Utilizing Ancillary Ligands to Optimize the Photophysical Properties of 4H‐Imidazole Ruthenium Dyes

Abstract

Correlations between structural features and photophysical properties offer the possibility to design dyes with tailor-made properties. In this respect, the photophysical properties of a series of 4H-imidazole (4H-im) ruthenium dyes with varying chemical and electronic structures of the complex fragments [(tpy)Ru(4H-im)X] {tpy=4,4',4''-tri-tert-butyl-2,2':6',2''-terpyridine; X=Cl(-), NCS(-)} and [(bpy)2Ru(4H-im)](+) {bpy=4,4'-di-tert-butyl-2,2'-bipyridine} were investigated. Variation of the π-donor/acceptor properties of the ancillary ligands offers the possibility to tune the relative energies of the d donor and π* acceptor orbitals of 4H-im, which results in a shift in the Ru→4H-im (1)MLCT (MLCT=metal-to-ligand charge transfer) absorption band in the visible range. Further, the energies of the excited states and also the interactions and mixing of the metal d orbital with the π* 4H-im orbital are sensitive to the chemical and electronic structures of the complex fragment. This causes subtle changes in the photoinduced processes; for example, the rate of the interim population of a planarized state increases with increasing/decreasing π-acceptor/donor character of the ancillary ligand. Although ground-state repopulation in the tpy species follows the energy-gap law, local symmetry-related effects have to be considered to account for the very short lifetime of the excitation in the bpy complex. Additionally, the ultrafast intramolecular relaxation processes leading from the initially excited states to the relaxed triplet states localized on the 4H-im ligand (internal conversion, internal vibrational energy redistribution, intersystem crossing) depend on the nature of the complex fragment. The high excitation-wavelength-dependent rate for the population of the relaxed triplet states in the bpy complex points to additional interligand electron-transfer contributions upon excitation into the higher lying Ru→bpy (1)MLCT states.

Photophysics of a Ruthenium 4H‐Imidazole Panchromatic Dye in Interaction with Titanium Dioxide

“…Finally, the system decays nonradiatively to the ground state. [13,14] Protonation of the imidazole ring stabilizes the excess charge on the imidazole ligand in the MLCT state. This stabilizing effect has a stronger impact on the nonplanarized state than on the planarized state (see Figure 1 C).…”

“…[13,14] Twisting of the tolyl rings with respect to the 4H-imidazole plane prevents the formation of a completely conjugated system. [13,14] A secondary, planarized 3 MLCT state [13,14] becomes accessible by photoinduced rotation of these terminal tolyl substituents. This excited state is more delocalized within the framework of the 4H-imidazole ligand, and thus electron density is shifted more towards the terminal tolyl rings compared to the initial photoexcited state.…”

“…[10,11,15] On excitation in the visible 1 MLCT absorption band, ultrafast population of the 3 MLCT manifold by intersystem crossing (ISC) and subsequent vibrational relaxation lead to population of a thermally relaxed 3 MLCT state. [13,14] This 3 MLCT state is mainly localized on the central part of the ligand, which consists of the imidazole ring and the directly connected phenyl group (see Figure 1 B). [13,14] Twisting of the tolyl rings with respect to the 4H-imidazole plane prevents the formation of a completely conjugated system.…”

“…[13,14] This 3 MLCT state is mainly localized on the central part of the ligand, which consists of the imidazole ring and the directly connected phenyl group (see Figure 1 B). [13,14] Twisting of the tolyl rings with respect to the 4H-imidazole plane prevents the formation of a completely conjugated system. [13,14] A secondary, planarized 3 MLCT state [13,14] becomes accessible by photoinduced rotation of these terminal tolyl substituents.…”

“…Figure 1. A) Structure of Ru1 in its monoprotonated form Ru1(ImCOOH) with a carboxyl group as anchoring moiety and reference complex Ru2 [13,15] in its nonprotonated form Ru2(Im). B) Calculated geometry and singly occupied molecular orbital of Ru2(ImH) in the nonplanarized 3 MLCT state.…”