation of ketyl radicals after photolysis of a number of BP and AQ derivatives in aqueous solutions, although water is considered to be a relatively “inert” hydrogen-donor solvent.

Recent studies conducted on some “meta effect” photochemical reactions focused on aromatic carbonyls having a substitution on one meta position of the benzophenone (BP) and anthraquinone parent compound. In this paper, two different substitutions were introduced with one at each meta position of the BP parent compound to investigate possible competition between different types of meta effect photochemistry observed in acidic solutions containing water. The photochemical pathways of 3-hydroxymethyl-3′-fluorobenzophenone (1) and 3-fluoro-3′-methylbenzophenone (2) were explored in several solvents, including acidic water-containing solutions, using time-resolved spectroscopic experiments and density functional theory computations. It is observed that 1 can undergo a photoredox reaction and 2 can undergo a meta-methyl deprotonation reaction in acidic water-containing solutions. Comparison of these results to those previously reported for the analogous BP derivatives that contain only one substituent at a meta position indicates the introduction of electron-donating (such as hydroxyl) and electron-withdrawing groups (such as F) on the meta positions of BP can influence the meta effect photochemical reactions. It was found that involvement of an electron-donating moiety facilitates the meta effect photochemical reactions by stabilizing the crucial reactive biradical intermediate associated with the meta effect photochemical reactions.

The experimental and theoretical results in neutral aqueous solutions reported here indicate that a proton-coupled electron transfer (PCET) from an alcohol C–H bond to the para-carbonyl is the initial and crucial process for the photoredox reaction of 2-(1-hydroxyethyl)-anthraquinone (HEAQ) to occur while the counterpart 3-(hydroxymethyl)-benzophenone (3-BPOH) compound displays a different PCET from an alcohol O–H bond to the carbonyl as the first step, followed by an intersystem crossing process that does not lead to the analogous photoredox, which is caused by a subtle charge-radical coupled effect between HEAQ and 3-BPOH. This can account for experimental results in the literature that HEAQ can undergo efficient photoredox but 3-BPOH does not under neutral aqueous conditions. These results have implications for the pH-dependent photochemical behavior of aromatic carbonyl compounds in aqueous media.

The efficient photosubstitution reaction of m-fluorobenzophenone and the related photohydration reactions were systematically investigated in acidic aqueous solutions. The mechanisms and intermediates were directly characterized by femtosecond transient absorption spectroscopy and nanosecond time-resolved resonance Raman spectroscopy, which is supported by density functional theory calculations. This photosubstitution was found to be a two-step process, based on the observation of a meta-hydration intermediate. The protonation of the ketone was confirmed as a crucial precursor step for further photochemical reactions as indicated by the observation of the absorption spectrum of an excited triplet protonated species. More interestingly, the efficient photosubstitution reaction could selectively occur under specific conditions. Control experiments on a series of halogen-substituted benzophenones were conducted to study the influence of the solution acidity, substituent positions, and the kind of substituted halogens on the efficiency in forming the corresponding hydroxyl photosubstitution product. Some practical conditions in predicting the efficiency of the photosubstitution reaction of interest are summarized, and they were successfully used to predict when the photosubstitution reaction takes place for some other halogen-substituted benzophenone derivatives. The driving force of this photosubstitution reaction may provide insights into several possible applications which are also briefly discussed.