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 molecular first hyperpolarizabilities inherent to (5,15-diethynylporphinato)metal-bridged donor−acceptor (D−A) molecules. β values relevant for hyper-Rayleigh experiments are calculated at 1.064 and 0.830 μm for a complex with such a structure, [5-((4‘-(dimethylamino)phenyl)ethynyl)-15-((4‘‘-nitrophenyl)ethynyl)-10,20-diphenylporphinato]zinc(II), and are 472 × 10-30 and 8152 × 10-30 cm5/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 electronic asymmetry manifest in such structures (derived from the strong bridge-mediated D−A coupling enabled by the largely porphyrin-based excited 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 state. Calculations on structurally related multiporphyrin systems suggest candidate chromophores with further enhanced optical nonlinearities.