New Nonlinear Optical Crystal of Rhodamine 590 Acid Phthalate

The
synthesis and crystal structure of rhodamine 590 acid phthalate
(RhAP) have been reported. This novel solid-state rhodamine derivative
not only has a longer fluorescence lifetime compared to rhodamine
solid-state matrixes where emission is quenched but also possesses
strong nonlinear optical characteristics. The static and dynamic first-
and second-order hyperpolarizabilities were calculated using the time-dependent
density functional theory at the B3LYP/6-31+G* level. The computed
static values of β and γ of RhAP by the X-ray diffraction
(XRD) structure were 31.9 × 10–30 and 199.0
× 10–36 esu, respectively. These values were
about 62 times larger than the corresponding values in urea, an already
well-known nonlinear optical material. The second-order hyperpolarizability
of the compound was determined experimentally by measuring the two-photon
absorption cross section using intensity-modulated light fields. The
reported compound, excitable at near-infrared, exhibited frequency
upconversion with the two-photon absorption coefficient enhanced by
two orders of magnitude compared to that of the dye solution. Hosting
the dye in the solid, at high concentrations, exploits the nonlinearity
of the dye itself as well as results in significant excitonic effects
including formation of broad exciton band and superradiance.