First Hyperpolarizability Dispersion of the Octupolar Molecule Crystal Violet: Multiple Resonances and Vibrational and Solvation Effects

Abstract

The first hyperpolarizability (β) dispersion curve is measured for the first time for an octupolar nonlinear optical (NLO) molecule (crystal violet, CV) and modeled theoretically, yielding an in-depth understanding of the electronic structure and vibronic and solvation effects on such octupolar conjugated systems. Tunable wavelength hyper-Rayleigh scattering (HRS) measurements were performed on this prototypical octupolar molecule in the broad fundamental wavelength range of 620-1580 nm, showing significant shortcomings of the commonly used β dispersion models. Three well-separated β resonances involving the lowest-energy state and several higher excited states are clearly observed, including a significant contribution from a nominally one-photon forbidden transition. The experimental results for second-harmonic wavelengths above 330 nm are successfully modeled by means of a vibronically coupled essential-state description for octupolar chromophores, developed by Terenziani et al. (J. Phys. Chem. B 2008, 112, 5079), which takes into account polar solvation effects. The relative intensities of the various resonances, including the one below 330 nm, are also quantified by quantum chemical calculations. Furthermore, interesting effects of inhomogeneous broadening due to polar solvation of the two-dimensional chromophore are recognized in both linear and nonlinear spectra, allowing us to quantitatively address the long-standing problem of the band shape of the linear absorption spectrum of CV. This clearly demonstrates that extensive wavelength-dependent HRS measurements, as presented in this work, are essential to the characterization and design of NLO materials and represent a powerful tool to gain valuable information on molecular excitations and environmental effects in general.

A three-state effective Hamiltonian for symmetric cationic diarylmethanes

“…This model will provide a framework for the interpretation of the quantum chemical results, so it is appropriate to introduce it now. The model is similar in spirit to classical resonance-theoretic models of cyanine-like (methine) dyes discussed by Moffitt, 18 Simpson, 19 Brooker, 22 and Platt, 20 and to recent essential-state models of polymethine, 23 donor-π -acceptor quadrupolar, 24 and triarylmethane 28 dyes that have been developed and applied by Terenziani, Painelli, and co-workers.…”

“…23,24,28 These models are built on a three-dimensional Hilbert space spanned by two "canonical" resonating structures, which are conjugate in the sense of flipped polarity and bond alternation (cf. Scheme 1), and by a third "intermediate" structure with charge on the bridge.…”

“…23,24,27,28 Resonance-theoretic models have been proven successful in the description of spectroscopic experiments. 19,20,[22][23][24][27][28][29] In order to learn the most from these successes, it would be useful to better understand the nature and justification of the approximate electronic structure implied by Scheme 1.…”

“…The diabatic states can be identified with the bonding structures invoked in resonance-based theories of the optical response of organic dyes. 19,20,[22][23][24][27][28][29] We will use the analogies established between the electronic structures for the dyes in Scheme 1 to infer a parametric relationship between the diabatic Hamiltonians for the different dyes. The Hamiltonians extracted from the quantum chemical calculations can be approximately described by a one-parameter family.…”

A two-state model of twisted intramolecular charge-transfer in monomethine dyes

“…35 It is in the spirit of recent "essential state" models that have successfully applied to model the spectra of organic chromophores. [65][66][67] The paper proceeds as follows. Section II describes the model Hamiltonian and the basic theoretical analytical framework, as well as providing specific details of the calculations used to parameterize it for different oxonol protonation states of the GFP chromophore (cf.…”