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.