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(LR)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(LH)2] (1) confirmed the expected N,O-bidentate coordination of LR, 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 (λexc = 340–360 nm). The intensity and λmax of the emission are both significantly influenced by the R-substituent, the emission maxima moving from blue (R = CF3, Zn(LCF3)2] (6)) to orange (R = NO2, Zn(LNO2)2] (7). Most of [Zn(LR)2] compounds are characterized by moderate-to-good absolute photoluminescence quantum yields, with a maximum of 0.33 for [Zn(LH)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 σp constants associated to the R-substituent, whereas the fluorescence behavior has been described in terms of HOMO-LUMO band gap.