Nanosecond transient absorption spectroscopy was used to study the photochemical ring-opening reaction for a 6-nitroindolinospiropyran (SP1) in solution and in nanocrystalline (NC) suspension at 298 K. We measured the kinetics in argon purged and air saturated acetonitrile and found that the presence of oxygen affected two out of the three components of the kinetic decay at 440 nm. These are assigned to the triplet excited states of the Z- and E-merocyanines (3Z-MC* and 3E-MC*). In contrast, a long-lived growth component at 550 nm and the decay of a band centered at 590 nm showed no dependence on oxygen and are assigned, respectively, to the ground state Z- and E-merocyanines (Z-MC0 and E-MC0). Laser flash photolysis studies performed in NC suspensions initially showed a very broad, featureless absorption spectrum that decayed uniformly for ca. 70 ns before revealing a more defined spectrum that persisted for greater than 4 ms and is consistent with a mixture of the more stable Z- and E-MC0 structures. We performed quantum mechanical calculations on the interconversion of E- and Z-MCs on the S0 and S1 potential energy surfaces. The computed UV-vis spectra for a scan along the Z → E interconversion reaction coordinate show substantial absorptivity from 300-600 nm, which suggests that the broad, featureless transient absorption spectrum results from the contribution of the transition structure and other high-energy species during the Z to E isomerization.

Herein, we report the use of fluorescence anisotropy decay for measuring the rotation of six shape-persistent molecular rotors with central naphthalene (2), anthracene (3a, 3b, and 3c), tetracene (4), and pentacene (5) rotators axially linked by triple bonds to bulky trialkylsilyl groups of different sizes. Steady-state and time-resolved polarization measurements carried out in mineral oil confirmed that the vibrationally resolved lowest-energy absorption bands are characterized by a transition dipole moment oriented along the short acene axes, in the direction of the alkyne linkers. Fluorescence lifetimes increased significantly with increasing acene size and moderately with a decrease in the size of the trialkylsilyl group. The fluorescence anisotropy decay for all compounds in mineral oil with a viscosity of ca. 21.6 cP at 40 °C was completed within the fluorescence lifetime, so that the rotational time constants could be obtained via their rotational correlation times, which increased with silyl protecting group size rather than acene size, indicating that polarization decay is determined by tumbling of the molecular rotor about the long acene axis. These results suggest that monitoring the rotational motion of bis­(silylethynyl)­acenes in restricted media should be possible for media with viscosity values on the order of 21.6 cP or greater.