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Steps To Demarcate the Effects of Chromophore Aggregation and Planarization in Poly(phenyleneethynylene)s. 2. The Photophysics of 1,4-Diethynyl-2-fluorobenzene in Solution and in Crystals
journal contribution
posted on 2001-04-04, 00:00 authored by Marcia Levitus, Gerardo Zepeda, Hung Dang, Carlos Godinez, Tinh-Alfredo V. Khuong, Kelli Schmieder, Miguel A. Garcia-GaribayCrystals of 1,4-bis(2-hydroxy-2-methyl-3-butynyl)-2-fluorobenzene 4 have a rich packing structure
with four distinct molecules in the unit cell. A complex hydrogen bonding network results in the
formation of cofacial trimers, cofacial dimers, and monomers within the same unit cell. Given a
remarkable opportunity to investigate the effect of aggregation on the photophysics of 1,4-diethynylbenzenes, we analyzed the absorption, diffuse reflectance, and emission spectra of
compound 4 in solutions and in crystals. Diffuse reflectance and fluorescence excitation revealed
a red-shifted absorption that is absent in dilute solution but becomes observable at high
concentrations and low temperatures. The fluorescence emission in the solid state is dual with
components assigned to monomers and aggregates. The excitation and emission assigned to the
monomer are nearly identical in crystals and dilute solutions. The absorption and emission bands
assigned to aggregates are broad and red-shifted by 60−80 nm. As expected for a sample with
absorbers and emitters with different energies and incomplete equilibration, efficient monomer-to-aggregate energy transfer was observed by a proper selection of excitation wavelengths. The
fluorescence quantum yield of 4 in solution is relatively low (ΦF = 0.15) and the singlet lifetime
short (τF = 3.8 ns). A lower limit for the triplet yield of ΦT = 0.64 was determined indirectly in
solution by 1O2 sensitization, and a relatively strong and long-lived phosphorescence was observed
in low-temperature glasses and in crystals at 77 K.