title: Enhanced intersystem crossing rate in polymethine-like molecules: sulfur-containing squaraines versus oxygen-containing analogues.
abstract: Two different approaches to increase intersystem crossing rates in polymethine-like molecules are presented: traditional heavy-atom substitution and molecular levels engineering. Linear and nonlinear optical properties of a series of polymethine dyes with Br- and Se-atom substitution, and a series of new squaraine molecules, where one or two oxygen atoms in a squaraine bridge are replaced with sulfur atoms, are investigated. A consequence of the oxygen-to-sulfur substitution in squaraines is the inversion of their lowest-lying ππ* and nπ* states leading to a significant reduction of singlet-triplet energy difference and opening of an additional intersystem channel of relaxation. Experimental studies show that triplet quantum yields for polymethine dyes with heavy-atom substitutions are small (not more than 10%), while for sulfur-containing squaraines these values reach almost unity. Linear spectroscopic characterization includes absorption, fluorescence, quantum yield, anisotropy, and singlet oxygen generation measurements. Nonlinear characterization, performed by picosecond and femtosecond laser systems (pump-probe and Z-scan measurements), includes measurements of the triplet quantum yields, excited state absorption, two-photon absorption, and singlet and triplet state lifetimes. Experimental results are in agreement with density functional theory calculations allowing determination of the energy positions, spin-orbital coupling, and electronic configurations of the lowest electronic transitions.
authors: CREOL: College of Optics and Photonics, University of Central Florida, Orlando, Florida 32816, United States.
authors: Davorin
authors: D
authors: Peceli
authors: Honghua
authors: H
authors: Hu
authors: Dmitry A
authors: DA
authors: Fishman
authors: Scott
authors: S
authors: Webster
authors: Olga V
authors: OV
authors: Przhonska
authors: Vladimir V
authors: VV
authors: Kurdyukov
authors: Yurii L
authors: YL
authors: Slominsky
authors: Alexey I
authors: AI
authors: Tolmachev
authors: Alexey D
authors: AD
authors: Kachkovski
authors: Andrey O
authors: AO
authors: Gerasov
authors: Artëm E
authors: AE
authors: Masunov
authors: David J
authors: DJ
authors: Hagan
authors: Eric W
authors: EW
authors: Van Stryland
mesh_terms: D001965:Bromides / Q000737:chemistry; D003503:Cyclobutanes / Q000737:chemistry*; D004583:Electrons; D007211:Indoles / Q000737:chemistry*; D015394:Molecular Structure; D010100:Oxygen / Q000737:chemistry*; D010636:Phenols / Q000737:chemistry*; D017785:Photons; D011789:Quantum Theory; D012643:Selenium / Q000737:chemistry; D026082:Singlet Oxygen / Q000737:chemistry*; D013050:Spectrometry, Fluorescence; D013455:Sulfur / Q000737:chemistry*
chemical_list: D001965:Bromides; D003503:Cyclobutanes; D007211:Indoles; D010636:Phenols; C480596:squaraine; D026082:Singlet Oxygen; C098209:VPM chloride; D013455:Sulfur; D012643:Selenium; D010100:Oxygen