Conformation and π-conjugation of olefin-bridged acceptor on the pyrrole β-carbon of nickel tetraphenylporphyrins: implicit evidence from linear and nonlinear optical properties

Abstract

This paper has scrutinized, both experimentally and theoretically, the conformation of an olefinic substituent on the pyrrole β-carbon of tetraphenylporphyrin. Compelling evidences collected from linear optical absorption and NOESY 1H NMR experiment strongly suggest that, in solution, it is highly possible for the olefinic acceptor on pyrrole β-carbon to have a coplanar or near coplanar conformation with the pyrrole of the porphyrin core. This conformation ensures effective π-conjugation between the pyrrole ring and its olefinic substituent which is reflected in the linear absorption spectroscopy as the red-shifting of both Soret and Q-bands and possibly the splitting of the absorption Soret band. The intensified long wavelength absorption Q-band of the porphyrin is also consistent with the proposed π-conjugation. Regarding nonlinear optical absorption, an active nonlinear optical (NLO) response has been observed for these porphyrins, indicative of efficient electronic coupling (resonance interaction) between the electronic acceptor and porphyrin macrocyclic ring, compatible with a coplanar or near coplanar conformation in order to achieve π-conjugation. Such conformation of the peripherally appended olefinic acceptor has been further confirmed by INDO/1 Hamiltonian in the ZINDO computational calculations. The INDO/S electron density calculation further reveals that the range of intramolecular charge-transfer (ICT) does not encompass the whole porphyrin core but is mostly confined to the pyrrole ring attached to the olefin-linked acceptor. Furthermore, ICT characteristics were found to be more heavily involved in the Soret absorption band than Q-band. Our analysis of the porphyrin π-conjugation is consistent with modest solvatochromic behavior and the moderate NLO activity of the porphyrins compared with NLO chromophores with many less π-electrons.