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Picosecond and Steady-State Emission of [Ru(phen)2dppz]2+ in Glycerol: Anomalous Temperature Dependence
journal contribution
posted on 2003-01-21, 00:00 authored by Björn Önfelt, Johan Olofsson, Per Lincoln, Bengt NordénThe excited-state deactivation of the “light-switch” compound [Ru(phen)2dppz]2+, where phen = 1,10-phenanthroline and dppz = dipyrido[3,2-a:2‘,3‘-c]phenazine, has been investigated in glycerol using single-photon counting at picosecond time resolution. Relaxation back to the ground state occurs in about 8 ns at
20 °C, which is much faster than previously reported in monohydric alcohols, though still slow compared to
that in water. Multivariate kinetic analysis reveals three distinct excited species involved in the relaxation
process in glycerol. Using a matrix exponential approach for the kinetic data analysis, including global fitting
of the relaxation data collected at many wavelengths, individual emission spectra for all three excited species
could be resolved. The resolved emission profile for the most short-lived species was found to resemble the
steady-state emission spectrum of [Ru(phen)3]2+ in glycerol whereas the emission profile of the intermediate
species resembled that of [Ru(phen)2dppz]2+ in ethanol. The spectrum of the third species is considerably
red-shifted compared to those of the other two. The longest lifetime as well as the emission quantum yield
show pronounced nonmonotonic variations with temperature in apparent conflict with the Arrhenius equation.
This anomalous temperature dependence can be accounted for by a model based on the equilibrium between
two excited species, corresponding to the two resolved emission spectra retrieved at 20 °C. Thermodynamic
data indicates that transfer to the fast-relaxing, red-shifted species is accompanied by a substantial lowering
in enthalpy. The thermodynamic data, as well as an abnormally high preexponential factor for the back reaction
from the third to the second excited species, could be explained in terms of the formation of two hydrogen
bonds, one to each of the aza nitrogens of the dppz moiety.