With various TEQ derivatives in hand, the photophysical properties of this new class of saddle-shaped molecules were studied. While TEQ derivatives showed inherently low fluorescence, the presence of a protic acid converted these into intensely emissive luminogens in a concentration-dependent fashion (Fig. ). The fluorescence of TEQ 8a in CH2Cl2 increased upon incremental addition of TFA and became saturated in the presence of 4 equiv of TFA, with intense emission at 461 nm (f = 0.63). Red and blue shifts were observed for phenylethynyl derivatives 8b (lmax = 452 nm; f = 0.24) and 8d (lmax = 495 nm; f = 0.71), which contain free phenolic hydroxyl groups. The structurally similar pyridine tetramer 11 exhibited no proton-dependent fluorescence, highlighting the unique photophysical nature of TEQ 8 with a rigid concatenated structure comprised of four quinoline units (Fig. ). While the fluorescence intensity and emission wavelength of these TEQs varied depending on the media, their relationship with the nature of the solvents remains unclear at this point (Fig. ). The counter-anion of the Brønsted acid was another factor to consider in the photoluminescence; mchlorobenzoic acid presumably interacts with photoexcited 8a to facilitate nonradiative decay, rendering protonated 8a barely emissive (Fig. ). Interestingly, the fluorescence of 8a was turned on by contact with a specific metal cation, i.e., Zn(II) (Fig. ). Among a series of first-row d-block metal cations, only the Zn(II) cation rendered 8a highly emissive in EtOH with an intensity difference of approximately two orders of magnitude relative to other metal cations. The fluorescence intensity was responsive to the Zn(II) concentration with a dissociation constant of 0.88 nM (H2O), and the isolated 8a/Zn(II) complex provided a high quantum yield (f = 0.43) for the emission at 474 nm with a large Stokes shift (132 nm) (Fig. , Supplementary Section 7). Although azacalix [4]pyridines have been reported to become emissive upon complexation of Zn(II), this involves only a three-fold increase in the fluorescence intensity with a minute quantum yield of (f = 0.010) . Given that the Zn(II) cation plays a pivotal role in biological systems, Zn(II)-based fluorescent probes have garnered sustained interest in the field of life sciences . In contrast to most Zn(II)-based fluorescent probes, which are comprised of a fluorophore and a chelating/quenching unit , 8a is inherently non-fluorogenic, promising prospective high-contrast imaging. While other quinoline-based acyclic Zn(II) probes can be found in the literature, they all suffer from low quantum yields or poor selectivity in the presence of Cd(II) . 8a is fluorescence-silent in the presence of other biologically abundant group-1 and group-2 metal cations such as Na(I), K(I), Mg(II), and Ca(II), and only 8a/Zn(II) exhibits the 100-fold increased fluorescence intensity, while the physicochemically similar Cd(II) does not induce such an increase (Fig. ). The fluorescent nature and C2-symmetric chirality of 8a suggest potential applications for circularly polarized luminescence (CPL) . Indeed, (+)-8a and (-)-8a displayed mirror-image CPL spectra in the presence of 4 eq of TFA or 1 eq of ZnCl2 with absolute luminescence dissymmetry factors (|glum|) of 5.61 x 10 -4 and 3.16 x 10 -4 , respectively (Supplementary Figs. 15-18).