Protonation effects on the resonance Raman properties of a novel (terpyridine)Ru(4H-imidazole) complex: an experimental and theoretical case study

Abstract

The optically active states in a novel (terpyridine)Ru(4H-imidazole) complex displaying an unusually broad and red-shifted absorption in the visible range are investigated experimentally and theoretically. Since this property renders the complex promising for an application as sensitizer in dye-sensitized solar cells, a detailed knowledge on the correlation between features in the absorption spectrum and structural elements is indispensable in order to develop strategies for spectroscopy/theory-guided design of such molecular components. To this aim, time-dependent density functional theory calculations, including solvent effects, are employed to analyze the experimental UV-vis absorption and resonance Raman (RR) spectra of the unprotonated and protonated forms of the complex. This provides a detailed photophysical picture for a complex belonging to a novel class of Ru-polypyridine black absorbers, which can be tuned by external pH stimuli. The complex presents two absorption maxima in the visible region, which are assigned by the calculations to metal-to-ligand charge transfer (MLCT) and intra-ligand states, respectively. RR simulations are performed in resonance with both bands and are found to correctly reproduce the observed effects of protonation. Finally, the examination of the molecular orbitals and of the RR spectra for the MLCT state shows that protonation favors a charge transfer excitation to the 4H-imidazole ligand.

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

“…In former studies on related 4H-imidazole systems, [10,11,15] resonance Raman spectroscopy [28] revealed its potential to discriminate between forms with nonprotonated and protonated imidazole rings. The resonance Raman spectrum of the monoprotonated species of Ru1 showed almost no signatures of the protonated imidazole ring ( Figure S2).…”

“…The theoretically possible double protonation of the imidazole ring was not considered, because it could not be observed for structurally similar 4H-imidazole complexes including Ru2. [10,11] In principle two modes of protonation of Ru1(ImCOO) are possible: The monoprotonated species can occur as Ru1(ImHCOO) with a protonated imidazole ring, or as Ru1(ImCOOH) with a protonated carboxylate group. The first protonation of Ru1(ImCOO) causes a small bathochromic shift of the longwavelength absorption band by 0.03 eV, from 556 to 563 nm.…”

“…[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).…”

“…In the family of ruthenium 4H-imidazole dyes, extinction coefficients of up to 28 100 M À1 cm À1 at 605 nm can be achieved. [10] To utilize the light-harvesting properties of the ruthenium 4H-imidazole complexes, the excitation energy must be transferred to an acceptor unit. The introduction of functional groups such as carboxyl groups into the ligand sphere allows the dye to be anchored to the surface of a semiconductor such as TiO 2 .…”

“…see Figure 1 A). [9][10][11][12][13][14][15] The spectral properties of these dyes can be tuned by structural alteration and protonation at the imidazole ring. In the family of ruthenium 4H-imidazole dyes, extinction coefficients of up to 28 100 M À1 cm À1 at 605 nm can be achieved.…”