Controlling the Excited State and Photosensitizing Property of a 2-(2-Pyridyl)benzo[b]thiophene-Based Cationic Iridium Complex through Simple Chemical Modification

Abstract

Bis-cyclometalated cationic iridium (Ir) complexes 1-4 comprising two 2-(2-pyridyl)benzo[b]thiophene (btp) ligands and one 2,2'-bipyridyl (bpy) ancillary ligand with different substituents were prepared as new visible light-absorbing sensitizers and examined for their photophysical and electrochemical properties. Complex 1 was prepared as a parent complex without any substituents. Complexes 2-4 contained methyl-, methoxy-, and trifluoromethyl groups at 4,4'-positions on the bpy ancillary ligand. Systematic investigation of these complexes revealed that such a simple chemical modification selectively controls the excited-state lifetime, while the absorption and emission spectral features remain unchanged. Specifically, the phosphorescence lifetimes of complexes 2 and 3 with electron-donating groups (τ = 3.50 μs, 3.90 μs) were found to be much longer than that of complex 1 (τ = 0.273 μs), and complex 4, possessing strong electron-withdrawing trifluoromethyl groups, did not exhibit detectable phosphorescence at room temperature. The large differences in excited-state lifetimes of complexes 1-3, as well as the nonemissive character of complex 4, are attributed to a strong influence of the substituents on the ligand field strength. The increased σ-donating ability of the ancillary ligand in complexes 2 and 3 destabilizes a short-lived, nonemissive triplet metal-centered ((3)MC) state and increases the energy separation between the (3)MC state and emissive triplet ligand-centered ((3)LC) state based on the btp ligand. For complex 4, however, the (3)MC state is close in energy to the (3)LC state because of the decreased σ-donating ability of the ancillary ligand. Additional evidence of the (3)MC state associated with the changeable excited state was also provided via low-temperature phosphorescence measurements and density functional theory calculations. Ir complexes 1-4 were tested as sensitizers in photoinduced electron-transfer reaction of triethanolamine and methylviologen chloride (MVCl2). As a result, complexes 2 and 3 exhibited much better photosensitizing property compared to complex 1 since their long-lived excited states promoted an oxidative quenching pathway. This Study has first demonstrated that simple substitution on the diimine ancillary ligand can control the (3)MC state of the bis-cyclometalated cationic Ir complex to finely tune the excited-state lifetime and photosensitizing property.

Cationic Iridium(III) Complexes with Two Carbene-Based Cyclometalating Ligands: Cis Versus Trans Isomers

“…In contrast to the archetype compounds [Ir-(ppy) 2 (L^L)]PF 6 (ppy = 2-phenylpyridine or 2-(2,4-difluorophenyl)pyridine and L^L = bis-carbene or pyridinecarbene), where the pyridine groups of the cyclometallating ppy ligands are always in trans orientation to each other, 31 in this series we observed cis and trans isomers. In both cases the C(1)−Ir(1)−C (19) angle deviates from linearity with 169.74 (7) 58 On the other hand, the described cathodic shift of 20 mV for the reduction potential was not found in our study. The minor differences in donor strength of the substituents on the 2,2′-bipyridine ligands seem to be overcompensated by the strongly donating carbene groups, resulting in an ∼150 mV cathodic shift of complexes trans-1 and trans-3 when compared to its corresponding 2-(2-pyridyl)benzo[b]thiophene iridium complexes.…”

“…The minor differences in donor strength of the substituents on the 2,2′-bipyridine ligands seem to be overcompensated by the strongly donating carbene groups, resulting in an ∼150 mV cathodic shift of complexes trans-1 and trans-3 when compared to its corresponding 2-(2-pyridyl)benzo[b]thiophene iridium complexes. 58 Both, oxidation and reduction processes show excellent reversibility throughout the entire series, as rarely observed for analogous luminescent iridium complexes. High stability can therefore be expected in electroluminescent devices.…”

Deep-Red Phosphorescent Iridium(III) Complexes Containing 1-(Benzo[b] Thiophen-2-yl) Isoquinoline Ligand: Synthesis, Photophysical and Electrochemical Properties and DFT Calculations

“…Intense absorption bands observed below 365 nm are attributed to the spin-allowed ligand-centered 1 π-π * transitions arising from both the cyclometalated and ancillary ligands. The weaker bands at lower energies (400-600 nm) are assigned to spin-allowed [35][36][37][38]. However, it is found that the electrondonating tert-butyl groups on the bpy ligand of complex 3c cause the absorption maximum red-shift of 10 nm in the latter low-energy region.…”

Tris-heteroleptic Iridium Complexes Based on Cyclometalated Ligands with Different Cores

“…A major drawback of these strategies is that they invariably result in concomitant variation of the emission colour and/or redox potentials of the complexes because of the intimate interplay between chemical structure and properties. [57][58][59] Therefore distinguishing the exact effect of a structural modification on specific properties is still a major challenge.…”