Optical Properties of Spiroconjugated Charge-Transfer Dyes

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.