Ultrafast solvation and anisotropy dynamics in a tri-branched molecule based on a triphenylamine core

Article abstract of DOI:10.1016/j.dyepig.2010.02.002

The steady-state spectra and the isotropic and anisotropic fluorescence dynamics of a tri-branched molecule comprising a triphenylamine core and 2,3-di(2-thienyl)acrylonitrile branches were studied in solution and solid film in the femtosecond regime. Iso

Full text of DOI:10.1016/j.dyepig.2010.02.002

Dyes and Pigments 87 (2010) 44e48
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Ultrafast solvation and anisotropy dynamics in a tri-branched molecule
based on a triphenylamine core
,
a
b
M. Fakis a , V. Giannetas , J. Mikroyannidis
*
a Department of Physics, University of Patras, 26504 Patras, Greece
b Department of Chemistry, University of Patras, 26504 Patras, Greece
a r t i c l e i n f o
a b s t r a c t
Article history:
The steady-state spectra and the isotropic and anisotropic fluorescence dynamics of a tri-branched
molecule comprising a triphenylamine core and 2,3-di(2-thienyl)acrylonitrile branches were studied
in solution and solid film in the femtosecond regime. Isotropic fluorescence dynamics revealed an
ultrafast decay component (ranging from 0.3 ps to 6.7 ps depending on solvent and emission wave-
length) that was observed only in solution and was attributed to solvation; a slow decay component
(w1 ns), attributed to radiative population decay, was also observed. The solvation process was studied
in three different solvents of different polarities and viscosities. The solvation efficiency increased with
increasing solvent polarity whilst solvation time was greatest in the solvent of highest viscosity.
Anisotropy dynamics showed an ultrafast decay in the fs regime indicating strong intramolecular
interactions among the branches.
Received 4 November 2009
Received in revised form
17 February 2010
Accepted 18 February 2010
Available online 1 March 2010
Keywords:
Femtosecond spectroscopy
Tri-branched molecules
Solvation
Ó 2010 Elsevier Ltd. All rights reserved.
Time resolved anisotropy
1. Introduction
especially for understanding the nature of the coupling between
the branches and for determining structureeproperty relationships
In the past decade a large amount of research has been devoted to
the design, synthesis and photophysical characterization of multi-
branched conjugated organic molecules owing to their potential
applications in non-linear optics (NLO), light emitting diodes, lasers
and solar cells [1e10]. The advantage of multi-branched molecules,
compared to the conventional linear conjugated compounds, is that
such compounds provide a large number of branched chromo-
phores in an ordered geometry. The non-linear optical properties of
multi-branched molecules can be greatly enhanced compared to
their linear counterparts due to a cooperative effect amongst the
branched chromophores imparted by electron coupling [11e14].
When this coupling is weak, the NLO parameters are the sum of
those of each chromophore. In order to exceed this additive behavior
[15,16], strong interbranch coupling leading to delocalized excita-
tions is required. The strength of coupling is greatly influenced by
the nature of the core and it has been shown that tri-branched
molecules with a nitrogen core favour excitation coupling among
branches compared to their counterparts having a carbon, phos-
phorus or benzene core [17].
[18,19]. Ultrafast isotropic and anisotropic fluorescence techniques
are valuable tools for probing energy migration and for differenti-
ating between coherent and incoherent energy transfer. Time
resolved isotropic fluorescence dynamics may probe different kinds
of dynamic processes such as intra- or inter-molecular energy
transfer, intramolecular rearrangement (e.g. twisting of a functional
group), solvation and radiative population decay. All of these
phenomena can occur within the time scale of tens of picoseconds
or less except for radiative population decay, which, typically,
occurs within hundreds of picoseconds. In addition, time resolved
anisotropic fluorescence dynamics may simultaneously probe
exciton migration and overall molecular rotation. However, these
processes typically occur over different time scales since exciton
migration occurs in the fs to ps regime whereas overall rotation
occurs in the ps to ns regime.
The photophysics and dynamics of conjugated molecules are
dramatically affected by the nature of the solvent. Understanding
solventesolute interactions is of great importance in photophysics
since by changing a solvent, the decay pathways of a molecule may
be completely altered. Especially, solvent polarity affects both
steady-state and time-resolved fluorescence spectra [20,21]. In
polar solvents an Intramolecular Charge Transfer (ICT) state is
formed especially when the solute molecule contains donor and
acceptor groups. In some cases, intramolecular charge separation
requires rotation of a group leading to a Twisted Intramolecular
Steady-state and time resolved spectroscopic techniques are
employed for the examination of multi-branched molecules,
* Corresponding author.
E-mail address: fakis@upatras.gr (M. Fakis).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.02.002

M. Fakis et al. / Dyes and Pigments 87 (2010) 44e48
45
Charge Transfer state (TICT) [20e22]; additionally, solvent viscosity
2.2. Techniques
affects reorientational motion and solvation dynamics [23,24].
From the above, it is apparent that combinatorial isotropic and
anisotropic fluorescence studies undertaken in various solvents
and in the solid state are required in order to thoroughly investigate
the photophysics of a molecule.
In this work, the steady-state spectra and ultrafast fluorescence
dynamics of a tri-branched molecule with triphenylamine central
core and 2,3-di(2-thienyl)acrylonitrile branches are studied in
solution and solid film. Both isotropic and anisotropic fluorescence
dynamics are presented. Three different solvents were used and the
effect of polarity and viscosity on the fluorescence dynamics is
discussed. Besides, ultrafast anisotropy dynamics is exhibited
indicating strong excitation delocalization among the branches.
The steady-state absorption and fluorescence spectra were
obtained with a Beckman DU-640 spectrometer and a Perkin Elmer
LS45 luminescence spectrometer respectively. The fluorescence
spectra were recorded by using an excitation wavelength of
400 nm. The fluorescence dynamics has been studied with a time
resolved fluorescence upconversion technique. The experimental
set-up has been described in details previously [26,27]. Briefly,
a mode-locked Ti:Sapphire femtosecond laser (80 fs pulse duration,
80 MHz repetition rate, tunable from 750 to 850 nm) has been
used as the fundamental laser source. The second harmonic of the
femtosecond laser was used for the excitation of the samples.
The excitation power was typically below 5 mW. The fluorescence
of the samples was collected and mixed with the delayed funda-
mental laser beam into a BBO crystal (type I) generating an
upconversion ultraviolet beam. This beam passed through appro-
priate filters and a monochromator and it was detected though
a photomultiplier as a function of the temporal delay between the
fundamental laser beam and the fluorescence. The temporal reso-
lution of the technique was 160 fs. Both isotropic and anisotropic
decays were measured. The isotropic decays were taken under
magic angle conditions while the anisotropic ones were detected
with polarization parallel and perpendicular to the excitation
beam. The polarized decays were used in order to determine the
time resolved anisotropy. The decays were measured for TPA(DTA)3
in three different solvents namely tetrahydrofuran (THF), aceto-
phenone (ACP) and dimethylformamide (DMF) as well as in a Poly
(methyl methecrylate) (PMMA) film. For the formation of the
film, appropriate amounts of TPA(DTA)3 and PMMA were initially
dissolved in THF and were magnetically stirred for 24 h. The films
were prepared by spin coating onto glass substrates, previously
rinsed with acetone. All samples were placed in a rotating holder in
order to avoid thermal degradation.
2. Experimental
2.1. Materials
The chemical structure of the tri-branched molecule studied is
shown in the inset of Fig. 1. It contains triphenylamine central core
(i.e. nitrogen atom at the center) and 2,3-di(2-thienyl)acrylonitrile
branches and is abbreviated as TPA(DTA)3. TPA(DTA)3 was prepared
by a three-step synthetic route; the detailed description of the
synthesis and chemical characterization has been published else-
where [25]. Briefly, tris(4-bromophenyl)amine reacted with
tributyl(thiophen-2-yl)stannane in the presence of PdCl2(PPh3)2 to
afford tris(4-(thiophen-2-yl)phenyl)amine. The latter was reacted
with N,N-dimethylformamide and POCl3 to yield 5,50500-(nitrilotri-4,1-
phenylene)tris-2-thiophenecarboxaldehyde. Finally, this compound
was reacted with 2-(thiophen-2-yl)acetonitrile in anhydrous ethanol
in the presence of NaOH to afford TPA(DTA)3. The chemical structure
of TPA(DTA)3 was confirmed by FT-IR and 1H NMR spectroscopy as
well as elemental analysis.
3. Results and discussion
FT-IR (KBr, cmꢀ1): 3026, 2210, 1658, 1594, 1576, 1528, 1494,
1442, 1322, 1293, 1272, 1253, 1184, 1054, 830, 800, 700. 1H NMR
(CDCl3, ppm): 7.46 (s, 3H, olefinic); 7.34e7.33 (m, 3H, thiophene
protons at position 2); 7.31e7.29 (m, 6H, phenylene protons meta
to nitrogen); 7.19e7.12 (m, 12H, thiophene protons at positions
3,4); 7.09e7.06 (m, 6H, phenylene protons ortho to nitrogen). Anal.
Calcd. for C51H30N4S6: C, 68.73; H, 3.39; N, 6.29. Found: C, 67.56; H,
3.30; N, 6.22.
3.1. Steady-state spectroscopy
The steady-state absorption and fluorescence spectra of TPA
(DTA)3 in solutions with different solvents are shown in Fig. 1.
Specifically, TPA(DTA)3 in THF exhibits an absorption peak at
460 nm corresponding to the
pe
p* excitation. The absorption
spectrum of TPA(DTA)3 was not sensitive to the change of the
solvent polarity. This is explained since the dipole moment in
the ground state is lower than in the excited one. For this reason the
absorption spectrum is shown in Fig. 1 only in THF. The fluores-
cence spectrum of TPA(DTA)3 in THF exhibits a structureless feature
with a peak at 528 nm. When the solvent is changed from THF,
which is a low-polarity solvent, to ACP and DMF, which are solvents
with higher polarities, the fluorescence spectrum shifts to longer
wavelengths. Specifically, TPA(DTA)3 in ACP and DMF exhibits
fluorescence peaks at 536 nm and 545 nm respectively. This red-
shift is expected, since, after excitation, the solvent molecules
reorient or relax around the dipole of the excited state lowering its
energy. This effect becomes more significant as the solvent polarity
increases [22].
1.2
S
CN
S
S
CN
1.0
0.8
0.6
0.4
0.2
0.0
S
N
S
NC
S
Absorption in THF
Fluorescence in THF
Fluorescence in ACP
Fluorescence in DMF
3.2. Fluorescence dynamics
The isotropic fluorescence dynamics of TPA(DTA)3 in THF solu-
tion (10ꢀ4M concentration) are shown in Fig. 2 together with the
fitting curves for four different emission wavelengths. The excita-
tion wavelength was 400 nm. It is clear that as the detection is
shifted from short towards long wavelengths, the decays become
300
350
400
450
500
550
600
650
Wavelength (nm)
Fig. 1. Absorption spectrum of TPA(DTA)3 in THF and fluorescence spectra in THF, ACP
and DMF. The inset shows the chemical structure of TPA(DTA)3.

46
M. Fakis et al. / Dyes and Pigments 87 (2010) 44e48
constant of the slow component was determined and it was found
w980 ps. The slow decay component showed no detectable
dependence on the emission wavelength.
1.0
0.8
0.6
0.4
0.2
0.0
500nm
The fluorescence transients shown in Figs. 2 and 3 have been
also observed in other similar tri-branched molecules in the past
[13,21,28e31]. They have been attributed to different kinds of
physical mechanisms such as inter-molecular energy transfer
towards aggregates, intramolecular energy migration, single-
tesinglet annihilation, solvation or photoinduced conformational
changes.
In order to examine whether the ultrafast dynamics is attributed
to aggregates or other concentration dependent species, the fluo-
rescene dynamics was detected in THF solutions with different
concentrations ranging from 0.5.10ꢀ4M up to 10ꢀ3M. The dynamics
are shown in Fig. 4(a) at excitation and detection wavelengths of
400 nm and 530 nm respectively. In all concentrations the decays
are similar meaning that aggregates are not formed and that no
other concentration dependent component affects the dynamics
(at least within the concentration range used).
0
20
40
60
80
100
1.0
0.8
0.6
0.4
0.2
0.0
540nm
0
20
40
60
80
100
1.0
0.8
0.6
0.4
0.2
0.0
Furthermore, in order to identify whether intensity dependent
phenomena occur, the fluorescence dynamics has been studied for
two different excitation powers and the results are shown in Fig. 4(b).
The dynamics are identical meaning that intensity dependent
effects like singletesinglet annihilation can be excluded within our
excitation power regime.
580nm
The fluorescence dynamics of TPA(DTA)3 has also been studied
in solid film. For the formation of the film, TPA(DTA)3 was dispersed
into PMMA matrix (0.1 wt%). The fluorescence decay of the TPA
(DTA)3ePMMA film at 530 nm is shown in Fig. 4(c) together with
the corresponding results of TPA(DTA)3 in THF (10ꢀ4M). It is clear
that in solid film, TPA(DTA)3 exhibits a relatively slow decay with
no ultrafast decay component.
From all the above results it is concluded that ultrafast fluores-
cence dynamics of TPA(DTA)3 occurs only in solution and that it is
not due to aggregates or singletesinglet annihilation. Therefore it
can be plausibly attributed to solvation. An influence from photo-
induced conformational changes on the dynamics could not be
excluded. In many cases, solvation and photoinduced conforma-
tional changes (such as twisting of a functional group around a bond
or cisetrans isomerization) co-exist and constitute competing
relaxation mechanisms occurring within the same temporal regime
[32,33]. However, the co-existence of these mechanisms would
probably require the introduction of an additional exponential
decay term to satisfactorily fit the fluorescence transients.
0
20
40
60
600nm
4
80
100
1.0
0.8
0.6
0.4
0.2
0.0
0
2
6
8
Time (ps)
Fig. 2. Fluorescence dynamics of TPA(DTA)3 in THF, in normalized units (n.u.), at
various detection wavelengths. The excitation wavelength was 400 nm while the
concentration was 10ꢀ4M.
slower. At short wavelengths, the decays are governed by an
ultrafast component followed by a slower one. The time constant of
the ultrafast component increases with the emission wavelength
and it ranges from 1.4 ps to 5.1 ps. Table 1 summarizes the ultrafast
decay parameters (time constant and pre-exponential factor) at
various emission wavelengths. At the longest wavelength detected
i.e. at 600 nm, the dynamics exhibits a slow rise meaning that there
is an accumulation process in the population of the low energy
states. In Fig. 2, the dynamics at 600 nm is shown within the first
6 ps so that the slow rise is clear. In Table 1 this rise is identified by
a negative pre-exponential factor. The slow decay component could
not be accurately fitted within the time scale shown in Fig. 2. Thus,
the fluorescence decays have been measured on a longer time scale
and are shown in Fig. 3 at 530 nm. Fitting these dynamics, the time
1.0
0.8
1: THF
2: ACP
3: DMF
0.6
0.4
0.2
0.0
Table 1
1
2
3
Fitting parameters of the ultrafast fluorescence decay component in different
solvents and detection wavelengths.
Solvent
l
¼ 500 nm
l
¼ 540 nm
l
¼ 580 nm
l
¼ 600 nm
0
100
200
300
400
500
600
700
s1 (ps)
A1
s1 (ps)
A1
s1 (ps)
A1
s1 (ps)
A1
Time (ps)
THF
ACP
DMF
1.4
2.2
0.3
0.80
0.86
0.95
2.5
4.3
1
0.58
0.63
0.75
5.1
6.7
2.9
0.23
0.33
0.67
0.6
1
0.4
ꢀ0.34
ꢀ0.2
Fig. 3. Long scale fluorescence dynamics of TPA(DTA)3 in THF, ACP and DMF at 530 nm
ꢀ0.45
(excitation wavelength: 400 nm, concentration: 10ꢀ4M).

M. Fakis et al. / Dyes and Pigments 87 (2010) 44e48
47
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.0
1: 10-3M
2: 10-4M
a
1:THF
2:ACP
3:DMF
0.8
500nm
3: 0,5.10-4M
0.6
0.4
0.2
0.0
1
1
2
3
2
3
0
20
40
60
80
100
1.0
0.8
0.6
0.4
0.2
0.0
0
20
40
60
80
100
540nm
1.2
1.0
0.8
0.6
0.4
0.2
0.0
b
1
1: 5mW
2: 25mW
2
3
0
20
40
60
80
100
2
1.0
0.8
0.6
0.4
0.2
0.0
1
580nm
1
0
20
40
60
80
100
2
3
1.0
0.8
0.6
0.4
0.2
0.0
c
0
20
40
60
80
100
Film
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1
2
3
THF solution
80
600nm
0
20
40
60
100
0
2
4
6
8
Time (ps)
Time (ps)
Fig. 4. The dependence of the fluorescence dynamics of TPA(DTA)3 on (a) concentra-
tion and (b) excitation power. In both figures the excitation wavelength was 400 nm
and the detection one was 530 nm. The curves in (a) and (b) have been slightly shifted
for clarity. Figure (c) shows the fluorescence dynamics of TPA(DTA)3 at 530 nm in
a PMMA film and in a THF solution. The excitation wavelength was 400 nm.
Fig. 5. Early fluorescence dynamics of TPA(DTA)3 in THF, ACP and DMF at four
wavelengths.
THF and DMF. At long wavelengths, the fluorescence in ACP, initially
decays similarly to THF. The difference in polarity alone cannot
explain the fluorescence dynamics observed in ACP. Since ACP has
an intermediate polarity we would expect the fluorescence decay in
this solvent to be faster than THF and slower than DMF. However, as
it is shown in Fig. 5, this is not observed. In order to explain the
results, the solvent viscosity has to be considered. ACP is the most
viscous solvent among the three solvents used in this work.
Solvation is considered as the reorientation (relaxing) of the solvent
molecules around the excited solute molecules lowering the energy
of the excited state. Since this reorientation is retarded in solvents
of high viscosity, the solvation dynamics is also retarded and
consequently the fluorescence decay due to solvation becomes
slower. However, the dynamics in ACP also shows another impor-
tant feature. Although this dynamics is slower in ACP than in THF,
the drop of the fluorescence intensity, because of the ultrafast
component, is bigger in ACP than in THF (the pre-exponential factor
for the ultrafast component is higher in ACP than in THF). Conse-
quently, the overall drop in fluorescence intensity because of the
solvation process (i.e. the solvation efficiency), follows the increase
of the solvent polarity meaning that it is enhanced as the polarity
increases. On the other hand, the solvation time is greatly affected
by the viscosity meaning that solvation becomes slower in the most
viscous solvent i.e. ACP.
The ultrafast process has been studied in two additional solvents
namely ACP and DMF and the role of solvent polarity and viscosity
was examined. In Fig. 5, the fluorescence dynamics of TPA(DTA)3 is
shown in these solvents at four wavelengths. The dynamics of TPA
(DTA)3 in THF is also shown for a comparison. As shown in Fig. 5
and in Table 1 the fluorescence in DMF (the most polar solvent)
initially decays much faster than in the other two solvents. Addi-
tionally, the ultrafast mechanism is more efficient in DMF as it is
evident by its very high pre-exponential factor. The decay dynamics
in DMF can be explained taking into account its higher polarity than
the other solvents. It is well known that in highly polar solvents, an
ICT state is formed becoming the energetically lowest lying state.
The ultrafast decay in DMF is therefore due to the relaxation from
the initially excited state to the ICT. The fluorescence of the relaxed
ICT state is detected at long wavelengths. Although, an ICT state
could also be the fluorescent state in the other solvents (i.e. THF and
ACP), the ultrafast relaxation is much more efficient in DMF
meaning that the activation barrier for ICT formation is decreased
in the polar solvent.
On the other hand, the decay features in ACP, which is a solvent
of intermediate polarity are more complicated. At short wave-
lengths, the fluorescence in ACP initially decays more slowly than in

48
M. Fakis et al. / Dyes and Pigments 87 (2010) 44e48
0.5
0.4
0.3
0.2
0.1
0.0
resolved fluorescence dynamics. The tri-branched molecule, in
solutions, exhibits ultrafast solvation decay. Solvation rate was
affected by both polarity and viscosity, being greatest in DMF,
a solvent of high polarity and lowest in ACP, a solvent of high
viscosity. In contrast, solvation efficiency was affected only by
solvent polarity and was enhanced as polarity increased. Anisotropy
dynamics revealed ultrafast decay indicating excitation delocaliza-
tion among the branches, which is desirable in the context of the
enhancement of non-linear optical properties.
2500
2000
1500
1000
500
Iparallel
Iperpendicular
anisotropy r(t)
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A tri-branched molecule with triphenylamine central core and
2,3-di(2-thienyl)acrylonitrile branches has been studied in solution
and solid film concerning the steady-state spectra and time