Luminescent Cyclometalated Rh^III, Ir^III, and (DIP)₂Ru^II Complexes with Carboxylated Bipyridyl Ligands: Synthesis, X-ray Molecular Structure, and Photophysical Properties

Abstract

Luminescent cyclometalated rhodium(III) and iridium(III) complexes of the general formula [M(ppy) 2(N (wedge)N)][PF 6], with N (wedge)N = Hcmbpy = 4-carboxy-4'-methyl-2,2'-bipyridine and M = Rh ( 1), Ir ( 2) and N (wedge)N = H 2dcbpy = 4,4'-dicarboxy-2,2'-bipyridine and M = Rh ( 3), Ir ( 4), were prepared in high yields and fully characterized. The X-ray molecular structure of the monocarboxylic iridium complex [Ir(ppy) 2(Hcmbpy)][PF 6] ( 2) was also determined. The photophysical properties of these compounds were studied and showed that the photoluminescence of rhodium complexes 1 and 3 and iridium complexes 2 and 4 originates from intraligand charge-transfer (ILCT) and metal-to-ligand charge-transfer/ligand-centered MLCT/LC excited states, respectively. For comparison purposes, the mono- and dicarboxylic acid ruthenium complexes [Ru(DIP) 2(Hcmbpy)][Cl] 2 ( 5) and [Ru(DIP) 2(H 2dcbpy)][Cl] 2 ( 6), where DIP = 4,7-diphenyl-1,10-phenanthroline, were also prepared, whose emission is MLCT in nature. Comparison of the photophysical behavior of these rhodium(III), iridium(III), and ruthenium(II) complexes reveals the influence of the carboxylic groups that affect in different ways the ILCT, MLCT, and LC states.

Theoretical investigation on the spectroscopic properties of cyclometallated iridium (III) complexes and the deprotonation influence on them in solution

“…These specialities lead to the red-shifted emission of 2. Moreover, this agrees to what has been reported experimentally [23]. The intensities for emission are not available in current investigation.…”

“…The calculated phosphorescence and the related frontier molecular orbital compositions are summarized in Tables V and VI, respectively. The calculated emission from A 3 A excited state is at 553 and 654 nm for 1 and 2, respectively, which can be comparable with the respective measured one at 565 and 624 nm [23]. This confirms, once again, the good performance of the hybrid scheme of TDDFT/CIS method for The calculated emission at 553 nm of 1 arises from the electron transition of MO 146a → 145a (see Table V).…”

“…When compared with other iridium complexes, Waern claimed [23] that the subligand COOH group may complicate the photophysical behavior of [Ir(NˆC) 2 …”

“…Waern et al [23] have tried to probe the influence of the COOH groups on the absorption and emission spectra by titration experiments but they suffered either solubility problems or degradation effects. However, we can model the deprotonation behavior theoretically.…”

Charge photo-accumulation and photocatalytic hydrogen evolution under visible light at an iridium(iii)-photosensitized polyoxotungstate

“…In this context, we recently reported the synthesis and photophysical properties of a series of heteroleptic carbocyclometalated iridium(III)-polyoxometalate conjugates 41,42 following mild reaction conditions developed earlier by some of us. [43][44][45][46] In such complexes, photoinduced chargeseparated excited states of various lifetimes (ranging from nanoseconds to hundreds of nanoseconds) were observed by transient absorption spectroscopy. Most importantly, the functionalization of the heteroleptic cyclometalated iridium(III) on the picolinate ligand provides directionality to the photoinduced electron transfer by enhancing charge separation and delaying charge recombination, which is an asset for preventing reverse charge transfer from proceeding (equation 1).…”