ACETYLENYL-LINKED, PORPHYRIN-BRIDGED, DONOR-ACCEPTOR MOLECULES - A THEORETICAL-ANALYSIS OF THE MOLECULAR FIRST HYPERPOLARIZABILITY IN HIGHLY CONJUGATED PUSH-PULL CHROMOPHORE STRUCTURES

We describe the theoretical basis for the exceptionally large molecula r first hyperpolarizabilities inherent to (5,15-diethynylporphinato)me tal-bridged donor-acceptor (D-A) molecules. beta values relevant for h yper-Rayleigh experiments are calculated at 1.064 and 0.830 mu m for a complex with such a structure, no)-phenyl)ethynyl)-15-((4''-nitrophen yl)ethynyl)- 10-20-diphenylporphinato]zinc(II), and are 472 x 10(-30) and 8152 x 10(-30) cm(5)/esu, respectively. The values are 1 order of magnitude larger than that calculated for any other porphyrin bridged donor-acceptor chromophore studied to date. The considerably enhanced hyperpolarizability arises from the significant excited-state electron ic asymmetry manifest in such structures (derived from the strong brid ge-mediated D-A coupling enabled by the largely porphyrin-based excite d state) and the large bridge-centered oscillator strength in this new class of D-bridge-A molecules, Our analysis of NLO properties (based upon INDO/SCI calculations within the sum over states formalism) shows a sensitivity to the degree of cumulenic character in the ground stat e. Calculations on structurally related multiporphyrin systems suggest candidate chromophores with further enhanced optical nonlinearities.