Ultrafast Spectroscopic Study of Oxindole Overcrowded Alkene Molecular Motors and Azo-Boronic Photoswitches

Abstract

Molecular motors are molecules which use an external energy source such as light or chemical energy to fuel directional motion on the molecular scale. They are ubiquitous in nature and many vital biological functions are dependent on them. Synthetic chemists have sought to create artificial molecular motors with the aim of producing elaborate molecular machines with a vast utility. This thesis reports the excited-state dynamics of the initial photochemical isomerization of the motor action for N-methyl oxindole motor molecules and ortho-boronic azo-photoswitches as candidates to be used as a modular unit of a novel autonomous light-driven rotational motor.

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 attention in particular 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 highlighted. Greater solvent viscosity increases the average excited-state lifetimes of the dark states by inhibiting rotation of the alkene bond. Coherent oscillations are observed in the excited-state absorption bands, attributed to coherent vibrations of the excited-state wave packet. The stable (P,P)-E and metastable (M,M)-Z diastereomer structures were optimized at the ωB97XD/6-31+g(d,p) level of theory, and the two geometries were interpolated using the internal coordinates to approximate the geometrical change of the isomerization reaction in the excited state. For each interpolated structure, the vertical transitions were calculated using TDDFT at the same level of theory to estimate the adiabatic potential energy surfaces of the S0, S1 and S2 electronic states. This interpolation study found that the excited-state dynamics are dictated by the S1 excited state, and found no involvement of higher singlet excited states. The poor quantum yield of isomerization was 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 of isomerization for all motors to range from 0.4 – 8.7%.

The excited-state dynamics of the ortho-boronic azo-photoswitches were studied for the first time using TA spectroscopy, utilizing the abundant reported time-resolved spectroscopy experiments and high-level calculations conducted on azobenzene to guide the interpretation of the excited-state behaviour. These experiments show that there is variable strength of the B-N interaction across the azo-photoswitches studied. However, the strength of this interaction does not impede the photoisomerization reaction. The excited state dynamics are characterized by four time constants: the S2(ππ*) excited-state lifetime (t1), the decay of molecules from the FC region on the S1(nπ*) adiabatic potential (t2), the S1(nπ*) excited-state lifetime (t3) and observed vibrational cooling of hot ground-state molecules after internal conversion to the ground state (tHGS).

UV-Vis spectroscopic experiments conducted in acetonitrile demonstrated that the azo-photoswitches show concentration-dependent absorption behaviour, which was assigned to the formation of aggregates in solution. The aggregation extends the excited-state lifetimes of the azo-photoswitches. This effect is mostly observed for the S1(nπ*)-excited TA experiments because these experiments require higher concentrations due to the poor transition moment of the S0 → S1(nπ*) transition.
The quantum yields for isomerization of azobenzene and two azo-boronic acids were estimated to be (15.9 ± 0.1)%, (22.2 ± 0.8)% and (8.7 ± 0.2)%, respectively. This analysis shows that the photochemical efficiency is partially dependent on the addition of the ortho-boronic and para-methoxy substituents.

Date of Award	1 Oct 2024
Original language	English
Awarding Institution	University of Bristol
Supervisor	Andrew J Orr-Ewing (Supervisor) & Beatrice Collins (Supervisor)