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Ultrafast Excited-State Dynamics of Hydrogen-Bonded Cytosine Microsolvated Clusters with Protic and Aprotic Polar Solvents
摘要： Microsolvation effects on the ultrafast excited-state deactivation dynamics of cytosine (Cy) were studied in hydrogen-bonded Cy clusters with protic and aprotic solvents using mass-resolved femtosecond pump-probe ionization spectroscopy. Two protic solvents, water (H2O) and methanol (MeOH), and one aprotic solvent, tetrahydrofuran (THF), were investigated, and transients of Cy·(H2O)1-6, Cy·(MeOH)1-3, and Cy·THF microsolvated clusters produced in supersonic expansions were measured. With the aid of electronic structure calculations, we assigned the observed dynamics to the low-energy isomers of various Cy clusters and discussed the microsolvation effect on the excited-state deactivation dynamics. With the protic solvents only the microsolvated clusters of Cy keto tautomer were observed. The observed decay time constants of Cy·(H2O)n are 0.5 ps for n=1 and ~0.2–0.25 ps for n=2–6. For Cy·(MeOH)n clusters, the decay time constant for n=1 cluster is similar to that of the Cy monohydrate, but for n=2 and 3 the decays are about a factor of two slower than the corresponding microhydrates. With the aprotic solvent, THF, hydrogen-bonded complexes of both keto and enol tautomers are present in the beam. The keto-Cy·THF shows a similar decay as the keto-Cy monomer, whereas the enol-Cy·THF exhibits a two-fold slower decay than the enol-Cy monomer, suggesting an increase in the barrier to excited-state deactivation upon binding of one THF molecule to the enol form of Cy.
关键词： Cytosine,Excited-state deactivation,Hydrogen-bonded clusters,Microsolvation,Ultrafast dynamics
更新于2025-09-23 15:21:01
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Solvation Controlled Excited-State Planarization in a Pusha??Pull Pyrene Dye
摘要： The excited state deactivation pathway of the push–pull pyrene derivative (EPP), a molecule emitting bright red fluorescence, has been studied in several solvents using quantum chemical calculations, femtosecond time-resolved fluorescence up-conversion spectroscopy and femtosecond transient absorption spectra (fs-TA). From the steady-state spectra, EPP has shown a strong negatively solvatochromic behavior of the absorption spectra and a non-solvatochromic emission spectra, revealing the decreased dipole moment upon excitation. Femtosecond up-conversion measurements have shown interesting rise-decay kinetics at red emission side and fs-TA spectra also feature the SE (Stimulated emission) formed and new ESA (Excited State Absorption) developed in all investigated solvents. The careful analysis of the data gathered from fluorescence up-conversion and fs-TA spectra, supported by DFT quantum chemical calculations and temperature-dependent fluorescence measurements, unambiguously pointed out the excited state relaxation pathway of EPP is controlled by solvent stabilized planar intramolecular charge transfer (PICT) dynamics. This finding confirms the surrounding solvents as a significant factor that affects the rate of excited state conformation changes.
关键词： solvation controlled,push–pull pyrene derivative,excited state deactivation,planar intramolecular charge transfer,femtosecond spectroscopy
更新于2025-09-19 17:13:59