Influence of the substitution position on spin communication in photoexcited perylene-nitroxide dyads

Abstract

By virtue of the modularity of their structures, their tunable optical and magnetic properties, and versatile applications, photogenerated triplet-radical systems provide an ideal platform for the study of the factors controlling spin communication in molecular frameworks. Typically, these compounds consist of an organic chromophore covalently attached to a stable radical. After formation of the chromophore triplet state by photoexcitation, two spin centres are present in the molecule that will interact. The nature of their interaction is governed by the magnitude of the exchange interaction between them and can be studied by making use of transient electron paramagnetic resonance (EPR) techniques. Here, we investigate three perylene-nitroxide dyads that only differ with respect to the position where the nitroxide radical is attached to the perylene core. The comparison of the results from transient UV-vis and EPR experiments reveals major differences in the excited state properties of the three dyads, notably their triplet state formation yield, excited state deactivation kinetics, and spin coherence times. Spectral simulations and quantum chemical calculations are used to rationalise these findings and demonstrate the importance of considering the structural flexibility and the contribution of rotational conformers for an accurate interpretation of the data.

Fig. 1. Structural building blocks of the three investigated perylene-based dyads and photoscheme of a chromophore in the absence and presence of an interacting stable radical. Nomenclature: per–1-eTEMPO: R¹ = eTEMPO, R² = R³ = H; per–2-eTEMPO: R² = eTEMPO, R¹ = R³ = H; per–3-eTEMPO: R³ = eTEMPO, R¹ = R² = H. The positions 1, 2, and 3 of the perylene core are referred to as the bay, ortho, and peri-positions, respectively. The numeric superscripts in the photoschemes indicate the spin multiplicity. Abbreviations: R-radical, EISC-enhanced intersystem crossing, EIC-enhanced internal conversion, EET-excitation energy transfer, ET-electron transfer.

Fig. 2. Normalised UV-vis absorption (solid lines) and fluorescence (dashed lines) spectra of the three bromoperylene precursors in toluene. The black vertical lines indicate the positions of the absorption and fluorescence intensity maxima of perylene. To the right, the optimised molecular structures are shown. The black arrow indicates the orientation of the transition dipole moment calculated at the CAM-B3LYP/def2-TZVP level of theory.

Fig. 3. Contour plots of the fsTA data recorded for the three perylene–eTEMPO dyads dissolved in toluene solution at room temperature after photoexcitation at 400 nm. (a) per–1-eTEMPO, (b) per–2-eTEMPO, (c) per–3-eTEMPO. The red and blue colour coding in the contour plot represents positive and negative signals, respectively. The vertical coloured lines indicate the positions corresponding to the kinetic traces shown in the left panel, while the dotted horizontal lines indicate the time delays associated with the spectra shown on the right.

Fig. 4. Transient continuous wave EPR (trEPR) spectra of per–1-eTEMPO, per–2-eTEMPO, and per–3-eTEMPO at 0.8 μs after laser excitation recorded at the X-band in frozen toluene solution at 80 K together with quartet state simulations using a spin Hamiltonian approach. The simulation is shown as a black solid line, the relative quartet state populations are shown next to the data and the complete set of simulation parameters can be found in the ESI.

Fig. 5. Spin echo decay for per–1-eTEMPO, per–2-eTEMPO and per–3-eTEMPO measured at the Q-band in frozen toluene solution at 80 K in the dark (a) and after photoexcitation at 435 nm (b). The spin coherence times (T_m), obtained from a stretched exponential fit, are indicated. Any residual dark state signal was suppressed during the quartet state T_m measurements by application of a pre-saturation pulse.

Fig. 6. Ground state conformations and rotational barriers calculated for per–1-eTEMPO, per–2-eTEMPO, and per–3-eTEMPO, employing a relaxed surface scan (BP86/def2-TZVP). The calculated excited state exchange interactions J_TR and relative occupation probabilities P (at two different temperatures) are indicated underneath the respective ground state structures.