SOLVENT, TEMPERATURE, AND EXCITONIC EFFECTS IN THE OPTICAL-SPECTRA OFPSEUDOISOCYANINE MONOMER AND J-AGGREGATES

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 t his system the J-aggregates have a single sharp band and there is prac tically no contamination with the monomeric dye, dimers, or II-aggrega tes. 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 tra nsition. 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 ener gy of exciton components (18195 +/- 15 cm(-1) for the blue form) was f ound to be very close to the 0-0 energy of the first strong site of PI C monomer (18223 cm(-1)) embedded in a 9-aza-PIC iodide matrix, which is transparent above 500 nm [Marchetti, A. P.; Scozzafava, M. Chem. Ph ys. Lett. 1976, 41, 87]. The 0-0 frequency of the nonsolvated PIC mono mer cation (nu(0)(0) = 19716 +/- 40 cm(-1) or 507.2 +/- 1 nm) was obta ined from the solvent shift measurements at room temperature. The abso rption bandwidths and shifts of both the PIC cation in poly(methyl met hacrylate) matrix and the aggregates were recorded in;the temperature range between 8 and 300 K. The thermal shift of band maxima was analys ed 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 phono n-induced contribution. The thermal shift and broadening behavior of m olecular and excitonic transitions reveals large differences in the me chanism and strength of the coupling-to low-frequency vibrations.