Ultrafast excited-state dynamics of ferrocene-bridge-acceptor system

Article abstract of DOI:10.1016/j.chemphys.2010.04.014

We employed both femtosecond fluorescence up-conversion and transient absorption techniques with ～150 fs time resolution to study the excited-state deactivation process of an intra-molecular charge transfer model compound, 4-(ferrocen-1-yl)benzylidene-malononitrile (Fc-ph-DCV), which consists of ferrocene (Fc) unit as an electron donor, dicyanovinly (DCV) as an electron acceptor and phenyl (ph) ring as the central bridge. The results showed that after photoexcitation into the higher excited S₂ state, ultrafast internal conversion into S₁ takes place. The rate of S₂ → S₁ internal conversion is markedly faster (with a typical time of 120 fs ± 20 fs) than the diffusive solvation process. On the other hand, the lifetime of the relaxed S₁ state was strongly dependent on the solvent polarity, changing from 40 to 50 ps in acetonitrile to ～20 ps in cyclohexane. Time-resolved fluorescence data also showed subpicosecond transient component that is attributable to the spectral relaxation caused by solvation and/or vibrational relaxation in the S₁ state.

Full text of DOI:10.1016/j.chemphys.2010.04.014

Chemical Physics 372 (2010) 17–21
Contents lists available at ScienceDirect
Chemical Physics
journal homepage: www.elsevier.com/locate/chemphys
Ultrafast excited-state dynamics of ferrocene-bridge-acceptor system
*
Omar F. Mohammed , Abdelwareth A.O. Sarhan
Chemistry Department, Faculty of Science, Assiut University, Assiut 71516, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
We employed both femtosecond fluorescence up-conversion and transient absorption techniques with
ꢀ150 fs time resolution to study the excited-state deactivation process of an intra-molecular charge
transfer model compound, 4-(ferrocen-1-yl)benzylidene-malononitrile (Fc-ph-DCV), which consists of
ferrocene (Fc) unit as an electron donor, dicyanovinly (DCV) as an electron acceptor and phenyl (ph) ring
as the central bridge. The results showed that after photoexcitation into the higher excited S2 state, ultra-
fast internal conversion into S1 takes place. The rate of S2 ? S1 internal conversion is markedly faster
(with a typical time of 120 fs 20 fs) than the diffusive solvation process. On the other hand, the lifetime
of the relaxed S1 state was strongly dependent on the solvent polarity, changing from 40 to 50 ps in ace-
tonitrile to ꢀ20 ps in cyclohexane. Time-resolved fluorescence data also showed subpicosecond transient
component that is attributable to the spectral relaxation caused by solvation and/or vibrational relaxa-
tion in the S1 state.
Received 12 March 2010
In final form 21 April 2010
Available online 10 May 2010
Keywords:
Charge transfer
Time-resolved fluorescence
Ultrafast internal conversion
Published by Elsevier B.V.
1. Introduction
charge transfer process containing metallocene unit and clarify
its photophysics.
The electron transfer (ET) is one of the most fundamental chem-
ical processes and has been studied intensively in the fields of
physical chemistry and biology [1–4]. Since the discovery of ferro-
cene (Fc), it has played an important role in the ET processes in
organometallic systems. For instance, electron-donating Fc deriva-
tives were frequently used as terminal building blocks in donor–
acceptor array systems and achieved extremely long charge sepa-
ration time [5–7]. It has also been shown that the organometallic
compounds containing Fc unit possess an appreciable hyperpolar-
izability which is desirable for nonlinear optics [8]. The observed
nonlinearity was attributed to intra-molecular charge transfer
In the present work, we report a femtosecond spectroscopic
study of the photophysical behavior of a newly synthesized do-
nor–acceptor system, 4-(ferrocen-1-yl)benzylidene-malononitrile
(Fc-ph-DCV), which has Fc as an electron donor, dicyanovinyl as
an electron acceptor, and a phenyl group as a bridge connecting
them. The ultrafast relaxation dynamics of Fc-ph-DCV was exam-
ined with femtosecond fluorescence up-conversion as well as tran-
sient absorption spectroscopy upon S0 ? S2 photoexcitation with
ꢀ150 fs time resolution.
2. Experimental section
from the Fc fragment to the acceptor moiety. The D–
the molecules, in which electron donor and acceptor are connected
via a -bridge, undergo ultrafast intra-molecular charge transfer
upon electronic excitation. In the -conjugated ferrocene-acceptor
p–A types of
Time-resolved fluorescence measurements were performed
using femtosecond fluorescence up-conversion. The experimental
setup has been described in detail elsewhere [13]. Excitation pulse
at 400 nm was generated with frequency-doubling of the funda-
mental output of a mode-locked Ti:sapphire laser (Coherent, MIRA)
using a BBO crystal (0.2 mm thickness). The blue pump pulse was
focused into a thin-film-like jet stream of the sample solution for
photoexcitation. The residual fundamental pulse after the second
harmonic generation was used as a gate pulse for the up-conver-
sion process. The sample fluorescence was mixed with a gate
pulses in a nonlinear crystal (BBO) and the sum-frequency signal
was detected by a photon-counting method. The magic-angle
condition was achieved by rotating the polarization of the pump
pulse with respect to the gate polarization. The time resolution
was evaluated as 140 fs by the up-conversion measurement for
the Raman signal from the solvent.
p
p
dyads, the highest occupied molecular orbital (HOMO) is localized
on the Fc unit and the lowest unoccupied molecular orbital (LUMO)
is largely localized on the acceptor with small delocalization onto
the p-bridge [9]. Thus, the ET process occurs in such systems with
photoexcitation. The electron transfer properties of such donor–
acceptor families have contributed significantly in the areas of
the light-induced ET chemistry and solar energy conversion [10–
12]. Therefore, it is important to investigate the intra-molecular
* Corresponding author. Present address: Arthur Amos Noyes Laboratory of
Chemical Physics, California Institute of Technology, Pasadena, CA 91125, USA.
E-mail address: omar3070@caltech.edu (O.F. Mohammed).
0301-0104/$ - see front matter Published by Elsevier B.V.
doi:10.1016/j.chemphys.2010.04.014

18
O.F. Mohammed, A.A.O. Sarhan / Chemical Physics 372 (2010) 17–21
OEt
CH2OH
NaBH4
EtOH
HCl/NaNO2
Fe
Fe
Fe
O
p-Aminoethyl
benzoate
2
3
1
MnO2
CHCl3
NC
CN
CH2(CN)2
CHO
EtOH/pip
Fe
Fe
H
reflux
4
5
Scheme 1.
Femtosecond time-resolved transient absorption measure-
intense absorption band around 360 nm (
and the less intense band peaked near 540 nm
e )
= 27000 Mꢁ1 cmꢁ1
ments were carried out with photoexcitation at 390 nm and prob-
ing with the white-light continuum. The 390-nm pump pulse was
produced by frequency-doubling of the fundamental output of the
Ti:sapphire regenerative amplifier (Legend, Coherent; 1 mJ, 100 fs,
1 kHz, 780 nm). The white-light continuum was generated by
focusing the 780-nm pulse into sapphire plate and it was split be-
fore the sample into the probe and reference beams. Both the probe
and reference spectra were analyzed by a spectrograph and de-
tected by a CCD that was synchronized with the laser system.
The pump polarization was rotated by a half-wave plate with re-
spect to the probe polarization to achieve the magic-angle condi-
tion. The Kerr gating signals of the solvents (cyclohexane,
dichloromethane and acetonitrile) have been measured with ex-
actly the same experimental conditions. The obtained Kerr data
were used to correct the effect of the chirp of the white-light con-
tinuum on the time-resolved spectra.
(e =
6000 Mꢁ1 cmꢁ1). These two absorption bands of Fc-ph-DCV are
substantially red-shifted compared to Fc moiety and, more impor-
tantly, the extinction coefficients of the absorption bands are dras-
tically increased. Such changes are understood in terms of the
significant changes of the molecular orbital relevant to the elec-
tronic transitions [8].
In general, the metallocene-p-bridge-acceptor systems show
two absorption bands in UV–Visible absorption. So far, two differ-
ent assignments have been given for the two transitions. In the first
assignment [9], the low energy transition is assigned to the d–d
transition and the high energy transition to metal–acceptor (d–
*
p
) transition. In the second one [9], the low energy transition is
The synthesis of 4-(ferrocen-1-yl)benzylidene-malononitrile
(Fc-ph-DCV) in which an Fc unit is separated from DCV by phenyl
group is shown in Scheme 1. The p-ferrocenylbenzaldehyde (4)
was prepared according to the literature procedure [14] from Fc
(1) followed by condensation with malononitrile in ethanol con-
taining few drops of piperidene as basic catalyst to give the corre-
sponding 4-(ferrocen-1-yl)benzylidene-malononitrile (5). The
system represents an intra-molecular charge transfer model which
is connected by donor and acceptor via molecular
p-bridge which
can enhance the delocalized movements of the -electrons in the
p
system. Such conjugated bridge contributes significantly to the
ET process.
1HNMR (500 MHz, CDCl3) d = 7.85–7.83 (d, J = 14 Hz, 2H, aro-
matic-H), 7.68 (s, 1H, arylidene-H), 7.58–7.57 (d, J = 14 Hz, 2H, aro-
matic-H), 4.77 (t, J = 3 Hz, 2H, ferrocene-H), 4.51 (t, J = 3 Hz, 2H,
ferrocene-H), 4.07 (s, 5H, ferrocene-H). 13CNMR (125 MHz, CDCl3)
d = 159.06 (CH), 148.78, 128.26, (aromatic-C), 131.16, 126.39 (aro-
matic-CH), 114, 113.31 (2 CN), 81.89 (ferrocene-C), 70.87, 70.12,
67.20 (ferrocene-CH). MS m/z [M+, 338 (100)], 339 (25), 336 (7),
290 (1), 273 (6), 221 (2), 217 (4), 190 (3), 169 (2), 139 (2), 121
(23), 95 (2), 81 (1), 56 (14).
3. Results and discussion
Fig. 1 shows UV–Visible absorption spectra of Fc and Fc-ph-
DCV. The Fc unit exhibits two weak absorption bands around
326 nm
(e
= 40 Mꢁ1 cmꢁ1
)
and 440 nm
(e )
= 90 Mꢁ1 cmꢁ1
(Fig. 1C), which are assigned to the metal-to-ligand charge transfer
and/or d–d transitions. The spectrum of Fc changes dramatically
upon substitution of hydrogen atom of cyclopentadienyl ring (Cp)
with a conjugated electron acceptor unit having dicyanovinyl moi-
ety. As shown in Fig. 1A, Fc-ph-DCV shows two bands, more
Fig. 1. Steady state absorption of Fc-ph-DCV in CHCl3, DCM and ACN (A), p-
ferrocenylbenzaldehyde in ACN (B), and Fc in ACN (C).

O.F. Mohammed, A.A.O. Sarhan / Chemical Physics 372 (2010) 17–21
19
*
assigned to metal–acceptor (d–
*
is Cp (ligand)-acceptor (p–p ) transition. As shown in Fig. 1B, the
p
) and the high energy transition
Femtosecond time-resolved fluorescence of Fc-ph-DCV was
measured at a series of detection wavelengths ranging from 480
to 590 nm after 400 nm excitation (the S2 S0 excitation (see
Fig. 2). The time-resolved fluorescence was recorded under the ma-
gic-angle conditions to avoid the effect of rotational motion. Fig. 2
displays the time-resolved fluorescence traces measured in ACN in
the time region up to 14 ps (A) and up to 250 ps (B). The represen-
tative time-resolved fluorescence traces in DCM at 480 and 590 nm
are also depicted in Fig. 2C. It can be clearly seen that the spike-like
fluorescence signal appears with photoexcitation in the whole
wavelength region. This extremely fast emission is attributable to
the S2 fluorescence. The long-lived fluorescence component was
seen in the wavelength region longer than 530 nm, which is
assignable to the fluorescence from the S1 state that is generated
by the internal conversion. The time-resolved fluorescence signals
were well fitted with three exponential functions with characteris-
low energy transition is strongly affected by replacing dicyanovinyl
with a weaker electron acceptor, CHO. This fact disagrees with the
first model because the d–d transition should not be very sensitive
to the acceptor strength. Therefore, we consider that the low en-
ergy transition is assignable to the metal–acceptor (d–
*
p ) CT band.
Both low and high energy bands of Fc-ph-DCV exhibit red-shifts
in dichrolomethane (DCM, n = 1.424) and chloroform (CHCl3,
n = 1.445) compared to that in acetonitrile (ACN, n = 1.344), n is
the solvent refractive index. This suggests the dipole–dipole inter-
action is not the predominant factor to determine the transition
energy because the absorption bands should be red-shifted in polar
ACN in such a case. The transition energy decreases with increasing
polarizability of the solvent molecules. This suggests that the Fc-
ph-DCV has only small dipole moment in the ground state and
the dipole–dipole interaction does not influence the transition en-
ergy. The observed solvatochromism is probably dominated by dis-
persion interactions and by dipole-induced-dipole interactions
[15,16] for which the polarizability of the solvent determines the
stabilization energy. A similar type of the solvatochromic shifts
has already been suggested for other CT systems [17].
tic times of 110 30 fs (
s1), 590 100 fs (
s
2), 49 3 ps (s3) in ACN
and 120 40 fs ( 1), 900 150 fs (
s
s2), 23 3 ps (s
3) in DCM. The s1
component was S2 fluorescence and the relevant time constant is
the S2 lifetime determined by the S2 ? S1 internal conversion.
The evaluated lifetime of 110–120 fs is a typical lifetime of the
highly excited state of large polyatomic molecules of this size
[18]. The slower component (s3) is assigned to the lifetime of the
relaxed S1 state. Importantly, the lifetime of the S1 state is signifi-
cantly longer in a more polar solvent than that in less polar ones,
indicating that the S1 state having CT character which is stabilized
in the polar medium. It is known that, in the conventional intra-
molecular CT dynamics, the CT state is more stabilized in solvents
having higher polarity because of higher barrier of the solvent reor-
ganization energy for back electron transfer reaction [19]. We note
that the torsional motion of the vinyl moiety may also contribute
to relatively short lifetime of S1/CT state. The effect of the twisting
motion on the non-radiative decay of the CT state has been dis-
cussed and it was suggested that it reduces the lifetime of the CT
state [19,20].
In regard to the s2 component, it is very likely that the solvation
process plays a very crucial role in the excited-state dynamics be-
cause the S1 state has CT character due to a substantial charge
transfer from the electron donor Fc to the acceptor (DCV). The sol-
vation process occurs on the subpicosecond to picosecond time
scales, depending on the solvent. In fact, the time scale of the dif-
fusive solvation process is reported to be 600 fs in ACN and 1 ps in
DCM [21], which are in good agreement with the time scale of the
s2 component (see Table 1). In this process, the solvent molecules
start relaxing and reorienting themselves to adapt the excited-
state molecules to achieve the equilibrium condition from the ini-
tial non-equilibrated one. Alternative possible assignment of the s2
component is intra-molecular vibrational relaxation process in the
S1 state following the ultrafast S2 ? S1 internal conversion. How-
ever, decisive assignment of the s2 component is difficult with
the available data, at the moment.
We note that the vibrational cooling (intermolecular vibrational
relaxation by energy transfer to the solvent molecules) does not
contribute to this subpicosecond component because the vibra-
tional cooling typically occurs on much longer time scales, in the
temporal range 5–30 ps [22–24].
We also carried out femtosecond time-resolved transient
absorption measurements with excitation at 390 nm and
Table 1
Fitted decay times of up-conversion profiles of Fc-ph-DCV in ACN and DCM at 590 nm
(limit of error 20%).
Fig. 2. Femtosecond time-resolved fluorescence signals of Fc-ph-DCV (cꢀ10ꢁ3 M)
in ACN measured at various wavelengths after 400 nm excitation in the short time
range (A) and in the longer time range (B), inset is the zoom of the ultrafast
component (lifetime of S2 which is about 100 fs). Time-resolved fluorescence
signals in DCM at the longer time range are also shown for comparison (C).
Solvent
s1 (fs)
s2 (fs)
s3 (ps)
ACN
DCM
110 30
120 40
590 100
900 150
49
23
3
3

20
O.F. Mohammed, A.A.O. Sarhan / Chemical Physics 372 (2010) 17–21
Fig. 3. Femtosecond time-resolved absorption spectra of Fc-ph-DCV in ACN
measured with 390 nm excitation. Early delay time range (A) and later delay time
range (B).
Fig. 4. Temporal profiles of the time-resolved absorption signal of Fc-ph-DCV
measured by 390-nm photoexcitation. Temporal profiles at 685 and 465 nm in ACN
(A) and in DCM (B). Temporal profiles at 460 nm in CHX (red), DCM (green), ACN.
(For interpretation of the references to colour in this figure legend, the reader is
referred to the web version of this article.)
confirmed the ultrafast excited-state relaxation of Fc-ph-DCV
observed by fluorescence up-conversion. Fig. 3 displays time-re-
solved absorption spectra observed in ACN. Time-resolved absorp-
tion traces at 465 and 685 nm in ACN are shown in Fig. 4A.
Immediately after excitation at 390 nm (S2 S0 excitation), a
broad positive transient signal assignable to the S2 state appears
within our time resolution (140 fs). The S0 bleaching should also
contribute to the early-time transient spectra in the region of
500–600 nm, but the S2 transient absorption dominates the spectra
and only positive transient signals were observed in total. Then,
the fast decay of the band around 650 nm was observed, being
accompanied with the increase of the transient absorption around
465 nm. As shown in Fig. 4A, the decay time at 650 nm and the rise
time at 460 nm match each other, and they agreed with ultrafast
S2 ? S1 internal conversion time (120 fs) that was determined by
fluorescence up-conversion experiments. In the spectra at later de-
lay time, the S0 bleaching was clearly seen as a negative band
peaked at 555 nm because of the weakness of the S1 transient
absorption in this wavelength region. The S1 absorption exhibits
two bands: one is a band peaked at 465 nm and the other is a broad
band in the wavelength region longer 600 nm. The S1 absorption,
as well as S0 bleaching, decays in a few hundred picoseconds.
The time-resolved absorption traces at 460 nm (the window
range of averaged signal is 425–770 nm), which represent the de-
cay of the S1 state, are shown in Fig. 4C for three different solvents.
The fitting analysis showed that the S1 lifetime of FC-ph-DCV in
ACN, DCM and CHX are 60, 27 and 19 ps, respectively. These
lifetimes quantitatively match the fluorescence lifetimes of the S1
state determined by the up-conversion experiments.
Scheme 2 summarizes the ultrafast relaxation process of the Fc-
ph-DCV after photoexcitation, which was clarified in this study.
The photoexcitation at 390 nm generates the S2 state. The
S2 ? S1 internal conversion occurs with
a time constant of
S2
100-120 fs
IC
S1
Solv.
0.5-1 ps
Relaxed S1
20-50 ps
390 or
400 nm
S0
Scheme 2. Ultrafast relaxation of Fc-ph-DCV after S0–S2 photoexcitation.

O.F. Mohammed, A.A.O. Sarhan / Chemical Physics 372 (2010) 17–21
21
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We thank Professor Tahei Tahara and Dr. Satoshi Takeuchi for
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