The photoisomerisation of 1,10-diethyl-2,20-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 S1 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 S1 path has been observed. Most importantly the global S1 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.

Barrierless photoisomerisation of the “simplest cyanine”: Joining computational and femtosecond optical spectroscopies to trace the full reaction path

POLLI, DARIO;CERULLO, GIULIO NICOLA;
2012

Abstract

The photoisomerisation of 1,10-diethyl-2,20-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 S1 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 S1 path has been observed. Most importantly the global S1 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.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11311/690655
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