Experimental and theoretical study on the excited-state dynamics of ortho-, meta-, and para-methoxy methylcinnamate
JChemPhys_2014_141_244313.pdf 2.66 MB
The S1 state dynamics of methoxy methylcinnamate (MMC) has been investigated under supersonic jet-cooled conditions. The vibrationally resolved S1-S0 absorption spectrum was recorded by laser induced fluorescence and mass-resolved resonant two-photon ionization spectroscopy and separated into conformers by UV-UV hole-burning (UV-UV HB) spectroscopy. The S1 lifetime measurements revealed different dynamics of para-methoxy methylcinnamate from ortho-methoxy methylcinnamate and meta-methoxy methylcinnamate (hereafter, abbreviated as p-, o-, and m-MMCs, respectively). The lifetimes of o-MMC and m-MMC are on the nanosecond time scale and exhibit little tendency of excess energy dependence. On the other hand, p-MMC decays much faster and its lifetime is conformer and excess energy dependent. In addition, the p-MMC-H2O complex was studied to explore the effect of hydration on the S1 state dynamics of p-MMC, and it was found that the hydration significantly accelerates the nonradiative decay. Quantum chemical calculation was employed to search the major decay route from S1(ππ*) for three MMCs and p-MMC-H2O in terms of (i) trans → cis isomerization and (ii) internal conversion to the 1nπ* state. In o-MMC and m-MMC, the large energy barrier is created for the nonradiative decay along (i) the double-bond twisting coordinate (～1000 cm-1) in S1 as well as (ii) the linear interpolating internal coordinate (～1000 cm-1) from S1 to 1nπ? states. The calculation on p-MMC decay dynamics suggests that both (i) and (ii) are available due to small energy barrier, i.e., 160 cm-1 by the double-bond twisting and 390 cm-1 by the potential energy crossing. The hydration of p-MMC raises the energy barrier of the IC route to the S1/1nπ* conical intersection, convincing that the direct isomerization is more likely to occur.
The Journal of Chemical Physics
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American Institute of Physics
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Graduate School of Science