Optical properties of spiroconjugated charge-transfer dyes

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A new type of intramolecular charge-transfer dye has been prepared. The LUMO of the acceptor part (1,3-indandione) in these compounds is spiroconjugated with the HOMO of the donor part (aromatic diamine or amino thiol). The interaction between the donor and acceptor is controlled by the energy and symmetry of the frontier orbitals. The ground state dipole moments of these compounds are aligned along the long molecular axes. In the solid state, distortions of structures are observed that are consistent with partial electron shift from the donor to the acceptor. Much more pronounced electron density shifts accompany the electronic transition that is observed in the visible region of the spectrum. These transitions are of the charge-transfer (CT) type, as shown by solvatochromic and electrooptical studies. The excited state dipole moments are in the direction opposite to that of the ground state. These observations are consistent with the excited state having radical ion pair character. The new dyes are modeled using a simple Mulliken charge-transfer theory. The mixing coefficient of neutral and ionic wave functions describing these systems is used as a measure of spiroconjugation between the subchromophores. The electrooptical data provide an estimate of the contribution of the CT transitions to the hyperpolarizabilities (β) within the two-state model. The measured values of β indicate, however, that spiro dyes have two opposing contributions to their hyperpolarizabilities, one from the CT transition and one due to the acceptor subchromophore.