Femtosecond broad-band pump−probe spectroscopy has been used to study intramolecular bichromophoric
coupling and structural relaxation in pyrene and aryl pyrene derivatives in solution. The influence of aryl
substituents on the S2 → S1 internal conversion process, which occurs with a time constant of ∼75 fs in
pyrene, has been investigated. While in 1-phenylpyrene the internal conversion is faster than 50 fs, it is
slower in 1-biphenyl-4-yl-pyrene (105 fs). The temporal evolution of the transient absorption spectrum indicates
strong mixing of several “zero-order” electronic configurationswhich evolve separately with timein the
S1 and the S2 states. The time-resolved spectra are interpreted within the framework of an adiabatic state
model which includes interchromophoric electronic coupling. In this paper we give a full description of the
experimental setup, the data acquisition procedure, and several experimental details about the characterization
of the broad-band femtosecond white light source.
The photophysical properties of several acceptor substituted 1-arylpyrene derivatives were investigated. The fluorescence spectra strongly depend on the nature of the aryl moiety and the position and number of methoxycarbonyl acceptor groups. Dual fluorescence, originating from a locally excited and a charge transfer state, was observed for the diester derivatives. The solvent dependence of the dual fluorescence and the slightly curved solvatochromic plots indicate a change of the character of the excited states from solvents of low to high polarity. The rate constants for fluorescence and nonradiative decay were calculated to reveal the nature of the excited-state relaxation, that is, the increase of the mesomeric interactions by geometrical flattening or stabilization of the CT state by further twisting toward perpendicularity. Flattening is the major relaxation pathway of the diester phenylpyrene derivative in nonpolar solvents whereas in highly polar solvents the low value for the fluorescence transition dipole moment (M f ) indicates stabilization of the charge transfer state by further twisting. The fluorescence of the diester biphenylpyrene derivative originates from a locally excited state (LE) in nonpolar solvents. The low value for M f in polar solvents and the change of the relative intensity of the dual fluorescence signals with the temperature indicate that the red-shifted fluorescence can be assigned to a twisted intramolecular charge transfer state.
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