Picosecond time-resolved kinetic studies on the formation of short-lived pseudoisocyanine iodide photoisomers in methanol and ethylene glycol

Upon exciting pseudoisocyanine iodide with 20 ps pulses at wavelengths between 490 and 530 nm within the absorption band, formation of a new transient absorption with a maximum at 545 nm was observed. It is attributed to an unstable photoisomer of pseudoisocyanine iodide. The quantum yield of the photoisomer is 31% in methanol and 6% in ethylene glycol     

PICOSECOND ABSORPTION SPECTROSCOPY OF CHLOROPHYLL A DIHYDRATE

Abstract— The solution of chlorophyll a in hexane with more than 70% of chlorophyll molecules in the form of the dihydrate (CHl-a.2H₂O)_n is investigated at room temperature by picosecond absorption technique. The transient difference spectra and the kinetic dependences of the absorption changes are measured for several excitation and probe wavelengths. Creation of a new state with blue-shifted absorption band is observed in the system after excitation. A model of the behaviour of the system after excitation is proposed and checked by comparison of the computer-simulated spectral and time dependences with the experimental data. According to the model, about 30% of excited molecules are in the state with the 0.046 eV shift of the absorption band in comparison with the ground state absorption. The lifetime of this state has 170-ps component. The rest of the excited molecules are in the first excited state with the decay time component of 37 ps.     

Absorption and fluorescent properties of pyrylium compounds: II. Spectra and cross sections for absorption from the ground and excited states of the structurally rearranged form

The nonstationary absorption of solutions of 2,4,6-substituted pyrylium salt is studied by the pump-probe method. The solutions in methylene chloride are studied upon picosecond pumping and probing with different time delays. The solutions in methyl ethyl ketone are studied upon pumping and synchronous probing by nanosecond pulses. It is shown that excitation of the initially nonplanar molecule causes its structural rearrangement. As a result, substituent 2 enters into the pyrylium ring plane and two nonstationary conformers are formed that differ in the position of substituent 6. The motion upon structural rearrangement has a small amplitude, and localization of molecular orbitals changes only slightly. The temperature dependence of the nonstationary absorption is studied in the range from 293 to 223 K. The spectra and cross sections are measured for transitions from the S₀ and S₁ states. In addition, the activation energies and kinetic parameters of the rearrangement in the ground and excited states are determined. Experimental data are compared with theoretical calculations of the transitions from the S₁ state. It is shown that a partial flattening of conformer molecules results in the long-wavelength shift of the bands and an increase in the cross sections for transitions localized on a flat part of the molecule and does not change parameters of the band localized on the molecular part that did not change upon the rearrangement but remained nonplanar.