Solvent, Temperature, and Excitonic Effects in the Optical Spectra of Pseudoisocyanine Monomer and J-Aggregates

Abstract

The absorption and fluorescence spectra of 1,1‘-diethyl-2,2‘-cyanine (pseudoisocyanine, PIC) aggregates have been studied between 8 and 293 K in water/glycerol glass containing 2−4 M of alkali halogenides. In this system the J-aggregates have a single sharp band and there is practically no contamination with the monomeric dye, dimers, or H-aggregates. This allowed us to better resolve the high-energy portion of the spectrum and to assign the middle 535 nm band to the upper exciton transition. The excitonic splitting at 8 K is the same for both the blue and the red forms of aggregates (1270 ± 10 cm-1). The average energy of exciton components (18195 ± 15 cm-1 for the blue form) was found to be very close to the 0−0 energy of the first strong site of PIC monomer (18223 cm-1) embedded in a 9-aza-PIC iodide matrix, which is transparent above 500 nm [Marchetti, A. P.; Scozzafava, M. Chem. Phys. Lett. 1976, 41, 87]. The 0−0 frequency of the nonsolvated PIC monomer cation (ν00 = 19716 ± 40 cm-1 or 507.2 ± 1 nm) was obtained from the solvent shift measurements at room temperature. The absorption bandwidths and shifts of both the PIC cation in poly(methyl methacrylate) matrix and the aggregates were recorded in the temperature range between 8 and 300 K. The thermal shift of band maxima was analysed in terms of the change in dispersive shift and excitonic splitting as a result of the expansion of the matrix and a pure thermal or phonon-induced contribution. The thermal shift and broadening behavior of molecular and excitonic transitions reveals large differences in the mechanism and strength of the coupling to low-frequency vibrations.