title: Barrierless photoisomerisation of the "simplest cyanine": joining computational and femtosecond optical spectroscopies to trace the full reaction path.
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.
authors.affiliation: Department of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-Strasse 2, 12489 Berlin, Germany.
authors.forename: Alexander
authors.initials: A
authors.lastname: Weigel
authors.forename: Matthias
authors.initials: M
authors.lastname: Pfaffe
authors.forename: Mohsen
authors.initials: M
authors.lastname: Sajadi
authors.forename: Rainer
authors.initials: R
authors.lastname: Mahrwald
authors.forename: Roberto
authors.initials: R
authors.lastname: Improta
authors.forename: Vincenzo
authors.initials: V
authors.lastname: Barone
authors.forename: Dario
authors.initials: D
authors.lastname: Polli
authors.forename: Giulio
authors.initials: G
authors.lastname: Cerullo
authors.forename: Nikolaus P
authors.initials: NP
authors.lastname: Ernsting
authors.forename: Fabrizio
authors.initials: F
authors.lastname: Santoro
mesh_terms: D000042:Absorption; D004058:Diffusion; D007536:Isomerism; D011789:Quantum Theory; D011804:Quinolines / Q000737:chemistry*; D013059:Spectrum Analysis, Raman; D013816:Thermodynamics; D013997:Time Factors
chemical_list: D011804:Quinolines; C100254:pseudoisocyanine