Acceleration of Photoinduced Electron Transfer by Modulating Electronegativity of Substituents in Stable Zr-Metal-Organic Frameworks to Boost Photocatalytic CO Reduction.

Photoreduction of CO with water into chemical feedstocks of fuels provides a green way to help solve both the energy crisis and carbon emission issues. Metal-organic frameworks (MOFs) show great potential for CO photoreduction. However, poor water stability and sluggish charge transfer could limit their application.

Pt-S Bond-Enabled Temperature-Dependent Phosphorescence in S-Heteroaryl Tetradentate Pt(S^C^N^O) Complexes.

This study presents the rare examples of S-heteroaryl tetradentate Pt(S^C^N^O) luminescent complexes ( and ) containing a Pt-S bond. The presence of the Pt-S bond allows the novel Pt(S^C^N^O) complexes to exhibit temperature-dependent phosphorescent emission behavior. The exhibits dual-emission phenomena and biexponential transient decay spectra above 250 K, indicating the presence of two minimal excited states in the potential energy surface (PES) of the T state.

Quantum mechanics/molecular mechanics studies on the excited-state decay mechanisms of cytidine aza-analogues: 5-azacytidine and 2'-deoxy-5-azacytidine in aqueous solution.

The excited state properties and deactivation pathways of two DNA methylation inhibitors, , 5-azacytidine (5ACyd) and 2'-deoxy-5-azacytidine (5AdCyd) in aqueous solution, are comprehensively explored with the QM(CASPT2//CASSCF)/MM protocol. We systematically map the feasible decay mechanisms based on the obtained excited-state decay paths involving all the identified minimum-energy structures, conical intersections, and crossing points driving the different internal conversion (IC) and intersystem crossing (ISC) routes in and between the ππ*, nπ*, ππ*, nπ*, and S states. Unlike the nπ* state below the ππ* state in 5ACyd, deoxyribose group substitution at the N1 position leads to the ππ* state becoming the S state in 5AdCyd.

Understanding the Excited-State Relaxation Mechanisms of Xanthophyll Lutein by Multi-configurational Electronic Structure Calculations.

The contradictory behaviors in light harvesting and non-photochemical quenching make xanthophyll lutein the most attractive functional molecule in photosynthesis. Despite several theoretical simulations on the spectral properties and excited-state dynamics, the atomic-level photophysical mechanisms need to be further studied and established, especially for an accurate description of geometric and electronic structures of conical intersections for the lowest several electronic states of lutein. In the present work, semiempirical OM2/MRCI and multi-configurational restricted active space self-consistent field methods were performed to optimize the minima and conical intersections in and between the 1Ag, 2Ag, 1Bu, and 1Bu states.

Quantum mechanics/molecular mechanics studies on mechanistic photophysics of cytosine aza-analogues: 2,4-diamino-1,3,5-triazine and 2-amino-1,3,5-triazine in aqueous solution.

The excited-state properties and photophysics of cytosine aza-analogues, , 2,4-diamino-1,3,5-triazine (2,4-DT) and 2-amino-1,3,5-triazine (2-AT) in solution have been systematically explored using the QM(MS-CASPT2//CASSCF)/MM approach. The excited-state nonradiative relaxation mechanisms for the initially photoexcited S(ππ*) state decay back to the S state are proposed in terms of the present computed minima, surface crossings (conical intersections and singlet-triplet crossings), and excited-state decay paths in the S, S, T, T, and S states. Upon photoexcitation to the bright S(ππ*) state, 2,4-DT quickly relaxes to its S minimum and then overcomes a small energy barrier of 5.

Quantum mechanics/molecular mechanics studies on excited state decay pathways of 5-azacytosine in aqueous solution.

In this work, we have used the QM(CASPT2//CASSCF)/MM approach to study the photophysical properties and relaxation mechanism of 5-azacytosine (5-AC) in aqueous solution. Based on the relevant minimum-energy structures and intersection structures, and excited-state decay paths in the S, S, T, T, and S states, several feasible excited-state nonradiative decay channels from the initially populated S(ππ*) state are proposed. Two major channels are singlet-mediated nonradiative pathways, in which the S system will internally convert (IC) to the S state directly or mediated by the nπ* state a ππ*/nπ* conical intersection.

Quantum mechanics/molecular mechanics studies on the mechanistic photophysics of sunscreen oxybenzone in methanol solution.

Herein, we have employed the QM(CASPT2//CASSCF)/MM method to explore the photophysical and photochemical mechanism of oxybenzone (OB) in methanol solution. Based on the optimized minima, conical intersections and crossing points, and minimum-energy reaction paths related to excited-state intramolecular proton transfer (ESIPT) and excited-state decay paths in the ππ*, nπ*, ππ*, nπ*, and S states, we have identified several feasible excited-state relaxation pathways for the initially populated S(ππ*) state to decay to the initial enol isomer' S state. The major one is the singlet-mediated and stretch-torsion coupled ESIPT pathway, in which the system first undergoes an essentially barrierless ππ* ESIPT process to generate the ππ* keto species, and finally realizes its ground state recovery through the subsequent carbonyl stretch-torsion facilitating S → S internal conversion (IC) and the reverse ground-state intramolecular proton transfer (GSIPT) process.

Theoretical Studies on the Excited-State Decay Mechanism of Homomenthyl Salicylate in a Gas Phase and an Acetonitrile Solution.

Here, we employ the CASPT2//CASSCF and QM(CASPT2//CASSCF)/MM approaches to explore the photochemical mechanism of homomenthyl salicylate (HMS) in vacuum and an acetonitrile solution. The results show that in both cases, the excited-state relaxation mainly involves a spectroscopically "bright" S(ππ*) state and the lower-lying T and T states. In the major relaxation pathway, the photoexcited S keto system first undergoes an essentially barrierless excited-state intramolecular proton transfer (ESIPT) to generate the S enol minimum, near which a favorable S/S conical intersection decays the system to the S state followed by a reverse ground-state intramolecular proton transfer (GSIPT) to repopulate the initial S keto species.

Mechanistic Photophysics of Tellurium-Substituted Uracils: Insights from Multistate Complete-Active-Space Second-Order Perturbation Calculations.

The photophysical mechanisms of tellurium-substituted uracils were studied at the multistate complete-active-space second-order perturbation level with a particular focus on how the position and number of tellurium substitutions affect their nonadiabatic relaxation processes. Electronic structure analysis reveals that the lowest several excited states are closely concerned with the n and π orbitals at the Te-C [Te-C] moiety of 2-tellurouracil (2TeU) [4TeU and 24TeU]. Both planar and twisted minima were optimized for 2TeU, whereas only planar ones were obtained for 4TeU and 24TeU, except for a twisted T minimum of 4TeU.

Hydrogen-Bond Network Determines the Early Photoisomerization Processes of Cph1 and AnPixJ Phytochromes.

Phytochrome proteins are light receptors that play a pivotal role in regulating the life cycles of plants and microorganisms. Intriguingly, while cyanobacterial phytochrome Cph1 and cyanobacteriochrome AnPixJ use the same phycocyanobilin (PCB) chromophore to absorb light, their excited-state behavior is very different. We employ multiscale calculations to rationalize the different early photoisomerization mechanisms of PCB in Cph1 and AnPixJ.

MS-CASPT2 studies on the mechanistic photophysics of tellurium-substituted guanine and cytosine.

Sulfur-substituted nucleobases are highly promising photosensitizers that are widely used in photodynamic therapy, and there are numerous studies exploring their unique photophysical behaviors. However, relevant photophysical investigations on selenium and tellurium substitutions are still rare. Herein, the high-level multistate complete-active-space second-order perturbation (MS-CASPT2) method was performed for the first time to explore the excited-state relaxation processes of tellurium-substituted guanine (TeG) and cytosine (TeC).

High-Lying 3A Dark-State-Mediated Singlet Fission.

Singlet fission (SF), the conversion of one high-energy singlet to two low-energy triplets, provides the potential to increase the efficiency of photovoltaic devices. In the SF chromophores with symmetry, exemplified by polyenes, singlet-to-triplet conversion generally involves a low-lying 2A dark state, which serves as either a multiexciton (ME) intermediate to promote the SF process or a parasitic trap state to shunt excited-state populations via internal conversion. This controversial behavior calls for a deep understanding of dark-state-related photophysics involving the higher-lying singlet state.

Quantum Mechanics/Molecular Mechanics Studies on the Photophysical Mechanism of Methyl Salicylate.

Methyl salicylate (MS) as a subunit of larger salicylates found in commercial sunscreens has been shown to exhibit keto-enol tautomerization and dual fluorescence emission via excited-state intramolecular proton transfer (ESIPT) after the absorption of ultraviolet (UV) radiation. However, its excited-state relaxation mechanism is unclear. Herein, we have employed the quantum mechanics(CASPT2//CASSCF)/molecular mechanics method to explore the ESIPT and excited-state relaxation mechanism of MS in the lowest three electronic states, that is, S, S, and T states, in a methanol solution.

Organic Thermometers Based on Aggregation of Difluoroboron β-Diketonate Chromophores.

We report several novel thermometers resulting from the temperature-induced aggregation of difluoroboron β-diketonate chromophores. These thermometers exhibit a much wider temperature-dependent fluorescence emission from 445 to 592 nm along with the color change from blue to red in a dilute chloroform solution. Spectroscopy measurements and theoretical calculations confirm that the thermochromic luminescence originates from the reversible change in the noncovalent intermolecular interactions and the abrupt volume shrinkage of the solvent at its melting point.

Theoretical studies on the photochemistry of 2-nitrofluorene in the gas phase and acetonitrile solution.

The photophysical and photochemical mechanisms of 2-nitrofluorene (2-NF) in the gas phase and acetonitrile solution have been studied theoretically. Upon ∼330 nm irradiation to the first bright state (ππ*), the 2-NF system can decay to triplet excited states via rapid intersystem crossing (ISC) processes through different surface crossing points or to the ground state via an ultrafast internal conversion (IC) process through the S/S conical intersection. The nπ* dark state will serve as a bridge when the system leaves the Franck-Condon (FC) region and approaches to the S minimum.

Selenium substitution effects on excited-state properties and photophysics of uracil: a MS-CASPT2 study.

The photophysics of selenium-substituted nucleobases has attracted recent experimental attention because they could serve as potential photosensitizers in photodynamic therapy. Herein, we present a comprehensive MS-CASPT2 study on the spectroscopic and excited-state properties, and photophysics of 2-selenouracil (2SeU), 4-selenouracil (4SeU), and 2,4-selenouracil (24SeU). Relevant minima, conical intersections, crossing points, and excited-state relaxation paths in the lowest five electronic states (i.

Nonadiabatic Dynamics Simulations on Early-Time Photochemistry of Spirobenzopyran.

Photoinduced ring-opening, decay, and isomerization of spirobenzopyran have been explored by the OM2/MRCI nonadiabatic dynamics simulations based on Tully's fewest-switches surface hopping scheme. The efficient S to S internal conversion as observed in experiments is attributed to the existence of two efficient excited-state decay pathways. The first one is related to the C-N dissociation, and the second one is done to the C-O dissociation.

Hydrogen-Bonding Interaction Regulates Photoisomerization of a Single-Bond-Rotation Locked Photoactive Yellow Protein Chromophore in Protein.

We have employed the QM(CASPT2//CASSCF)/MM method to explore the excited-state isomerization and decay mechanism of a single-bond-rotation locked photoactive yellow protein (PYP) chromophore in wild-type and mutant proteins. The S state is a spectroscopically bright state in the Franck-Condon region. In this state, there exist two excited-state isomerization pathways separately related to the clockwise and anticlockwise rotations of the C=C bond.

Stereoselective Excited-State Isomerization and Decay Paths in -Cyclobiazobenzene.

Herein, we have employed OM2/MRCI-based full-dimensional nonadiabatic dynamics simulations to explore the photoisomerization and subsequent excited-state decay of a macrocyclic cyclobiazobenzene molecule. Two S/S conical intersection structures are found to be responsible for the excited-state decay. Related to these two conical intersections, we found two stereoselective photoisomerization and excited-state decay pathways, which correspond to the clockwise and counterclockwise rotation motions with respect to the N═N bond of the azo group.

QM/MM studies on the excited-state relaxation mechanism of a semisynthetic dTPT3 base.

Semisynthetic alphabets can potentially increase the genetic information stored in DNA through the formation of unusual base pairs. Recent experiments have shown that near-visible-light irradiation of the dTPT3 chromophore could lead to the formation of a reactive triplet state and of singlet oxygen in high quantum yields. However, the detailed excited-state relaxation paths that populate the lowest triplet state are unclear.

Short-time dynamics and decay mechanism of 2(1H)-pyridinone upon excitation to the light-absorbing S(2𝝅𝝅) state.

The excited-state structural dynamics and the decay mechanism of 2(1H)-pyridinone (NHP) after excitation to the S(2ππ) light-absorbing state were studied using resonance Raman spectroscopy and complete-active space self-consistent field (CASSCF) calculations. The B-band absorption cross-section and the corresponding absolute resonance Raman cross-sections were simulated using a simple model based on time-dependent wave-packet theory. The geometric structures of the singlet electronic excited states and their curve-crossing points were optimized at the CASSCF level of theory.

Short-time dynamics of 2-thiouracil in the light absorbing S2(ππ(∗)) state.

Ultrahigh quantum yields of intersystem crossing to the lowest triplet state T1 are observed for 2-thiouracils (2TU), which is in contrast to the natural uracils that predominantly exhibit ultrafast internal conversion to the ground state upon excitation to the singlet excited state. The intersystem crossing mechanism of 2TU has recently been investigated using second-order perturbation methods with a high-level complete-active space self-consistent field. Three competitive nonadiabatic pathways to the lowest triplet state T1 from the initially populated singlet excited state S2 were proposed.