title: Communication: the dark singlet state as a doorway state in the ultrafast and efficient intersystem crossing dynamics in 2-thiothymine and 2-thiouracil.
abstract: Femtosecond broadband transient absorption experiments are reported for 2-thiothymine and 2-thiouracil in aqueous buffer solution and in acetonitrile. It is shown that the S1(nπ*) state acts as a doorway state in the ultrafast and efficient population of the T1(ππ*) state upon 316 nm excitation. A sequential kinetic model is presented to explain the excited-state dynamics in 2-thiothymine and 2-thiouracil upon UVA excitation: S2(ππ*) → S1(nπ*) → T1(ππ*). The experimental results are also used to scrutinize the excited-state relaxation pathways recently predicted for 2-thiouracil at the CASPT2//CASSCF level of theory [G. Cui and W. Fang, J. Chem. Phys. 138, 044315 (2013)]. The efficient population of the T1(ππ*) state for both 2-thiothymine and 2-thiouracil in a few hundreds of femtoseconds lends further support to the emerging idea that thiobase derivatives exhibit photo-toxic properties that can be effectively harnessed in photo-chemotherapeutic applications.
authors.affiliation: Department of Chemistry and Center for Chemical Dynamics, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, USA.
authors.forename: Marvin
authors.initials: M
authors.lastname: Pollum
authors.affiliation: Department of Chemistry and Center for Chemical Dynamics, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, USA.
authors.forename: Carlos E
authors.initials: CE
authors.lastname: Crespo-Hernández
mesh_terms: D000097:Acetonitriles / Q000737:chemistry; D002021:Buffers; D004583:Electrons; D007700:Kinetics; D011789:Quantum Theory; D013056:Spectrophotometry, Ultraviolet; D013889:Thiouracil / Q000737:chemistry*; D013941:Thymine / Q000031:analogs & derivatives* / Q000737:chemistry
chemical_list: D000097:Acetonitriles; D002021:Buffers; D013889:Thiouracil; C101233:2-thiothymine; D013941:Thymine; C032159:acetonitrile