Fast intramolecular charge transfer with a planar rigidized electron donor/acceptor molecule

Article abstract of DOI:10.1021/ja049809s

The planar rigidized molecule fluorazene (FPP) undergoes fast reversible intramolecular charge transfer (ICT) in the excited state, with a reaction time of 12 ps in the polar solvent ethyl cyanide at -45 °C. The ICT state of FPP has a dipole moment μ_e(ICT) of 13 D, much larger than that of the locally excited state LE (1 D). The ICT behavior of FPP is similar to that of its flexible counterpart N-phenylpyrrole (PP), for which μ_e(ICT) = 12 D. These results show that intramolecular charge transfer to a planar ICT state can occur efficiently. In designing ICT systems capable of rapid switching, it is therefore important to realize that large amplitude motions such as those necessary for the formation of a twisted intramolecular charge transfer (TICT) state are not required.

Full text of DOI:10.1021/ja049809s

Published on Web 06/18/2004
Fast Intramolecular Charge Transfer with a Planar Rigidized
Electron Donor/Acceptor Molecule
†
Toshitada Yoshihara, Sergey I. Druzhinin, and Klaas A. Zachariasse*
Contribution from the Max-Planck-Institut f u¨ r biophysikalische Chemie, Spektroskopie und
Photochemische Kinetik, 37070 G o¨ ttingen, Germany
Received January 12, 2004; E-mail: kzachar@gwdg.de
Abstract: The planar rigidized molecule fluorazene (FPP) undergoes fast reversible intramolecular charge
transfer (ICT) in the excited state, with a reaction time of 12 ps in the polar solvent ethyl cyanide at -45
°
C. The ICT state of FPP has a dipole moment µ
state LE (1 D). The ICT behavior of FPP is similar to that of its flexible counterpart N-phenylpyrrole (PP),
for which µ (ICT) ) 12 D. These results show that intramolecular charge transfer to a planar ICT state can
e
(ICT) of 13 D, much larger than that of the locally excited
e
occur efficiently. In designing ICT systems capable of rapid switching, it is therefore important to realize
that large amplitude motions such as those necessary for the formation of a twisted intramolecular charge
transfer (TICT) state are not required.
Introduction
Electron donor(D)/acceptor(A) molecules such as 4-amino-
DMABN, for example, is considered to be twisted to a
configuration perpendicular to the plane of the phenyl ring,
whereas in the planar intramolecular charge transfer (PICT)
benzontriles1 and N-phenylpyrroles,
-3
1,3-6
can emit fluorescence
1
3-15
model,
the dimethylamino and benzonitrile moieties have
from two relaxed singlet excited states (dual fluorescence). Upon
photoexcitation a locally excited state (LE) is formed, from
which an intramolecular charge transfer (ICT) state with a larger
dipole moment is produced. With 4-(dimethylamino)benzonitrile
a largely coplanar structure.
The experiments initially used to support the TICT model
were based on compounds having either a rigid planar structure
(
1-methyl-5-cyanoindoline and 1-methyl-6-cyano-1,2,3,4-
(DMABN), for example, the dipole moment increases from 6.6
tetrahydroquinoline) and only LE emission or with a strongly
twisted amino group in the ground state (3,5-dimethyl-4-
D in the electronic ground state (µg) via 10 D for LE to 17 D
for the ICT state (µe(ICT)).7,8 In the case of N-phenylpyrrole
3
6
(dimethylamino)benzonitrile) and only ICT fluorescence. Re-
(
PP), µg ) 1.4 D and µe(ICT) ) 12 D. Since its discovery
9
cent experimental evidence in favor of TICT comes from
around 1960, dual fluorescence (LE + ICT) has been observed
with a large number of D/A molecules in dilute solution,
crystals, and also in the gas phase. It is now well-established
1
6,17
1
-9
vibrational (infrared and Raman) spectroscopy.
In the
and N-(4-cyanophe-
nyl)pyrrole (PP4C) the frequency attributed to the N-phenyl
in
1
6,17
10
11
spectrum of the ICT state of DMABN
1
6
that this process is an intramolecular phenomenon and that the
-
1
LE and ICT states are structural conformers.1-11
bond stretch is shifted to lower energies by about 160 cm
relative to that of the ground state. This was considered to be
caused by a lengthening of this bond, as predicted by the TICT
model (electronic decoupling of the D and A subsystems). In
conflict with this interpretation, the quinoidal character of the
phenyl ring of the ICT state is larger than that in the ground
state, whereas a decrease in quinoidality should occur for a TICT
A controversy still exists, however, concerning the molecular
structure of the ICT state.1
2,13
In the twisted intramolecular
the dimethylamino group of
charge transfer (TICT) model,1
,3,12
†
Present address: Department of Chemistry, Gunma University, Kiryu,
Gunma 376-8515, Japan.
(
(
1) Rettig, W. Angew. Chem., Int. Ed. Engl. 1986, 25, 971.
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Zachariasse, K. A. Chem. Phys. Lett. 2000, 323, 351.
1
6
structure. Definite structural conclusions on bond lengths and
molecular structure can therefore not be derived from the
vibrational spectra of DMABN or PP4C until reliable quan-
(
(
(
(
3) Grabowski, Z. R.; Rotkiewicz, K.; Rettig, W. Chem. ReV. 2003, 103, 3899.
4) Rettig, W.; Marschner, F. New J. Chem. 1990, 14, 819.
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K. A. Photochem. Photobiol. Sci. 2003, 2, 342.
18
tumchemical calculations of the transient spectra are available.
(
(
(
7) Schuddeboom, W.; Jonker, S. A.; Warman, J. M.; Leinhos, U.; K u¨ hnle,
(14) Il’ichev, Yu. V.; K u¨ hnle, W.; Zachariasse, K. A. J. Phys. Chem. A 1998,
102, 5670.
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Yu. V.; Jiang, Y.-B.; Morawski, O.; K u¨ hnle, W. J. Photochem. Photobiol.,
A 1996, 102, 59. Erratum: J. Photochem. Photobiol., A 1998, 115, 259.
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W. T.; Towrie, M. J. Phys. Chem. A 2002, 106, 3294 and references cited
therein.
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A. Bull. Chem. Soc. Jpn. 2002, 75, 957.
W.; Zachariasse, K. A. J. Phys. Chem 1992, 96, 10809.
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Rotkiewicz, K. J. Photochem. Photobiol., A 1992, 64, 49.
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Blankenstein, I. Angew. Chem. 1961, 73, 695.
(
10) Druzhinin, S. I.; Demeter, A.; Zachariasse, K. A. Chem. Phys. Lett. 2001,
3
47, 421.
(
11) Daum, R.; Druzhinin, S. I.; Ernst, D.; Rupp, L.; Schroeder, J.; Zachariasse,
K. A. Chem. Phys. Lett. 2001, 341, 272.
(
(
12) Rettig, W.; Bliss, B.; Dirnberger, K. Chem. Phys. Lett. 1999, 305, 8.
13) Zachariasse, K. A. Chem. Phys. Lett. 2000, 320, 8.
10.1021/ja049809s CCC: $27.50 © 2004 American Chemical Society
J. AM. CHEM. SOC. 2004, 126, 8535-8539
9
8535

A R T I C L E S
Yoshihara et al.
A further experiment interpreted as supporting a TICT structure
is the syn-anti photoisomerization observed with a cyanopy-
ridine derivative in the protic solvent methanol at low temper-
ature.19
Chart 1
The main arguments in favor of PICT are based on the finding
that the amino and benzonitrile parts in the ICT state of a series
of 4-(dialkylamino)benzonitriles show a substantial electronic
coupling (considered to be absent in a TICT state),14 as well as
from the observation that efficient ICT occurs in planarized
4
-aminobenzonitriles with a seven-membered (1-methyl-7-
These measurements allow a direct conclusion regarding the
structure of the ICT state in these dual fluorescent molecules.
cyano-2,3,4,5-tetrahydro-1H-1-benzazepine) or a six-membered
(1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline, NTC6) ali-
1
5,20,21
Experimental Section
cyclic ring.
As mentioned above, the absence of dual
fluorescence with the 1-methyl and also the 1-ethyl derivative
of NTC6 was considered to be one of the main experimental
arguments leading to the TICT model. Further support for the
N-Phenylpyrrole (PP; Chart 1) was purchased from Aldrich. In the
3
0,31
synthesis of 9H-pyrrolo-[1,2-a]-indole (fluorazene, FPP),
1-(2-
3
carboxyphenyl)pyrrole (Maybridge) was reacted with PCl in toluene
5
4
under addition of SnCl . The 9H-pyrrolo-[1,2-a]-indol-9-one thus
PICT model comes from picosecond X-ray diffraction measure-
ments with crystalline 4-(diisopropylamino)benzonitrile
obtained was treated with semicarbazide hydrochloride, giving the
corresponding semicarbazone, which upon heating to 220-230 °C in
diethylene glycol produced fluorazene (mp 88.6-89.6 °C. Literature
9-90 °C (ref 30)). With PP and FPP, HPLC was the last purification
step. The identity of FPP was established by NMR and mass
(
DIABN).22 This molecule was chosen because it undergoes
10
efficient ICT in the crystal. The dihedral angle of the amino
group relative to the phenyl plane of DIABN was found to be
8
1
0° in the ICT state, an effectively coplanar configuration, as
spectroscopy. The assignment is based on two-dimensional NMR
22
compared with 14° in the ground state.
spectra (HH-COSY, HSQC, HMBC). The
1
H and 13C NMR spectra
1
were measured with a Varian Mercury 300 spectrometer. H NMR (300
In view of these results, it can be concluded that the absence
TICT) or presence (PICT) of electronic coupling between the
MHz, CDCl
3
, in ppm) for FPP: 3.83 (s, br, CH
m, H4), 7.27 (m, H5), 7.26 (m, H6), 7.09 (m, H8), 7.08 (t, H9), 6.1
m, H10). The alkylcyanide solvents acetonitrile (MeCN, Merck,
2
), 7.38 (d, H3), 7.08
(
(
(
D and A groups in the ICT state of a D/A molecule, such as
that between the phenyl and pyrrole moieties in PP, clearly is
a more significant difference between the two ICT models than
the presence of an exclusively perpendicular (90°) or coplanar
Uvasol), ethyl cyanide (EtCN, Fluka, for analysis), n-propyl cyanide
PrCN, Fluka, for analysis), and n-butyl cyanide (BuCN, Fluka, pure)
were chromatographed over Al . The solutions, with an optical
(
2 3
O
(0°) configuration. In other words, electronic coupling between
density between 0.4 and 0.6 for the maximum of the first band in the
absorption spectrum, were deaerated with nitrogen (15 min).
The fluorescence spectra were measured with quantum-corrected
Shimadzu RF-5000PC or ISA-SPEX Fluorolog 3-22 spectrofluoro-
the D and A subgroups is more important than their twist angle.
Quantumchemical calculations of dual fluorescent D/A
molecules such as DMABN and PP, at various levels of
sophistication, practically all came to the conclusion that the
meters. Fluorescence quantum yields Φ
ibility of 2%, were determined with quinine sulfate in 1.0 N H
a standard (Φ
) 0.546 at 25 °C).32 The fluorescence decay times were
obtained with a picosecond laser system (excitation wavelength λexc
76 nm) consisting of a mode-locked titanium-sappire laser (Coherent,
f
, with an estimated reproduc-
2
SO as
4
3,23-29
ICT state has a TICT structure.
The possibility of a planar
f
ICT state was, however, generally not taken into account. In
:
28
recent calculations, a PICT state was investigated for DMABN,
PP, and PP4C, but it was likewise concluded that a TICT state
is the most likely molecular structure for the lowest-energy and
hence fluorescing ICT state. On the basis of time-dependent
density functional theory (TDDFT) calculations with DMABN,
it was even concluded that final evidence was provided in favor
of the perpendicular twist interpretation and that the TICT-
PICT controversy, mainly originating from a discussion of
2
MIRA 900F) pumped by an argon ion laser (Coherent, Innova 415) or
with a nanosecond (λexc: 296 nm) flashlamp single-photon counting
(SPC) setup. These setups and the procedure used for the analysis of
3
3,34
the fluorescence decays have been described previously.
The
instrument response function of the laser SPC system has a half-width
of 18 ps.
Results and Discussion
experimental evidence, thereby should become a part of
history.29
Fluorescence Spectra. The fluorescence spectrum of FPP
in the polar solvent acetonitrile (MeCN) at -45 °C (Figure 1a)
consists of two well-separated bands, similar to the LE and ICT
emissions of PP (Figure 1b). The two bands are separated by
subtraction with the fluorescence spectrum of N-(4-methylphe-
nyl)pyrrole (PP4M) adopted as the LE model compound, as
Here we report on ICT and dual fluorescence observed with
fluorazene (FPP; Chart 1), a rigidized planar derivative of PP.
(
19) Dobkowski, J.; W o´ jcik, J.; Ko z´ mi n´ ski, W.; Kołos, R.; Waluk, J.; Michl, J.
J. Am. Chem. Soc. 2002, 124, 2406.
(
20) Zachariasse, K. A.; Grobys, M.; von der Haar, Th.; Hebecker, A.; Il’ichev,
Yu. V.; Morawski, O.; R u¨ ckert, I.; K u¨ hnle, W. J. Photochem. Photobiol.,
A 1997, 105, 373.
6
PP4M does not undergo an ICT reaction. Analogous to the
ICT/LE fluorescence quantum yield ratio Φ′(ICT)/Φ(LE) of PP,
the band intensity ratio of FPP becomes smaller with decreasing
(21) Zachariasse, K. A.; Druzhinin, S. I.; Bosch, W.; Machinek, R. J. Am. Chem.
Soc. 2004, 126, 1705.
(
(
(
(
(
22) Techert, S.; Zachariasse, K. A. J. Am. Chem. Soc. 2004, 126, 5593.
23) Dreyer, J.; Kummrow, A. J. Am. Chem. Soc. 2000, 122, 2577.
24) Parusel, A. B. J. Phys. Chem. Chem. Phys. 2000, 2, 5545.
25) Jamorski J o¨ dicke, C.; L u¨ thi, H. P. J. Chem. Phys. 2002, 117, 4146.
26) Proppe, B.; Merch a´ n, M.; Serrano-Andr e´ s, L. J. Phys. Chem. A 2000, 104,
(30) Laschtuvka, E.; Huisgen, R. Chem. Ber. 1960, 93, 81.
(31) Sydney Bailey, A.; Scott, P. W.; Vandrevala, M. N. J. Chem. Soc., Perkin
Trans. 2, 1980, 97.
(32) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991.
(33) Zachariasse, K. A.; Yoshihara, T.; Druzhinin. S. I. J. Phys. Chem. A 2002,
106, 6325. Erratum: J. Phys. Chem. A 2002, 106, 8978.
(34) Il’ichev, Yu. V.; K u¨ hnle, W.; Zachariasse, K. A. Chem. Phys. 1996, 211,
441.
1
608.
(
(
(
27) Mennucci, B.; Toniolo, A.; Tomasi, J. J. Am. Chem. Soc. 2000, 122, 10621.
28) Zilberg, S.; Haas, Y. J. Phys. Chem. A 2002, 106, 1.
29) Rappoport, D.; Furche, F. J. Am. Chem. Soc. 2004, 126, 1277.
8536 J. AM. CHEM. SOC.
9
VOL. 126, NO. 27, 2004

Fast Intramolecular Charge Transfer
A R T I C L E S
Figure 1. Fluorescence spectra of (a) fluorazene (FPP) and (b) N-
phenylpyrrole (PP) in acetonitrile (MeCN) at -45 °C. The fluorescence
spectra consist of emissions from a locally excited (LE) and an intramo-
lecular charge transfer (ICT) state. The spectral subtraction is carried out
by adopting the fluorescence spectrum of N-(4-methylphenyl)pyrrole to
represent the LE emission. Excitation wavelength: 270 nm.
Table 1. Fluorescence Quantum Yield Ratio Φ′(ICT)/Φ(LE) of
FPP and PP in Alkyl Cyanides at -45 °C
MeCN
EtCN
PrCN
BuCN
a
ꢀ
50.2
5.48
1.25
39.0
2.75
0.35
35.1
0.93
0.22
27.7
0.59
0.16
Φ′(ICT)/Φ(LE) (FPP)
Φ′(ICT)/Φ(LE) (PP)
Figure 2. Fluorescence and absorption spectra of FPP in n-hexane (HEX),
diethyl ether (DEE), and acetonitrile (MeCN) at 25 °C. In n-hexane and
diethyl ether, the fluorescence spectra consist of an emission from a locally
excited (LE) state, whereas in acetonitrile a dual emission from an LE and
an intramolecular charge transfer (ICT) state is observed. The spectral
subtraction in MeCN is carried out by adopting the fluorescence spectrum
of N-(4-methylphenyl)pyrrole to represent the LE emission. Excitation
wavelength: 270 nm.
a
Dielectric constant at -45 °C (ref 35).
polarity35 in the solvents MeCN, ethyl cyanide (EtCN), n-propyl
cyanide (PrCN), and n-butyl cyanide (BuCN) at this temperature
Table 1). With FPP, as well as with PP, only LE emission is
observed in the nonpolar n-hexane and the slightly polar diethyl
ether (Figure 2). These results show that the efficiency of the
excited-state reaction of FPP indeed increases when the solvent
becomes more polar, as should be the case for an ICT reaction.
6
(
Scheme 1
Φ′(ICT)/Φ(LE) as a Function of Temperature. Stevens-
Ban Plots of FPP and PP in MeCN. To get information on
the enthalpy change occurring during the LE f ICT reac-
tion, the temperature dependence of the ratio Φ′(ICT)/Φ(LE)
was determined for FPP and PP in MeCN (eq 1 and
Scheme 1).
In Scheme 1, ka and kd are the rate constants of the forward
and backward ICT reaction, τ0(LE) and τ′0(ICT) are the
fluorescence lifetimes, and kf(LE) and k′f(ICT) are the radiative
rate constants.
From the Stevens-Ban plots in Figure 3, it follows that for
both molecules in MeCN the high-temperature limit (HTL, kd
Φ′(ICT)/Φ(LE) ) k′ /k {k /(k + 1/τ ′ )}
(1)
f
f
a
d
0
(
35) Landolt-B o¨ rnstein, Numerical Data and Functional Relationships in Science
and Technology, New Series; Madelung, O., Ed.; Springer: Berlin, 1991;
Group IV, Vol. 6.
.
1/τ ′0(ICT)) holds (Scheme 1) because the data points in these
36,37
plots do not deviate from the linear HTL dependence.
From
J. AM. CHEM. SOC.
9
VOL. 126, NO. 27, 2004 8537

A R T I C L E S
Yoshihara et al.
Figure 4. Plot of the ICT emission maxima ν˜ max (ICT) of fluorazene (FPP)
2
at 25 °C, against the solvent polarity parameter f(ꢀ) - 1/2 f(n ) (eq 3).
Figure 3. Plots of Φ′(ICT)/Φ(LE) versus 1000/T for fluorazene (FPP) and
N-phenylpyrrole (PP) in acetonitrile (MeCN). The value of the change in
Solvents: (1) acetonitrile, (2) ethyl cyanide, (3) n-propyl cyanide, (4) n-butyl
cyanide.
∆
H for the LE f ICT reaction is indicated at the plots.
a
Table 2. Dipole Moments µg and µe(ICT) (in D) of FPP
the slope of the HTL lines in Figure 3, the following values for
H in MeCN are calculated: -15.1 kJ/mol for FPP as compared
with -8.8 kJ/mol for PP.
(ICT)b
(ICT)c
(ICT)d
µ
e
µ
g
µ
e
µ
e
∆
FPP
1.7
13 ( 1
11 ( 0.4
11 ( 1
a
ICT and LE Excited-State Dipole Moments. The excited-
state dipole moment µe of the state emitting the red-shifted
additional band of FPP, an indication for the extent of charge
separation (ICT) when compared with the ground-state dipole
moment µg, can be determined by the solvatochromic method
from the solvent polarity dependence of the energy of the
The dipole moment µe(ICT) has a direction opposite to that in the
b d
ground state (µg); see text. See Figure 4. c See Figure 5a. See Figure
b.
5
as the impact of the choice of the magnitude of F in the
comparison of FPP and PP can be minimized, as has been
6
discussed in detail elsewhere.
max
fluorescence maximum ν˜ (fl) of the new emission band (eqs
From the value of 13 D obtained for the red-shifted
fluorescence band of FPP (Figure 4), it follows that this band
indeed originates from an ICT state, with a dipole moment
,8,34,38
2
and 3).6
max
2
2
6
ν˜ (fl) ) -
µ (µ - µ )(f(ꢀ) - (1/2) f(n )) + constant
slightly larger than that of PP (12 D), likewise determined
e
e
g
3
hcF
max
2
from a plot of ν˜
(fl) versus f(ꢀ) - 1/2 f(n ). For the LE
(2)
state of FPP, a dipole moment of around 1 D was obtained in
a similar manner. These results clearly show that in the rigidized
planar molecule FPP, not able to form a perpendicularly twisted
TICT state, an efficient excited-state reaction takes place to an
ICT state with properties similar to those of its flexible
counterpart PP. This is an indication that the ICT state of PP is
also planar.
As large amplitude motions are not likely to take place
during the ICT reaction of FPP, it is concluded that the
structural changes between the LE and ICT state, i.e., the
reaction coordinate, mainly involve changes in the bond
lengths of PP in these excited states. An example of
bond length changes occurring in electron-transfer reac-
tions can be found in the transformation of N,N,N′,N′-tetra-
methyl-p-phenylenediamine (TMPD) to its radical cation
where
2
1
2
2
(ꢀ - 1)
(2ꢀ + 1)
1 (n - 1)
f(ꢀ) - f(n ) )
-
(3)
2
2
(
2n + 1)
In eq 2, F is the Onsager radius of the solute, h is the Planck
constant, and c is the speed of light, whereas ꢀ and n are the
dielectric constant and refractive index of the solvent.
max
By plotting ν˜
(fl) against the solvent polarity parameter
2
f(ꢀ) - 1/2 f(n ) in Figure 4 (eq 2), a dipole moment µe of 13 (
1
D is calculated. In this procedure, the ground-state dipole
6
moment µg (1.7 D) and the Onsager radius F (4.15 Å) of FPP
are employed. The dipole moment µg was determined by
measuring ꢀ and n as a function of FPP concentration. On the
3
9
+
40,41
TMPD .
6
basis of what has been established for PP, the dipole moment
LE and ICT Fluorescence Decays. To investigate the LE
and ICT fluorescence dynamics of FPP, time-correlated single-
photon counting measurements were made. A global analysis
of the LE and ICT fluorescence decays of FPP in EtCN at -45
°C is shown in Figure 6, with common decay times τ2 and τ1
µg is assumed to have a direction opposite to that in the excited
state. The radius F for FPP was calculated in a manner analogous
2,10
6
to the procedure described previously for PP. The emission
max
maxima ν˜ (fl) of FPP are also plotted against the correspond-
6
ing maxima of DMABN and PP4C (Figure 5), resulting in a
(
eqs 4 and 5). Two decay times (τ2 ) 11 ps and τ1 ) 13.18 ns)
somewhat smaller dipole moment of 11 D (Table 2). In this
manner, the influence of specific solvent interactions as well
are sufficient to fit the LE decay, whereas only in the ICT decay
a minor contribution from an additional time of 32 ps appears,
which is attributed to a photochemical degradation product
emitting in the same spectral region as the ICT state. The
(
(
(
(
36) Leinhos, U.; K u¨ hnle, W.; Zachariasse, K. A. J. Phys. Chem. 1991, 95,
2
013.
37) Druzhinin, S.; Demeter, A.; Niebuer, M.; Tauer, E.; Zachariasse, K. A.
Res. Chem. Intermed. 1999, 25, 531.
38) Liptay, W. In Excited States; Lim, E. C., Ed.; Academic Press: New York,
(40) Ikemoto, I.; Katagiri, G.; Nishimura, S.; Yakushi, K.; Kuroda, H. Acta
Crystallogr. 1979, B35, 2264.
(41) De Boer, J. L.; Vos, A. Acta Crystallogr. 1972, B28, 835.
1
974; Vol. 1, p 129.
39) Hedestrand, G. Z. Phys. Chem. B 1929, 2, 428.
8
538 J. AM. CHEM. SOC. VOL. 126, NO. 27, 2004
9

Fast Intramolecular Charge Transfer
A R T I C L E S
Figure 5. Plot of the ICT emission maxima ν˜ max (ICT) of fluorazene (FPP) at 25 °C, against the corresponding ν˜ max (ICT) data of 4-(dimethylamino)-
benzonitrile (DMABN) and of N-(4-cyanophenyl)phenylpyrrole (PP4C) (eq 2). Solvents: (1) acetonitrile, (2) ethyl cyanide, (3) n-propyl cyanide, (4) n-butyl
cyanide.
Table 3. Decay and ICT Parameters of FPP and PP in Ethyl Cyanide at - 45 °C (eqs 4 and 5 and Scheme 1)
(1010 s-1
)
k
d
(1010 s-1
)
τ′
(ns)
∆G (kJ/mol)
a
τ
2
(ns)
τ
1
(ns)
A12/A11
τ
0
(ns)
k
a
0
FPP
PP
0.011
0.016
13.18
6.52
9.03
1.35
11.7
11.7
8.2 ( 0.7
3.7 ( 0.8
0.9 ( 0.1
2.7 ( 0.3
13.4
4.9
-4.2 ( 0.3
-0.6 ( 0.4
a
From ∆G ) -RT ln(ka/kd). The standard deviations of ka, kd, and ∆G were calculated by adopting a reproducibility error of 1 ps for τ2, 50 ps for τ0
and τ1, and 10% for A.
excitation of the ground state, similar to what has been observed
1
4,36
with DMABN and related molecules.
i (LE) ) A exp(-t/τ ) + A exp(-t/τ )
(4)
(5)
f
11
1
12
2
i (ICT) ) A exp(-t/τ ) + A exp(-t/τ )
f
21
1
22
2
This decay analysis of FPP corresponds to that for a two-
state model (Scheme 1) consisting of an LE state from which
an ICT state is formed after photoexcitation.
From the decay times τ1, τ2, and the amplitude ratio A12/A11
() 9.03) of the LE decay of FPP in EtCN at -45 °C in Figure
6
(eq 4), together with the lifetime τ0 (11.7 ns) of the LE model
6
compound PP4M, the ICT rate constants ka and kd and the
1
4,36
lifetime τ′0 of the ICT state can be determined.
The results
are: ka ) 8.2 × 10 s , kd ) 0.9 × 10 s-1, and τ′0(ICT) )
1
0
-1
10
1
)
3
3.4 ns (Table 3). For PP, the following data were obtained: ka
1
0
-1
10 -1
3.7 × 10 s , kd ) 2.7 × 10 s , and τ′0 ) 4.9 ns (Table
). It is seen from these data that in the planarized molecule
FPP the ICT reaction rate constant ka has become faster than
that of PP by a factor of 2. The ICT efficiency can be controlled
by changing the solvent polarity and temperature.
Conclusions
With the rigidized planar D/A molecule FPP, efficient ICT
occurs in a few picoseconds, without the involvement of large
amplitude motions. This means that fast and efficient ICT is
possible in planar D/A molecules, showing that perpendicular
twisting (TICT) of the D and A moieties is not required in this
process. The photophysical similarity of FPP and PP indicates
that a TICT state also does not play a role with PP.
Figure 6. LE and ICT fluorescence response functions of FPP in ethyl
cyanide (EtCN) at -45 °C. The LE and ICT decays are analyzed
simultaneously (global analysis). The decay times (τ1,τ2) and their preex-
ponential factors A1i and A2i are given (eqs 4 and 5). The shortest decay
time τ2 is listed first. The time in parentheses, only present as a minor
contribution in the ICT decay, is attributed to a photoproduct (see text).
The weighted deviations, expressed in σ (expected deviations), the auto-
2
correlation functions A-C, and the values for ø are also indicated.
Excitation wavelength: 279 nm. Emission wavelengths: 310 nm (LE), 430
nm (ICT).
Acknowledgment. Many thanks are due to Mr. Wilfried
Bosch for the synthesis of FPP. We are grateful to Mr. R.
Machinek, G o¨ ttingen University, for the analysis and measure-
ment of the NMR spectra.
presence of a negative amplitude for the shortest ICT decay
time τ2 with an amplitude ratio A22/A21 close to -1 (eq 5)
indicates that the ICT state of FPP cannot be formed by direct
JA049809S
J. AM. CHEM. SOC.
9
VOL. 126, NO. 27, 2004 8539