Ultrafast charge transfer in amino-substituted boron dipyrromethene dyes and its inhibition by cation complexation: A new design concept for highly sensitive fluorescent probes
M. Kollmannsberger et al., Ultrafast charge transfer in amino-substituted boron dipyrromethene dyes and its inhibition by cation complexation: A new design concept for highly sensitive fluorescent probes, J PHYS CH A, 102(50), 1998, pp. 10211-10220
The photophysical behavior of a newly synthesized aza crown-substituted bor
on-dipyrromethene (BDP) dye and its dimethylamino analogue were investigate
d with steady-state and time-resolved fluorometry and compared to a referen
ce compound. In solvents more polar than hexane, excitation of the dyes lea
ds to a fast charge transfer from the locally excited (LE) state to a weakl
y emissive charge-transfer (CT) state. The donor-substituted compounds show
dual emission from the LE and CT state, both fluorescence quantum yields b
eing low. The rate constant of excited-stare charge separation, calculated
from the global analysis of time-resolved emission data, was determined to
1.6 x 10(11) s(-1) in 1,4-dioxane. The crowned compound forms 1:1-complexes
with various alkali and alkaline-earth metal ions, which exist as two conf
ormers in solution. In these complexes, coordination of the cation to the n
itrogen donor atom of the crown inhibits the charge-transfer process, leadi
ng to a cation-dependent enhancement of the LE emission and the fluorescenc
e lifetimes by factors >10(3). This efficient "switching on" of the fluores
cence renders the crowned BDP dye an extremely sensitive fluorescent probe
for metal ions.