We describe the theoretical basis for the exceptionally large molecular first hyperpolarizabilities inherent to (5,15-diethynylporphinato)metal-bridged donor-acceptor (D-A) molecules. beta values relevant for hyper-Rayleigh experiments are calculated at 1.064 and 0.830 mu m for a complex with such a structure, [5-((4’-(dimethylamino)-phenyl)ethynyl)-15-((4"-nitrophenyl)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 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.