Low-cost photo-switches based on stilbene-appended Zn(II)–terpyridine complexes

Abstract

We report herein the synthesis, characterization, photophysics, and photo-isomerization behaviors of three Zn(II)-terpyridine complexes of the type [Zn(tpy-pvp-X) 2 ] 2+ (X = H, Me, and NO 2 ) covalently tethered with stilbene moiety. The complexes exhibit absorption bands stretching up to the edge of the visible domain due to ligand → ligand charge transfer (LLCT) transitions and strong emission at room temperature in the visible due to radiative deactivation of 3 LLCT state having lifetime within 1.0-3.0 ns. The stilbene motifs in the complexes undergo trans to cis isomerization upon irradiating with UV and visible light accompanied by significant alteration of their absorption, emission, and 1 H NMR spectral profiles. Apart from the variation of electron donating and electron withdrawing substituent (X), the isomerization studies were also carried out in three different solvents (DCM, MeCN, and DMSO) to further tune their kinetic and thermodynamic parameters. The rate, rate constant and quantum yield of isomerization were estimated in all the solvents. The reverse process (cis to trans) also occurs very slowly on keeping but could be accelerated upon heating. Trans to cis photoisomerization leads to quenching of emission in case of 1 and 2, whereas backward thermal cis to trans conversion leads to restoration of emission. By contrast, for the nitro-derivative (3) forward process induces emission enhancement, while backward process gives rise to emission quenching. In essence, "on-off" and "off-on" emission switching is feasible for 1 and 2, whereas "off-on" and "on-off" emission switching occurs in case of 3. Emission spectral responses upon successive action of photonic and thermal input lead to the fabrication of INHIBIT and IMPLICATION logic gates. DFT and TD-DFT computational investigations were also undertaken to visualize their electronic structures, correct assignment of the spectral bands, and mode of isomerization process.

A Bichromophoric Approach to Induce Luminescence – A Blend of Experimental and Theoretical Studies

“…Hence the emission in case of 1 might be attributed to an admixture of 3 LC and 3 MLCT. On the other hand complex 2 was found to be non‐luminescent which was in agreement with the previous literature reports for Fe [20,21] . Hence the strategy of using the ligand having a phenyl spacer between the An and the tpy moieties was found to be successful in order to induce luminescence in complex 1 which was earlier absent in its structural analogue R1 .…”

“…The photoluminescence quantum yield (PLQY, Φ) of 1 was measured in MeCN at RT under degassed conditions and was found to be 0.03. On the other hand, complex 2 was found to be non‐luminescent at RT which was in agreement with the literature complex R2 and other Fe‐tpy‐based complexes (Figure 5, Chart 2 and see the section of potential energy diagram for a relative comparison of emitting states in 1 and 2 ) [21,22] . The Fe(II) complexes, in contrast to their low spin d 6 analogues (Ru(II) and Os(II)), are not good sensitizers and are reluctant to exhibit emission.…”

“…An investigation into the excited state photophysical properties of the complexes 1 and 2 , revealed the fact that the strategy of incorporating Ph spacer was useful in case of 1 which exhibited NIR emission at ~700 nm with an excited state lifetime components of 1.33 and 6.52 ns. However, complex 2 was found to be non‐emissive at RT which complimented with the reported Fe(II)‐tpy complexes in which the excited electron usually deactivates through the cascade 3 MLCT→ 3 MC→ 5 MC→ 1 GS pathway [20,21] . Additional experiments such as generation of photocurrent and theoretical calculations at the level of DFT also supported the fact of emissive and non‐emissive natures of complexes 1 and 2 , respectively.…”