We then systematically investigated the PL response of TPAPyBP (1.3´10 -4 M) in toluene upon gradual addition of ZnCl2 (0.10 M) in ethanol. As shown in Figure , the PL intensity of TPAPyBP at 550 nm decreases progressively upon addition of ZnCl2 with concomitant increase of a new emission band at 680 nm. This leads to a stark spectral response where the emission changes from greenish yellow to deep-red Figure , with corresponding the Commission International de L'Éclairage (CIE) coordinates from (0.44, 0.55) to (0.61, 0.38), Figure . The time-resolved photoluminescence (TRPL) of TPAPyBP with 10 equiv. of ZnCl2 still shows monoexponential decay kinetics; however, the lifetime is shorter at 2.9 ns compared to 4.9 ns in the absence of ZnCl2 (Figure ). Similarly, there are distinct spectral changes in the UV/vis absorption spectrum whereupon gradual addition of ZnCl2, the absorption band at 338 nm was bathochromically shifted to 358 nm while a new CT band appeared at 505 nm, probably due to the formation of a Zn complex (Figure ). An isosbestic point at 487 nm and the 1:1 stoichiometry identified in the Job plot indicates that only a single ZnCl2 is coordinated to TPAPyBP (Figure ). Single crystals were grown by slow evaporation of a saturated toluene solution of the complex at room temperature. The structure of Zn(TPAPyBP)Cl2 is shown in Figure and reveals that the Zinc ion adopts a distorted tetrahedral geometry, coordinated through the pyridyl nitrogen of TPAPyBP (N1), two chlorido ligands and a molecule of ethanol solvent (N-Zn-Cl bond angle of 107.6(2) and 116.0(2)º and N-Zn-O bond angle of 96.5(2)º). This, or a structurally related tetrahedral complex, is the likely putative species in solution. The 1 H NMR spectrum of TPAPyBP with increasing concertation of ZnCl2 in CDCl3 revealed that the resonances at positions 1 and 3 (Figure ) of TPAPyBP were the most perturbed upon addition of ZnCl2, suggesting a possible coordination of Zn 2+ ion through pyrido [3,4-b]pyrazine core of the acceptor (Figure ).