N N N M Copyright © 2007 Society of Porphyrins & Phthalocyanines Published on web 02/19/2008 Journal of Porphyrins and Phthalocyanines Published at http://www.u-bourgogne.fr/jpp/ J. Porphyrins Phthalocyanines 2007; 11: 857-873 Conformation and π-conjugation of olein-bridged acceptor on the pyrrole β-carbon of nickel tetraphenylporphyrins: implicit evidence from linear and nonlinear optical properties Hsiu-Chih Yeh a , Chin-Ti Chen* a , Jenwei Yu* b , Pei-Chang Tsai c and Juen-Kai Wang* c a Institute of Chemistry, Academia Sinica, Taipei, Taiwan 11529, Republic of China b Department of Chemistry, National Kaohsiung Normal University, Kaohsiung, Taiwan 824, Republic of China c Center for Condensed Matter Sciences, National Taiwan University and Institute of Atomic and Molecular Science Academia Sinica, Taipei, Taiwan 106, Republic of China Received 20 July 2007 Accepted 24 January 2008 ABSTRACT: This paper has scrutinized, both experimentally and theoretically, the conformation of an oleinic substituent on the pyrrole β-carbon of tetraphenylporphyrin. Compelling evidences collec- ted from linear optical absorption and NOESY 1 H NMR experiment strongly suggest that, in solution, it is highly possible for the oleinic 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-conjuga- tion between the pyrrole ring and its oleinic substituent which is relected 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 intensiied 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 eficient 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 oleinic acceptor has been further conirmed 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 conined to the pyrrole ring attached to the olein-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. Copyright © 2007 Society of Porphyrins & Phthalocyanines. KEYWORDS: tetraphenylporphyrin, pyrrole β-carbon, conformation, π-conjugation, electron acceptor, optical nonlinearity, UV-visible spectra. INTRODUCTION Due to their extended π-conjugation system, por- phyrins or their metal-chelated complexes have long been appealing molecular constructs in the design of molecular devices involving photoelectronic appli- cations, such as light-harvesting and molecular pho- tonic porphyrin arrays [1]. In addition to the edge- fused porphyrin arrays [2], an alkynyl linker on either the meso or β-pyrrole carbon of the porphyrin core *Correspondence to: Chin-Ti Chen, email: cchen@chem. sinica.edu.tw; Jenwei Yu, email: t2064@nknucc.nknu.edu. tw and Juen-Kai Wang, email: jkwang@ntu.edu.tw  SPP full member in good standing