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The photoinduced intramolecular charge separation (CS) and charge recombination (CR) phenomena in a series of donor-bridge-acceptor (D-B-A) molecules are intensively investigated as a means of understanding electron transport through the π-B. Pyrene (Pyr) and triarylamine (TAA) moieties connected via phenylene Bs of various lengths are studied because their CS and CR behaviors can be readily monitored in real time by femtosecond transient absorption (fs-TA) spectroscopy. By combining the steady-state and fs-TA spectroscopic measurements in a variety of solvents together with chemical calculations, the parameters that govern the CS behaviors of these dyads were obtained, such as the solvent effects on free energy and the B-length-dependent electronic coupling ( V DA ) between D and A. We observed the sharp switch of the CS behavior with the increase of the solvent polarity and B-linker lengths. Furthermore, in the case of the shortest distance between D and A when the electron coupling is sufficiently large, we observed that the CS phenomenon occurs even in low-polar solvents. Upon increasing the length of B, CS occurs only in strong polar solvents. The distance-dependent decay constant of the CS rate is determined as ∼0.53 Å -1 , indicating that CS is governed by superexchange tunneling interactions. The CS rate constants are also approximately estimated using Marcus electron transfer theory, and the results imply that the V DA value is the key factor dominating the CS rate, while the facile rotation of the phenylene B is important for modulating the lifetime of the charge-separated state in these D-B-A dyads. These results shed light on the practical strategy for obtaining a high CS efficiency with a long-lived CS state in TAA-B-Pyr derivatives.

In the past decades, tremendous efforts have been invested into organic molecules involved in the excited-state intramolecular proton transfer (ESIPT) reaction due to their enormously Stokes-shifted fluorescence and distinctive photophysical properties. The alterations of the environmental medium can effectively adjust the luminous performance of ESIPT molecules, which inspires us to unravel the solvent effect on the ESIPT mechanism. Here, we report the solvent-dependent excited-state properties of two new seven-membered ring pyrrole-indole ESIPT molecules, g -PPDBI and e -PPDBI , by steady-state spectra, picosecond transient fluorescence spectra, femtosecond transient absorption spectra, and theoretical calculations. The bathochromic-shifted normal fluorescence and the negligibly shifted tautomer fluorescence suggest the occurrence of an excited-state intramolecular proton-coupled charge transfer reaction. Thus, the solvent effect plays a vital role in stabilizing the intramolecular charge transferred state, resulting in a higher ESIPT reaction barrier in more polar solvents. Additionally, the observation of the slight dynamic difference between PPDBIs with different π-conjugation positions provides a new strategy to adjust the performance of ESIPT molecules.

Intensified research interests are posed with the thionucleobase 4-thiouracil (4-TU), due to its important biological function as site-specific photoprobe to detect RNA structures and nucleic acid-nucleic acid contacts. By means of time-resolved IR spectroscopy and density functional theory (DFT) studies, we have examined the unique photophysical and photochemical properties of 4-TU. It is shown that 4-TU absorbs UVA light and results in the triplet formation with a high quantum yield (0.9). Under N2-saturated anaerobic conditions, the reactive triplet undergoes mainly cross-linking, leading to the (5-4)/(6-4) pyrimidine-pyrimidone product. In the presence of O2 under aerobic conditions, the triplet 4-TU acts as an energy donor to produce singlet oxygen (1)O2 by triplet-triplet energy transfer. The highly reactive oxygen species (1)O2 then reacts readily with 4-TU, leading to the products of uracil (U) with a yield of 0.2 and uracil-6-sulfonate (U(SO3)) that is fluorescent at ~390 nm. The product formation pathways and product distribution are well rationalized by the joint B3LYP/6-311+G(d,p) calculations. From dynamics and mechanistic point of views, these results enable a further understanding for 4-TU acting as reactive precursors for photochemical reactions relevant to (1)O2, which has profound implications for photo cross-linking, DNA photodamage, as well as photodynamic therapy studies.

We have explored the potential energy profiles of TpT dinucleotides toward formation of a DNA photolesion product, spore photoproduct (SP), along the S(0), S(1), and T(1) states, by means of density functional theory and time-dependent density functional theory. Together with the spin density analysis, the consecutive mechanism for the SP formation can be established. The detailed reaction pathways have been revealed. All the adiabatic reaction pathways proceeding though S(1), T(1), or S(0) alone are shown to be energetically infeasible, while the nonadiabatic pathway involving both the T(1) and S(0) states corresponds to the lowest-energy path and is the most favorable in energy. The nonadiabatic pathway is rate-limited by the step of the hydrogen atom transfer proceeding in the T(1) state with a barrier of 14.2 kcal mol(-1) (11.9 kcal mol(-1) in bulk solution), whereas the subsequent C5-CH(2) bond formation toward the final SP formation occurs readily in S(0) after intersystem crossing from T(1) to S(0) via the singlet-triplet interaction. The results provide a rationale for the experimentally observed kinetic isotope effect after deuterium substitution at the 3'-T methyl group of TpT.