No abstract

The photophysical properties of a series of laser-dye-labeled poly(aryl ether) dendrimers, generations 1-4, have been determined. The dendrimers act as extremely efficient light-harvesting antennae capable of transferring light energy through space from their periphery to their core. The light-harvesting ability of these molecules increases with generation due to an increase in the number of peripheral chromophores. The energytransfer efficiency was found to be quantitative for generations 1-3, with only a slight decrease observed for the fourth generation (∼93%). Due to the high extinction coefficients and fluorescence quantum yields of the chromophores and the efficient intramolecular energy transfer of the dendritic assemblies, these macromolecules have the potential to become integral components of molecular photonic devices.

Ultrafast pump−probe studies of room-temperature solutions of 3-hydroxyflavone (3-HF) and some 4‘-substituted derivatives have been undertaken. Transient absorption attributable to the two zwitterionic forms of the excited tautomer arising from excited-state intramolecular proton transfer (ESIPT) was observed across most of the visible spectral region. For 3-HF in methylcyclohexane and acetonitrile, the ESIPT was found to be so rapid that it was only possible to assign a time constant of 35 fs to the process. In ethanol, however, a time constant of 60 fs was determined. The slower ESIPT in this solvent was attributed to the greater strength of the solute−solvent interactions. For the derivatives of 3-HF in all three solvents, the ESIPT step was also found to be instrument-limited. In addition to the femtosecond kinetics, there was also a picosecond component of the kinetics that is attributed to ESPT in molecules that are intermolecularly hydrogen bonded to the solvent.

Transient spectra for 4‘-(N,N-dimethylamino)-3-hydroxy- (1a) and 4‘-(N,N-dimethylamino)-3-methoxyflavone (2a) in several solvents at room temperature are reported following excitation with pulses of approximately 100 fs fwhm. 1a exhibits emission from both the normal excited state and from the excited tautomer produced following excited-state intramolecular proton transfer (ESIPT). Rate constants for the ESIPT process are found to vary between 3.3 × 1011 and 3 × 1010 s-1 depending on the solvent. Polar solvents are found to play a significant role in stabilizing the normal excited state of both 1a and 2a.

Excited state intramolecular proton transfer (ESIPT) in 1-chloroacetylaminoanthraquinone (CAAQ) and 1,8dihydroxyanthraquinone (DHAQ) in solution is investigated at room temperature by means of femtosecond transient absorption spectroscopy. While ESIPT is practically instantaneous in DHAQ, in CAAQ the same process occurs with measurable kinetics, with a time constant of 100 fs. The energy diagrams for the ground and excited states of the proton-transferred and non-proton-transferred species, compatible with these observations, are discussed and an estimate is given for the activation energy of the process.