Exploring the photophysical properties of unusual π-conjugated porphyrin nanohoops

Abstract

π-Conjugated nanohoops containing porphyrins, a long desirable target, have become synthetically accessible thus far. Exploring their origin of photophysical properties at the electronic microstructure level is rather important for performance improvement and application. Herein, we investigated the photophysical properties of a series of π-conjugated nanohoops, consisting of two porphyrins linked by terphenyl bridges, via density functional theory calculations. Compound 1 exhibits a π–π* electron transition character within the porphyrins, and intrinsic chirality primarily originating from exciton-coupling between the porphyrin moieties and terphenyls. Based on the parent compound 1, seven cyclic porphyrin-based compounds 2–8 with various central metals and electron donor/acceptor substituents were designed. These subtle structural modifications have significant impact on the photophysical properties such as UV-vis absorption intensity, electron transition properties, and second-order nonlinear optical (NLO) responses, indicating an obvious adjustability of electron properties. Notably, the second-order polarization value of compound 8 is up to 76.6 × 10⁻³⁰ esu, which is about 14 times that (cal. 5.30 × 10⁻³⁰ esu) of the typical organic molecule p-nitroaniline. Thus, the studied compounds would be potential candidates for second-order NLO materials.