Ultrafast photophysics of para-substituted 2,5-bis(arylethynyl) rhodacyclopentadienes: thermally activated intersystem crossing

Abstract

2,5-Bis(phenylethynyl) rhodacyclopentadienes (RCPDs), as a type of Rh(iii) complex, exhibit unusually intense fluorescence and slow intersystem crossing (ISC) due to weak metal-ligand interactions. However, details on their ultrafast photophysics and ISC dynamics are limited. In this work, electronic relaxation upon photoexcitation of two substituted RCPDs with two -CO₂Me (A-RC-A) or -NMe₂/-CO₂Me (D-RC-A) end groups are comprehensively investigated using femtosecond transient absorption spectroscopy and theoretical analysis. Upon ultraviolet and visible excitation, dephasing of vibrational coherence, charge transfer, conformation relaxation, and ISC are observed experimentally. By calculating the spin-orbit coupling, reorganization energy, and adiabatic energy gap of plausible ISC channels, semi-classical Marcus theory revealed the dominance of thermally activated ISC (S₁ → T₂) for both D-RC-A and A-RC-A, while S₁ → T₁ channels are largely blocked due to high ISC barriers. With weak spin-orbit coupling, such differences in plausible ISC channels are predominately tuned by energetic parameters. Singlet oxygen sensitization studies of A-RC-A provide additional insight into the excited-state behavior of this complex.

Scheme 1. Chemical structures of investigated D-RC-A and A-RC-A. The Rh(iii)–ligand core is illustrated in black while the peripheral groups are in gray. The phosphine ligand is PMe₃.

Fig. 1. (a and b) Static UV/visible absorption spectra of D-RC-A ((a), blue) and A-RC-A ((b), red) in THF solution (10⁻⁵ mol L⁻¹), overlapped with spectra of UV (violet shaded) and visible (green shaded) excitation pulses employed in the fs-TA experiments. (c and d) TD-DFT-calculated vertical excitation energies and oscillator strengths of D-RC-A (c) and A-RC-A (d).

Fig. 2. Spectro-temporal maps of λ_ex-dependent fs-TA signals of D-RC-A upon UV (a) and visible (b) excitation as well as of A-RC-A upon identical UV (c) and visible (d) excitation. The spectral signatures of ISC-generated triplet states are highlighted by white dashed lines.

Fig. 3. Target-analysis-extracted species-associated spectra (SAS) of λ_ex-dependent fs-TA signals of D-RC-A upon 295 nm UV (a) and 513 nm visible (b) optical excitation, as well as SAS of A-RC-A upon identical UV (c) and visible (d) excitation. Four or five independent species were employed to reproduce the fs-TA signals.

Fig. 4. Electronic relaxation of D-RC-A (left two panels) and A-RC-A (right two panels) revealed by λ_ex-dependent fs-TA measurements.

Fig. 5. (a) Simplified energetic diagram for calculating the ISC barrier of the S₁ → T₂ transition in the framework of Marcus theory, and energy diagrams for plausible ISC channels of D-RC-A (b) and A-RC-A (c), in which both T₁ and T₂ states are considered.

Fig. 6. (a) Integrated intensity of the O₂(a¹Δ_g) phosphorescence signal upon irradiation at 417 nm, normalized by the sensitizer absorbance and the O₂(a¹Δ_g) lifetime, plotted as a function of laser power and collected over a range of O₂ concentrations for both A-RC-A and for the reference standard, phenalenone (PN), in toluene-d₈. The O₂ concentration is represented as the percent of O₂ in the O₂/N₂ gas mixture bubbled through the solution. The slopes of the linear fits are proportional to the O₂(a¹Δ_g) quantum yield. Representative time-resolved O₂(a¹Δ_g) phosphorescence traces used to obtain the A-RC-A data are shown in (b and c). (b) The data from 100 μs to 2000 μs were fitted by a single exponential decay function (solid lines) to obtain the lifetime of O₂(a¹Δ_g) (i.e., 1/k_Δ). (c) Using k_Δ as a fixed parameter, eqn (5) was used as a fitting function (solid lines) to obtain values of k_T for a time domain where O₂(a¹Δ_g) was formed in the photosensitized reaction. (d) Plot of k_T, obtained from the fits shown in (c) for the A-RC-A data, against the concentration of dissolved oxygen. The slope, (1.3 ± 0.1) × 10⁹ M⁻¹ s⁻¹, is the bimolecular rate constant for oxygen quenching of the O₂(a¹Δ_g) precursor. The intercept yields a value of ∼9 μs for the lifetime of this O₂(a¹Δ_g) precursor in the absence of oxygen. Both of these numbers are consistent with expectation for oxygen quenching of a triplet state.