Excited-state χ3 enhancement for a p-oligophenylene derivative

A substantial enhancement in the non-resonant χ³ response from a dilute solution of the molecule p-quaterphenyl [(Ph)₄] after it has been raised to its first excited singlet electronic state [(Ph)₄*]. (Ph)₄ displays many desirable characteristics for excited-state NLO applications. These include large absorption at the pump wavelength (~320 nm), separation of the fluorescence and triplet absorption spectral regions from the probe wavelength (~640 nm), and very high photochemical stability. (Ph)₄ and its higher homologues (Ph)_n suffer, however, in their limited solubilities. To achieve a larger figure of merit, one must increase this concentration substantially. This follows because, in the unsaturated regime, we expect the enhancement effect to increase with the solution concentration. We have accomplished this increase in concentration by synthesis of a derivatized form of (Ph)₄, diethyl ester of quaterphenylenediacetic acid (QPDA) which is substantially more soluble than the parent molecule. Concentrations greater than 2×10^-3 M were readily achieved. Here we show that QPDA displays large nonlinearity in its excited-state (QPDA*) similar to (Ph)₄. We will also discuss the transient behavior of this nonlinearity as probed by femtosecond time-resolved degenerate four-wave mixing (DFWM); its behavior will be compared to that obtained for the underivatized molecule