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 ] (HL R : (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 (λ exc = 340–360 nm). The intensity and λ max of the emission are both significantly influenced by the R-substituent, the emission maxima moving from blue (R = CF 3 , Zn(L CF3 ) 2 ] (6)) to orange (R = NO 2 , 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 σ p constants associated to the R-substituent, whereas the fluorescence behavior has been described in terms of HOMO-LUMO band gap.