Tuning the Fluorescence Emission and HOMO-LUMO Band Gap in Homoleptic Zinc(II) Complexes with N,O-Bidentate (Imidazo[1,5-a]pyrid-3-yl)phenols

A series of homoleptic zinc(II) complexes of the general formula [Zn(L-R)(2)] (HLR: (imidazo[1,5-a]pyrid-3-yl)phenol; R: para-substituent to the phenol) have been synthesized. The single-crystal X-ray structure analysis of complex [Zn(L-H)(2)] (1) confirmed the expected N,O-bidentate coordination of L-R, via the pyridine-like nitrogen of the imidazo[1,5-a]pyridine skeleton and the phenolate oxygen. The photophysical properties of the complexes have been investigated in dichloromethane solution, showing fluorescence emission when excited with UV light ((exc) = 340-360 nm). The intensity and (max) of the emission are both significantly influenced by the R-substituent, the emission maxima moving from blue (R = CF3, Zn(L-CF3)(2)] (6)) to orange (R = NO2, Zn(L-NO2)(2)] (7). Most of [Zn(L-R)(2)] compounds are characterized by moderate-to-good absolute photoluminescence quantum yields, with a maximum of 0.33 for [Zn(L-H)(2)] (1). Density functional calculations allowed to identify the Natural Transition Orbitals involved in the electronic transitions and define the main transition as being HOMO-LUMO (>95%) in character. A good linear correlation was found between the HOMO energy and the Hammett sigma(p) constants associated to the R-substituent, whereas the fluorescence behavior has been described in terms of HOMO-LUMO band gap.