Solvent-Dependent Ultrafast Photochemical Dynamics of N-Methyl Oxindole Overcrowded Alkene Molecular Motors

Overcrowded alkenes are a class of rotational molecular motor that operate via alternating photochemical and thermal relaxation processes. Although the performances of various designs of molecular motors have been extensively studied, in general their photo-induced isomerization efficiencies remain low. Ultrafast time-resolved spectroscopy can explore the excited-state dynamics and investigate the photoisomerization mechanisms. Herein, we study a series of visible-light activated overcrowded alkene motors with N-methyl oxindole functionality using transient absorption (TA) and time-resolved infrared (TRIR) spectroscopies. The motors are examined in cyclohexane, DMSO and methanol to probe the solvent environmental effects on the photoisomerization, paying particular attention to polarity and viscosity. Four dynamical processes are identified: relaxation from the Franck-Condon region of the bright excited state to a region of different electronic character (< 120 fs) that is not directly optically accessible from the ground state; prompt (0.5 – 4 ps) and indirect (4 – 14 ps) depopulation of this dark state via conical intersections with the ground state; and vibrational cooling of hot ground-state (HGS) molecules (10 – 15 ps). The timescales for decay of the dark state are both solvent polarity and viscosity dependent. In nonpolar cyclohexane solutions, only direct depopulation of the dark state is observed, but in the DMSO and methanol solutions both prompt and indirect depopulation are identified. Greater solvent viscosity increases the average excited-state lifetimes of the dark states by inhibiting rotation of the alkene bond. Oscillations observed in the excited-state absorption bands are attributed to coherent vibrations of the excited-state wave packet. Density functional theory (DFT) calculations of the stable (P,P)-E and metastable (M,M)-Z diastereomer structures, optimized at the ωB97XD/6-31+G(d,p) level of theory and interpolated between the two geometries using internal coordinates, are used to approximate the geometrical change of the isomerization reaction in the excited state. For each interpolated structure, the vertical excitation energies are calculated using time-dependent (TD) DFT calculations at the same level of theory to track the adiabatic potential energy surfaces of the S₀, S₁ and S₂ electronic states. This interpolation study shows that the excited-state dynamics are dictated by the S₁ state, with no involvement of higher-lying singlet states. The poor quantum yield of isomerization is confirmed using the degree of ground-state bleach recovery of the carbonyl stretch in the recorded TRIR spectra, finding an upper estimate of the quantum yield for isomerization of the five molecular motors studied to range from 0.4 – 8.7%.