Enhanced Intersystem Crossing Rate in Polymethine-Like Molecules: Sulfur-Containing Squaraines versus Oxygen-Containing Analogues

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2013-03, Vol.117 (11), p.2333-2346
• Main Authors: Peceli, Davorin, Hu, Honghua, Fishman, Dmitry A, Webster, Scott, Przhonska, Olga V, Kurdyukov, Vladimir V, Slominsky, Yurii L, Tolmachev, Alexey I, Kachkovski, Alexey D, Gerasov, Andrey O, Masunov, Artëm E, Hagan, David J, Van Stryland, Eric W
• Format: Article
• Language: English
• Subjects: Anisotropy; Atomic and molecular physics; Bromides - chemistry; Calculations and mathematical techniques in atomic and molecular physics (excluding electron correlation calculations); Channels; Cyclobutanes - chemistry; Density-functional theory; Dyes; Electronic structure of atoms, molecules and their ions: theory; Electronics; Electrons; Exact sciences and technology; Fluorescence and phosphorescence; radiationless transitions, quenching (intersystem crossing, internal conversion); Indoles - chemistry; Intensities and shapes of molecular spectral lines and bands; Inversions; Molecular properties and interactions with photons; Molecular Structure; Nonlinearity; Oscillator and band strengths, lifetimes, transition moments, and franck-condon factors; Oxygen - chemistry; Phenols - chemistry; Photons; Physics; Quantum Theory; Selenium - chemistry; Singlet Oxygen - chemistry; Spectrometry, Fluorescence; Spectroscopy; Sulfur; Sulfur - chemistry
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp400276g

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.