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 (ν0 0 = 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.

Absorption spectrum of monomeric pseudoisocyanine: A new perspective and its implications for formation and spectral response of J-aggregates in solution and in thin films

“…The low T residuals are likely to enclose an additional band E495 nm [7,8]. In addition, the hypothesized spectrum is not expected to be extremely temperature dependent [6]. Therefore, at low T further conversion of monomers to J-aggregates is implied.…”

“…Despite extensive investigation, the spectral changes following self-aggregation and the kinetics of aggregate formation in solution/thin films are still debated [3][4][5][6][7][8].…”

“…The Frenkel exciton picture [9] is well agreed as the basis for understanding their optical response [3][4][5][6][7][8]10]. The prerequisite to reveal the aggregate optical response within the Frenkel picture is a well-defined monomeric absorption spectrum.…”

“…The prerequisite to reveal the aggregate optical response within the Frenkel picture is a well-defined monomeric absorption spectrum. The spectrum in water at highly dilute dye concentration and moderate temperatures is commonly assumed to represent the absorption of monomeric PIC [6][7][8]. Under these conditions the absorption is characterized by two peaks E525 and E490 nm, and a shoulder E460 nm.…”

Formation of Pseudoisocyanine J-Aggregates in Poly(vinyl alcohol) Fibers by Electrospinning

“…Previously, it was shown that PIC dissolved in glycerol exhibits a band around 530 nm, assigned as monomers in solution at room temperature. 28 Approximately a 20 nm red-shift of the monomer band in our result can be due to the environmental change from solution to the solid phase. A shoulder at 620 nm in the spectrum of the electrospun mat with the lowest PIC weight percent is apparent, and its nature will be discussed below.…”