Emission Switching in the Near-Infrared by Reversible Trans–Cis Photoisomerization of Styrylbenzene-Conjugated Osmium Terpyridine Complexes

Abstract

A new array of homoleptic osmium(II) complexes based on styrylbenzene-conjugated terpyridine ligands (tpy-pvp-X) were synthesized and their photophysical, electrochemical, and photoisomerization behaviors thoroughly investigated in this work. Both electron-donating and -withdrawing substituents were incorporated onto a tpy-pvp-X (X = H, Me, Cl, NO 2 , and Ph) moiety to tune the optical properties and also the rate of photoisomerization behaviors in the complexes. All complexes display strong spin-allowed singlet metal-to-ligand charge-transfer bands in the visible (495−506 nm) and weak singlet ground state to triplet metal-to-ligand charge-transfer ( 3 MLCT) broad bands within the 600−700 nm range. The complexes also exhibit strong phosphorescence emission from their 3 MLCT state in the nearinfrared domain (737−752 nm) at room temperature with excited-state lifetimes spanning between 107 and 165 ns. Two styrylbenzene units promote reversible trans−trans to trans−cis/cis−cis isomerization induced by light. The rate constants and quantum yields of photoisomerization were found to vary linearly with the Hammett σ p parameters of the substituents. The rate and quantum yields were also found to decrease with increasing polarity of the solvents. Considerable modulation of the optical behavior along with luminescence switching in the complexes has been achieved upon photoisomerization. Moreover, the optical outputs as a function of two photonic stimuli inputs were used to demonstrate the binary function of a two-input IMPLICATION logic gate. In conjunction with the experimental study, computational investigations were also carried out in all three conformations of the complexes (trans−trans, trans−cis, and cis−cis) to have a perception of their electronic structures and for correct assignment of their absorption and emission spectral bands.

Efficient photoswitchable organometallic complexes with azobenzene and stilbene units: the case of Au(i)

“…Preservation of the photoisomerisation quantum yield value for {metallocentre + photoswitch} molecular systems in comparison with the native photoswitch molecule is rare, 40,72 and the enhancement of this value is unique. 53 The literature is dominated by examples where photoisomerisation is partially 13,15,22,23,25,36,48 or completely 31,33,42,45,46 inhibited by the presence of the metal due to energy transfer, 34 competition with MLCT processes 16,18,24,32,54,55 and/or luminescence of the metallocentre, 52 redox process, 50 or steric hindrances. 29,30,38 Compounds LN, LC and complexes 1N-5N, 1C and 5C are completely non-emissive in solution upon UV irradiation, while 6N demonstrates very weak luminescence at ca.…”

“…To date, a number of complexes containing azobenzene as a reversible photoswitch have been reported in the literature for Pt( ii / iv ), 13–25 Ni( ii ), 14,26–28 Pd( ii ), 14,23,29,30 Re( i ), 31–33 Co( ii / iii ), 28,34 Fe( ii / iii ), 34–38 Zn( ii ), 36,38–43 Cu( i / ii ), 28,36,42,44 Cd( ii ), 38 Ir( iii ), 45,46 Rh( iii ), 47 and Ln( iii ), 48 and for ferrocene derivatives. 49–51 In contrast to azobenzene, stilbene organometallic derivatives are more often used not as photoswitches 52–55 but as building blocks for assembly of super- 56–60 and supramolecular systems, 61–64 as well as of π-conjugated systems which can show nonlinear optical effects. 65–71 However, examples of the combination of the Au( i ) centre with an azobenzene 72–76 or stilbene 56,57,66–68,70,77 photochromic moiety are still rare in the literature and most have not been studied as organometallic photoswitches.…”

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

“…Nishihara's group reported some terpyridine complexes of Ru(II) and Rh(III) coupled with photoactive azo unit [68]. Recently, our group initiated the isomerization studies of stilbene-appended terpyridine complexes of Fe(II), Ru(II) and Os(II) metals [30,31,33,59]. Photoisomerization behaviors of various heavier transition metal complexes (Re, Co, Rh, Ir, etc.)…”

“…Even though lots of studies were performed with purely organic stilbene derivatives, but analogous studies with their coordination complexes are comparatively limited in the literature [18][19][20][21][22]. Compared to organic compounds, metal complexes offer a greater variety of electronic structures, in relation to the coordination sphere, electronic configuration and oxidation state of the metal [23][24][25][26][27][28][29][30][31][32][33]. To mimic the function of photochemical molecular devices (such as photo switches) in metal complexes, one of the most effective strategies is to reversibly alter their properties by inducing conformational changes on the photo-responsive component appended on the ligands.…”

“…To mimic the function of photochemical molecular devices (such as photo switches) in metal complexes, one of the most effective strategies is to reversibly alter their properties by inducing conformational changes on the photo-responsive component appended on the ligands. A majority of the studies to this end are primarily focused on noble metals which are very expensive and the synthetic procedures often demand very drastic conditions [23][24][25][26][27][28][29][30][31][32][33]. Our primary objective in this work is to design suitable basemetal complexes which could show off similar behavior and can lead to the construction of a new class of low-cost and easily synthesizable photo-switches.…”