Femtosecond broadband transient absorption setup with temporal detection up to hundreds of microseconds. The lettered components are defined as follows: (a) dichroic beam splitter, (b) Glan–Taylor polarizer, (c) neutral density filter, (d) depolarizing plate, (e) tunable beam splitter, (f) sample cell with magnetic stirrer or flow cell, (g) variable neutral density filter, (h) dichroic all-reflective telescope, () curved mirrors with radius of circular curvature (j) and 100 mm , and (R) gold coated retroreflector.
Dependence of the transient absorption signal of 1NN on the pump intensity (in energy units) at the specified probe wavelength. The transient signals were recorded at a fixed time delay of 100 ps in methanol solution. The linear response is lost at pump intensities higher than under the experimental conditions used in this work (see Experimental Methods for details).
Structures of the cis (left) and trans (right) isomers of the complex optimized at the B3LYP/IEFPCM/6–311G(d,p) level of theory. The trans isomer is more stable than the cis by 0.02 kcal/mol in methanol solutions at the B3LYP/IEFPCM/6–311++G(d,p) level of theory. The asterisks define the nitro-aromatic torsion angle.
Ground- and excited-state potential energies of 1NN as a function of the nitro-aromatic torsion angle defined in Fig. 2. Single-point energies in (a) cyclohexane, (b) acetonitrile, and (c) for the trans complex in methanol are reported at the and level of theory for the ground- and excited-states, respectively (see Computational Methods for details). The upper graph in each panel represents the change in the oscillator strength, while the lower graph in each panel represents the ground-state potential energy as a function of torsion angle. The horizontal black line corresponds to the thermal energy available at 298 K.
Steady-state absorption spectra of 1NN: (a) in nonpolar and polar aprotic solvents and (b) in polar protic solvents.
Contour plots of the femtosecond broadband transient absorption spectra of 1NN in cyclohexane, acetonitrile, and pentanol solutions.
Femtosecond transient absorption spectra of 1NN in nonpolar and polar aprotic solvents: (a) short time dynamics and (b) long time dynamics. Note that the transient absorption spectra cease to change after a time delay of .
Transient absorption signals of 1NN in (a) cyclohexane, (b) acetonitrile, and (c) pentanol at the specified probe wavelengths. Best global-fit curves are shown by solid lines (see text for details).
Normalized transient absorption spectra of 1NN in (a) cyclohexane, (b) acetonitrile, and (c) pentanol showing the band narrowing and blueshift characteristic of VC dynamics. Bottom panel (d) shows the dynamic Stoke shift of the transient absorption band of 1NN in the time delay window of 40–530 ps in pentanol solution.
Femtosecond transient absorption spectra of 1NN in polar protic solvents: (a) short time dynamics and (b) long time dynamics.
Picosecond to microsecond transient absorption spectra of 1NN in (a) cyclohexane, (b) acetonitrile, and (c) pentanol.
Normalized transient absorption signals in the picosecond to microsecond time scale of 1NN in cyclohexane, acetonitrile, and pentanol solvents: (a) in -saturated conditions and (b) in air-saturated conditions. Best-fit curves are shown by solid lines (see text for details). The reported transient signal is the average of several monoexponential-decay probe wavelengths.
Correlation of the fourth time constant of 1NN recorded in primary alcohols (methanol, ethanol, n-propanol, n-butanol, and n-pentanol) as a function of (a) viscosity of the solvent, (b) characteristic correlation times reported in Ref. 44, and (c) static dielectric constant . Symbols are connected by solid lines to guide the eye.
Plausible kinetic mechanisms explain the excited-state dynamics and photochemistry of 1NN in nonpolar, aprotic, and protic solvents. Decay pathways in red describe solvation dynamics and intersystem crossing in 1NN observed in primary alcohol solvents. The kinetic model to the right explains satisfactorily the experimental and computational data available to date.
Lifetimes from global fits to the transient absorption signals from femtoseconds to the nanoseconds. Errors are reported as two times the standard deviation of three or more individual global fit measurements.
Lifetimes from global fits to the transient absorption signals from nanoseconds to the microseconds. Errors are reported as two times the standard deviation of three or more individual global fit measurements.
Vertical excitation energies of the fully optimized 1NN structure in different solvents determined at level of theory.
Singlet-triplet and triplet-triplet energy gaps for the fully optimized 1NN structure in different solvents determined at level of theory.
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