TEMPERATURE EFFECT ON THE FORMATION OF THE LOWEST EXCITED TRIPLET-STATES OF 9-BROMOANTHRACENE AND 9,10-DIBROMOANTHRACENE

Fluorescence spectra due to the lowest excited singlet (S-1) states and triplet-triplet (T'<--T-1) absorption spectra due to the lowest excited triplet (T-1) states of 9-bromoanthracene (BA) and 9,10-dibromoanthracene (DBA) have been measured in 3-methylpentane and EPA (diethyl ether-isopentane-ethanol, 5:5:2 in volume ratio) over a temperature range from 90 to about 300 K. Plots of the integrated intensities of fluorescence and T'<--T-1 absorption spectra against temperature (T) reveal that the S-1 decay constant (k(F)(T)) can be expressed in the form k(F)(T)=k(r)+k(isc)(o)+A(isc)(1-Phi(TS))exp(-Delta E/kT), where kr is the rate constant for the S-1--> ground state radiative transition, A(isc) exp(-Delta E/kT) and k(isc)(o) are the rate constants for the temperature-dependent S-1-->T-n (n greater than or equal to 2) and temperature-independent S-1-->T-x(1 less than or equal to x<n) intersystem crossings respectively and Phi(TS) is the quantum yield for the temperature-independent Tn(n greater than or equal to 2)-->S-1 inverse intersystem crossing; A(isc) and Delta E are the Arrhenius terms of frequency factor and activation energy respectively and k is the Boltzmann constant. Since the values of k(isc)(o) ((1.2-1.7)x10(7) s-1 for BA and 1.9x107 s-1 for DBA) are comparable with those of kr ((5.2-5.7)x10(7) s(-1) for BA and 6.7x10(7) s(-1) for DBA), the temperature-dependent S-1-->T-1 or S-1-->T-x(1<x<n)-->T-1 intersystem crossing populating the lowest excited triplet states of bromoanthracenes (BA and DBA) is very important, especially at low temperatures where the temperature-dependent S-1-->T-n(n greater than or equal to 2)-->T-1 intersystem crossing is negligible. This conclusion is consistent with detection of the T'<--T-1 absorption spectra even at 77 K.