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| - Singlet Excited-State Dynamics of 5-Fluorocytosine and Cytosine: An Experimental andComputational Study
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| - The photophysics of singlet excited 5-fluorocytosine (5FC) was studied in steady-state and time-resolvedexperiments and theoretically by quantum chemical calculations. Femtosecond transient absorption measurements show that replacement of the C5 hydrogen of cytosine by fluorine increases the excited-state lifetimeby 2 orders of magnitude from 720 fs to 73 ± 4 ps. Experimental evidence indicates that emission in bothcompounds originates from a single tautomeric form. The lifetime of 5FC is the same within experimentaluncertainty in the solvents ethanol and dimethyl sulfoxide. The insensitivity of the S1 lifetime to the proticnature of the solvent suggests that proton transfer is not the principal quenching mechanism for the excitedstate. Excited-state calculations were carried out for the amino-keto tautomer of 5FC, the dominant speciesin polar environments, in order to understand its longer excited-state lifetime. CASSCF and CAS−PT2calculations of the excited states show that the minimum energy path connecting the minimum of the 1π,π*state with the conical intersection responsible for internal conversion has essentially the same energetics forcytosine and 5FC, suggesting that both bases decay nonradiatively by the same mechanism. The dramaticdifference in lifetimes may be due to subtle changes along the decay coordinate. A possible reason may bedifferences in the intramolecular vibrational redistribution rate from the Franck−Condon active, in-planemodes to the out-of-plane modes that must be activated to reach the conical intersection region.
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