Control of Photoisomerization Kinetics via Multistage Switching in Bimetallic Ru(II)–Terpyridine Complexes

Abstract

In this study, we carried out detailed experimental and theoretical investigation on photophysical, electrochemical, and photoisomerization behaviors of a new array of luminescent binuclear Ru(II) complexes derived from a phenylene−vinylene-substituted terpyridyl ligand possessing RT lifetimes within 60.3−410.5 ns. The complexes experienced trans-to-cis isomerization in MeCN on irradiation with visible light, accompanied by significant changes in their absorption and emission spectral profiles. The reverse cis-to-trans process is also possible with the use of ultraviolet (UV) light. On conversion from trans to cis isomers, the emission intensity increases substantially, while for the reverse process, luminescence quenching occurs. Thus, "off−on" and "on−off" emission switching is facilitated upon treatment with visible and UV light alternatively. By the use of chemical oxidants (ceric ammonium nitrate and potassium permanganate) and reductants (metallic sodium) as well as light of appropriate wavelengths, multistate switching phenomena involving reversible oxidation−reduction and trans−cis isomerization have been achieved. Interestingly, the rate of this multistate photoswitching process becomes much faster compared to only two-state trans−cis isomerization of these complexes. Density functional theory (DFT) and time-dependent-DFT (TD-DFT) calculations are also performed to obtain a clear picture of the electronic environment of the complexes and also for the appropriate assignment of absorption and emission spectral bands.

Advancing Molecular-Scale Logic Devices through Multistage Switching in a Luminescent Bimetallic Ru(II)-Terpyridine Complex

“…The synthesis, characterization, and photoisomerization behavior of 1 4+ ( t – t ) have just been reported by our group . A concise summary of the spectral changes under the influence of a chemical oxidant (Ce 4+ ), a reductant (Na metal), and light of different wavelengths is again provided here for the benefit of the readers.…”

“…The synthesis and characterization of the bimetallic Ru(II) complex of composition, [(Me 2 bip)Ru(tpvpvpt)Ru(Me 2 bip)] 4+ , have been executed by our recently reported literature procedure …”

“…In this study, we employed our recently reported bimetallic Ru(II) complex, [{(Me 2 bip)Ru(tpvpvpt)Ru(Me 2 bip)} 4+ ] { 1 4+ ( t–t )}, featuring consecutive phenylene–vinylene motifs in-between the two terpyridine units within the ligand architecture (Chart ). The phenylene-vinylene units facilitate trans – trans ( t–t ) to cis – cis ( c–c ) isomerization upon visible light irradiation while reverse c – c to t – t isomerization upon exposure to UV light with remarkable change in its absorption and emission spectral profiles in both ways. − Furthermore, the rate of photoisomerization of the complex has been significantly enhanced in the presence of a chemical oxidant (Ce 4+ ) as well as a reductant (metallic sodium). Inclusion of Ce 4+ leads to the oxidation of the t – t form of the complex from its 4+ to 6+ state { 1 4+ ( t – t ) → 1 6+ ( t – t )}, which upon light irradiation leads to simultaneous reduction as well as isomerization from the t – t to the c – c form { 1 6+ ( t – t ) → 1 4+ ( c – c )}.…”