DEVELOPMENT OF ANTENNA CHROMOPHORES FOR SENSITISED LANTHANIDE EMISSION Filip Kielar, David Parker

1
DEVELOPMENT OF ANTENNA CHROMOPHORES FOR
SENSITISED LANTHANIDE EMISSION Filip Kielar,
David Parker
Department of Chemistry, University of Durham,
South Road, Durham, DH1 3LE, United Kingdom

• Introduction
• Lanthanide luminescence is a phenomenon that has
  been intensively studied in last 30 years. The
  understanding of underlying principles is now
  sufficiently advanced that luminescent lanthanide
  complexes can be developed for cellular imaging.

Tb3 complexes
The forbidden nature of lanthanide ff transitions
leads to the need of incorporation of sensitizing
chromophores into such complexes. There are
several conditions that an organic molecule has
to meet to be considered as a suitable candidate
for lathanide sensitization. One of most
fundamental of these is a readily populated
triplet energy state of sufficiently high energy.
There are other criteria derived from
practicalities of cellular imaging. An absorption
wavelength of the chromophore above 350 nm is one
of them.
Figure 1. Sensitized lanthanide emission
Two classes of chromophores were chosen for this
study. The first is derived from the
tetraazatriphenylene structure, which is well
known to sensitise lanthanide emission.1 The
other is based on thioazaxanthone structure,
which exhibits promising photochemical
properties.2 The nature of these chromophores was
modified by incorporation of carboxylic acid
groups into their structure as a possible
conjugation points in further work.
Scheme 3. Synthesis of Tb3 complex 1
Figure 2. Target sensitizer core structures
Chromophore synthesis
Figure 3. Absorption spectrum of Tb3 complex
1 (?mas 370 nm)
Figure 4. Total emission spectrum of Tb3 complex
1 ??Tb 2.2 ms)
Fluorescence quenching Quenching of lanthanide
luminescence by electron rich species (iodide,
citrate, ascorbate) was observed for complexes
containing tetraazatriphenylene chromophore.1 A
transient reduction of the chromophore by the
quencher is postulated as a step in this process.
The carboxylic substituents were introduced into
the tetraazatriphenylene chromophore with the aim
of inhibiting this quenching by electrostatic
repulsion. The quenching experiments are carried
out in solution buffered to pH 7.4, where the
carboxylic groups are dissociated and carry a
negative charge. The chromophore carrying a
negative charge was expected to be less
susceptible to reduction, thus decreasing the
feasibility of the quenching. Quenching by
iodide was however observed in a preliminary
experiment. This result may be attributed to the
fact that the negative charge is mainly localised
in the carboxylate groups and the pyrazine ring
and thus can provide only Coulombic shielding of
the chromophore. Moreover the charge transferred
to the chromophore is expected to be localised
initially on the pyridine rings coordinated to
the metal, whereas the carboxylate groups are
situated on the pyrazine ring, relatively far
from proposed site of reduction
Scheme 1. Tetraazatriphenylene chromophore
synthesis
Scheme 4. Synthesis of Tb3 complex 2
Scheme 2. Thiaxanthone chromophore synthesis
Chromophore properties
Table 1. Chromophore photochemical properties
1. ?abs, ?, ?em were measured in methanol at
298K. 2.The triplet energy of each chromophore
was measured at 77 in frozen glass
(Ether/Pentane/Ethanol 552). The values
obtained refer to the zero-zero transition,
considered as the observed maximum or as the
intercept (starred values).
Figure 5. Absorption spectrum of Tb3 complex 2
(?max 370 nm)
Figure 6. Total emission spectrum of Tb3 complex
2 (?Tb 0.9 ms)
Conclusions The synthesis of two new
chromophores based on tetraazatriphenylene and
thioazaxanthone structures has been accomplished.
These sensitizing groups have been and covalently
attached to DO3AtBu and the corresponding Tb3
complexes prepared. Both chromophores were shown
to be capable of sensitizing Tb3 emission.
Preliminary quenching experiment was carried out
with complex containing the tetraazatriphenylene
sensitizer. Quenching by iodide was observed,
implying a need of further adjustment of the
chromophore structure so that lower or much
reduced sensitivity to anion quenching is
achieved.

• References
• R. A. Poole, G. Bobba, M. J. Cann, J. C. Frias,
  D. Parker, R. D. Peacock, Org. Biomol. Chem.,
  2005, 3, 1013.
• D. Parker, J. Yu, Chem Comm., 2005, 3141