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Theoretical study of photoreactions between oxidized pterins and molecular oxygen
摘要： Pterins are low molecular weight heterocyclic compounds that are widely distributed in living organisms, primarily in the form of reduced coenzyme forms. Oxidized pterins are present in the cells for the most part as oxidation products of tetrahydropterins and dihydropterins. Oxidized pterins are known to cause DNA photodamage under UV-irradiation. Photosensitization of oxidized pterins may cause oxidative stress in the human skin depigmentation disorder vitiligo. There have been made attempts to use oxidized pterins as sensitizers in photodynamic therapy (PDT) of cancer. The photoreactions between a set of six pterin compounds and molecular oxygen were explored using density functional theory, a time-dependent formalism for excited states (TD-DFT) and a continuum COSMO model to include the effects of H2O solvation. Both acid and base forms of pterins were taken into consideration. The computed singlet and triplet excitation energies are in agreement with the experimental data. We showed that compounds with the most electronegative lateral substituent (formyl, carboxyl) at C6 position have the highest value of both S0 and T1 state ionization potential. We conclude that pterin molecules do participate in photoreactions with molecular oxygen. Oxidized pterins are able to generate singlet oxygen and may also produce superoxide-anion radicals indirectly through autoionization reactions. Direct electron transfer reactions between pterins and oxygen were also studied.
关键词： TDDFT,DFT,Pterins,Singlet oxygen,Triplet states
更新于2025-09-23 15:23:52
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Progress toward third-order parametric down-conversion in optical fibers
摘要： Optical ?bers have been considered an optimal platform for third-order parametric down-conversion since they can potentially overcome the weak third-order nonlinearity by their long interaction length. Here we present, in the ?rst part, a theoretical derivation for the conversion rate both in the case of spontaneous generation and in the presence of a seed beam. Then we review three types of optical ?bers and we examine their properties in terms of conversion ef?ciency and practical feasibility.
关键词： optical ?bers,phase matching,nonlinearity,photon triplet states,third-order parametric down-conversion
更新于2025-09-23 15:19:57
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Spin-Controlled Charge Recombination Pathways across the Inorganic/Organic Interface
摘要： Charge transfer and recombination across the inorganic/organic interface in nanocrystal or quantum dot (QD)-molecule hybrid materials has been extensively studied. Principles of controlling charge transfer and recombination via energetics and electronic coupling have been established. However, the use of electron spin to control transfer and recombination pathways in such systems remains relatively underexplored. Here we use CdS QD-alizarin (AZ) as a model system to demonstrate this principle. Using time-resolved spectroscopy, we found that the charge separated states (QD--AZ+) created by selectively exciting AZ molecules mostly recombined to regenerate ground state complexes, whereas the apparently “same” charge separated states created by exciting QDs recombined to produce AZ molecular triplet states. Such a difference can be traced to the distinct spin configurations between excited QDs (QD*, with an ill-defined spin) and AZ (1AZ*, spin singlet) and the asymmetric electron and hole spin-flip rates in II-VI group QDs. The transferability of such a principle was confirmed by similar observations obtained for CdS QD-tetracene complexes. Opening an avenue of controlling charge transfer and recombination pathways via electron spin is potentially important for applications such as artificial photosynthesis.
关键词： CdS QD-alizarin,artificial photosynthesis,quantum dot,inorganic/organic interface,triplet states,electron spin,recombination,time-resolved spectroscopy,Charge transfer
更新于2025-09-23 15:19:57
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Screening mixing GW/Bethe–Salpeter approach for triplet states of organic molecules
摘要： We recently proposed a screening mixing many-body ansatz (Ziaei and Bredow 2017 Phys. Rev. B 96 195115) to decrease the typical overestimation of the Hartree–Fock based GW/Bethe–Salpeter equation (BSE) singlet–singlet excitation energies in molecular systems. Now we have evaluated the accuracy of the proposed scheme for triplet states of a set of 20 organic molecules known as the Thiel set. We show that by mixing different screenings into GW and BSE calculated within random phase approximation (in order to ensure best gap and an optimal exciton binding energy), the total mean absolute error of 0.59 eV in the standard Hartree–Fock based eigenvalue GW/BSE approach is reduced to 0.26 eV for 63 triplet states. We further demonstrate that the quasi-particle self-consistent GW/BSE approach in which orbitals and energies are updated in the Green’s function and the dynamically screened interaction mostly and considerably underestimates the excitation energies as shown for a few molecules.
关键词： modified electronic structure method,triplet states,optical properties
更新于2025-09-19 17:15:36
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Insights into the Efficient Intersystem Crossing of Bodipy-Anthracene Compact Dyads with Steady State and Time-Resolved Optical/Magnetic Spectroscopies and Observation of the Delayed Fluorescence
摘要： Spin-orbit charge transfer induced intersystem crossing (SOCT-ISC) is of particular interest for preparation of heavy atom-free triplet photosensitizers. Up to now examples for SOCT-ISC dyads are limited and electron donor/acceptor SOCT-ISC dyads showing strong visible light-harvesting are rare. Herein we studied the photophysics of a series of Bodipy-anthracene (BDP-An) compact dyads, especially the triplet state electron spin dynamics with the time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The electronic coupling matrix elements (VDA*) between the 1CT (charge transfer) state and 1LE (locally excited) state are in the range 773 cm?1 – 1545 cm?1. For one dyad, we observed three triplet states simultaneously with TREPR, i.e. the triplet states confined on the anthracene (3An) and the Bodipy (3BDP) moieties, as well as a 3CT state. Based on the electron spin polarization (ESP) of these three triplet states and the optical experiments, the SOCT-ISC mechanism is confirmed and the RP-ISC mechanism as the main ISC channel was excluded. Triplet-triplet annihilation (TTA) induced delayed fluorescence was observed for the dyads, which is rare for Bodipy fluorophores.
关键词： intersystem crossing,Bodipy-anthracene dyads,Spin-orbit charge transfer,time-resolved electron paramagnetic resonance,delayed fluorescence,triplet states
更新于2025-09-09 09:28:46