Neutral Radical and Singlet Biradical Forms of Meso-Free, -Keto, and -Diketo Hexaphyrins( Effects on Aromaticity and Photophysical Properties

We have investigated the electronic structures and photophysical properties of 5,10,20,25-tetrakis(pentafluorophenyl)-substituted hexaphyrin( (<b>1</b>) and its meso-keto (<b>2</b>) and meso-diketo derivatives (<b>3</b>) using various spectroscopic measurements. In conjunction with theoretical calculations, these analyses revealed fundamental structure–property relationships within this series, including unusual ground-state electronic structures with neutral, monoradical, and singlet biradical character. The meso-free species <b>1</b> is a representative 26 π-electron aromatic compound and shows characteristic spectroscopic features, including a sharp Soret band, well-defined Q-like bands, and a moderately long excited state lifetime (τ = 138 ps). In contrast, the meso-keto derivative <b>2</b> displays features characteristic of a neutral monoradical species at the ground state, including the presence of lower energy absorption bands in the NIR spectral region and a relatively short excited-state lifetime (13.9 ps). The meso-diketo <b>3</b> exhibits features similar to those of <b>2</b>, specifically NIR absorptions and a short excited-state lifetime (9.7 ps). Compound <b>3</b> is thus assigned as being a ground-state singlet biradicaloid. Two photon absorption (TPA) measurements revealed comparatively large σ<sup>(2)</sup> values of 600 GM for <b>2</b> and 1600 GM for <b>3</b> with excitation at λ<sub>ex</sub> =1600 nm as compared to that observed for <b>1</b> (σ<sup>(2)</sup>: 360 GM). The enhanced nonlinear optical properties of <b>2</b> and <b>3</b> are rationalized in terms of the open-shell electronic configuration allowing a large, field-induced fluctuation in the electron density (i.e., a large polarization). This interpretation is supported by theoretical evaluations of the static second hyperpolarizabilities (γ) and γ density analyses. Furthermore, nucleus-independent chemical shift (NICS) and harmonic oscillator model of aromaticity (HOMA) values and anisotropy of the induced current density (AICD) plots revealed a clear distinction in terms of the aromatic character of <b>1</b>–<b>3</b>. Importantly, the open-shell radicaloid <b>2</b> and singlet biradicaloid <b>3</b> can be formally regarded as 27 π-electron nonaromatic and 26 π-electron aromatic species, respectively, constrained within a dominant 28 π-electron conjugated network. On the basis of the combined experimental and theoretical evidence, it is concluded that the meso-carbonyl groups of <b>2</b> and <b>3</b> play an important role in perturbing the macrocyclic π-conjugation of the parent hexaphyrin structure <b>1</b>. In particular, they lead to the imposition of intrinsic radical and biradical character on the molecule as a whole and thus easy-to-discern modifications of the overall electronic effects.