Effect of the polymethine chain length, the polarity and temperature of the medium on the spectroscopic properties of merocyanine dyes

We have studied the luminescent spectral properties and calculated the quadratic polarizability of dimethine, tetramethine, and hexamethine merocyanines based on 1,3-dihydro-1,3,3-trimethyl-(2H)-indol-2-ylidene and malonodinitrile in solutions and polymer films at room temperature and 77 K. We have shown that increasing the polarity of the solvent and lengthening the polymethine chain in the studied merocyanines lead to an increase in fluorescence quantum yields and narrowing of the fluorescence bands. This is explained by the decrease in bond length alternation and weakening of vibronic interactions in the singlet-excited fluorescent state. The mirror symmetry in the structured absorption and fluorescence spectra of merocyanine dyes in ethanol at 77 K, observed here for the first time, is due to enhancement of electrostatic interactions of the merocyanine and solvent molecules. Increased stiffness of the medium considerably decreases the probability of nonradiative processes, and the major channel for deactivation of the electronic excitation energy at 77 K is fluorescence. The higher values of the quadratic polarizability (6.0·10⁻²⁸ esu in ethanol and 3.3·10⁻²⁸ esu in chloroform) for hexamethine merocyanine, estimated by the solvatochromic method, lets us consider this compound as a promising material for designing nonlinear optical converters for harmonics of laser radiation. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 1, pp. 26–35, January–February, 2006.