Influence of Multiple Protonation on the Initial Excitation in a Black Dye

Abstract

Solvent pH influences electronic and optical properties of photoactive systems by protonation/deprotonation of basic/acidic positions. In this study a joint experimental and theoretical approach is presented to analyze the acid/base properties of a new 4H-imidazole ruthenium(II) complex (Ru). The imidazole ligand is substituted by two dimethylamino groups and hence offers four positions for protonation. To identify the species present in certain acid concentration ranges calculated absorption and resonance Raman (RR) spectra are compared to experimental results. It is shown that three different protonated species can be prepared separately from each other by varying the acid concentration in solution and the character of the substituent can be switched from electron donating to electron withdrawing by protonation, which plays an important role in the further analysis of pH-dependent photoinduced processes in these systems.

In situ spectroelectrochemical and theoretical study on the oxidation of a 4H-imidazole-ruthenium dye adsorbed on nanocrystalline TiO2thin film electrodes

“…Due to the direct and strong interaction between each chromophore with the metal center the complexes exhibit the photoelectrochemical and photophysical properties related to the ruthenium(II)polypyridine fragment while displaying broad and strong absorption in the visible range due to the presence of strong 1 MLCT and ligand centered ( 1 LC) transitions. [29][30][31][32][33][34] Furthermore, the redox and photophysical properties of the 4H-imidazole coordinating ruthenium(II) complexes can be easily tuned by varying the substituent pattern and the protonation state of the ligand, as has been recently shown. 30,31,33,34 These properties render complexes like RuIm with promising structures for use as molecular sensitizers in DSSCs, in which the electron injection from the photoexcited light harvesting unit into the wide bandgap semiconductor occurs via an anchoring group, typically a carboxylate group (COO À ).…”

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

“…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.…”

“…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).…”

“…It was shown by others that B3LYP provides a balanced description of the absorption features of such complexes. [10,11,15,60] The absorption spectra were simulated by determining the excitation energies and oscillator strengths of the 120 lowest singlet excited states. The effects of the interaction with the solvent on the geometry, frequencies, excitation energies, and excited-state gradients were taken into account by means of the integral equation formalism of the polarizable continuum model (acetonitrile, dielectric constant e = 35.688, refractive index n = 1.344).…”

“…[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).…”