Tuning Electronic Structure, Redox, and Photophysical Properties in Asymmetric NIR-Absorbing Organometallic BODIPYs

Abstract

Stepwise modification of the methyl groups at the α positions of BODIPY 1 was used for preparation of a series of mono- (2, 4, and 6) and diferrocene (3) substituted donor-acceptor dyads in which the organometallic substituents are fully conjugated with the BODIPY π system. All donor-acceptor complexes have strong absorption in the NIR region and quenched steady-state fluorescence, which can be partially restored upon oxidation of organometallic group(s). X-ray crystallography of complexes 2-4 and 6 confirms the nearly coplanar arrangement of the ferrocene groups and the BODIPY π system. Redox properties of the target systems were studied using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). It was found that the first oxidation process in all dyads is ferrocene centered, while the separation between the first and the second ferrocene-centered oxidation potentials in diferrocenyl-containing dyad 3 is ∼150 mV. The density functional theory-polarized continuum model (DFT-PCM) and time-dependent (TD) DFT-PCM methods were used to investigate the electronic structure as well as explain the UV-vis and redox properties of organometallic compounds 2-4 and 6. TDDFT calculations allow for assignment of the charge-transfer and π → π* transitions in the target compounds. The excited state dynamics of the parent BODIPY 1 and dyads 2-4 and 6 were investigated using time-resolved transient spectroscopy. In all organometallic dyads 2-4 and 6 the initially excited state is rapidly quenched by electron transfer from the ferrocene ligand. The lifetime of the charge-separated state was found to be between 136 and 260 ps and demonstrates a systematic dependence on the electronic structure and geometry of BODIPYs 2-4 and 6.

Photophysics, Redox Processes, and Electronic Structures of Ferrocenyl-Containing BODIPYs, aza-BODIPYs, BOPHYs, Transition-Metal Dipyrromethenes and aza-Dipyrromethenes

“…[16,44] Electrochemical data on ferrocene-merocyanine compounds 24 -25 and their non-ferrocene precursors are indicate that the first irreversible oxidation is localized on the BODIPY's π-system and DFT calculations correlate well with experimental data (Figure 3). [35] In the case of tetraferrocene-containing aza-BODIPY 8 and the corresponding aza-dipyrromethene 4, the DFT-predicted Comparison between experimental and TDDFTpredicted UV-Vis-NIR spectra of BODIPY 17.…”

“…[6,9,10,15,16,18,22,[25][26][27][33][34][35] In a typical situation for this class of compounds, the DFT-predicted HOMO is dominated by ferrocene contribution with substantial (in some cases close to 50%) contribution from the BODIPY of aza-BODIPY π-system (compounds 4, 5, 7, 8, Figure 1). [6,22] It seems that an extension of the π-system in BODIPY's core with electron-rich N,N-dimethyl-p-C 6 H 4 -CH=CH-(compound 17) [16] or merocyanine fragments (compounds 24 and 25) [35] results is a situation when the DFT-predicted HOMO is predominantly localized at the BODIPY's core, with ferrocene-centered orbitals predicted just in close proximity to the HOMO (Figure 1). Electrochemical data on compound 17 and its non-ferrocene-containing precursor, however, indicate that the ferrocene fragment in 17 should be oxidized first, and this assignment was further confirmed by the spectroelectrochemical and chemical oxidation data.…”

“…Electrochemical data on compound 17 and its non-ferrocene-containing precursor, however, indicate that the ferrocene fragment in 17 should be oxidized first, and this assignment was further confirmed by the spectroelectrochemical and chemical oxidation data. [16] The most likely explanation for the discrepancy between electrochemical and DFT-predicted nature of the HOMO in 17 is the choice of the B3LYP exchangecorrelation functional, which is a common problem in DFT and TDDFT calculations of the electronic structures and optical properties of ferrocene-containing BODIPYs and aza-BODIPYs. Indeed, as was pointed out in refs.…”

“…Reproduced with permission from the American Chemical Society. [16] HOMO has contributions from all four ferrocene fragments with higher contribution from the ferrocene groups located at the α-pyrrolic positions (Figure 4). [6] In all cases, when DFT data are available, the HOMO has no electron-density at the meso-atom position (8-position of BODIPY's core) and resembles half of the "a 1u " (in Gouterman's four-orbital model) orbital of the porphyrin core.…”

“…In the most of case, DFT and TDDFT calculations using standard hybrid (20 % of Hartree-Fock exchange) B3LYP exchange-correlation functional coupled with doubleor triple-ζ quality basis sets provide a good agreement between theoretical and experimental data. [6][7][8][9]11,13,16,19,21,[23][24][25]29,32,35] In several cases of the ferrocenyl-containing azaBODIPYs and BOPHY, however, it was shown that use of the hybrid TPSSh (10 % of Hartree-Fock exchange) results in better correlation between theoretical and experimental data. Modified from reference [6] .…”