From solutions to Langmuir-Blodgett films: Spectroscopic stydy of a zwitterionic dye

We present a thorough spectroscopic study of γ-(n-hexadecyl)quinolinium tricyanoquinodimethanide, C16H33Q-3CNQ, a very well-known zwitterionic donor-π-acceptor chromophore, that behaves as a molecular rectifier. Absorption and Raman spectra are collected from solution and solid samples, as well as from Langmuir−Blodgett (LB) monolayer and multilayers. Whereas Raman spectra confirm the zwitterionic nature of the molecule in all phases, the absorption spectra show a complex evolution with the molecular environment. The impressive inverse solvatochromic behavior (hypsochromism) shown by C16H33Q-3CNQ is analyzed, based on the Mulliken−Holstein model that was previously used to describe spectral properties of normal solvatochromic (bathochromic) dyes. The same model is extended here to describe the molecular units in the LB monolayer. Intermolecular interactions in the film are introduced as classical electrostatic forces. The molecular parameters entering the model are extracted from the analysis of absorption and emission spectra in solution in a bottom-up modeling that allows the transfer of information among the different phases. The polarity of the ground state is estimated as ρ ≈ 0.89, 0.90, and 0.93 in CHCl3, CH2Cl2, and CH3CN, respectively. The ground and first excited-state electric dipole moment at infinite dilution in CH2Cl2 are estimated as 36 and 4 D, respectively, in good agreement with previous estimates. The large blue-shift of the absorption band in the LB film with respect to solution is ascribed to the increased polarity of the environment, rather than to excitonic effects.