S. Dhami et al., PHTHALOCYANINE FLUORESCENCE AT HIGH-CONCENTRATION - DIMERS OR REABSORPTION EFFECT, Photochemistry and photobiology, 61(4), 1995, pp. 341-346
For tetrasulfonated aluminum phthalocyanine (AlPcS(4)), dimer formatio
n is characterized in the absorption spectrum by a broadening of the Q
-band and the appearance of a new band at the red edge of the spectrum
. The high concentrations required to produce dimers, however, often l
eads to anomalous observations in fluorescence spectroscopy. In the pr
esent study, we have examined the photophysical characteristics of two
dye systems; AlPcS(4) in a 66% ethanol/water mixture and disulfonated
aluminum phthalocyanine in methanol. Using absorption spectroscopy, t
he formation of dimers is shown to be prevalent only in the case of Al
PcS(4). The fluorescence emission spectra in both cases, however, exhi
bit similar spectral changes with increasing dye concentration. The me
asured fluorescence decay profiles for both dyes also show similar tre
nds: They are monoexponential, invariant with emission wavelength and
have decay times that increase with dye concentration. These distortio
ns are sometimes incorrectly attributed to dimer fluorescence. We find
no evidence for the existence of dimer fluorescence and demonstrate t
hat these data can be readily explained, by taking into consideration
the effects of reabsorption of fluorescence.