Neutral Radical and Singlet Biradical Forms of Meso-Free, -Keto, and -Diketo Hexaphyrins(220.127.116.11.1.1): Effects on Aromaticity and Photophysical Properties
journal contributionposted on 05.10.2011, 00:00 by Masatoshi Ishida, Jae-Yoon Shin, Jong Min Lim, Byung Sun Lee, Min-Chul Yoon, Taro Koide, Jonathan L. Sessler, Atsuhiro Osuka, Dongho Kim
We have investigated the electronic structures and photophysical properties of 5,10,20,25-tetrakis(pentafluorophenyl)-substituted hexaphyrin(18.104.22.168.1.1) (1) and its meso-keto (2) and meso-diketo derivatives (3) 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 1 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 2 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 3 exhibits features similar to those of 2, specifically NIR absorptions and a short excited-state lifetime (9.7 ps). Compound 3 is thus assigned as being a ground-state singlet biradicaloid. Two photon absorption (TPA) measurements revealed comparatively large σ(2) values of 600 GM for 2 and 1600 GM for 3 with excitation at λex =1600 nm as compared to that observed for 1 (σ(2): 360 GM). The enhanced nonlinear optical properties of 2 and 3 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 1–3. Importantly, the open-shell radicaloid 2 and singlet biradicaloid 3 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 2 and 3 play an important role in perturbing the macrocyclic π-conjugation of the parent hexaphyrin structure 1. 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.
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138 pselectron densityenergy absorption bandsoscillator modelTPAPhotophysical PropertiesWespectroscopic measurementstermγ density analysesNeutral RadicalHOMA1600 GMCompound 3singlet biradicaloid 3photophysical propertiesmonoradical speciesSinglet Biradical Forms600 GMparent hexaphyrin structure 1.2 displays featuresground statebiradical characterspectroscopic featureslifetimeAICDphoton absorptionNICSNIR absorptionssinglet biradical characterSoret band