Dissociative Intramolecular Charge Transfer after Local Two-Photon Ionization of Ferrocene Derivatives with Aromatic Chromophores

Article abstract of DOI:10.1021/jp992963u

Experimental evidence for dissociative intramolecular charge transfer processes is found for gas-phase ferrocene-aniline derivatives after local ionization at the aniline chromophore site via intermediate states with predominant aniline character. The direct ionization at the ferrocene site can be excluded since there is a rapid dissociation of ferrocene in the intermediate state prior to ionization so that no ferrocene ions would be produced in this case. Because of the higher local ionization energy of the chromophore, charge transfer takes place after local ionization. The released energy and additional photon absorption leads to breaking of bonds and results in a dissociation into intact ferrocene or ferrocene-containing cations and neutral fragments.

Full text of DOI:10.1021/jp992963u

J. Phys. Chem. A 2000, 104, 2013-2019
2013
Dissociative Intramolecular Charge Transfer after Local Two-Photon Ionization of
Ferrocene Derivatives with Aromatic Chromophores
J. E. Braun and H. J. Neusser*
Institut fu¨r Physikalische und Theoretische Chemie, Technische UniVersita¨t Mu¨nchen, Lichtenbergstrasse 4,
D-85748 Garching, Germany
P. Ha1rter and M. Sto1ckl
Institut fu¨r Anorganische Chemie, Technische UniVersita¨t Mu¨nchen, Lichtenbergstrasse 4,
D-85748 Garching, Germany
ReceiVed: August 20, 1999; In Final Form: December 15, 1999
Experimental evidence for dissociative intramolecular charge transfer processes is found for gas-phase ferrocene-
aniline derivatives after local ionization at the aniline chromophore site via intermediate states with predominant
aniline character. The direct ionization at the ferrocene site can be excluded since there is a rapid dissociation
of ferrocene in the intermediate state prior to ionization so that no ferrocene ions would be produced in this
case. Because of the higher local ionization energy of the chromophore, charge transfer takes place after
local ionization. The released energy and additional photon absorption leads to breaking of bonds and results
in a dissociation into intact ferrocene or ferrocene-containing cations and neutral fragments.
1. Introduction
Organometallic compounds, especially ferrocene and its
derivatives, have attracted great interest as they represent a link
between inorganic and organic chemistry and offer numerous
applications.1 Ferrocene, [bis(cyclopentadienyl)]iron(II); Fe-
(C5H5)2 or Fe(cp)2 (see Figure 1, 1a), was discovered in 1951,2
it is an organometallic sandwich compound, whose photophysi-
cal properties have been investigated by various groups (see
ref 1 and references therein). Due to its remarkable chemical
stability and the well-established methods for its incorporation
into more complex structures, ferrocene has become a versatile
building block for the synthesis of novel substances with unusual
physical properties. Ferrocene-containing substances have been
used for the investigation of charge-transfer processes in the
solid and liquid phase.3 In this work, photoinduced charge-
transfer experiments in a supersonic molecular beam, carried
out with special tailor-made ferrocene derivatives, are described.
In a series of multiphoton dissociation (MPD)/multiphoton
ionization (MPI) studies of ferrocene4-12 with nanosecond (ns)
laser pulses at different wavelengths in the visible and near UV
(UV/VIS) it has been shown that the cyclopentadienyl (cp)
ligands are lost prior to ionization, and as a consequence, no
intact ferrocene cations can be observed in the mass spectra. In
the intensive laser radiation field the central iron atom loses its
cp rings in one or more MP steps, and the uncaged iron atom
can be ionized and detected by a MPI process when the laser
wavelength is close to an atomic resonance. The detailed
mechanism of photon absorption, energy redistribution, and
dissociation has not been unequivocally established. It was
concluded that the dynamics of the dissociation process depends
on the chosen photon energy as well as the laser fluence. Kimura
and co-workers8 found that the conversion of electronic to
vibrational energy occurs on a faster time scale than a cp
elimination. On the other hand, in two earlier publications a
fast dissociation mechanism via electronic repulsive states was
Figure 1. Ferrocene derivatives investigated in this work: 1a)
Ferrocene. 1b) [3]Ferrocenophane. 2a) n-(Ferrocenylmethyl)aniline. 2b)
2-(n-Phenyl)amino-[3]-Ferrocenophane.
proposed by Liou et al.9,10 So, two competing dissociation
mechanisms, one via statistical redistribution, the other via direct
dissociation, are thought to be possible.
From thermochemical studies5,13,14 the dissociation energy E2,
for the decomposition of ferrocene into a neutral iron atom and
two cp radicals, is 6.2 eV. Elimination of only one cp ring
requires an energy of E1 ) 4 eV5 which is noticeably more
than necessary for removing the remaining second cp ring.
While resonance-enhanced two-photon ionization with typical
photon energies hV e 6.2 eV failed to produce parent ions, one-
photon vacuum ultraviolet (VUV) ionization of ferrocene leads
10.1021/jp992963u CCC: $19.00 © 2000 American Chemical Society
Published on Web 02/18/2000

2014 J. Phys. Chem. A, Vol. 104, No. 10, 2000
Braun et al.
to intact parent (ferrocene) ions. Determination of the IE of
ferrocene by one-photon VUV ionization or electron impact
yielded slightly different values of 6.75 eV,7 6.72 eV (adiabatic)
and 6.86 eV (vertical),15 and 6.99 eV.12 In our work, the value
of 6.75 eV for the adiabatic ionization energy (AIE) is adopted.
This value is 0.55 eV above the dissociation energy E2.
We would like to point out that soft ionization of ferrocene
with a two-photon process because of a fast dissociation in the
intermediate state is not possible but that the detailed mechanism
of dissociation is of minor importance in our experiment.
Therefore, in the ferrocene derivatives containing a chromophore
used in this work, we can exclude any direct local ionization at
the ferrocene site. The investigated ferrocene derivatives (see
Figure 1, parts 2a and 2b) basically consist of a ferrocene or
[3]ferrocenophane (1,1′-(1,3-propandiyl)ferrocene) (see Figure
1, parts 1a and 1b, respectively) unit attached to a chromophore
molecule with distinct spectroscopic properties. The clear
distinction of different excitation pathways at the ferrocene site
and the chromophore site is crucial for our investigations of
charge transfer processes after local ionization of a chromophore
moiety with the higher local IE.
H 5.40, Fe 17.54, N 4.58%. IR (KBr, cm-1): ν ) 1646 (s, ν
(CdN)); MS (CI, %): m/z ) 315 (100, M+); 212 (68, M+-
CNPh); UV/VIS (CH2 Cl2, nm): λmax (ꢀ) ) 442 (250); 354
1
(2600); 284 (27 000); 232 (38 000); H NMR (C6D6, 400.13
3
MHz, ppm): δ ) 7.19 (t, 2H, m-Ph, JHH ) 7.4 Hz); 6.94 (t,
3
3
1H, p-Ph, JHH ) 7.4 Hz); 6.83 (d, 2H, o-Ph, JHH ) 7.5 Hz);
4.03 (s, 4H, C5H4); 3.95 (“t”, 2H, C5H4); 3.81 (“t”, 2H, C5H4);
2.92 (s, 2H, CH2); 2.54 (s, 2H, CH2); 13C NMR (C6D6, 100.63
MHz, ppm): δ ) 171.4 (CdN); 151.6 (i-Ph); 129.3 (m-Ph);
123.3 (p-Ph); 119.9 (o-Ph); 76.6; 76.3 (i-C5H4); 70.2; 70.0; 70.0;
69.9 (C5H4); 37.1; 27.7 (CH2).
2-(N-Phenyl)amino-[3]-Ferrocenophane. To 38 mg (1 mmol)
LiAlH4 in 10 mL of dry diethyl ether a solution of 315 mg (1
mmol) [3]-ferrocenophan-2-phenylimine in 50 mL dry diethyl
ether is added dropwise. After addition of the substrate the
reaction is stirred for 2 h at ambient temperature. For the workup
the reaction mixture is cooled in an ice bath, and very carefully
ice water is added dropwise to the reaction mixture. After the
H2 evolution has ceased the organic phase is decanted and the
water phase is extracted twice with diethyl ether. The combined
ether extracts are washed with 20 mL of a saturated NaCl
solution, separated, and dried. After evaporation of the solvent
the residue is recrystallized from n-hexane yielding 301 mg
(95%) of orange crystals.
2. Experimental Section
2.1. Ferrocene Derivatives. The ferrocene derivatives have
to fulfill several conditions to be suitable for our purposes to
detect charge transfer processes. The chromophore molecule has
to show strong absorption in a spectral range where the
absorpion coefficient of ferrocene is relatively weak. Moreover
the chosen ferrocene derivative should have sufficient gas
pressure at moderate temperatures and decompose only negli-
gibly when heated and vaporized. Among various synthesized
derivatives we found two suitable compounds with an aniline-
like chromophore. In Figure 1 the structures of different
ferrocenes are shown. In Figure 1, part 1a, the ferrocene
molecule is displayed, and in Figure 1, part 1b, the ferro-
cenophane (bridged ferrocene) is shown. The chemical structures
of the derivatives with chromophore aniline are shown in Figure
1, parts 2a, and 2b respectively: (2a) n-(ferrocenylmethyl)aniline
(2b) 2-(n-phenyl)amino-[3]-ferrocenophane. The main difference
between the two molecules 2a and 2b in Figure 1 is that in 2a
the aniline chromophore is attached to one cp ring whereas in
the case of 2b the aniline is bound to the handle that connects
the two cp rings. The idea behind this selection of structures
was that the charge produced after local ionization of the aniline
site moves along different paths between the moieties of the
respective molecules and, thus, any effects caused by the
different migration paths can be examined.
Mp. 165-167 °C. Mw: 317.21 g/mol (C19H19FeN) EA (%):
calcd.: C 71.94, H 6.03, Fe 17.60, N 4.42, found: C 71.98, H
6.10, Fe 17.48, N 4.63; IR (KBr, cm-1): ν ) 3379 (s, ν(NH),
st); 3077 (m, ν(CH), st, Cp); 1599 (s, ν(CdC), st); 1503 (s,
ν(CdC), st); MS (CI, %): m/z ) 317 (100, M+); 238 (41, M+-
MeCp); 214 (5, M+-CNPh); 158 (2, M+-CNPh-Fe); 135 (12,
M+-CNPh-MeCp); UV/VIS (CH2Cl2, nm): λmax (ꢀ) ) 442
(240); 298 (4600); 284 (7200); 250 (29 000); 232 (38 000); 1H
NMR (CD2Cl2, 400.13 MHz, ppm): ꢀ ) 7.19 (t, 2H, m-Ph,
3JHH ) 7.2 Hz); 6.68 (m, 3H, o- + p-Ph); 4.11 (s, 4H, C5H4);
4.07 (“t”, 2H, C5H4); 4.03 (“t”, 2H, C5H4); 3.91 (br, 1H, NH);
3
3.82 (tt, 1H, CH, JHH ) 3.0/11.1 Hz); 2.71 (dd, 2H, CHHcis,
3
2
2JHH ) 13.8 Hz, JHH ) 2.6 Hz); 1.71 (dd, CHHtrans, JHH
)
3
1
13.8 Hz, JHH ) 11.2 Hz). 3C NMR (CD2Cl2, 100.63 MHz,
ppm): ꢀ ) 147.6 (i-Ph); 129.7 (m-Ph); 117.6 (p-Ph); 113.3 (o-
Ph); 82.0 (i-C5H4); 71.6; 69.6; 68.4; 68.3 (C5H4); 60.6 (CH);
32.1 (CH2).
2.1.3. Laser Mass Spectrometry. The experimental setup used
was described in detail elsewhere.18 Briefly, it consists of two
dye lasers yielding light pulses with a bandwidth of ∼0.3 cm-1
(fwhm) and a duration of ∼15 ns (fwhm) (FL 3002, and LPD
3000; Lambda Physik). The dye lasers are pumped synchro-
nously by a XeCl excimer laser (EMG 1003i, Lambda Physik).
The two counter-propagating laser beams intersect a skimmed
supersonic molecular beam perpendicularly 15 cm downstream
from the nozzle orifice. The light pulses overlap in time and
space in the acceleration region of a linear reflecting time-of-
flight mass spectrometer.19 The supersonic jet is obtained by
expanding argon carrier gas with a backing pressure of 3 bar
into the vacuum with a heated, pulsed (10 Hz repetition rate)
valve. The samples of the investigated ferrocenes were heated
to 160 °C in a sample chamber close to the nozzle of the valve.
At this temperature, little thermal decomposition of the inves-
tigated ferrocene derivatives takes place. After the experiment,
the remaining substances were removed and analyzed by NMR
and electron impact mass spectrometry. No products originating
from thermal decomposition could be found. To check if
sufficient amounts of the substances were evaporated, mass
spectra with sharply focused laser light at the highest available
laser power were recorded, thus forcing complete fragmentation
2.1.1. Synthesis of n-(ferrocenylmethyl)aniline. This com-
pound was synthesized according to the literature16 and purified
by recrystallization from n-hexane prior to use.
2.1.2. 2-(n-Phenyl)amino-[3]-Ferrocenophane ([3]-Ferro-
cenophan-2-Phenylamine). [3]-Ferrocenophane-2-Phenylimine
(precursor). 600 mg (2.5 mmol) of [3]-ferrocenophan-2-one17
are placed in a 100 mL round-bottom flask together with ca.
50 mg anhydrous ZnCl2 and 5 g of molecular sieves (4 Å).
Subsequently, 0.45 mL (5 mmol) aniline and 40 mL dry toluene
are added. The reaction mixture is refluxed for 12 h and filtered
after cooling to room temperature. After extraction of the residue
twice with 10 mL of toluene the combined extracts are freed of
solvent. The resulting yellow powder is recrystallized from
CH2Cl2/n-hexane yielding 615 mg (78%) of yellow crystals.
Mp. 174 °C; elementary analysis (mw (C19H17FeN): 315.20)
calcd.: C 72.40, H 5.44, Fe 17.72, N 4.44%. Found: C 72.19,

Charge Transfer in Ferrocene Derivatives
J. Phys. Chem. A, Vol. 104, No. 10, 2000 2015
Figure 3. Resonance enhanced two-photon ionization spectrum of
[3]ferrocenophane measured at the mass of Fe+ (56 u). The various
lines indicate transitions in neutral Fe atoms produced by dissociation
of photoexcited [3]ferrocenophane subsequently ionized by resonant
two-photon ionization.
Figure 2. Multiphoton mass spectra of ferrocene recorded at different
wavelengths. Note that no ferrocene cations at mass 186 u (marked by
an arrow) were detected. The strong signal at 56 u is from Fe atoms
produced by a fast dissociation of the neutral ferrocene after photo-
excitation.
of the ferrocene derivatives and detecting the subsequent
production of iron cations. Excitation and ionization of the
ferrocene derivatives was achieved in a resonantly enhanced
two-photon two-color process. The produced cations were
accelerated in an electric potential of 1000 V toward the ion
reflector, where they are reflected back toward the multi-
channel plate ion detector. REMPI spectra are recorded mass
selectively with a gated integrator/microcomputer system, and
mass spectra were recorded by a LeCroy transient recorder
system (TR8828).
Figure 4. Schematic absorption spectra of ferrocene and aniline in
solution. The absorption maximum of aniline is close to a minimum
of absorption of ferrocene. The absorption coefficient ꢀ is given in units
of liter mol-1 cm-1. The dotted line denotes the S1 W S0 transition of
aniline at about 34 000 cm-1
.
transitions in the atomic Fe resulting from the resonance-
enhanced two-photon ionization of the neutral Fe atoms
produced after dissociation of the photoexcited ferroceno-
phane.
3. Results
3.1. Ferrocene and [3]Ferrocenophane. Both ferrocene and
[3]ferrocenophane (1,1′-trimethyleneferrocene) (see Figure 1,
parts 1a and 1b) are building blocks of the investigated
derivatives and act as electron donors in our experiment. The
UV/VIS absorption spectra of both substances20-23 are nearly
identical. In solution and in the gas phase both substances absorb
light in a broad wavelength range below 400 nm, with some
smooth features that have been assigned to d-d transitions in
Fe, metal-to-ligand or ligand-to-metal charge transfer (MLCT
or LMCT) transitions, and to electronic transitions within the
cp rings.20
To rule out the possibility of locally photoionizing the
ferrocene or the [3]ferrocenophane molecule in a multiphoton
process under the conditions of our experiment, we recorded
laser mass spectra at different wavelengths, and resonant two-
photon ionization (R2PI) spectra of the two compounds. Figure
2 displays a series of time-of-flight (TOF) mass spectra of
ferrocene for several wavelengths of the excitation laser light.
Clearly no intact parent molecule cations at the mass 186 u
(ferrocene) but only Fe cations at the mass 56 u are found. The
latter are produced in a subsequent multiphoton ionization
process of Fe after decomposition of the photoexcited ferrocene.
The inset of Figure 2 displays a magnified portion of the mass
spectrum with a 56Fe peak and the accompanying smaller
isotopic 54Fe peak.
3.2. Derivatives. 3.2.1. n-(Ferrocenylmethyl)aniline. In Figure
4 the absorption spectrum of aniline,22 which acts as a
chromophore in the investigated derivatives, is schematically
drawn and compared to a spectrum of ferrocene and ferro-
cenophane in the visible and near UV.20 In the figure only the
ferrocene spectrum is shown, because the spectra of ferrocene
and ferrocenophane in solution and in the gas phase of these
ferrocene compounds are very similar. At the left side to higher
photon energies an absorption shoulder in the ferrocene spectrum
can be identified, where the absorption coefficient increases by
at least 1 order of magnitude within a relatively narrow
frequency range of 2000 cm-1. To lower energy a smaller
maximum exists indicating the Fe d-d transition. The aniline
S1 W S0 transition maximum is located at about 34 000 cm-1
at the right side of the absorption shoulder of pure ferrocene.
At this frequency the absorption coefficient of ferrocene is
substantially smaller than that of aniline.
A series of mass spectra of n-(ferrocenylmethyl)aniline is
shown in Figure 5. They were recorded with the first (excitation)
laser wavelength tuned to different values between 250 and 285
nm and the second (ionization) laser wavelength tuned to 350
nm. All mass peaks could be assigned unambiguously, as
discussed below.
The most prominent feature in all traces is at mass 93 u. It is
identified as aniline by its spectral fingerprint in a R2PI spectrum
recorded at this mass. The aniline originates from minor thermal
decomposition of the derivative in the heated nozzle. Because
of the high sensitivity of laser mass spectrometry, the merest
For the [3]ferrocenophane we found the same results, i.e.,
strong Fe+ peaks but no signal at the parent mass (226 u). No
mass spectrum is displayed, but in Figure 3 the recorded R2PI
spectrum measured at mass 56 u (Fe+). The sharp lines are

2016 J. Phys. Chem. A, Vol. 104, No. 10, 2000
Braun et al.
Figure 7. Multiphoton mass spectra of 2-(n-phenyl)amino-[3]-ferro-
cenophane (2b in Figure 1) recorded at different laser wavelengths.
The signal at mass 93 u originates from aniline after thermal
decomposition of the ferrocene derivative. Top trace: Fe cations at
mass 56 u are present after fragmentation of the neutral ferrocene
derivative by sharply focused laser light, tuned to atomic resonance of
Fe at 271.9 nm. Lower traces: A mass peak at 220 u appears, marked
with asterisks. For explanation, see text.
Figure 5. Multiphoton mass spectra of n-(Ferrocenylmethyl)aniline
(2a in Figure 1) recorded at different laser wavelengths. Note that no
Fe cations at mass 56 u were detected. The strong aniline signal at
mass 93 u is produced by thermal decomposition of the ferrocene
derivative. The signal at mass 186 u marked with an asterisk originates
from ferrocene cations produced by an intramolecular dissociative
charge transfer. For explanation of the wavelength dependence, see
text.
spectrum is a fingerprint of the absorption pathway via the
aniline part of the ferrocene derivative.
3.2.2. 2-(n-Phenyl)amino-[3]-Ferrocenophane. Similar ex-
periments as described above were performed with the bridged
ferrocene-aniline derivative. As before, a series of mass spectra
was recorded and the result is shown in Figure 7. Again, the
strongest mass peaks originate from aniline. The total signal
from this compound was weaker, since the 2-(n-phenyl)amino-
[3]-ferrocenophane has a smaller vapor pressure than n-
(ferrocenylmethyl)aniline. In contrast to the results obtained for
n-(ferrocenylmethyl)aniline, here no signal at the mass 186 u
(ferrocene) could be detected. Again, no parent ions of the intact
derivative at the mass of 317 u could be observed. Instead, a
signal at mass 220 u is detected. A large mass such as this can
be only observed for a molecule or fragment containing a
ferrocene subunit, since combinations of fragment or cluster
masses do not yield a mass of 220 u.
Figure 6. Resonance enhanced two-photon ionization spectra of
n-(ferrocenylmethyl)aniline (2a in Figure 1), recorded at the mass of
ferrocene (186 u, upper trace) and aniline (93 u, lower trace). Note the
similarity of the position of the maximum in both spectra. For
explanation, see text.
traces of aniline generate considerable signals. For the same
reason, additional signals are observed originating from impuri-
ties and residuals of previous experiments in the nozzle.
We could not observe parent ions of the intact ferrocene
derivative at the mass of 291 u, but, more important, a signal
at the mass 186 u of ferrocene was found. There is no other
reasonable combination of fragments and molecules which could
explain the formation of a signal at this mass. So it is clear that
this peak originates from ferrocene cations. The intensity of the
ion current could be varied by tuning the laser intensity of the
excitation or the ionization laser, so that this signal must
originate from a true two-color process.
In addition to the mass spectra, resonant two-photon ioniza-
tion (R2PI) spectra in the vicinity of the S1 W S0 transition of
aniline (294 nm) were recorded, varying the excitation wave-
length while recording the signal at a mass of 93 u (aniline)
and 186 u (ferrocene). The resulting two spectra in Figure 6
display some similarity in the position of the maximum. On
the other hand the pure ferrocene absorption spectrum is
completely different in the observed spectral range. The
spectrum recorded at 186 u, i.e., the excitation spectrum of the
ferrocene derivative, is substantially broadened and no distinct
structure can be found. This is what we would expect when
assuming that the energy levels of the chromophore aniline are
disturbed and shifted by some intramolecular interactions with
the ferrocene part of the molecule. Thus the intermediate state
4. Discussion
The main result of our experiments is that a signal at the
ferrocene mass or at a mass larger than ferrocene is observed
for the case of ferrocene-chromophore derivatives. This is
completely different from the experiment with ferrocene mol-
ecules where absorption takes place in ferrocene itself leading
to a strong dissociation and production of Fe+ ions but no signal
at all at the ferrocene mass. From this we conclude that the
excitation and ionization mechanism is different for both
experiments. In principle two excitation mechanisms are possible
for our experimental conditions: (i) Ferrocene cations are
produced by a charge transfer from aniline cations to ground
state ferrocene molecules in a collision, similar to chemical
ionization. Free aniline cations are produced by thermal
decomposition of the derivatives in our experiment. It is likely
that ferrocene molecules are produced in the same way. The
aniline molecules are easily detected by REMPI and yield strong
peaks at 93 u in the mass spectra of Figure 5. On the other
hand ferrocene molecules present in the gas jet cannot be
identified in the mass spectrum since they dissociate into Fe
and cp rings in a MPD process before ionization. To check this
process, special experiments with a mixture of ferrocene
(Fe(cp)2) and aniline in the molecular beam were carried out
for the same laser conditions as used before. The mass spectra
obtained after resonance-enhanced two-photon excitation at λ1

Charge Transfer in Ferrocene Derivatives
J. Phys. Chem. A, Vol. 104, No. 10, 2000 2017
Figure 8. Excitation and dissociation pathway of n-(ferrocenylmethyl)aniline (2a in Figure 1). After local ionization of the aniline site electron
transfer takes place from the ferrocene part to the aniline part followed by fragmentation of the compound. The excess energy released leads to a
hydrogen transfer from the aniline to the ferrocene part, resulting in a formation of ferrocene+ as detected and a concomitant elimination of phenylimine.
in the range between 250 nm to 285 nm and λ2 ) 350 nm did
not display any peak at the mass of ferrocene (186 u) and only
iron (56 u) and aniline (93 u) cations were produced. From this
result we conclude that an intermolecular charge transfer from
aniline cations to ferrocene does not take place with sufficient
efficiency and can be ruled out as a source for the appearance
of ferrocene cations in the experiments with ferrocene-aniline
derivatives described above. (ii) Therefore, we conclude that
the ferrocene signal observed at 186 u originates from an
intramolecular charge transfer following the excitation and
ionization of the ferrocene derivative at the aniline site.
At this point we briefly discuss the site selective excitation
in a ferrocene derivative containing a ferrocene and a chro-
mophore site. If absorption of the first photon takes place at
the absorption maximum of the aniline site with ns laser pulses
the excitation of long-lived aniline zero order states is possible
which are weakly coupled to the broad dissociative zero order
states of the ferrocene site. For weak coupling the time evolution
is so slow that the absorption of the second photon takes place
from the aniline zero order state to the ionization continuum of
the aniline. On the other hand an absorption from excited
ferrocene zero order states is not possible since they rapidly
decay by dissociation. The emitted electron is the π* electron
of the aniline leading to a local ionization of the aniline part of
the molecule, i.e., an ionic wave function which is located at
the aniline site and thus similar to the aniline+ MO function.
The so produced ion zero order wave functions with aniline
character are not eigenstates and develop in time to a final
eigenstate of the system with positive charge localized at the
ferrocene part of the molecule.24 After this rapid charge transfer
process, the difference of the local IEs of ferrocene and aniline,
which is about 1 eV, is released.
If the dissociation rate constant is fast, fragments are produced
during the laser pulse which in turn can absorb further photons
as was demonstrated first for the ladder switching process for
benzene cations.25,26
In Figures 8 and 9 possible pathways of the dissociative
intramolecular charge-transfer reactions are shown. The primary
step in the reaction of n-(ferrocenylmethyl)aniline (Figure 8) is
the transfer of an electron from the ferrocene part to the ionized
aniline part. Subsequently, the fragmentation of the compound
proceeds. As a key step the excess energy released after the
electron transfer leads to a hydrogen transfer from the aniline
to the ferrocene part. Most likely the excess energy is sufficient
for the hydrogen transfer and no energy takeup by absorption
of further photons is necessary. This results in the formation of
ionic ferrocene as detected and the concomitant elimination of
phenylimine. Such a decomposition pathway is not found in
the electron impact mass spectrum of the compound,27 but it is
reported as a minor route with other ferrocenylmethylamines
and ferrocenylmethanol.27,28 The fragmentation pathway of
complex 2b (see Figure 1) should not be as simple as in complex
2a (see Figure 1) because of its more complex structure. After
ionization of the aniline, electron transfer is again assumed as
the primary step to take place. This primary electron transfer
step initiates a series of reactions. The aniline is eliminated by
takeup of a hydrogen atom presumably from the alkyl bridge.
The resulting ferrocene A (see Figure 9) could in principle
rearrange to compound B which is known as a stable com-
pound.17 A mass peak which corresponds to B is not found.
The different fragmentation pathway leading to a cation of mass
220 u can be obtained by eliminating 4 hydrogen atoms from
A and rearrangement to complex C. The formation of an alkyne
residue and a fulvene ligand seems to be feasible since fulvene
ligands are quite widespread in ferrocenylcarbenium chemistry.29
As the charge transfer process is fast it occurs during the
laser pulse duration (some ns). After the charge transfer
additional photons can be absorbed, leading to a higher internal
energy of the cation which is sufficient for forming finally the
products shown in Figure 9, C and D, which are less stable
than the initial structures A and B.
It is also known from mass spectra of [3]-ferrocenophanes,
that the interannular bridge is preferentially cleaved between
the first and second carbon atom of the bridge and not between

2018 J. Phys. Chem. A, Vol. 104, No. 10, 2000
Braun et al.
Figure 9. Excitation and dissociation pathway of 2-(n-phenyl)amino-[3]-ferrocenophane (2b in Figure 1). An electron transfer step initiates a
series of reactions. Aniline is eliminated by take up of a hydrogen atom presumably from the alkyl bridge, resulting in ferrocene A. There are two
possible fragmentation pathways. (i) Rearranging to compound B which is known as a stable compound,17 not found in our mass spectra. (ii)
Elimination of 4 hydrogen atoms from A and rearrangement to complex C. Formation of an alkyne residue and a fulvene ligand seems to be
feasible since fulvene ligands are quite widespread in ferrocenylcarbenium chemistry.29 As an alternative one could think of the formation of an
aryne ligand as described in structure D.
the cp-ligand and the first carbon atom.30 As an alternative one
could think of the formation of an aryne ligand as described in
structure D.
Such ring expansions from cp to benzene derivatives are often
found in mass spectra of ferrocenes.30 From thermochemical
estimates for structures C and D one can expect their formation
to be only slightly endothermic, so that the absorption of one
or more additional photons can deliver the energy needed for
eliminating the hydrogen atoms.
The ferrocene peaks at 186 u in the mass spectra are not
broadened. This would be the case for a dissociation time similar
to the acceleration time in the ion optics. So we conclude that
the dissociation occurs on a time scale much faster than 1 µs,
which is the typical time resolution of our mass spectrometer.
In addition, from the energetic considerations above we conclude
that the charge transfer process rate is faster than 108 s-1 and
thus occurs during the laser pulse.
novel ferrocene derivatives were investigated by resonance-
enhanced time-of-flight laser mass spectrometry to study
dissociative charge-transfer processes in isolated molecules, after
optical excitation. The site-selective ionization of these mol-
ecules at the chromophore site results in a subsequent electron
transfer from the ferrocene part to the chromophore part. The
energy difference of about 1 eV between the local IEs and
additional energy takeup by absorption of further laser photons
lead to an excess energy sufficient for breaking the chemical
bond between the two functional moieties. The appearance of
ferrocene or ferrocene-containing cations is therefore explained
by a fast (<10-8 s) charge transfer process between the two
parts of the ferrocene derivatives after local ionization and a
subsequent dissociation.
Future experiments will include the dependence of the charge
transfer rate on the distance of the moieties and the difference
of the ionization energies. This information is expected by
varying the distance between the ferrocene and the chromophore
or by the use of specially synthesized ferrocene-chromophore
derivatives with varying IEs and absorption spectra.
5. Summary and Conclusion
Ferrocene molecules are typical examples for a fast dissocia-
tion in the excited electronic state. As a consequence in previous
work no parent ions have been observed after resonance-
enhanced two-photon ionization with UV light. In this work
Acknowledgment. This work was supported by the SFB
377, “Photoionisation und Ladungstrennung in groâen Mole-
ku¨len, Clustern und kondensierter Phase” and the Fonds der

Charge Transfer in Ferrocene Derivatives
J. Phys. Chem. A, Vol. 104, No. 10, 2000 2019
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