Barrierless photoisomerisation of the "simplest cyanine": joining computational and femtosecond optical spectroscopies to trace the full reaction path

Phys Chem Chem Phys. 2012 Oct 14;14(38):13350-64. doi: 10.1039/c2cp41522d.

Abstract

The photoisomerisation of 1,1'-diethyl-2,2'-pyridocyanine, regarded by Brooker as the simplest cyanine, is examined in methanol by time-resolved experiments and PCM/TD-CAM-B3LYP calculations. Femtosecond transient absorption, fluorescence upconversion, and stimulated Raman scattering, all with broadband coverage, provide a panoramic view of the photoreaction. On the computational side, evolving distributions on an S(1) minimum-energy path are obtained by solving the Smoluchowski equation for drift and diffusion of torsional motion. Absorption and fluorescence bandshapes are calculated and compared to the observations; near-quantitative agreement implies that the entire S(1) path has been observed. Most importantly the global S(1) minimum, i.e. the perpendicular "phantom state" P*, can be identified and characterized in this way. Internal conversion of P* (3.7 ps), assisted by solvent equilibration, leads to the hot ground state. Within 5 ps, vibrational bands of cis and trans isomers are recognized with the help of calculated Raman spectra. The differences between observed and simulated spectra are discussed.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Absorption
  • Diffusion
  • Isomerism
  • Quantum Theory
  • Quinolines / chemistry*
  • Spectrum Analysis, Raman
  • Thermodynamics
  • Time Factors

Substances

  • Quinolines
  • pseudoisocyanine