Effect of deprotonation on absorption and emission spectra of Ru(ii)-bpy complexes functionalized with carboxyl groups

Abstract

Changes in the ground and excited state electronic structure of the [Ru(bpy)(3)](2+) (bpy = 2,2'-bipyridine) complex induced by functionalization of bpy ligands with carboxyl and methyl groups in their protonated and deprotonated forms are studied experimentally using absorption and emission spectroscopy and theoretically using density functional theory (DFT) and time dependent DFT (TDDFT). The introduction of the carboxyl groups shifts the metal-to-ligand-charge-transfer (MLCT) absorption and emission bands to lower energies in functionalized complexes. Our calculations show that this red-shift is due to the stabilization of the lowest unoccupied orbitals localized on the substituted ligands, while the energies of the highest occupied orbitals localized on the Ru-center are not significantly affected. Consistent with previously observed trends in optical spectra of related Ru(II) complexes, deprotonation of the carboxyl groups results in a blue shift in the absorption and phosphorescence spectra. The effect originates from interplay of positive and negative solvatochromism in the protonated and deprotonated complexes, respectively. This results in more delocalized character of the electron transition orbitals in the deprotonated species and a strong destabilization of the three lowest unoccupied orbitals localized on the substituted and unsubstituted ligands, all of which contribute to the lowest-energy optical transitions. We also found that owing to the complexity of the excited state potential energy surfaces, the calculated lowest triplet excited state can be either weakly optically allowed (3)MLCT or optically forbidden Ru (3)d-d transition depending on the initial wavefunction guess used in TDDFT calculations.

Electronic Properties and Structure of Assemblies of CdSe Nanocrystal Quantum Dots and Ru‐Polypyridine Complexes Probed by Steady State and Time‐Resolved Photoluminescence

“…[ 35 ] We propose that the deprotonation of complex 2 in the presence of NQDs occurs as a result of its attachment to the NQD surface during the formation of the NQD-2 assembly. The details of the adsorption mechanism will be discussed in a separate publication.…”

The Frenkel exciton Hamiltonian for functionalized Ru(II)–bpy complexes

“…Similarly, compound C1 can be treated as a symmetric combination of three bipyridine ligands connected through the Ru center. For low-energy MLCT excited states, the hole is localized on the ruthenium with a small contribution from ligands while the electron wavefunction is localized on the ligands [39]. Consequently, the Hamiltonian (1) for C1 in the ligand-localized basis becomes…”

“…[39], namely using the hybrid B3LYP functional, the LANL2DZ basis set for Ru, and the 6-31G n basis set for the ligand atoms. This combination of functional and basis has been one of the most ''tried-and-true'' methods for calculating properties of Ru-polypyridine complexes, reproducing absorption spectra in past studies of similar systems [39,[65][66][67]. However, while our previous work discusses how various solvation models can lead to different ground state orbital energies (Fig.…”

“…Pairing experimental excited-state studies with TDDFT modeling, we have found that addition of the carboxyl/methyl groups to [Ru(bpy) 3 ] 2 þ red-shifts the MLCT absorption and emission bands due to stabilization of the lowest unoccupied orbitals localized on the substituted ligands [39].…”

“…We have recently analyzed [39] changes in electronic structure and optical response of [Ru(bpy) 3 ] 2 þ complexes that occur when the bpy ligands are functionalized with methyl groups and protonated/de-protonated carboxyl groups-a typical modification in photochemical cells. Pairing experimental excited-state studies with TDDFT modeling, we have found that addition of the carboxyl/methyl groups to [Ru(bpy) 3 ] 2 þ red-shifts the MLCT absorption and emission bands due to stabilization of the lowest unoccupied orbitals localized on the substituted ligands [39].…”