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.
12 Figures and Tables
Figure 1. Chemical structure of the donor-acceptor bridged systems (DABS) and of TMBDY. The stars define the dihedral angle, θd.
TABLE 1: Photophysical Properties of the DABS and of TMBDY in Toluene
Figure 2. Absorption and fluorescence spectra of 1 and TMBDY in toluene (excitation at 480 nm).
TABLE 2: Fluorescence Lifetime, τf (in ps), Measured with 1-3 in Various Polar Solvents and Time Constants of Charge Separation, τCS (in ps), Determined from Eq 2
Figure 3. Time profiles of the fluorescence of 1 in ACN and VaCN measured at 525 nm after excitation at 400 nm and best singleexponential fits (solid lines).
TABLE 3: Fluorescence Anisotropy Decay Times, τr, of TMBDY in Solvents of Various Viscosities, η
Figure 4. Time profile of the fluorescence of TMBDY in nitrobenzene measured at 540 nm after excitation at 400 nm and best triexponential fit.
Figure 5. Transient absorption spectra measured with (A) 1 in BuCN and (B) TMBDY in nitrobenzene at different time delays after excitation at 500 nm.
Figure 6. Time evolution of the polarized fluorescence measured with TMBDY in ACN at 525 nm after excitation at 400 nm. The inset shows the decay of the polarization anisotropy.
Figure 7. Viscosity dependence of the fluorescence anisotropy decay time of TMBDY and best linear fit (solid gray line).
Figure 8. Correlation between the CS time constant of 1, τCS, and the fluorescence anisotropy decay time of TMBDY, τr, and best linear fits (solid gray lines).
Figure 9. Frontier molecular orbitals of 1 computed at the B3LYP/ [3s2p1d] level of theory.
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