Synthesis, Photophysics, and Switchable Luminescence Properties of a New Class of Ruthenium(II)–Terpyridine Complexes Containing Photoisomerizable Styrylbenzene Units

We
report here the synthesis and structural characterization of
a new class of homoleptic terpyridine complexes of Ru­(II) containing
styrylbenzene moieties to improve room-temperature luminescence properties.
Solid-state structure determination of 2 was done through
single-crystal X-ray diffraction. Tuning of photophysical properties
was done by incorporating both electron-donating and electron-withdrawing
substituents in the ligand. The complexes exhibit strong emission
having lifetimes in the range of 10.0–158.5 ns, dependent on
the substituent and the solvent. Good correlations were also observed
between Hammett σp parameters with the lifetimes
of the complexes. Styrylbenzene moieties in the complexes induce trans–trans
to trans–cis isomerization accompanied by huge alteration of
their spectral profiles upon treating with UV light. Reversal of trans–cis
to trans–trans forms was also achieved on interacting with
visible light. Change from trans–trans to the corresponding
trans–cis form leads to emission quenching, whereas trans–cis
to the corresponding trans–trans form leads to restoration
of emission. In essence, “on–off” and “off–on”
photoswitching of luminescence was observed. Calculations involving
density functional theory (DFT) and time-dependent-DFT methods were
performed to understand the electronic structures as well as for appropriate
assignment of the absorption and emission bands.