Excited-State Dynamics of Donor−Acceptor Bridged Systems Containing a Boron−Dipyrromethene Chromophore:  Interplay between Charge Separation and Reorientational Motion

Abstract

The excited-state dynamics of a series of electron donor-acceptor bridged systems (DABS) consisting of a boron-dipyrromethene chromophore covalently linked to a dinitro-substituted triptycene has been investigated using femtosecond time-resolved spectroscopy. The chromophores differ by the number of bromine atom substituents. The fluorescence lifetime of the DABS without any bromine atom is strongly reduced when going from toluene to polar solvents, this shortening being already present in chloroform. This effect is about 10 times weaker with a single bromine atom and negligible with two bromine atoms on the chromophore. The excited-state lifetime shortening is ascribed to a charge transfer from the excited chromophore to a nitrobenzene moiety, the driving force of this process depending on the number of bromine substituents. The occurrence of this process is further confirmed by the investigation of the excited-state dynamics of the chromophore alone in pure nitrobenzene. Surprisingly, no correlation between the charge separation time constant and the dielectric properties of the solvents could be observed. However, a good correlation between the charge separation time constant and the diffusional reorientation time of the chromophore alone, measured by fluorescence anisotropy, was found. Quantum chemistry calculations suggest that quasi-free rotation about the single bond linking the chromophore to the triptycene moiety permits a sufficient coupling of the donor and the acceptor to ensure efficient charge separation. The charge separation dynamics in these molecules is thus controlled by the reorientational motion of the donor relative to the acceptor.

Local Viscosity of Binary Water+Glycerol Mixtures at Liquid/Liquid Interfaces Probed by Time-Resolved Surface Second Harmonic Generation

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Effect of Salt on the Excited-State Dynamics of Malachite Green in Bulk Aqueous Solutions and at Air/Water Interfaces: a Femtosecond Transient Absorption and Surface Second Harmonic Generation Study

“…The femtosecond TA setup has already been described in detail elsewhere. 28 Briefly, TA measurements were performed by using two different pump wavelengths, 615 and 400 nm. Excitation at 615 nm was achieved with the tunable output of a home-built noncollinear optical parametric amplifier (NOPA) fed by the 800 nm output of a standard 1kHz amplified Ti:Sapphire system (Spectra-Physics).…”

Intramolecular Charge-Transfer Dynamics in Covalently Linked Perylene−Dimethylaniline and Cyanoperylene−Dimethylaniline

“…The experimental setup has been described in detail earlier. 31 Excitation was performed either at 400 nm by using the frequency-doubled output of a standard 1 kHz amplified Ti:Sapphire system (Spectra-Physics) or at 490 nm with a home-built two-stages noncollinear optical parametric amplifier. The pump intensity on the sample was around 2 mJ · cm -2 .…”