Charge-transfer or excimeric state? Exploring the nature of the excited state in cofacially arrayed polyfluorene derivatives

Article abstract of DOI:10.1016/j.jphotochem.2019.01.030

It is well known that upon electronic excitation various π-stacked dimers readily exhibit excimer formation, facilitated by a perfect sandwich-like arrangement between the chromophores. However, it is unclear whether such a dimer is also capable of electron transfer upon excitation, if a strong electron-donating group is covalently attached. In this work, we probe the nature of the excited state in a series of cofacially arrayed polyfluorene derivatives with electron-rich aromatic donor attached via a methylene linker. Our studies show that in all cases excimer formation is energetically favorable, and promotion of a charge-transfer state in such systems is possible but requires a free energy for electron transfer far exceeding 1 V. These findings shed light on important design principles for molecular scaffolds capable of stabilizing both excimeric and charge-transfer states upon their excitation.

Full text of DOI:10.1016/j.jphotochem.2019.01.030

Accepted Manuscript
Title: Charge-transfer or excimeric state? Exploring the nature
of the excited state in cofacially arrayed polyfluorene
derivatives
Authors: Ainur Abzhanova, Lena V. Ivanova, Denan Wang,
Tushar S. Navale, Sameh H. Abdelwahed, Maxim V. Ivanov,
Sergey Lindeman, Rajendra Rathore, Scott A. Reid
PII:
S1010-6030(18)31290-5
DOI:
Reference:
https://doi.org/10.1016/j.jphotochem.2019.01.030
JPC 11689
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Accepted date:
4 September 2018
28 January 2019
Please cite this article as: Abzhanova A, Ivanova LV, Wang D, Navale TS, Abdelwahed
SH, Ivanov MV, Lindeman S, Rathore R, Reid SA, Charge-transfer or excimeric
state? Exploring the nature of the excited state in cofacially arrayed polyfluorene
derivatives, Journal of Photochemistry and amp; Photobiology, A: Chemistry (2019),
https://doi.org/10.1016/j.jphotochem.2019.01.030
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Charge-transfer or excimeric state? Exploring the nature of the excited state
in cofacially arrayed polyfluorene derivatives
Ainur Abzhanova, Lena V. Ivanova, Denan Wang, Tushar S. Navale, Sameh H.
†
Abdelwahed, Maxim V. Ivanov, Sergey Lindeman, Rajendra Rathore and Scott A.
Reid*
Marquette University, 53233
Highlights

Nature of the excited state is probed in a series of cofacially arrayed
polyfluorene derivatives with electron-rich aromatic donor attached via a
methylene linker

We examine whether an excimer to charge-transfer transition can be
observed


In all cases we find that excimer formation is energetically favorable
Promotion of a charge-transfer state in such systems is possible but requires
a free energy for electron transfer far exceeding 1 volt.
These findings shed light on important design principles for molecular
scaffolds capable of stabilizing both excimeric and charge-transfer states
upon their excitation.

Abstract
It is well known that upon electronic excitation various -stacked dimers readily
exhibit excimer formation, facilitated by a perfect sandwich-like arrangement
between the chromophores. However, it is unclear whether such a dimer is also
capable of electron transfer upon excitation, if a strong electron-donating group is
covalently attached. In this work, we probe the nature of the excited state in a series
of cofacially arrayed polyfluorene derivatives with electron-rich aromatic donor
attached via a methylene linker. Our studies show that in all cases excimer
formation is energetically favorable, and promotion of a charge-transfer state in
such systems is possible but requires a free energy for electron transfer far
exceeding 1 volt. These findings shed light on important design principles for
molecular scaffolds capable of stabilizing both excimeric and charge-transfer states
upon their excitation.
Introduction

Understanding the nature of the excited state in -stacked assemblies is critically
important for development of efficient artificial light harvesting systems and
1-3
photovoltaic devices. It is now established that extensive exciton delocalization is
a crucial factor in achieving efficient energy transfer in various molecular
4,5
assemblies. However, coupling to the internal and external vibrational modes
leads to exciton localization, heavily restricting the size of exciton and reducing the
6
,7
rate of exciton migration. In such a scenario it is assumed that exciton transfer
follows an incoherent hopping mechanism, which allows several hops before
8
emission.
As an example, photoexcitation of cofacially-arrayed polyfluorenes (Fn, Figure 1A)
leads to the formation of an excimeric state where exciton delocalization is limited
to two fluorenes for all n as evident from the characteristic band at 395 nm in the
9
,10
emission spectra of F2-F6 (Figure 1B). This is understood as occurring due to the
rapid decrease in stabilization with increasing n in comparison with the geometrical
penalty for reorganization into multiply stacked structures. Furthermore, we have
recently shown that an identical band is observed in the emission spectrum of the
C
rigid cyclophane-like bifluorene F2, providing further support that in the Fn series
exciton delocalization is limited to two fluorenes with a sandwich-like arrangement
11
(
Figure 1C).
Figure 1. A. Structures of cofacially-arrayed polyfluorenes (Fn, n = 1-6). B. Emission spectra of F1-F6 in CH
2
2
Cl at
C
2
2 °C. C. Emission spectrum of F2 CH
2
2
Cl at 22 °C.
While it is usually assumed that excimer formation acts as a trap, resulting in
dissipation of the excitonic energy and concomitant loss of efficient charge
separation, a recent study by Würthner and co-workers showed that the electron
transfer step in covalently linked perylene bisimide dimers (PBI) occurs via
12
formation of the excimeric state.
There, it was demonstrated that the decay of
excimer emission is accompanied by the rise of transient bands assigned to the PBI
cation and anion radicals. This observation calls for the studies to investigate the

excited state dynamics in systems capable of stabilizing both excimeric and charge-
transfer states.
Motivated by these findings, we synthesized a series of polyfluorene derivatives
Ar
(
Fn, n = 1-3, Chart 1) with an aromatic electron donor (i.e., Ar = 2,5-dimethoxy-p-
xylene or DMX) attached via a methylene linker. On may expect that upon excitation
Ar
of Fn, the aryl group would act as an electron donor (Eox[DMX] = 1.12 V vs SCE)
and polyfluorene moiety as an electron acceptor (Eox[F1] = 1.73 V and Ered[F1] ~ 2.2
V vs SCE), thus favoring a charge-transfer state. On the other hand, accessibility of
the sandwich-like arrangement in the polyfluorene moiety beginning from n = 2
might favor stabilization of the excimeric state. In this manuscript, we probed the
Ar
nature of the excited state in the Fn derivatives with the aid of steady-state optical
spectroscopy and DFT calculations. We demonstrate that excimer formation is
energetically favorable, and in order to promote the charge-transfer state in such
systems, free energy of an electron transfer must far exceed 1 volt. Details of the
Ar
preliminary findings on the nature of the excited state in Fn are discussed below.
Ar
Chart 1. Structures of ( Fn, n = 1-3, Ar = 2,5-dimethoxy-p-xylene)
Results and Discussion
Synthesis
Ar
A series of polyfluorene derivatives Fn (Ar = 2,5-dimethoxy-p-xylene or DMX, and
n = 1−3) were synthesized from readily available fluorene using the synthetic route
1
13
illustrated in Scheme 1A. All compounds were characterized by H/ C NMR and
Ar
MALDI-TOF mass spectroscopy. The structure of F3 was further characterized by
X-ray crystallography (Scheme 1B). Complete experimental details and
characterization data are summarized in the Supporting Information.

Scheme 1. A. Synthetic scheme of ArFn (Ar = 2,5-dimethoxy-p-xylene and n = 1−3). B. ORTEP diagram (50%
Ar
probability) and space-filling representation of Fn.
Ar
Electrochemistry of Fn and spectroscopy of their cation radicals
Ar
The electron donor strengths of Fn series (Ar = 2,5-dimethoxy-p-xylene or DMX,
and n = 1−3) were evaluated by electrochemical oxidation at a platinum electrode as
a 2 mM solution in CH
2
Cl
2
containing 0.1 M n-Bu
4
NPF as the supporting electrolyte.
6
Ar
Cyclic voltammograms (CVs) of Fn showed one reversible oxidation wave in each
case, with a minor lowering of the oxidation potential (Eox) upon increasing number
of fluorenes (Figure 2A). In contrast, unsubstituted polyfluorenes (Fn) display a
significant lowering of the Eox with increasing n, signifying extended delocalization
of the cationic charge (i.e., hole) across the cofacially-arrayed polyfluorenes (Figure
Ar
2
A). As the Eox of Fn spans a narrow range of 0.66-0.63 V that includes Eox of DMX,
1
3
Ar
the hole delocalization must be limited to the aryl moiety of Fn, irrespective of
the number of fluorenes. Indeed, calculations using CAM-B3LYP-D3/6-31G(d) 1
4-16
confirmed that: 1) the free energy of oxidation does not depend on the number of
Ar
+•
fluorenes, and 2) the hole is localized at the aryl moiety of Fn as visualized using
the isovalue spin-density plots in Figure 2A.

Figure 2. Cyclic and square-wave voltammograms of 2 mM ArFn in CH
Cl
at 22 °C; isovalue spin-density plots of
2
2
Ar
+•
Fn calculated using CAM-B3LYP-D3/6-31G(d)+PCM(CH
2
2
Cl ) and plots of experimental oxidation potentials of
Ar
Fn (blue) and Fn (red) against 1/n.
Ar
The electrochemical stability of the Fn series prompted us to generate their cation
+
•
-
radicals using a stable cation-radical salt [NAP SbCl
6
] (NAP = 1,1,4,4,7,7,10,10-
+
octamethyl-1,2,3,4,7,8,9,10-octahydrotetracene, Ered = 0.94 V vs Fc/Fc , λmax = 673
nm, εmax = 9300 cm M ) as a one-electron aromatic oxidant in CH
-1
-1
.17 The redox
Cl
2 2
titration experiment was carried out by an incremental addition of sub-
Ar
+•
stoichiometric amounts of Fn to the solution of NAP . The one-electron oxidation
Ar
Ar
+•
+•
of Fn to Fn and reduction of NAP to NAP can be described by the equilibrium
shown in eq. 1.
+•
Ar
Ar
Ar
+•
NAP + Fn ⇄ Fn + Fn
(eq. 1)
+
•
Ar
Treatment of NAP with increments of Fn led to disappearance of the absorption
+•
bands of NAP at 672 nm and growth of a new band at 468 nm (Figure 3B).
Numerical deconvolution
1
8,19
of the absorption spectrum at each increment further
confirmed that the redox reaction follows a 1:1 stoichiometry as indicated in eq. 1.
Ar
+•
+•
Comparison of the generated electronic spectra of Fn with that of model DMX
confirmed that the sharp band at 468 nm arises due to the hole localization at the
Ar
+•
aryl moiety of Fn in consistency with the electrochemical data and DFT
calculations (Figure 3A).

We emphasize that observation of a hole localization exclusively on the aryl group of
Ar
+•
Fn may seem unexpected, and further suggests that the electronic coupling
between aryl group and adjacent fluorene is relatively minor. Yet, the electronic
coupling in a structurally similar scaffold was found to be sufficient to promote the
20
efficient energy transfer as has been shown in previous studies of triplet energy
transfer in the donor-bridge-acceptor system displayed in Scheme 2. In this study,
the cofacially arrayed polyfluorenes acted as the bridge that was connected to a
benzophenone triplet energy donor (Bp) and naphthalene acceptor (Nap) via the
methylene linker. This study showed that the polyfluorenes act as an efficient
medium for the energy transfer with the mechanism of the triplet energy transfer
switching from tunneling to hopping when the number of fluorenes is changed from
1
to 2.
Scheme 2. Bp-Fn-Nap structures used to probe triplet energy transfer in cofacially arrayed polyfluorenes.
Ar
Optical spectroscopy of Fn
To probe the nature of the (singlet) excited state in ArFn we recorded their
absorption and emission spectra in solvents with varied polarity. Absorption
Ar
spectra of Fn display characteristic bands with the wavelength of the absorption
maximum invariant to the solvent polarity (Figure S1 in the Supporting
Ar
Information). In contrast, emission spectrum of F1 show a strong dependence on
solvent polarity, with the wavelength of the emission maximum evolving from λem
=
322 nm in hexane to λem = 404 nm in acetonitrile (Figure 3A). Calculations using
time-dependent (TD) DFT with CAM-B3LYP-D3/6-31G(d) with state-specific
Ar
solvation further confirm that emission of F1 at the equilibrium excited state
geometry involves a transition between HOMO and LUMO, with the HOMO residing
on the aryl group, and LUMO centered on the fluorene (Figure 3B). Furthermore,
emission energies calculated using TD-DFT showed a perfect linear correlation with
the experimental values (Figure 3C). Natural population analysis (NPA)2 of the
excited state wavefunction showed that a charge of 0.95 e is transferred upon
excitation, which together with the experimentally observed strong solvent effect,
1
Ar
clearly points to the charge-transfer nature of the excited state in F1.

Figure 3. A. Normalized emission spectra of ArF1 in various solvents. B. Isovalue plots of HOMO and LUMO of
Ar
F1 calculated using CAM-B3LYP-D3/6-31G(d)+PCM(CH
2
Cl
2
). C. Correlation plot between emission energies
Ar
calculated using DFT and experimental energies of maximum emission of F1 in various solvents.
In contrast, emission spectra of ArF2 and ArF3 derivatives show a characteristic
excimeric band centered at 395 nm whose position is invariant to the solvent
polarity (Figure 4A and 4B). TD-DFT calculations confirmed that in ArF2 exciton
delocalization is spread over the two fluorenes, which form an ideal sandwich-like
arrangement as represented in the transition density plots in Figure 4C.
Accordingly, excimeric emission involves transition from LUMO to HOMO, both
centered on the same two-fluorene moieties. Finally, NPA of the excited state
Ar
Ar
wavefunction in F2 and F3 showed that only a negligible amount of charge (i.e.,
0.04 e) is transferred to the bifluorene moiety upon excitation.
Figure 4. Emission spectra of (A) ArF2 and (B) ArF3 in various solvents. C. Isovalue plots of transition density,
HOMO and LUMO calculated using CAM-B3LYP-D3/6-31G(d)+PCM(CH Cl ).
2
2
The striking transition in nature of the excited state from being purely charge-
Ar
Ar
transfer in F1 to the excimeric in Fn when n = 2-3 provides important
information on the stability of excimer formation in polyfluorenes. In particular,

comparison of the redox potentials of the Ar group (Eox[DMX] = 1.12 V vs SCE) and
fluorene moiety (Ered[F1] ~ 2.2 V vs SCE) provides an estimated free energy of the
electron transfer, i.e., ΔGET = Eox[DMX] - Ered[F1] = -1.1 V. Thus, a significant energy
gain in the electron transfer is responsible for the observed charge transfer upon
Ar
Ar
excitation of F1. In contrast, while the presence of additional fluorenes in F2 and
Ar
22
F3 is expected to further stabilize the anionic charge and favor charge-transfer,
Ar
Ar
the observation of an excimeric state in F2 and F3 suggests that the stabilization
energy of the excimer exceeds that of the energetic gain of charge transfer (ΔGET).
These preliminary findings show that energy of excimer formation is significant and
a much larger difference in the redox properties between the Ar group and
Ar
polyfluorene moiety is needed in order to promote the charge transfer in Fn
scaffolds. The preparation of such systems is currently under investigation and will
be presented in due course.
Conclusions
Motivated by the recent reports on the stabilization of the electron-transfer state via
excimer formation in perylene bisimide dimers (PBI), we explored the nature of the
Ar
excited state in a series of cofacially arrayed polyfluorene derivatives ( Fn) with an
electron-rich aromatic donor attached via methylene linker. Our preliminary studies
Ar
showed that F1 exhibits a charge-transfer state upon excitation, as evidenced in
the strong dependence of the emission on the solvent polarity and supported by
Ar
Ar
DFT calculations. In contrast, emission spectra of F2 and F3 show a characteristic
excimeric band without any solvent effect, suggesting that an excimeric state is
formed, an observation also supported by DFT calculations. The transition of the
Ar
Ar
nature of the excited state from charge-transfer like in F1 to excimer like in F2
Ar
and F3 suggests a significant energetic gain associated with excimer formation.
Finally, comparison of the redox potentials of aromatic group and fluorene moiety
suggest that the free energy of the electron transfer must exceed 1.1 V in order to
favor formation of the charge-transfer state in polyfluorene scaffolds. These
findings provide important guidance into the design and synthesis of molecular
scaffolds capable of stabilizing both excimeric and charge-transfer states upon their
excitation.
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