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Locating Cytosine Conical Intersections by Laser Experiments and <i>Ab Initio</i> Calculations
摘要： The decay mechanism of S1 excited cytosine (Cyt) and the effect of substitution are studied combining jet-cooled spectroscopy (nanosecond resonant two-photon ionization (R2PI) and picosecond lifetime measurements) with CASPT2//CASSCF computations for eight derivatives. For Cyt and five derivatives substituted at N1, C5, and C6, rapid internal conversion sets in at 250?1200 cm?1 above the 000 bands. The break-off in the spectra correlates with the calculated barriers toward the "C5?C6 twist" conical intersection, which unambiguously establishes the decay mechanism at low S1 state vibrational energies. The barriers increase with substituents that stabilize the charge shifts at C4, C5, and C6 following (1ππ*) excitation. The R2PI spectra of the clamped derivatives 5,6-trimethyleneCyt (TMCyt) and 1-methyl-TMCyt (1M-TMCyt), which decay along an N3 out-of-plane coordinate, extend up to +3500 and +4500 cm?1.
关键词： CASPT2//CASSCF computations,jet-cooled spectroscopy,internal conversion,substitution effect,R2PI spectra,decay mechanism,cytosine,conical intersection
更新于2025-09-23 15:19:57
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Unveiling the photophysics of thiourea from CASPT2/CASSCF potential energy surfaces and singlet/triplet excited state molecular dynamics simulations
摘要： This work describes the decay mechanism of photoexcited thiourea, both in gas phase and in solution, from the information inferred from the topography of the excited and ground state potential energy surfaces and mixed singlet/triplet quantum classical molecular dynamics simulations. Our gas phase results reveal T1/S0 intersystem crossing as the dominant (49%) intrinsic decay channel to the ground state, which reaches a population of 0.28 at the final time of our simulations (10 ps). Population of the T1, would occur after internal conversion to the S1 from the spectroscopic S2 electronic state, followed by S1->T2 intersystem crossing and T2->T1 internal conversion processes. Minor decay channels occurring exclusively along the singlet manifold, i.e. S2->S0 (33%) and S1->S0 (18%), were also observed to play a role in the relaxation of photoexcited thiourea in the gas phase. The explicit incorporation of water-thiourea interactions in our simulations was found to provoke a very significant delay in the decay to the ground state of the system, with no transitions to the S0 being registered during the first 10 ps of our simulations. Intermolecular vibrational energy redistribution and explicit hydrogen bond interaction established between water molecules and the NH2 group of thiourea were found to structurally or energetically hamper the access to the intersystem crossing or internal conversion funnels with the S0.
关键词： intersystem crossing,internal conversion,CASPT2 calculations,thiourea,molecular dynamics simulations
更新于2025-09-19 17:15:36
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Unveiling coupled electronic and vibrational motions of chromophores in condensed phases
摘要： The quest for capturing molecular movies of functional systems has motivated scientists and engineers for decades. A fundamental understanding of electronic and nuclear motions, two principal components of the molecular Schr?dinger equation, has the potential to enable the de novo rational design for targeted functionalities of molecular machines. We discuss the development and application of a relatively new structural dynamics technique, femtosecond stimulated Raman spectroscopy with broadly tunable laser pulses from the UV to near-IR region, in tracking the coupled electronic and vibrational motions of organic chromophores in solution and protein environments. Such light-sensitive moieties hold broad interest and significance in gaining fundamental knowledge about the intramolecular and intermolecular Hamiltonian and developing effective strategies to control macroscopic properties. Inspired by recent experimental and theoretical advances, we focus on the in situ characterization and spectroscopy-guided tuning of photoacidity, excited state proton transfer pathways, emission color, and internal conversion via a conical intersection.
关键词： emission color,excited state proton transfer,electronic and vibrational motions,chromophores,photoacidity,femtosecond stimulated Raman spectroscopy,conical intersection,internal conversion
更新于2025-09-11 14:15:04
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Mechanical Insights into Aggregation-Induced Delayed Fluorescence Materials with Anti-Kasha Behavior
摘要： Organic materials with aggregation-induced delayed fluorescence (AIDF) have exhibited impressive merits for improving electroluminescence efficiency and decreasing efficiency roll-off of nondoped organic light-emitting diodes (OLEDs). However, the lack of comprehensive insights into the underlying mechanism may impede further development and application of AIDF materials. Herein, AIDF materials consisting of benzoyl serving as an electron acceptor, and phenoxazine and fluorene derivatives as electron donors are reported. They display greatly enhanced fluorescence with increased delayed component upon aggregate formation. Experimental and theoretical investigations reveal that this AIDF phenomenon can be rationally ascribed to the suppression of internal conversion and the promotion of intersystem crossing in solid. Moreover, the theoretical calculations disclose that the efficient solid-state delayed fluorescence originates from the higher energy electronic excited state (e.g., S2) rather than the lowest energy-excited state (S1), demonstrating an anti-Kasha behavior. The excellent AIDF property allows high exciton utilization and thus superb performance of OLEDs using these new materials as light-emitting layers.
关键词： aggregation-induced delayed fluorescence,anti-Kasha emission,intersystem crossing,internal conversion,organic light-emitting diodes
更新于2025-09-09 09:28:46