Two-photon absorption of styryl-quinolinium, -pyridinium, and -barbituric acid derivatives, and intramolecular charge transfer

A series of new chromophores, styryl-parent end-capped with various donors, and with barbituric acid, methyl-pyridinium, and methyl-quinolinium as the acceptors, has been synthesized and characterized by element analysis or X -ray diffraction. Using the Z-scan system, their two-photon absorption (TPA ) cross-section values (delta) have been determined under excitation with 1 0 Hz, and 1064 nm, 35 ps mode-locked Nd:YAG laser pulse in DMF with d(o)= 0 .05 M. The effective delta value is as high as 10.9 x 10(48) cm(4) s per ph oton for trans-4-(4'-N,N-diphenyl amino) styryl-N-methyl quinolinium iodide (DPASQI). The delta value increases from barbituric acid- to pyridinium- t o quinolinium-derivatives apparently due to the increase in both the conjug ated degree and planarity; however, when the acceptor is fixed, the delta v alue increases from dialkyl amino groups to diphenyl amino groups even thou gh the latter is a weaker donor than the dialkyl amino groups. Theoretical calculations confirm that the increased distortion from planarity for the b arbituric acid derivative makes its delta value decrease. The relatively la rge delta value for quinolinium- or pyridinium-derivatives originates from larger intramolecular charge transfer, which can be characterized by the di fference of dipole moment (Delta mu (ge)) between the S-0 and S-1, and the transition dipole moment (M-ee') between S-1 and S-2.