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
M. Levitus et al., 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, J ORG CHEM, 66(9), 2001, pp. 3188-3195
Crystals 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. 4 com
plex hydrogen bonding network results in the formation of cofacial trimers,
cofacial dimers, and monomers within the same unit cell, Given a remarkabl
e opportunity to investigate the effect of aggregation on the photophysics
of 1,4-diethynylbenzenes,we analyzed the absorption, diffuse reflectance, a
nd emission spectra of compound 4 in solutions and in crystals. Diffuse ref
lectance 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 em
ission assigned to the monomer are nearly identical in crystals and dilute
solutions. The absorption and emission bands assigned to aggregates are bro
ad and red-shifted by 60-80 nm, As expected for a sample with absorbers and
emitters with different energies and incomplete equilibration, efficient m
onomer-to-aggregate. energy transfer was observed by a proper selection of
excitation wavelengths. The fluorescence quantum yield of 4 in solution is
relatively low (Phi (F) = 0.15) and the singlet lifetime short (tau (F) = 3
.8 ns, A lower limit for the triplet yield of Phi (T) = 0.64: was determine
d indirectly in solution by O-1(2) sensitization, and a relatively strong a
nd long-lived phosphorescence was observed in low-temperature glasses and i
n crystals at 77 K.