Ultrafast Investigation of Excited-State Dynamics in Trans-4-methoxyazobenzene Studied by Femtosecond Transient Absorption Spectroscopy

The ultrafast photoisomerization and excited-state dynamics of trans-4-methoxyazobenzene (trans-4-MAB) in solutions were investigated by femtosecond transient absorption spectroscopy and quantum chemistry calculations. After being excited to the S2 state, the two-dimensional transient absorptions spectra show that cis-4-MAB is produced and witnessed by the permanent positive absorption in 400-480 nm. Three decay components are determined to be 0.11, 1.4 and 2.9 ps in ethanol, and 0.16, 1.5 and 7.5 ps in ethylene glycol, respectively. The fast component is assigned to the internal conversion from the S2 to S1 state. The other relaxation pathways are correlated with the decay of the S1 state via internal conversion and isomerization, and the vibrational cooling of the hot S0 state of the cis-isomer. Comparing of the dynamics in different solvents, it is demonstrated that the photoisomerization pathway undergoes the inversion mechanism rather than the rotation mechanism.

Ultrafast proton transfer dynamics of 2-(2'-hydroxyphenyl)benzoxazole dye in different solvents

The excited-state intramolecular proton transfer of 2-(2-hydroxyphenyl)benzoxazole dye in different solvents is investigated using ultrafast femtosecond transient absorption spectroscopy combined with quantum chemical calculations.Conformational conversion from the syn-enol configuration to the keto configuration is proposed as the mechanism of excited-state intramolecular proton transfer.The duration of excited-state intramolecular proton transfer is measured to range from 50 fs to 200 fs in different solvents.This time is strongly dependent on the calculated energy gap between the N-S;and T-S;structures in the S;state.Along the proton transfer reaction coordinate,the vibrational relaxation process on the S;state potential surface is observed.The duration of the vibrational relaxation process is determined to be from8.7 ps to 35 ps dependent on the excess vibrational energy.

Hydrogen Bonding Effects on the Photophysical Properties of 2,3-dihydro-3-keto- 1H-pyrido[3,2,1-kl] phenothiazine

Time-dependent density functional theory （TDDFT） and femtosecond transient absorption spectroscopy were used to investigate the photophysical properties of 2,3-dihydro-3-keto-lH- pyrido[3,2,1-kl]phenothiazine （PTZ4） and 3-keto-lH-pyrido[3,2,1-kl]phenothiazine （PTZ5）. The calculated results obtained from TDDFT suggest that the red-shifts of the absorption spectra of these two fluorophores in methanol are due to the formation of hydrogen-bonded complexes at the ground state. Four conformers of PTZ4 were obtained by TDDFT. The two fluorescence peaks of PTZ4 in tetrahydrofuran （THF） came from the ICT states of the four conformers. The fluorescence of PTZ4 in THF showed a dependence on the excitation wavelength because of butterfly bending. The excited state dynamics of PTZ4 in THF and methanol were obtained by transient absorption spectroscopy. The lifetime of the excited PTZ4 in methanol was 53.8 ps, and its relaxation from the LE state to the ICT state was completed within several picoseconds. The short lifetime of excited PTZ4 in methanol was due to the formation of out-of-plane model hydrogen bonds between PTZ4 and methanol at the excited state.

Rebinding kinetics of dissociated amino acid ligand and carbon monoxide to ferrous microperoxidase-11 in aqueous solution

The rebinding kinetics of an amino acid ligand to ferrous microperoxidase-11 (MP11) after photolysis of aggregated ferrous MP11 was measured in aqueous solution with femtosecond transient visible absorption spectroscopy. The kinetics of CO rebinding to ferrous MP11 after photolysis of MP11CO was also measured in aqueous solution with femtosecond transient visible absorption spectroscopy. From these measurements, we found that either Val-11 or Lys-13 rebinds to ferrous MP11 exponentially with an 8 picosecond time constant in aggregated ferrous MP11 solution and that CO rebinds to ferrous MP11 nonexponentially with subnanosecond time scale in MP11CO solution. The kinetics of both the amino acid and CO rebinding to ferrous MP11 in MP11 system mimics that in carbon monoxide oxidation activator protein (CooA) or carboxymethyl cytochrome c (CmCytC) system. We also measured the kinetics of CO rebinding to ferrous MP11 in aqueous solution at different MP11CO concentrations and found that MP11CO concentration has an obvious effect on the kinetics of CO rebinding to ferrous MP11, where both the germinate yield and rate of CO rebinding to ferrous MP11 increase with the increase of MP11CO concentration. These findings suggested that the picosecond amino acid ligand rebinding process could disturb the proximal heme-ligand structure that possibly leads to the subnanosecond CO rebinding kinetics in MP11CO, CooACO and CmCytCCO systems.