Excited-State Switching in Rhenium(I) Bipyridyl Complexes with Donor–Donor and Donor–Acceptor Substituents

Abstract

The optical properties of two Re­(CO)3(bpy)Cl complexes in which the bpy is substituted with two donor (triphenylamine, TPA, ReTPA2) as well as both donor (TPA) and acceptor (benzothiadiazole, BTD, ReTPA-BTD) groups are presented. For ReTPA2 the absorption spectra show intense intraligand charge-transfer (ILCT) bands at 460 nm with small solvatochromic behavior; for ReTPA-BTD the ILCT transitions are weaker. These transitions are assigned as TPA → bpy transitions as supported by resonance Raman data and TDDFT calculations. The excited-state spectroscopy shows the presence of two emissive states for both complexes. The intensity of these emission signals is modulated by solvent. Time-resolved infrared spectroscopy definitively assigns the excited states present in CH2Cl2 to be MLCT in nature, and in MeCN the excited states are ILCT in nature. DFT calculations indicated this switching with solvent is governed by access to states controlled by spin–orbit coupling, which is sufficiently different in the two solvents, allowing to select out each of the charge-transfer states.

Light‐Driven Multi‐Charge Separation in a Push‐Pull Ruthenium‐Based Photosensitizer – Assessed by RASSCF and TDDFT Simulations

“…The first aim of the present contribution is to establish a computational protocol to assess the excited‐state properties of the low‐lying transitions in Ru – as given by the experimental absorption spectrum – in an unbiased fashion. The standard B3LYP [43,44] functional in combination with a double‐ζ basis set has been shown to provide a good compromise between computational cost and accuracy for the MLCT excitations present in 4d and 5d transition metal complexes [34–42] . However, in the case of organic push‐pull chromophores, long‐range corrected XC functionals are known to provide a better description of their ILCT states [28,29,45–49] .…”

“…This 3 ILCT thia state represents a potentially long-lived charge-separated triplet state, as charge recombination -either via phosphorescence or radiationless decay -is commonly hampered due to small spin-orbit couplings involving 3 ILCT states. [34,42,59] This triplet state is predicted at the TDDFT level of theory 1.41 eV above the singlet ground state, while a singlet-triplet gap of merely 1.21 eV is obtained at the DFT level of theory. Experimentally, two main emission bands at 1.97 and 1.68 eV (627 and 736 nm) were observed for Ru upon 400-nm excitation.…”

“…The standard B3LYP [43,44] functional in combination with a double-ζ basis set has been shown to provide a good compromise between computational cost and accuracy for the MLCT excitations present in 4d and 5d transition metal complexes. [34][35][36][37][38][39][40][41][42] However, in the case of organic push-pull chromophores, long-range corrected XC functionals are known to provide a better description of their ILCT states. [28,29,[45][46][47][48][49] The difficulty for the present ruthenium complex is that both types Figure 1.…”

“…However, hybrid functionals with lower amount of exact exchange are known to provide a reasonable compromise between computational cost and accuracy to assess the photophysics of transition metal complexes, i. e., for MLCT transition. [34][35][36][37][38][39][40][41][42] The difficulty for the present black dye is that both types of excitations (MLCT and ILCT) contribute significantly to its photophysics, which prevents a straightforward application of standard TDDFT methods, while multiconfigurational simulations are hampered by the size of the chemical systems.…”

Coupling of photoactive transition metal complexes to a functional polymer matrix**

“…Still, our DFT calculations yield a HOMO-LUMO gap of the Ru photosensitizer that is by roughly 1 eV larger than the experimentally derived value which has also been found in similar systems. [64,65] Time-dependent density functional theory, on the other hand, is well established to access and predict the light-driven processes in transition metal complexes [44,66] and soft matter embedded light-harvesting units. [67] Through TDDFT, individual excited states and their properties such as oscillator strength and electron density become accessible, which allows fundamental insights into the mechanisms of light-driven processes such as electron or hole injection.…”