C. E. Finlayson, P. H. J. Kouwer et al.
Zn–Pc tetrakis
(PDI) by Li et al.[7] In these molecules, and in
Finally, these octad molecules demonstrate the ability to
form the kinds of aggregated structures that may be condu-
cive to the delocalisation of generated charges in the solid
state. Whilst being beyond the scope of this present study,
further work is also in progress to fully characterise the
physical and photophysical properties in thin films. We an-
ticipate the development of these materials as model sys-
tems of multichromophoric aggregates, exhibiting delocal-
ised states. Adjustments to the rigidity of the linkers, which
connect the various moieties, and to the arrangement and
side substitution of the molecules may in the future enable
the realisation of optimised hierarchical molecular struc-
tures, for pseudo-biological light-harvesting antennae.[6,38] In
addition, further tuning of the energy levels of moieties
might allow efficient charge generation and extraction along
the stacks for use in solar cells.[39–42]
the “octads” under consideration herein, there is no well-de-
fined phase-separated heterojunction, across which charge
separation may occur, as there is in a blend system. A perti-
nent comparison may be drawn with the work of Kim
et al.,[36] who studied the photophysical differences between
i) blend heterojunctions of two conjugated polymers and
ii) analogous random co-polymers; observing the predomi-
nance of inter-chain charge separation in the former case,
but rapid intra-chain energy transfer in the latter. Further-
more, in the low dielectric constant solvent media in which
the octads are soluble, the critical distances for energy trans-
fer (R0) have large values of several nm,[37] implying that
FRET processes are predominant over short-range Dexter-
like charge tunnelling processes. Commensurately, it is likely
that any charge separation between moieties will be unsta-
ble with respect to the excitonic states.
Experimental Section
Conclusion
For the TA experiments, solutions of the materials in chloroform (ꢀ10–
100 mgmLꢁ1) were excited at l=490–500 or 650 nm by using the output
of a commercial TOPAS (Light Conversion) pumped by an amplified Ti:
sapphire laser system (Spectra-Physics Tsunami and Spitfire system) with
a pulse duration of approximately 100 fs. More concentrated solutions
were also studied, but no changes in kinetics were observed. The excita-
tion power density was varied between 50 and 300 mJcmꢁ2 by using con-
ventional neutral density filters. The TA behaviour was then probed with
a white-light continuum, derived from a home-built, non-collinear optical
parametric amplifier (NOPA) setup.[43] The NOPA generates an ampli-
fied white-light continuum in the range of l=500–800 nm. A fraction
This paper describes the synthesis and photophysical charac-
terisation of a series of Pc–PDI octad molecules in which
eight perylene moieties are attached to a Pc core with alkyl-
chain linkers. Our experiments illustrate how the photophys-
ical properties may be altered by changes within the struc-
ture and the arrangement of the molecules; in particular, a
correlation between intermolecular aggregation and the
presence of long-lived excitations on the Pc core is ob-
served. There is clear spectroscopic evidence, as well as
direct physical evidence from DLS measurements, that these
octads can exist as monomers or form aggregates along the
Pc cores, depending on the type of Pc and the solvent
medium used.
Photophysical characterisation illustrates how relaxation
pathways may be altered by changes within the structure
and the arrangement of the molecules. In the low dielectric
constant solvents, in which the octads are soluble, photoexci-
tation of the PDI units leads to rapid energy transfer to the
Pc centre, rather than a charge separation between moieties.
In octad monomers, the Pc singlet excited-state decays
within tens of ps, whereas the excitons are stabilised in the
aggregated form of the molecules, typically with lifetimes in
the order of 1–10 ns. By contrast, in an octad design in
which p–p interactions and intermolecular aggregation are
suppressed by the steric hindrance of a corona of incompati-
ble glycol tails around the molecule, a more straightforward
photophysical interaction of FRET between the PDI moiet-
ies and non-radiative Pc core is observed. The much higher
fluorescence QY of this octad, relative to the other designs,
is in good agreement with the reduced fluorescence lifetime
of the PDI moieties in this case. In the case of an octad
A
2 mm thick sapphire plate for white-light seed generation, which is then
amplified in a BBO crystal (cut at 328) by overlapping with a focused
400 nm pump (ꢀ50 mJ) under a non-collinear angle, which allows for
broadband phase matching. Only reflective optics are used to focus and
collimate the white-light seed and the NOPA output to reduce the chirp
of the white light. Both the 500 nm pump and the white-light probe are
focused into the sample solution, which is held in a 1 mm thick quartz
glass cuvette. Optimal spatial overlap is guaranteed by a significantly
smaller probe beam spot. The probe beam arrival is varied in time, rela-
tive to the pump pulse, by the use of an optical-delay line.
Normal-incidence UV/Vis absorption measurements were made by using
a Hewlett–Packard 8453 spectrophotometer. PL spectra were measured
by using a calibrated fluorescence spectrophotometer (Cary Eclipse). Fi-
nally, time-resolved PL for octad 2 was measured, using a technique of
time-correlated single photon counting (TCSPC). In brief, the samples
were excited by using a l=470 nm ps pulsed diode-laser source, with the
emitted photons being detected and binned based on their lag from the
triggering pulse, with the limiting time-resolution of the experiment
being around 100 ps. The PL lifetimes were then fitted by using linear re-
gression curve-fitting software.
Acknowledgements
This work was supported by the ESF-SONS II-SUPRAMATES project
and Marie Curie Research Training Network CHEXTAN (MRTN-CT-
2004-512161). S.A.-S. thanks the Ernest Oppenheimer Foundations for
funding. C.E.F. acknowledges the Leverhulme Trust (UK) for an Early
Career Fellowship. F.L. thanks the Deutsche Forschungsgemeinschaft
(DFG) for a postdoctoral research fellowship. P.H.J.K. thanks the EU
(LC-energy) for financial support. A.E.R. thanks the Vidi and Vici. The
based on a metallated form of the Pc core, the Cu–PcACTHNUTRGNEUNG(PDI)8
system, we find that aggregation and long-lived delocalised
excitations are observed, even in solvents with a high solu-
bility. Ultrafast energy transfer is followed by slow non-radi-
ative decay of the Pc singlet excitons, with tꢀ8 ns.
10028
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 10021 – 10029