Polar Dyes in Solution:  A Joint Experimental and Theoretical Study of Absorption and Emission Band Shapes

Abstract

Absorption and steady-state emission spectra of two fluorescent dyes are measured in a series of aprotic solvents with similar refractive index and different polarity. The spectra are interpreted in terms of a two-state electronic model accounting for the coupling to internal vibrations and to an effective solvation coordinate. The proposed approach naturally accounts not only for solvatochromic shifts of absorption and emission bands but also for the evolution of band shapes with solvent polarity and for the observation of nonspecular absorption and fluorescence bands. The good agreement between experimental and calculated spectra confirms the validity of a two-state picture for the low-energy spectral properties of these donor−acceptor molecules, provided that the molecular polarizability is fully accounted for. The role of conformational degrees of freedom in flexible chromophores is also addressed.

Correlating the motion of electrons and nuclei with two-dimensional electronic–vibrational spectroscopy

“…The other proposed model (ii), illustrated in Fig. 1F, is that the S 1 state is CT in character and therefore has an anharmonically displaced potential compared with the S 0 , which gives rise to the large vibrational Stokes shift (26,35). The one prevailing conclusion from all of these studies is that the emissive state has some CT character.…”

“…To demonstrate this new experimental technique, we apply it to the laser dye 4-(di-cyanomethylene)-2-methyl-6-p-(dimethylamino)styryl-4H-pyran (DCM), a model push-pull emitter (26). The calculated ground-state structure of DCM is displayed in Fig.…”

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

“…131 This represents a crude model of the twisting channels, because it does not describe relaxation on the excited state surface along degrees of freedom orthogonal to the twisting modes -for example, the bridge bond length alternation modes. These modes are known to play an important role in the photophysics of conjugated dyes generally 65,66,[87][88][89][90] and in fluorescent protein chromophores particularly. [91][92][93][94] The relevance of bridge stretching modes to the development of charge-transfer behavior is implicit in Scheme 2.…”

Cyanines Bearing Quaternary Azaaromatic Moieties

“…The inverse solvatochromism of 11 was analyzed using the Mulliken-Holstein model, usually serving to describe spectral properties of normal solvatochromic dyes (Scheme 4). 45 Dye 12 is a molecular rectifier, its LB films show asymmetric current-voltage characteristics. However, it should be noted that its analogue 13 behaves differently.…”