第30回化学反応討論会, 2014年6月4日-6日, イーグレひめじ(姫路

Ebata, Takayuki

Ehara, M.

Inokuchi, Yoshiya

Miyazaki, Y.

Hiroshima University Institutional Repository

3ES03 S1 excited-state dynamics of OMpCA and its hydrogen-bonded complexes(Hiroshima Univ.1 IMS2) Y. Miyazaki1, Y. Inokuchi1, T. Ebata1 and M. Ehara2The photocycle of photoactive yellow protein (PYP) has drawn attention to understand thenegative phototaxis behavior of Halorhodospira halophila. Upon photoexcitation at 446 nm (λmax),p-coumaric acid embedded in PYP initiates the photocycle by trans→cis isomerization1. Althoughvarious experimental and computational studies have been published, a nonradiative relaxation processthat leads trans→cis isomerization is still a matter of discussion2-5. Recently, the S1 lifetime of ajet-cooled Oxyester Methyl p-Coumaric Acid (OMpCA) and its hydrogen-bonded complexes wasmeasured to elucidate the role of the hydrogen bond by employing picosecond time-resolvedpump-probe spectroscopy5. It unveiled the following characteristics: 1) the lifetime of OMpCA at theS1 band origin is 9 ps while that of OMpCA-H2O is 930 ps, and 2) OMpCA-H2O exhibits excessenergy dependence of nonradiative decay that is best fitted with a biexponential decay curve of fastand slow components. In the present work, we adapted the kinetic argument that was used to describethe excited-state dynamics of trans-stilbene6, and estimated an energy barrier height on the S1 potentialsurface by the RRK calculation.A gas mixture of OMpCA and solvent diluted with Ne carrier gas was expanded into vacuumthrough the pulsed nozzle, and skimmed by a skimmer to generate a supersonic molecular beam. Apicosecond tunable UV laser, obtained by second harmonic generation of the optical parametricgeneration/optical parametric amplifier (OPG/OPA) system of Nd:YAG laser, crossed with amolecular beam. Electronic spectrum was acquired by mass-resolved resonance 2-photon ionization(R2PI) method via S1 state. Pump-probe scheme was used to record the decay curve as a function ofthe time delay between pump and probe lasers. The S1-lifetime was determined by the convolutionfitting with a laser width of 12 ps.The R2PI excitationspectra of OMpCA and itshydrogen-bonded complexesin the 31000 – 33100 cm-1region are shown in Fig 1.Blue and red lines are thespectra recorded bynanosecond laser andpicosecond laser, respectively.It is clearly seen that aspectral red-shift ofhydrogen-bonded complexesincreases i.e. 633 cm-1, 921cm-1, 1396 cm-1 as solvent Fig 1. R2PI spectra of (a) OMpCA and (b)-(d) complexesbasicity increases from H2O, NH3 to N(CH3)3, respectively. The electronic excited-state is morestabilized than the ground state due to a slightly extended π-conjugation.As mentioned, the excess energy dependence of thenonradiative decay is observed for OMpCA-H2O. Thereexists potential energy barrier as a result of forming thehydrogen bond at the phenolic OH group. Fig 2. displaysthe time evolution excited at 310 cm-1 above the S1-S0origin. The curve is fitted by a double exponential curvewith a fast component (80 ps) and a slow component (220ps). We applied a kinetic model involving IVR andnonradiative decay to describe the temporal behavior of thecomplex.Fig 3 shows the schematic picture of S1-state dynamics for OMpCA complexes, where λ is thewavelength, σpump is the absorption cross section, σprobe is the ionization cross section, β is thedimentionless parameter that indicates the relative strength of the absorption from the S1 state. In short,an excess energy from the S1 band origin is first redistributed into the S1 vibrational manifold by IVR.A nonradiative process, e.g. isomerization and/or internal conversion, proceeds when an excess energyovercomes the barrier. Thus, a relative value of the two components could be observed in accord withan excess energy. In fact, the plot of decay rate constant as a function of excess energy forOMpCA-H2O in Fig 4. shows the validity of this kinetic argument, indicating that the fast componentis IVR and the slow component is a nonradiative process. The extrapolated line is based on the RRKfitting with the barrier height of 205 cm-1. A detail of the analysis and dynamics associated withpotential energy barrier for other OMpCA-complexes will be presented.References: 1. Y. O. Jung et al., Nat. Chem. 5. 212 (2013) 2. E. M. M. Tan et al., Faraday Disucc. 163, 321(2013) 3. E. V. Gromov et al., J. Phys. Chem. A 109, 4623 (2005) 4. M. Promkatkaew et al., Photochem.Photobiol. A 13, 583 (2014) 5. D. Shimada et al., Phys. Chem. Chem. Phys. 14, 8999 (2012) 6. J. S. Baskinet al., J. Phys. Chem. 100, 11920 (1996)Fig 3. Schematic picture of the two-step kineticmodel of the excited OMpCA complexS0S1IPS1IPisomerizationorinternal conversitonS0kIV R knrσ pum pλ pum pσ 0βλ probeσ 0(β-1)λ probefluo. fluo.Fig 4. Plot of the observed decay rateconstant of S1 state vs excess energy forOMpCA-H2OFig 2. The observed decay curve ofOMpCA-H2O above 310 cm-1 from theS1-S0 band origin.

S1 excited-state dynamics of OMpCA and its hydrogen-bonded complexes

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S1 excited-state dynamics of OMpCA and its hydrogen-bonded complexes

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