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Photocatalytic CO2 reduction is one of the best solutions to solve the global energy crisis and to realize carbon neutralization. The tetradentate phosphine‐bipyridine (bpy)‐phosphine (PNNP)‐type Ir(III) photocatalyst, Mes‐IrPCY2, was reported with a high HCOOH selectivity but the photocatalytic mechanism remains elusive. Herein, we employ electronic structure methods in combination with radiative, nonradiative, and electron transfer rate calculations, to explore the entire photocatalytic cycle to either HCOOH or CO, based on which a new mechanistic scenario is proposed. The catalytic reduction reaction starts from the generation of the precursor metal‐to‐ligand charge transfer (3MLCT) state. Subsequently, the divergence happens from the 3MLCT state, the single electron transfer (SET) and deprotonation process lead to the formation of one‐electron‐reduced species and Ir(I) species, which initiate the reduction reaction to HCOOH and CO, respectively. Interestingly, the efficient occurrence of proton or electron transfer reduces barriers of critical steps. In addition, nonadiabatic transitions play a nonnegligible role in the cycle. We suggest a lower free‐energy barrier in the reaction‐limiting step and the very efficient SET in 3MLCT are cooperatively responsible for a high HCOOH selectivity. The gained mechanistic insights could help chemists to understand, regulate, and design photocatalytic CO2 reduction reaction of similar function‐integrated molecular photocatalyst. [ABSTRACT FROM AUTHOR]

A dinuclear Pt(II) compound was reported to exhibit thermally activated delayed fluorescence (TADF); however, the luminescence mechanism remains elusive. To reveal relevant excited‐state properties and luminescence mechanism of this Pt(II) compound, both density function theory (DFT) and time‐dependent DFT (TD‐DFT) calculations were carried out in this work. In terms of the results, the S1 and T2 states show mixed intraligand charge transfer (ILCT)/metal‐to‐ligand CT (MLCT) characters while the T1 state exhibits mixed ILCT/ligand‐to‐metal CT (LMCT) characters. Mechanistically, a four‐state (S0, S1, T1, and T2) model is proposed to rationalize the TADF behavior. The reverse intersystem crossing (rISC) process from the initial T1 to final S1 states involves two up‐conversion channels (direct T1→S1 and T2‐mediated T1→T2→S1 pathways) and both play crucial roles in TADF. At 300 K, these two channels are much faster than the T1 phosphorescence emission enabling TADF. However, at 80 K, these rISC rates are reduced by several orders of magnitude and become very small, which blocks the TADF emission; instead, only the phosphorescence is observed. These findings rationalize the experimental observation and could provide useful guidance to rational design of organometallic materials with superior TADF performances. [ABSTRACT FROM AUTHOR]

Developing Type‐I photosensitizers is considered as an efficient approach to overcome the deficiency of traditional photodynamic therapy (PDT) for hypoxic tumors. However, it remains a challenge to design photosensitizers for generating reactive oxygen species by the Type‐I process. Herein, we report a series of α,β‐linked BODIPY dimers and a trimer that exclusively generate superoxide radical (O2−.) by the Type‐I process upon light irradiation. The triplet formation originates from an effective excited‐state relaxation from the initially populated singlet (S1) to triplet (T1) states via an intermediate triplet (T2) state. The low reduction potential and ultralong lifetime of the T1 state facilitate the efficient generation of O2−. by inter‐molecular charge transfer to molecular oxygen. The energy gap of T1‐S0 is smaller than that between 3O2 and 1O2 thereby precluding the generation of singlet oxygen by the Type‐II process. The trimer exhibits superior PDT performance under the hypoxic environment. [ABSTRACT FROM AUTHOR]

We have taken (dA), (dT), and (dA)·(dT) as model systems to study concerted effects of base pairing and stacking on excited-state nature of DNA oligonucleotides using density functional theory (DFT) and time dependent DFT methods. The spectroscopic states are determined to be of a partial → charge-transfer nature in the oligonucleotides. The → charge-transfer transitions produce dark states, which are hidden in the energy region of the steady-state absorption spectra. This is different from the previous assignment that the → charge-transfer transition is responsible for a shoulder at the red side of the first strong absorption band. The → charge-transfer states were predicted to have relatively high energies in the oligonucleotides. The present calculations predict that the → charge-transfer states are not involved in the spectra and excited-state dynamics of the oligonucleotides. In addition, the influence of base pairing and stacking on the nature of the nπ* and ππ* states are discussed in detail. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011 [ABSTRACT FROM AUTHOR]