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

TL;DR

This study investigates how solvents affect the ultrafast photochemical behavior of N-methyl oxindole overcrowded alkene molecular motors using advanced spectroscopy techniques.

Contribution

The study reveals solvent-dependent dynamics of dark states and identifies the role of S1 electronic states in isomerization without higher singlet state involvement.

Findings

Four dynamical processes were identified in the photoisomerization of overcrowded alkene motors.

Solvent polarity and viscosity significantly influence the decay time scales of dark states.

The quantum yield of isomerization for the motors ranges from 0.4–8.7%.

Abstract

Overcrowded alkenes are a class of rotational molecular
motors
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 photoinduced
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 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 molecules (10–15
ps). The time scales 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
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%.

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