Abstract
In the current work, comprehensive photophysical and electrochemical studies were performed for eight rhenium(I) complexes incorporating 2,2′:6′,2″-terpyridine (terpy) and 2,6-bis(pyrazin-2-yl)pyridine (dppy) with appended 1-naphthyl-, 2-naphthyl-, 9-phenanthrenyl, and 1-pyrenyl groups. Naphthyl and phenanthrenyl substituents marginally affected the energy of the MLCT absorption and emission bands, signaling a weak electronic coupling of the appended aryl group with the Re(I) center. The triplet MLCT state in these complexes is so low lying relative to the triplet 3ILaryl that the thermal population of the triplet excited state delocalized on the organic chromophore is ineffective. The attachment of the electron-rich pyrenyl group resulted in a noticeable red shift and a significant increase in molar absorption coefficients of the lowest energy absorption of the resulting Re(I) complexes due to the contribution of intraligand charge-transfer (ILCT) transitions occurring from the pyrenyl substituent to the terpy/dppy core. At 77 K, the excited states of [ReCl(CO)3(Ln-κ2N)] with 1-pyrenyl-functionalized ligands were found to have predominant 3ILpyrene/3ILCTpyrene→terpy character. The 3IL/3ILCT nature of the lowest energy excited state of [ReCl(CO)3(4′-(1-pyrenyl)-terpy-κ2N)] was also evidenced by nanosecond transient absorption and time-resolved emission spectroscopy. Enhanced room-temperature emission lifetimes of the complexes [ReCl(CO)3(Ln-κ2N)] with 1-pyrenyl-substituted ligands are indicative of the thermal activation between 3MLCT and 3IL/3ILCT excited states. Deactivation pathways occurring upon light excitation in [ReCl(CO)3(4′-(1-naphthyl)-terpy-κ2N)] and [ReCl(CO)3(4′-(1-pyrenyl)-terpy-κ2N)] were determined by femtosecond transient absorption studies.
Highlights
Transition-metal complexes with 2,2′:6′,2′′-terpyridines and their structural analogues have been receiving widespread attention from scientists due to their optical, electrochemical, catalytic, and medicinal properties, making these compounds appealing for potential applications in biological imaging,[1,2] catalysis,[3−7] and organic light-emitting devices.[8−10]Most of the Re(I) carbonyl complexes [ReCl(CO)3(Lnκ2N)] bearing substituted terpy-like ligands coordinated to the metal center in a bidentate way that have been developed so far[11−20] emit from the triplet excited state of metal to ligand charge-transfer character (3MLCT)
Exploring remote substituent effects in [ReCl(CO)3(Ln-κ2N)] with 4′-(4-substituted phenyl)terpyridine ligands, Fernań dez-Terań and Severy demonstrated that the introduction of the strongly electron donating −NMe2 group leads to the switching from 3MLCT to 3ILCT, which is accompanied by significant lengthening of the excited-state lifetime (380 vs 1.5 ns)
It is worth noting that the signals of the central pyridine protons of 2A show significant downfield shifts relative to those recorded for complex 1A, bearing a more sterically hindered 1
Summary
Transition-metal complexes with 2,2′:6′,2′′-terpyridines and their structural analogues have been receiving widespread attention from scientists due to their optical, electrochemical, catalytic, and medicinal properties, making these compounds appealing for potential applications in biological imaging,[1,2] catalysis,[3−7] and organic light-emitting devices.[8−10]. From 1A−4A to 1B−4B, the low-energy band experiences a bathochromic shift (Figure S34), which can be rationalized by stronger electron-withdrawing abilities of Ar-dppy due to the presence of additional nitrogen atoms in the peripheral rings, leading to a decrease in the LUMO energy level of [ReCl(CO)3(4-Arn-dppy-κ2N)] in relation to [ReCl(CO)3(4′-Arn-terpy-κ2N)] In both 1A−4A and 1B−4B series, the attachment of the pyrenyl group to the central pyridine ring of the terpy or dppy core leads to a noticeable red shift of the lowest energy absorption of the resulting Re(I) complexes, while the effect of naphthyl and phenanthrenyl units on the position of the lowest-energy absorption of [ReCl(CO)3(4Arn-dppy-κ2N)] and [ReCl(CO)3(4′-Arn-terpy-κ2N)] is rather marginal. Significantly red shifted relative to the emission of the parent Re(I) complexes, we can assume that their emission at 77 K occurs predominately from the excited state of 3ILpyrene with an admixture of 3ILCTpyrene→terpy character (Figures S50 and S51) Such an assignment is supported by lifetimes in milliseconds, 1−3 orders of magnitude longer than those for 1A−3A to 1B− 3B chromophores at 77 K (Table 2). DAS4 corresponds to the absorption spectrum of the fully relaxed lowest triplet state 3IL/3ILCT
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