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.

The two-photon absorption (2PA) spectrum of an organic single crystal is reported. The crystal is grown by self-nucleation of a subsaturated hot solution of acetonitrile, and is composed of an asymmetrical donor-π-acceptor cyanine-like dye molecule. To our knowledge, this is the first report of the 2PA spectrum of single crystals made from a cyanine-like dye. The linear and nonlinear properties of the single crystalline material are investigated and compared with the molecular properties of a toluene solution of its monomeric form. The maximum polarization-dependent 2PA coefficient of the single crystal is 52 ± 9 cm/GW, which is more than twice as large as that for the inorganic semiconductor CdTe with a similar absorption edge. The optical properties, linear and nonlinear, are strongly dependent upon incident polarization due to anisotropic molecular packing. X-ray diffraction analysis shows π-stacking dimers formation in the crystal, similar to H-aggregates. Quantum chemical calculations demonstrate that this dimerization leads to the splitting of the energy bands and the appearance of new red-shifted 2PA bands when compared to the solution of monomers. This trend is opposite to the blue shift in the linear absorption spectra upon H-aggregation.

Combined quantum‐chemical and spectral studies of the features of electron transitions and absorption spectra of symmetrical and unsymmetrical polymethine dyes, derivatives of aminocoumarin, have been performed. It was established that the lowest two electron transitions are splitting transitions, involving solitonic level and two low positioned acceptor levels, in contrast to the cationic cyanine dyes, where two lowest transitions are connected with two splitting donor levels and solitonic level. The considerable interaction between two acceptor levels in symmetrical dyes provides an additional decrease of the first transition energy and hence leads to the relatively deep and intensive color, whereas the second transition with its negligible oscillator strength is practically unobserved. The higher electron transitions involve orbitals located mainly at the coumarin residue; the corresponding spectral bands appear in the short‐wavelength region. Introduction of dialkylamino group in coumarin heterocycle shifts bathochromically the long‐wavelength absorption band and considerably increases the intensity of S0 → S1 and S0 → S3 electron transitions. Copyright © 2008 John Wiley & Sons, Ltd.