Twisted Intramolecular Charge Transfer Molecules Research Articles

Lighting Up the Invisible Twisted Intramolecular Charge Transfer State by High Pressure.

The twisted intramolecular charge transfer (TICT) state plays an important role in determining the performance of optoelectronic devices. However, for some nonfluorescent TICT molecules, the "invisible" TICT state could only be visualized by modifying the molecular structure. Here, we introduce a new facile pressure-induced approach to light up the TICT state through the use of a pressure-related liquid-solid phase transition of the surrounding solvent. Combining ultrafast spectroscopy and quantum chemical calculations, it reveals that the "invisible" TICT state can emit fluorescence when the rotation of a donor group is restricted by the frozen acetonitrile solution. Furthermore, the TICT process can even be effectively regulated by the external pressure. Our study offers a unique strategy to achieve dual fluorescence behavior in charge transfer molecules and is of significance for optoelectronic and biomedical applications.

Evidence for the TICT mediated nonradiative deexcitation process for the excited coumarin-1 dye in high polarity protic solvents

Photophysical properties of coumarin-1 (C1) dye in different protic solvents have been investigated using steady-state and time-resolved fluorescence measurements. Correlation of the Stokes’ shifts ( Δ ν ¯ ) with the solvent polarity (Δ f) suggests the intramolecular charge transfer (ICT) character for the dye fluorescent state. Fluorescence quantum yields ( Φ f) and lifetimes ( τ f) of the dye show an abrupt reduction in high polarity solvents having Δ f >∼0.28. In these solvents τ f is seen to be strongly temperature dependent, though it is temperature independent in solvents with Δ f <∼0.28. It is inferred that in high polarity protic solvents there is a participation of an additional nonradiative decay process via the involvement of twisted intramolecular charge transfer (TICT) state. Unlike present results, no involvement of TICT state was observed even in strongly polar aprotic solvent like acetonitrile. It is indicated that the intermolecular hydrogen bonding of the dye with protic solvents in addition with the solvent polarity helps in the stabilization of the TICT state for C1 dye. Unlike most TICT molecules, the activation barrier (Δ E a) for the TICT mediated nonradiative process for C1 dye is seen to increase with solvent polarity. This is rationalized on the basis of the assumption that the TICT to ground state conversion is the activation-controlled rate-determining step for the present system than the usual ICT to TICT conversion as encountered for most other TICT molecules.

Photophysical Properties of Coumarin‐30 Dye in Aprotic and Protic Solvents of Varying Polarities¶

ABSTRACTExperimental results on various photophysical properties of coumarin‐30 (C30) dye, namely, Stokes' shift (Δv), fluorescence quantum yield (τf), fluorescence lifetime (τf), radiative rate constant (kf) and nonradiative rate constant (knr), as obtained using absorption and fluorescence measurements have been reported. Though in most of the solvents the properties of C30 show more or less linear correlation with the solvent polarity function, Δf= [(ε ‐ 1)/(2ε+ 1) ‐ (n2 ‐ 1)/ (2n2+ l)], they show unusual deviations in nonpolar solvents at one end and in high‐polarity protic solvents at the other end. From the solvent polarity and temperature effect on the photophysical properties of the dye, following inferences have been drawn: (1) in nonpolar solvents, the dye exists in a nonpolar structure, where its 7‐NEt2 substituent adopts a pyramidal configuration and the amino lone pair is out of resonance with the benzopyrone π cloud; (2) in medium to higher polarity solvents, the dye exists in a polar intra‐molecular charge transfer structure, where the 7‐NEt2 group and the 1,2‐benzopyrone moiety are in the same plane and the amino lone pair is in resonance with the benzopyrone π cloud; (3) in protic solvents, the dye‐solvent intermolecular hydrogen bonding influences the photophysical properties of the dye; and (4) in high‐polarity protic solvents, the excited C30 undergoes a new activation‐controlled nonradiative deexcitation process because of the involvement of a twisted intra‐molecular charge transfer (TICT) state. Contrary to most other TICT molecules, the activation barrier for this deexcitation process in C30 is observed to increase with solvent polarity. A rational for this unusual behavior has been given on the basis of the solvent polarity‐dependent stabilization and crossing of relevant electronic states and the relative propensity of interconversion among these states.

The use of Fraga's potential with AM1 atomic point charges in the evaluation of spectral shifts: Application to TICT molecules

AbstractFraga potential calculations with atomic point charges and geometrical parameters calculated from AM1 calculations have been used to calculate spectral shifts upon electronic excitation in twisted intramolecular charge transfer (TICT) systems due to intermolecular interactions. Changes of atomic polarizabilities have also been taken into account. Present calculations deal with absorption transitions of the p‐N,N‐dimethylaminobenzonitrile (DMABN) surrounded by methane, water, acetone, or acetonitrile solvent molecules. The methodology permits us to evaluate the influence of the solvent molecule on DMABN dimethylamino motions and to find the most stable conformation of a cluster configuration which can lead to a blue or red shift. The results have been compared with the experimental work of Warren et al.7 and confirm their analysis. © 1994 by John Wiley &amp; Sons, Inc.

Modeling hyperpolarizabilities of some TICT molecules and their analogs

Ground-state hyperpolarizabilities of a prototypical twisted intramolecular charge transfer (TICT) molecule (DMABN) and its analogues have been calculated by the finite-field SCF method at the CNDO/S level of approximation. The ground-state geometry of each molecule has been optimized by the MNDO metbod. Excited-state hyperpolarizabilities have been calculated by invoking the finite-field SCF single excitation CI method at the CNDO/S level of approximation. The study focuses attention on the possibly important role played by the TICT state in generating large quadratic polarizability in some of these molecules

Photophysical and photochemical switches based on twisted intramolecular charge transfer (TICT) states

TICT states, accessible in twisted multichromophoric systems, are nonradiative funnels to the ground state in many dyes. By controlling these funnels, faster saturable absorbers for subpicosecond laser pulses can be developed. Oriented assemblies of TICT molecules, as in liquid crystalline polymers, are expected to exhibit light-induced macroscopic charge separation. Chemical approaches to supramolecular bistable species are also shown.