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On the Nature of the Long‐Lived “Dark” State of Isolated 1‐Methylthymine
Author(s) -
Kunitski Maksim,
Nosenko Yevgeniy,
Brutschy Bernhard
Publication year - 2011
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.201000985
Subject(s) - excited state , thymine , chemistry , photochemistry , ionization , dark state , ground state , relaxation (psychology) , triplet state , atomic physics , physics , ion , organic chemistry , dna , social psychology , biochemistry , psychology
The photoinduced excited‐state relaxation dynamics of gaseous thymine and 1‐methylthymine are studied by both femtosecond and nanosecond pump–probe ionization spectroscopy on the sub‐picosecond to microsecond timescale. A threefold exponential decay is observed with time constants of 80±40 fs, 4.8±2 ps, and 280±30 ns for thymine and 70±40 fs, 3.4±1.1 ps, and 310±30 ns for 1‐methylthymine using a 267 nm excitation and subsequent 800 nm multiphoton ionization. In addition, a vibrational spectrum in the NH stretch region of the long‐lived “dark” electronic state of isolated 1‐methylthymine is reported for the first time. This spectrum, in combination with the dependence of the dark‐state ionization rate on the laser intensity, allows assignment of the dark state of 1‐methylthymine to the lowest triplet state of the keto tautomer, thus excluding enol tautomers as well as the nπ* excited state and a hot electronic ground state from the consideration. Very similar excited‐state relaxation dynamics of thymine and 1‐methylthymine justify the conclusion that the long‐lived dark state of isolated thymine is also of triplet nature.