Planarizing cytosine: The S 1 state structure, vibrations, and nonradiative dynamics of jet-cooled 5,6-trimethylenecytosine

Trachsel, Maria A.; Lobsiger, Simon; Schär, Tobias; Blancafort, Lluís; Leutwyler, Samuel (2017). Planarizing cytosine: The S 1 state structure, vibrations, and nonradiative dynamics of jet-cooled 5,6-trimethylenecytosine. The Journal of Chemical Physics, 146(24), p. 244308. American Institute of Physics 10.1063/1.4989465

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We measure theS0→S1spectrum and time-resolvedS1state nonradiative dynamics of the “clamped”cytosine derivative 5,6-trimethylenecytosine (TMCyt) in a supersonic jet, using two-color resonanttwo-photon ionization (R2PI), UV/UV holeburning, and ns time-resolved pump/delayed ionization.The experiments are complemented with spin-component scaled second-order approximate cou-pled cluster (SCS-CC2), time-dependent density functional theory, and multi-state second-orderperturbation-theory (MS-CASPT2)ab initiocalculations. While the R2PI spectrum of cytosine breaksoff∼500 cm1above its 000band, that of TMCyt extends up to +4400 cm1higher, with over a hun-dred resolved vibronic bands. Thus, clamping the cytosine C5–C6bond allows us to explore theS1state vibrations andS0→S1geometry changes in detail. The TMCytS1state out-of-plane vibra-tionsν′1,ν′3, andν′5lie below 420 cm1, and the in-planeν′11,ν′12, andν′23vibrational fundamentalsappear at 450, 470, and 944 cm1.S0→S1vibronic simulations based on SCS-CC2 calculationsagree well with experiment if the calculatedν′1,ν′3, andν′5frequencies are reduced by a factorof 2–3. MS-CASPT2 calculations predict that the ethylene-typeS1S0conical intersection (CI)increases from +366 cm1in cytosine to>6000 cm1in TMCyt, explaining the long lifetime andextendedS0→S1spectrum. The lowest-energyS1S0CI of TMCyt is the “amino out-of-plane”(OPX) intersection, calculated at +4190 cm1. The experimentalS1S0internal conversion rateconstant at theS1(v′=0) level iskIC=0.98–2.2·108s1, which is∼10 times smaller than in1-methylcytosine and cytosine. TheS1(v′=0) level relaxes into theT1(3ππ∗) state by intersystemcrossing withkISC=0.41–1.6·108s1. TheT1state energy is measured to lie 24 580±560 cm1above theS0state. TheS1(v′=0) lifetime isτ=2.9 ns, resulting in an estimated fluorescencequantum yield ofΦfl=24%. Intense two-color R2PI spectra of the TMCyt amino-enol tautomersappear above 36 000 cm1. A sharpS1ionization threshold is observed for amino-keto TMCyt,yielding an adiabatic ionization energy of 8.114±0.002 eV.

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