Two-photon spectroscopy of conjugated organic chromophores

Abstract

The study of two-photon absorbing (TPA) chromophores has previously shown that intramolecular charge transfer (ICT) between electron donors (D) and acceptors (A) symmetrically substituted on a π-conjugated bridge can result in large values of the two photon cross section (δ). This work addresses the role of torsion in the π-backbone on δ, modulation of δ by solvent polarity, and development of two-photon excitable molecular sensors. Investigations of the one- and two-photon properties of di(styryl)benzene derivatives with terminal donor groups and cyano acceptors substituted either on the central phenyl ring or on the adjacent vinyl groups show that the position of the cyano group has a significant effect on the geometry and optical properties of the molecules. δ for vinyl substituted molecules is half that for phenylene substituted molecules and is similar to the values obtained for molecules with no acceptor groups. The one- and two-photon spectroscopic differences can be related to the donor-acceptor distance in these molecules and to the degree of torsion in conjugated backbone. Torsional effects on the electronic coupling of multi-chromophore molecules are also investigated. For quadrupolar TPA chromophores, solvent polarity weakly affects the linear absorption but strongly modifies the fluorescence spectral position and quantum yield (η). The TPA peak position does not shift with solvent polarity, however the magnitude of δ increases by up to a factor of two in acetonitrile relative to toluene. Analysis of the data in terms of the stabilization of intramolecular charge transfer by polar solvents is explored. Two-photon absorbing molecular sensors are investigated in which an ion binding group is incorporated as one of the donor groups (D-A-D' ) in a TPA molecule. When one monoaza-15-crown-5-ether macrocycle is bound to the chromophore, the two-photon induced fluorescence signal (ηδ) decreases seven-fold upon addition of magnesium ions, in part as delta is modulated, due to decreased ICT from the nitrogen lone pair involved in ion binding. In molecules incorporating 1,2-bis(o-aminophenoxy)-ethane- N,N,N',N'-tetraacetic acid, a 5-fold enhancement of ηδ is observed upon calcium ion binding in water at 720 nm. Changes in the TPA spectrum upon binding of Ca²⁺ in micellar systems are also observed.