Intramolecular Charge Transfer with the Planarized 4-Aminobenzonitrile 1-tert-Butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6)

Article abstract of DOI:10.1021/ja037544w

Fast and efficient intramolecular charge transfer (ICT) and dual fluorescence is observed with the planarized aminobenzonitrile 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6) in a series of solvents from n-hexane to acetonitrile and methanol. Such a reaction does not take place for the related molecules with 1-isopropyl (NIC6) and 1-methyl (NMC6) groups, nor with the 1-alkyl-5-cyanoindolines with methyl (NMC5), isopropyl (NIC5), or tert-butyl (NTC5) substituents. For these molecules, a single fluorescence band from a locally excited (LE) state is found. The charge transfer reaction of NTC6 is favored by its relatively small energy gap ΔE(S₁,S ₂), in accordance with the PICT model for ICT in aminobenzonitriles. For the ICT state of NTC6, a dipole moment of around 19 D is obtained from solvatochromic measurements, similar to μ_e(ICT) = 17 D of 4-(dimethylamino)benzonitrile (DMABN). For NMC5, NIC5, NTC5, NMC6, and NIC6, a dipole moment of around 10 D is determined by solvatochromic analysis, the same as that of the LE state of DMABN. For NTC6 in diethyl ether at -70 °C, the forward ICT rate constant (1.3 × 10⁹ s^-1) is much smaller than that of the back reaction (5.9 × 10⁹ s ^-1), showing that the equilibrium is on the ICT side. The results presented here make clear that ICT can very well take place with a planarized molecule such as NTC6, when ΔE(S₁,S₂) is sufficiently small, indicating that a perpendicular twist of the amino group relative to the rest of the molecule is not necessary for reaching an ICT state with a large dipole moment. The six-membered alicyclic ring in NMC6, for example, prevents ICT by increasing ΔE(S₁,S₂) relative to that of DMABN.

Full text of DOI:10.1021/ja037544w

Published on Web 01/27/2004
Intramolecular Charge Transfer with the Planarized
4-Aminobenzonitrile
1-tert-Butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6)
Klaas A. Zachariasse,*,† Sergey I. Druzhinin, Wilfried Bosch, and
†
†
‡
Reinhard Machinek
Contribution from the Max-Planck-Institut f u¨ r Biophysikalische Chemie, Spektroskopie und
Photochemische Kinetik, 37070 G o¨ ttingen, Germany, and Institut f u¨ r Organische Chemie,
UniVersit a¨ t G o¨ ttingen, Tammannstrasse 2, 37077 G o¨ ttingen, Germany
Received July 25, 2003; E-mail: kzachar@gwdg.de
Abstract: Fast and efficient intramolecular charge transfer (ICT) and dual fluorescence is observed with
the planarized aminobenzonitrile 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6) in a series of
solvents from n-hexane to acetonitrile and methanol. Such a reaction does not take place for the related
molecules with 1-isopropyl (NIC6) and 1-methyl (NMC6) groups, nor with the 1-alkyl-5-cyanoindolines with
methyl (NMC5), isopropyl (NIC5), or tert-butyl (NTC5) substituents. For these molecules, a single
fluorescence band from a locally excited (LE) state is found. The charge transfer reaction of NTC6 is favored
by its relatively small energy gap ∆E(S
For the ICT state of NTC6, a dipole moment of around 19 D is obtained from solvatochromic measurements,
similar to µ (ICT) ) 17 D of 4-(dimethylamino)benzonitrile (DMABN). For NMC5, NIC5, NTC5, NMC6, and
NIC6, a dipole moment of around 10 D is determined by solvatochromic analysis, the same as that of the
1 2
,S ), in accordance with the PICT model for ICT in aminobenzonitriles.
e
1
1
-1
LE state of DMABN. For NTC6 in diethyl ether at -70 °C, the forward ICT rate constant (1.3 × 10
s )
9
-1
is much smaller than that of the back reaction (5.9 × 10 s ), showing that the equilibrium is on the ICT
side. The results presented here make clear that ICT can very well take place with a planarized molecule
1 2
such as NTC6, when ∆E(S ,S ) is sufficiently small, indicating that a perpendicular twist of the amino group
relative to the rest of the molecule is not necessary for reaching an ICT state with a large dipole moment.
The six-membered alicyclic ring in NMC6, for example, prevents ICT by increasing ∆E(S ,S ) relative to
1 2
that of DMABN.
Introduction
configuration in LE to a mutually perpendicular structure in
the ICT state. The planar intramolecular charge transfer (PICT)
In the investigation of intramolecular charge transfer (ICT)
with dual fluorescent electron donor(D)/acceptor(A)-substituted
aromatic hydrocarbons such as 4-(dimethylamino)benzonitrile
7
,11-13
model,
on the other hand, assumes that the ICT state of
1
4-16
DMABN is largely planar.
In this model, the magnitude
of the energy gap ∆E(S1,S2) between the two lowest excited
(DMABN), the molecular structure of the ICT state is an
7
singlet states plays a crucial role.
1-7
important point of discussion.
charge transfer (TICT) model,8
dimethylamino group of DMABN and its derivatives undergoes
a twist of 90° relative to the benzonitrile moiety, from a coplanar
In the twisted intramolecular
Until the advent of direct methods to determine excited-state
-10
it is postulated that the
1
7,18
structures, such as picosecond X-ray analysis,
planarized
derivatives of DMABN fixed by a methylene bridge between
(
11) Il’ichev, Yu. V.; K u¨ hnle, W.; Zachariasse, K. A. J. Phys. Chem. A 1998,
†
Max-Planck-Institut f u¨ r Biophysikalische Chemie.
Universit a¨ t G o¨ ttingen.
102, 5670.
‡
(12) Zachariasse, K. A.; Grobys, M.; von der Haar, Th.; Hebecker, A.; Il’ichev,
Yu. V.; Jiang, Y.-B.; Morawski, O.; K u¨ hnle, W. J. Photochem. Photobiol.,
A: Chem. 1996, 102, 59. Erratum: J. Photochem. Photobiol., A: Chem.
1998, 115, 259.
(13) 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: Chem. 1997, 105, 373.
(
1) Ma, C.; Kwok, W. M.; Matousek, P.; Parker, A. W.; Phillips, D.; Toner,
W. T.; Towrie, M. J. Phys. Chem. A 2002, 106, 3294 and references cited
therein.
2) Okamoto, H.; Inishi, H.; Nakamura, Y.; Kohtani, S.; Nakagaki, R. J. Phys.
Chem. A. 2001, 105, 4182.
3) Okamoto, H.; Kinoshita, M.; Kohtani, S.; Nakagaki, R.; Zachariasse, K.
A. Bull. Chem. Soc. Jpn. 2002, 75, 957.
4) Zilberg, S.; Haas, Y. J. Phys. Chem. A 2002, 106, 1.
5) Dobkowski, J.; W o´ jcik, J.; Ko z´ minski, W.; Kołos, R.; Waluk, J.; Michl, J.
J. Am. Chem. Soc. 2002, 124, 2406.
(
(
(14) The use of the name LE state in the ICT reaction of D/A-substituted
molecules such as DMABN obviously originates from the formal kinetic
similarity of the ICT reaction scheme consisting of an initially excited LE
state and an ICT state (Scheme 1) and the schemes applicable to excimer
and exciplex formation.
(
(
(
(
(
6) Rettig, W.; Bliss, B.; Dirnberger, K. Chem. Phys. Lett. 1999, 305, 8.
7) Zachariasse, K. A. Chem. Phys. Lett. 2000, 320, 8.
8) Grabowski, Z. R.; Rotkiewicz, K.; Siemiarczuk, A.; Cowley, D. J.;
Baumann, W. NouV. J. Chim. 1979, 3, 443.
(15) Heine, A.; Herbst-Irmer, R.; Stalke, D.; K u¨ hnle, W.; Zachariasse, K. A.
Acta Crystallogr. 1994, B50, 363.
(16) Berden, G.; van Rooy, J.; Meerts, W. L.; Zachariasse, K. A. Chem. Phys.
Lett. 1997, 278, 373.
(
9) Rettig, W. Angew. Chem., Int. Ed. Engl. 1986, 25, 971.
(
10) Rettig, W. In Topics in Current Chemistry, Electron-Transfer I; Mattay,
(17) Techert, S.; Schotte, F.; Wulff, M. Phys. ReV. Lett. 2001, 86, 2030.
(18) Techert, S.; Zachariasse, K. A. J. Am. Chem. Soc., submitted.
J., Ed.; Springer: Berlin, 1994; Vol. 169, p 253.
10.1021/ja037544w CCC: $27.50 © 2004 American Chemical Society
J. AM. CHEM. SOC. 2004, 126, 1705-1715
9
1705

A R T I C L E S
Zachariasse et al.
the amino nitrogen and the benzene ring have played a major
Chart 1
role in the investigations aimed at the elucidation of the ICT
structure.8
-10,13,19-25
The first example of a model compound
for a planar LE state that cannot form a TICT state was
1
1
-methyl-5-cyanoindoline (NMC5).24 Later, 1-methyl-6-cyano-
2
3
,2,3,4-tetrahydroquinoline (NMC6) and 1-ethyl-5-cyano-
21
indoline (NEC5) were introduced as other model substances
for LE.
Chart 2
With NMC5, NEC5, and NMC6, dual fluorescence was not
observed: even in the strongly polar solvents acetonitrile and
methanol, the fluorescence spectrum consists of a single LE
band.2
2-24
The absence of ICT fluorescence in the case of model
compounds such as NMC6 was considered to be the most
convincing evidence for the TICT model.8-10,26
A closer inspection of the absorption spectra of NMC5 and
especially NMC6 reveals, however, that the planarization of
these compounds by incorporating the amino nitrogen into an
alicyclic ring results in a larger energy gap ∆E(S1,S2) as
compared with more flexible aminobenzonitriles such as
DMABN.13 ICT does not take place with molecules having
Experimental Section
To obtain NTC6, 1,2,3,4-tetrahydroquinoline in dimethylformamide
was formylated with POCl , resulting in 3,4-dihydro-2H-quinoline-1-
carbaldehyde (NCAC6).33 NCAC6 was converted into 1,2,3,4-tetrahy-
droquinoline-6-carbaldehyde with POCl
with hydroxylamine hydrochloride in 1-methylpyrrolidone, giving
3
relatively large values for ∆E(S1,S2), such as with 4-(methy-
lamino)benzonitrile (MABN), 4-aminobenzonitrile (ABN), and
34
3
. This compound was reacted
4
-(dimethylamino) phenylacetylene.7
,12,13,27-29
In support of the
PICT model, fast and efficient ICT was observed with the
planarized aminobenzonitrile 1-methyl-7-cyano-2,3,4,5-tetra-
hydro-1H-1-benzazepine (NMC7) in diethyl ether and aceto-
35
1,2,3,4-tetrahydroquinoline-6-carbonitrile (NHC6). NHC6 in tetrahy-
drofuran (THF) was added slowly to a suspension consisting of CuI
and tert-butyllithium in THF at -60 °C, followed by insertion of
1
2,13
36
nitrile.
For this molecule NMC7 with a seven-membered
oxygen. The product NTC6 was purified by flash chromatography
alicyclic ring, the ∆E(S1,S2) value is smaller than that for NMC6.
The replacement of the methyl substituents in DMABN,
giving 4-(diisopropylamino)benzonitrile (DIABN), leads to a
reduction of ∆E(S1,S2) and a strong increase in the efficiency
of the ICT reaction results, to such an extent that ICT and dual
and HPLC, checked by mass spectra. The molecular structure was
established by NOE experiments. NTC5 was synthesized in an
analogous manner. The detailed synthesis procedures and the NMR
data are available as Supporting Information. For all aminobenzonitriles,
HPLC was the last purification step. In the case of NTC6, for example,
a purity of better than 99.5% is achieved, as no contaminants were
visible in the HPLC after separation.
The solvents n-hexane and isopentane (Merck, Uvasol) were used
2 3
as received. The other solvents were chromatographed over Al O . The
solutions, with an optical 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
fluorescence occur in the nonpolar solvent n-hexane, in the
crystalline state, and even in the gas phase.3
0-32
On the basis
of the observed enhancement of the ICT efficiency by replacing
methyl with isopropyl amino substituents, it was therefore
attempted to synthesize derivatives of NMC5 and NMC6 with
isopropyl and tert-butyl groups instead of methyl. Results
obtained with these molecules, 1-isopropyl-5-cyanoindoline
(
1
1
NIC5), 1-isopropyl-6-cyano-1,2,3,4-tetrahydroquinoline (NIC6),
-tert-butyl-5-cyanoindoline (NTC5), and 1-tert-butyl-6-cyano-
,2,3,4-tetrahydroquinoline (NTC6), are reported in this paper
spectrofluorometers. Fluorescence quantum yields Φ
reproducibility of 2%, were determined with quinine sulfate in 1.0 N
SO as a standard (Φ
) 0.546 at 25 °C).37
The fluorescence decay times were determined with a picosecond
laser system (excitation wavelength λexc: 276 nm) or a nanosecond
f
, with an estimated
H
2
4
f
(Chart 1). For comparison, the 1-methyl derivatives NMC5 and
NMC6 are also investigated.
1
1,21,29
(λexc: 296 nm) flashlamp single-photon counting (SPC) setup.
The instrument response function of the laser SPC system has a half-
width of 18-20 ps, with an estimated time resolution of 3 ps.
(
(
(
19) Jamorski J o¨ dicke, C.; L u¨ thi, H. P. J. Chem. Phys. 2002, 117, 4146.
2
9,30,32
20) Lommatzsch, U.; Brutschy, B. Chem. Phys. 1998, 234, 35.
21) Leinhos, U.; K u¨ hnle, W.; Zachariasse, K. A. J. Phys. Chem. 1991, 95,
2
013.
Results and Discussion
(
(
22) G u¨ nter, W.; Rettig, W. J. Phys. Chem. 1984, 88, 2729.
23) Visser, R. J.; Varma, C. A. G. O. J. Chem. Soc., Faraday Trans. 2 1980,
Absorption and Fluorescence Spectra of 4-Aminoben-
zonitriles in n-Hexane at 25 °C. Influence on ICT and ∆E
7
6, 453.
(24) Rotkiewicz, K.; Grabowski, Z. R.; Kr o´ wczy n´ ski, A.; K u¨ hnle, W. J. Lumin.
1
976, 12/13, 877.
(S1,S2) by Replacing Amino Methyl Groups with Isopropyl
(
25) Baumann, W.; Bischof, H.; Brittinger, J.-C.; Rettig, W.; Rotkiewicz, K. J.
Photochem. Photobiol., A: Chem. 1992, 64, 49.
or tert-Butyl Substituents. The replacement of both amino
methyl substituents in DMABN (Chart 2) by isopropyl groups
results in the appearance of efficient intramolecular charge
(
(
26) Grabowski, Z. R. Acta Phys. Pol. 1987, A71, 743.
27) Zachariasse, K. A.; von der Haar, Th.; Hebecker, A.; Leinhos, U.; K u¨ hnle,
W. Pure Appl. Chem. 1993, 65, 1745.
(
(
(
(
(
28) Zachariasse, K. A.; Grobys, M.; Tauer, E. Chem. Phys. Lett. 1997, 274,
3
72.
29) Zachariasse, K. A.; Yoshihara, T.; Druzhinin, S. I. J. Phys. Chem. A 2002,
(33) Lukevits, EÄ .; Zablotskaya, A.; Segal, I. Chem. Heterocycl. Compd. 1995,
31, 374.
1
06, 6325. Erratum: J. Phys. Chem. A 2002, 106, 8978.
30) Demeter, A.; Druzhinin, S.; George, M.; Haselbach, E.; Roulin, J.-L.;
Zachariasse, K. A. Chem. Phys. Lett. 2000, 323, 351.
(34) Gawinecki, R.; Sylwia, A.; Puchala, A. Org. Prep. Proced. Int. 1998, 30,
455. Private communication: Prof. R. Gawinecki.
31) Daum, R.; Druzhinin, S. I.; Ernst, D.; Rupp, L.; Schroeder, J.; Zachariasse,
K. A. Chem. Phys. Lett. 2001, 341, 272.
(35) Sampath Kumar, H. M.; Subba Reddy, B. V.; Tirupathi Reddy, P.; Yadav,
J. S. Synthesis-Stuttgart 1999, 586.
32) Druzhinin, S. I.; Demeter, A.; Zachariasse, K. A. Chem. Phys. Lett. 2001,
(36) Yamamoto, H.; Maruoka, K. J. Org. Chem. 1980, 45, 2739.
(37) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991.
3
47, 421.
1706 J. AM. CHEM. SOC.
9
VOL. 126, NO. 6, 2004

1-tert-Butyl-6-cyano-1,2,3,4-tetrahydroquinoline
A R T I C L E S
of an LE emission band, with a relatively small value of ∼0.2
for Φ′(ICT)/Φ(LE). Introduction of a tert-butyl group instead
of isopropyl, however, results for 4-(tert-butylmethylamino)-
benzonitrile (tBMABN) in a much larger reduction of the
∆E(S1,S2) energy gap as compared with DMABN and IMABN,
see Figure 1c. For tBMABN in n-hexane, the absorption
spectrum is similar to that of DIABN (Figure 1d), and the
fluorescence spectrum likewise predominantly consists of an
ICT emission band (Figure 1c).
The results presented in Figure 1 show empirically that the
replacement of one of the methyl groups in DMABN by tert-
butyl leads to a substantial decrease of the ∆E(S1,S2) energy
gap and the appearance of efficient ICT in n-hexane at 25 °C,
similar to what is observed when both methyl groups of
DMABN are replaced by isopropyl in the case of DIABN. This
is in clear contrast to what results from replacing a methyl group
by isopropyl, which in IMABN only leads to a minor reduction
of ∆E(S1,S2) and the appearance of a small amount of ICT
fluorescence. This pronounced influence of the introduction of
a tert-butyl substituent as compared with isopropyl on the
enhancement of ICT was the basis for the investigations
presented in this paper for two series of planarized aminoben-
zonitriles, the 1-alkyl-6-cyano-1,2,3,4-tetrahydroquinolines NXC6
(NMC6, NIC6, NTC6) and the 1-alkyl-5-cyanoindolines NXC5
(NMC5, NIC5, NTC5) with methyl (M), isopropyl (I), and tert-
butyl (T) alkyl substituents.
Absorption Spectra of NXC6 and NXC5 in n-Hexane and
Acetonitrile at 25 °C. The absorption spectra of the 1-alkyl-
6-cyano-1,2,3,4-tetrahydroquinolines NMC6, NIC6, NTC6 and
the 1-alkyl-5-cyanoindolines NMC5, NIC5, and NTC5 in
n-hexane and acetonitrile at 25 °C are shown in Figure 2. The
max
molar extinction coefficients ꢀ
measured at the maxima of
the main absorption bands of NMC5, NEC5, NIC5, NMC6,
NEC6, NIC6, and NTC6 in n-hexane all have a value around
-
1
-1
2
2 000 M cm (Table 1), which means that the structure of
the amino group is similar in these compounds. This is based
on the observation that for 4-aminobenzonitriles the amino twist
max
angle θ in the ground state determines ꢀ
according to the
max
2
38,39
relation ꢀ
is proportional to cos θ.
In the case of NMC6 and NIC6 in n-hexane, the absorption
spectrum is similar to that of DMABN (Figure 1a) with well-
separated S1 and S2 bands. Replacing the isopropyl substituent
of NIC6 by a tert-butyl group (NTC6) leads, however, to a
reduction of the ∆E(S1,S2) energy gap caused by a red-shift of
the S2 band, with a difference between the absorption spectra
of NTC6 and NIC6 comparable to that found for the pair
DIABN/DMABN, see Figure 1. For the three indolines NMC5,
NIC5, and NTC5 in n-hexane, an influence of the 1-alkyl
substituent on ∆E(S1,S2) is found to be similar to that for the
three corresponding tetrahydroquinolines. The comparison of
the spectra of NTC6 and NTC5 reveals that the S1 band becomes
more visible in the indoline derivative, indicating that the
reduction of the size of the alicyclic ring leads to an overall
increase in the ∆E(S1,S2) energy gap. The absorption spectra
of NXC6 and NXC5 in acetonitrile (Figure 2, dashed lines) do
not reveal a clearly separated S1 band, due to the preferential
red-shift of the main S2 absorption band.
Figure 1. Fluorescence and absorption spectra in n-hexane at 25 °C of (a)
-(dimethylamino)benzonitrile (DMABN), (b) 4-(isopropylmethylamino)-
4
benzonitrile (IMABN), (c) 4-(tert-butylmethylamino)benzonitrile (tBMABN),
and (d) 4-(diisopropylamino)benzonitrile (DIABN). The fluorescence
spectrum of DMABN consists of a single LE emission, whereas IMABN,
tBMABN, and DIABN are dual fluorescent (LE and ICT). Excitation in
the maximum of the absorption spectra.
transfer (ICT) for DIABN in n-hexane, in contrast to DMABN,
for which only an emission from a locally excited (LE) state is
observed in this solvent,30 see Figure 1a and 1d.
This difference has been attributed to the considerable
decrease in the energy gap ∆E(S1,S2) between the S1 and S2
states of DIABN as compared with DMABN, as seen by
3
0
inspecting the absorption spectra in Figure 1a and 1d.
When only one methyl group of DMABN is replaced by
isopropyl, giving 4-(isopropylmethylamino)benzonitrile (IMABN),
a much smaller reduction of ∆E(S1,S2) is observed (Figure 1b)
than for DIABN and the fluorescence spectrum mainly consists
(38) Burgers, J.; Hoefnagel, M. A.; Verkade, P. E.; Visser, H.; Wepster, B. M.
Recl. TraV. Chim. Pays-Bas 1958, 77, 491.
(
39) R u¨ ckert, I.; Hebecker, A.; Parusel, A. B. J.; Zachariasse, K. A. Z. Phys.
Chem. 2000, 214, 1597.
J. AM. CHEM. SOC.
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A R T I C L E S
Zachariasse et al.
Figure 2. Fluorescence and absorption spectra at 25 °C in n-hexane (HEX, solid line) and acetonitrile (MeCN, dashed line) of (a) 1-methyl-6-cyano-
,2,3,4-tetrahydroquinoline (NMC6), (b) 1-isopropyl-6-cyano-1,2,3,4-tetrahydroquinoline (NIC6), (c) 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6),
d) 1-methyl-5-cyanoindoline (NMC5), (e) 1-isopropyl-5-cyanoindoline (NIC5), and (f) 1-tert-butyl-5-cyanoindoline (NTC5). The fluorescence spectrum of
1
(
all molecules except NTC6 consists of a single LE emission in both solvents, whereas NTC6 is dual fluorescent (LE and ICT) in n-hexane as well as in
acetonitrile. Excitation in the maximum of the absorption spectra (not shown).
Table 1. Extinction Coefficients ꢀmax Measured at the Maximum λmax of the Main Absorption Band of Compounds NXC5 and NXC6 in
n-Hexane at 25 °C, See Figure 2
NMC5
NEC5
NIC5
NMC6
NEC6
NIC6
NTC6
max
[M-1 cm-1
ꢀ
]
21 120
287.7
21 200
289.6
23 660
291.0
21 800
287.2
22 840
289.2
22 970
290.0
21 310
298.8
λmax [nm]
Fluorescence Spectra of NXC6 and NXC5 in n-Hexane
and Acetonitrile at 25 °C. The fluorescence spectra of NXC6
and NXC5 in n-hexane and acetonitrile are shown in Figure 2.
The spectrum of NIC6 in n-hexane (Figure 2b) resembles that
of NMC6 (Figure 2a) and DMABN (Figure 1a), which means
that the replacement of the single methyl substituent at the amino
nitrogen of NMC6 by isopropyl does not lead to dual fluores-
cence and ICT in n-hexane at 25 °C, contrary to what is found
for the aminobenzonitriles in Figure 1. The introduction of a
tert-butyl group (NTC6), however, does in fact result in a
substantial broadening of the spectrum. This broadening and
red-shift of the fluorescence spectrum of NTC6 as compared
with that of NIC6 and NMC6 has become clearly larger in the
polar solvent acetonitrile (Figure 2c). The emission band of
NTC6 is strongly red-shifted as compared with NMC6 and
between the maxima of the absorption and fluorescence spectra
-
1
max
of 9000 cm , considerably larger than ∆ ν˜ (ST) for NIC6
-
1
-1
(5400 cm ) and NMC6 (5300 cm ). These spectral results
are interpreted as indications of the occurrence of an ICT
reaction in the case of NTC6, the interpretation of which will
be further elaborated in the following sections.
For the indolines NMC5, NIC5, and NTC5 in n-hexane and
acetonitrile (Figure 2d-f), the fluorescence spectra consist of
a single LE band, showing that when the size of the alicyclic
ring is reduced from six to five, an ICT reaction does not occur
upon replacing the isopropyl in NIC5 by a tert-butyl substituent
(NTC5). This reduction in ring size leads to an increase of
∆E(S1,S2), see Figure 2, as discussed in the previous section.1
The Energy Gap ∆E(S1,S2) and Intramolecular Charge
Transfer. The strong increase in the efficiency of the ICT
reaction in n-hexane for DIABN as compared with that for
2,13
NIC6, with a Stokes shift ∆ ν˜ max(ST) ) ν˜ (abs) - ν˜ (flu)
max
max
1708 J. AM. CHEM. SOC.
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VOL. 126, NO. 6, 2004

1-tert-Butyl-6-cyano-1,2,3,4-tetrahydroquinoline
A R T I C L E S
Table 2. Solvent Parameters and Fluorescence Maxima ν˜ max (in 1000 cm-1) for NTC6, ICT of NTC6, and LE for NIC6, NMC6, NTC5, NIC5,
NMC5
NTC6
NIC6
LE
NMC6
LE
NTC5
LE
NIC5
LE
NMC5
LE
solvent
n-hexane (1)
f(ꢀ) − f(n2)a
f(ꢀ) − 1/
2
f(n )
2 a
LE + ICT
ICT
0
0.092
0.097
0.102
0.113
0.168
0.177
0.192
0.207
0.253
0.266
0.281
0.292
0.308
0.307
0.319
0.326
0.366
0.375
0.383
0.393
0.393
28.21
28.00
28.07
27.87
27.09
26.97
26.86
26.67
26.10
25.20
25.01
24.73
24.34
24.90
24.19
24.08
23.85
23.75
23.53
23.13
22.60
(26.55)
(26.63)
(26.70)
(26.69)
26.89
26.86
26.70
26.52
26.05
25.04
24.87
24.66
24.34
24.80
24.19
24.08
23.85
23.75
23.53
23.13
22.60
28.56
28.52
28.53
28.40
28.26
28.25
28.22
28.20
28.07
27.73
27.71
27.65
27.59
27.65
27.54
27.53
27.39
27.37
27.29
27.24
27.21
29.06
28.17
28.20
28.06
cyclopentane (2)
n-hexadecane (3)
cis-decaline (4)
-0.001
-0.001
0.002
0.067
0.077
0.095
0.113
0.165
0.170
0.187
0.200
0.218
0.208
0.218
0.221
0.269
0.281
0.293
0.306
0.309
di(n-hexyl) ether (5)
di(n-pentyl) ether (6)
di(n-butyl) ether (7)
di(n-propyl) ether (8)
diethyl ether (9)
n-butyl acetate (10)
n-propyl acetate (11)
ethyl acetate (12)
27.88
27.90
27.86
27.66
27.57
27.56
27.55
27.39
28.61
28.40
27.68
27.41
27.94
27.99
27.99
27.28
27.30
27.21
27.05
27.05
26.99
27.05
27.01
27.06
methyl acetate (13)
tetrahydrofuran (14)
dichloromethane (15)
1
,2-dichloroethane (16)
n-butyl cyanide (17))
n-propyl cyanide (18)
ethyl cyanide (19)
acetonitrile (20)
27.01
26.75
27.56
26.49
26.66
methanol (21)
a
2
2
2
f(ꢀ) ) (ꢀ - 1)/(2ꢀ + 1), f(n ) ) (n - 1)/(2n + 1).
DMABN has been attributed to the decrease of the energy gap
E(S1,S2), which leads to a lowering of the energy of the ICT
state relative to the equilibrated LE(S1) state, as outlined
parameter f(ꢀ,n), see eqs 1-3, where ꢀ is the dielectric constant,
n is the refractive index, and F is the Onsager radius of the
∆
2
5,41,42
solute.
For an LE state, the applicable polarity function
1
2,13,30
2
1
2
above.
Inspection of the fluorescence and absorption
f(ꢀ,n) is f(ꢀ) - f(n ), whereas f(ꢀ,n) ) f(ꢀ) - /2f(n ) is used for
an ICT state.
4
1-45
spectra in Figure 2 shows that a similar situation holds for the
pairs NTC6/NIC6 and NTC6/NTC5. It is therefore concluded
that the reduction of ∆E(S1,S2) caused by the replacement of
methyl by isopropyl (DMABN/DIABN) as well as that of
isopropyl by tert-butyl (NIC6/NTC6) is the reason for the strong
enhancement of the ICT efficiency in the case of DIABN and
NTC6. A similar effect on ∆E(S1,S2) is brought about by the
increase of the alicyclic ring size from five (NTC5) to six
max
1
ν˜ (flu) ) -
µ (µ - µ )f(ꢀ,n) + const.
(1)
(2)
(3)
e
e
g
3
2hcF
(ꢀ - 1)
f(ꢀ) )
(2ꢀ + 1)
2
(NTC6).
(n - 1)
2
f(n ) )
Fluorescence Spectra of NXC6 and NXC5 as a Function
of Solvent Polarity at 25 °C. The fluorescence spectra of the
2
(
2n + 1)
three tetrahydroquinolines NXC6 and the three indolines NXC5
Solvatochromic Plots for NIC6 and NTC6. The maxima
ν˜ max(flu) of the fluorescence bands of NIC6 and NTC6 (Table
(X ) M, I, T) were measured at 25 °C in a series of solvents
with increasing polarity, from n-hexane to acetonitrile and
methanol, see Table 2. The red-shift of the spectra, gradually
becoming larger with increasing solvent polarity, is much larger
for NTC6 than for NIC6 and NMC6 and NXC5 (Table 2 and
Figure 2). These findings support our interpretation that of the
NXC6 and NXC5 compounds only NTC6 undergoes ICT. At
first sight, there is, however, no direct spectral indication for
two separate emission bands (dual fluorescence) in the spectra
of NTC6. By a solvatochromic analysis of the maxima of the
fluorescence bands,40 the excited-state dipole moment can be
determined, from which the ICT or LE character of an emission
spectrum can be decided. This analysis is presented for NXC6
and NXC5 in the next section.
2
) are plotted in Figure 3 against the solvent polarity parameter
1
2
f(ꢀ) - /2f(n ). It is seen that the solvent polarity dependence of
ν˜ (flu) is much more pronounced for NTC6 than for NIC6.
The maxima of NIC6 undergo a red-shift of 1350 cm with
increasing solvent polarity from n-hexane (28 560 cm ) to
max
-1
-1
-1
methanol (27 210 cm ). For NTC6, a much larger red-shift
-1
-1
(5610 cm ) is observed, from 28 210 cm in n-hexane to
-1
2
2 600 cm in methanol. For NMC6 and the three NXC5
max
molecules, only a relatively small red-shift of ν˜ (flu) with
increasing solvent polarity is found (Table 2), similar to that
obtained with NIC6.
Nonlinear Solvatochromic Plot for NTC6. Dual Fluores-
cence. Whereas a single straight line can be used to fit the data
points for NIC6 in Figure 3a, two straight lines with different
Solvatochromic Measurements. Excited-State Dipole Mo-
ments µe(LE) and µe(ICT) of NXC6 and NXC5 at 25 °C.
To determine the excited-state dipole moment µe of the
(
41) Liptay, W. In Excited states; Lim, E. C., Ed.; Academic Press: New York,
974; Vol. 1, p 129.
(42) Il’ichev, Yu. V.; K u¨ hnle, W.; Zachariasse, K. A. Chem. Phys. 1996, 211,
1
max
molecules NXC6 and NXC5, the energies ν˜ (flu) of the
441.
maxima of their emission bands are plotted against the polarity
(43) Schuddeboom, W.; Jonker, S. A.; Warman, J. M.; Leinhos, U.; K u¨ hnle,
W.; Zachariasse, K. A. J. Phys. Chem. 1992, 96, 10809.
(
40) Suppan, P.; Ghoneim, N. SolVatochromism; The Royal Society of Chem-
(44) Yoshihara, T.; Galievsky, V.; Druzhinin, S. I.; Saha, S.; Zachariasse, K.
istry: Cambridge, UK, 1997; Chapter 7.
A. Photochem. Photobiol. Sci. 2003, 2, 342.
J. AM. CHEM. SOC.
9
VOL. 126, NO. 6, 2004 1709

A R T I C L E S
Zachariasse et al.
Figure 3. Fluorescence emission maxima ν˜ max(flu) of (a) 1-isopropyl-6-
cyano-1,2,3,4-tetrahydroquinoline (NIC6) and (b) 1-tert-butyl-6-cyano-
1
,2,3,4-tetrahydroquinoline (NTC6) at 25 °C against the solvent polarity
1
2
parameter f(ꢀ) - /2f(n ), see eqs 1-3. The numbers refer to the solvents in
Table 2.
slopes are necessary to fit the data in the case of NTC6, see
Figure 3b. This nonlinear polarity dependence of ν˜ max(flu) shows
that the fluorescence spectrum of NTC6 consists of two bands.
The difference in polar character of the excited states responsible
for the fluorescence bands of NIC6 and NTC6 can be established
by determining their dipole moments, as will be discussed in
subsequent sections. The observation of a nonlinear solvato-
chromic plot for NTC6 is in accord with the conclusion
presented in the previous sections that ICT takes place with
NTC6, but not with NIC6, NMC6, and NXC5 molecules.
Nonlinear solvatochromic plots have been observed before, such
46
as with 6-cyanobenzoquinuclidine (CBQ) and the 6-(phenyl-
amino)-2-naphthalenesulfonate (6,2-ANS),47 interpreted in the
case of 6,2-ANS as originating from the presence of emission
bands from two excited states with different charge distributions.
LE Dipole Moments µe(LE) of NIC6, NMC6, and NXC5.
From the much smaller solvent polarity dependence of NIC6
as compared with NTC6 (Figure 3), it can already be concluded
that the excited state of NIC6 will have a relatively small dipole
moment and hence will be an LE state. Its dipole moment
µe(LE) is determined by plotting ν˜ (flu) against f(ꢀ) - f(n ),
see eqs 1-3. The same procedure is followed for NMC6 and
the three NXC5 compounds. The plots are presented in Figure
Figure 4. LE fluorescence emission maxima ν˜ max(LE) of 1-isopropyl-6-
cyano-1,2,3,4-tetrahydroquinoline (NIC6), 1-methyl-6-cyano-1,2,3,4-tet-
rahydroquinoline (NMC6), 1-tert-butyl-5-cyanoindoline (NTC5), 1-isopropyl-
5
-cyanoindoline (NIC5), and 1-methyl-5-cyanoindoline (NMC5) in a series
2
of solvents at 25 °C against the solvent polarity parameter f(ꢀ) - f(n ), see
eqs 1-3. The numbers refer to the solvents in Table 2.
max
2
dipole moments µe(LE) are determined: 10.8 ( 0.1 D (NIC6),
10.6 ( 0.2 D (NMC6), 10.7 ( 0.2 D (NTC5), 10.9 ( 0.2 D
(NIC5), and 10.4 ( 0.2 D (NMC5). These values are similar to
4. In all cases, a single straight line adequately fits the data
points. From the slope of these plots (Table 3), the following
4
3,44
that for µe(LE) of DMABN (10 D),
indicating that the
(
45) Beens, H.; Weller, A. Acta Phys. Pol. 1968, 34, 593.
46) K o¨ hler, G.; Rechthaler, K.; Grabner, G.; Luboradzki, R.; Suwi n´ ska, K.;
Rotkiewicz, K. J. Phys. Chem. A 1997, 101, 8518.
excited state of NIC6, NMC6, and NXC5 indeed has the
character of an LE state, meaning that ICT does not take place
with these molecules.
(
(
47) Dodiuk, H.; Kosower, E. M. J. Am. Chem. Soc. 1977, 99, 859.
1710 J. AM. CHEM. SOC.
9
VOL. 126, NO. 6, 2004

1-tert-Butyl-6-cyano-1,2,3,4-tetrahydroquinoline
A R T I C L E S
Table 3. Dipole Moments for the Ground State (µg), for the ICT State of NTC6, and for the LE State of NIC6, NMC6, NTC5, NIC5, and
NMC5 (µe), Derived from Solvatochromic Measurements, See Eqs 1-3
max
∆ ν˜ max(NTC6)/
∆ ν˜ max(model)
∆
ν˜ /∆f(ꢀ,n)
e g
µ (µ ) of
model [D]
1
substance (model)
band (model)
F [Å]
µ
g
[D]
[1000 cm-
]
e
µ [D]
NTC6
NTC6
NTC6
NTC6 (DIABN)
NTC6 (DMABN)
NTC6 (PP4C)
NTC6 (PP3C)
NTC6 (NIC6)
NIC6
NMC6
NTC5
NIC5
NMC5
ICT
total
total
ICT (ICT)
ICT (ICT)
ICT (ICT)
ICT (ICT)
ICT (LE)
LE
LE
LE
LE
LE
4.78
4.78
4.78
6.80
6.80
6.80
6.80
6.80
6.80
6.80
6.80
6.80
6.80
6.39
6.39
6.39
-17.47 ( 0.91
-18.01 ( 1.82
-16.75 ( 0.53
17.6 ( 0.4
17.8 ( 0.7
17.3 ( 0.2
19.2 ( 0.2
18.9 ( 0.2
18.5 ( 0.2
17.5 ( 0.2
16.8 ( 0.1
10.8 ( 0.1
10.6 ( 0.2
10.7 ( 0.2
10.9 ( 0.2
10.4 ( 0.2
a
b
0.945 ( 0.018
0.881 ( 0.023
0.614 ( 0.016
0.607 ( 0.015
3.52 ( 0.007
19.2 (6.78)
17 (6.6)
18.3 (3.18)
17.4 (3.56)
10.9 (6.80)
4.67
4.32
4.67
4.56
4.44
-4.27 ( 0.16
-5.02 ( 0.38
-4.61 ( 0.33
-5.21 ( 0.33
-4.75 ( 0.32
a
Total band, not subtracted, in polar solvents 9-20, see Table 2, mainly ICT emission. b Total band, not subtracted, in solvents 1-20, see Table 2.
Table 4. Fluorescence Quantum Yields Φ(LE) of NMC6, NIC6,
NMC5, NIC5, and NTC5 at 25 °C
Table 5. Fluorescence Quantum Yields and ICT/LE Quantum
Yield Ratios Φ′(ICT)/Φ(LE) of NTC6 at 25 °C
solvent
NMC6
NIC6
NMC5
NIC5
NTC5
solvent
Φ(total)
Φ′(ICT)
Φ(LE)
Φ′(ICT)/Φ(LE)
n-hexane (1)
0.244
0.228
0.188
0.191
0.205
0.223
0.230
n-hexane (1)
cyclopentane (2)
n-hexadecane (3)
cis-decaline (4)
0.22
0.086
0.134
0.64
0.79
0.91
0.87
2.3
2.8
2.9
3.6
6.8
14
diethyl ether (9)
tetrahydrofuran (14)
n-propyl cyanide (18)
acetonitrile (20)
0.237
0.233
0.236
0.22
0.105
0.115
0.229
0.196
0.229
0.202
di(n-hexyl) ether (5)
di(n-pentyl) ether (6)
di(n-butyl) ether (7)
di(n-propyl) ether (8)
diethyl ether (9)
n-butyl acetate (10)
n-propyl acetate (11)
ethyl acetate (12)
methyl acetate (13)
tetrahydrofuran (14)
toluene (23)
0.43
0.39
0.317
0.290
0.113
0.100
In accordance with this conclusion, the fluorescence quantum
yields of NMC6, NIC6, and the three NXC5 molecules do not
strongly depend on solvent polarity when going from n-hexane
or diethyl ether to acetonitrile, see Table 4, in contrast to what
0.51
0.58
0.445
0.065
16
20
27
17
0.552
0.548
0.028
0.032
8,9,28
is observed with dual fluorescent molecules such as DMABN.
0.58
0.43
Spectral Subtraction of Overlapping LE and ICT Bands
of NTC6 and Dipole Moment µe(ICT). From the slope of the
line for the solvents diethyl ether to acetonitrile in the solva-
tochromic plot for NTC6 (Figure 3b and eq 1), a dipole moment
µe of 17.8 ( 0.7 D is obtained (Table 3). On the basis of this
value of 18 D and the nonlinear character of the solvatochromic
plot of NTC6, it is therefore concluded that the fluorescence
spectra of NTC6 consist of contributions from an LE and an
ICT state. These LE and ICT emissions can be separated by
spectral subtraction with NIC6 as the model compound, as
shown in Figure 5. This molecule only differs from NTC6 in
the N-alkyl substituent, and its emission spectrum consists of a
single LE band (Figures 2 and 4). It is seen from Table 5 that
the relative contribution of ICT fluorescence to the total emission
spectrum, the ICT/LE fluorescence quantum yield ratio Φ′(ICT)/
Φ(LE), of NTC6 at 25 °C increases when the solvent polarity
becomes larger, from 0.64 in n-hexane, via 17 in tetrahydrofuran,
to 121 in acetonitrile and 192 in methanol.
dichloromethane (15)
52
50
43
57
72
1,2-dichloroethane (16)
n-butyl cyanide (17)
n-propyl cyanide (18)
ethyl cyanide (19)
acetonitrile (20)
0.58
0.55
0.54
0.570
0.542
0.536
0.010
0.075
0.004
121
192
methanol (21)
points are the maxima of composite LE and ICT bands. It is
then clear that the presence of the LE emission band with a
smaller solvatochromic slope µ (µ - µ ) (eq 1) reduces the
effective slope of this plot. For this reason, the solvatochromic
analysis to determine µ (ICT) of NTC6 in the previous section
was limited to the solvents diethyl ether to acetonitrile.
ICT Dipole Moment µe(ICT) of NTC6 Relative to DIABN,
DMABN, PP4C, and PP3C. The ICT dipole moment of NTC6
can also be obtained from a plot of ν˜ (ICT) against the ICT
maxima of a model compound such as DIABN, DMABN, N-(4-
e
e
g
e
max
The data for ν˜ max(ICT) of the separated ICT band of NTC6
cyanophenyl)pyrrole (PP4C), or N-(3-cyanophenyl)pyrrole
3
0,44
in the solvents n-hexane to tetrahydrofuran are listed in Table
(PP3C),
see Figure 7, Table 6, and ref 44 for PP3C. By
max
2
. For the other solvents, ν˜ (ICT) does not differ from
employing this procedure, the scatter in the data points as
appearing in Figure 3b, generally observed when plotting ν˜
(ICT) against the solvent polarity parameter f(ꢀ) - / f(n ),
max
max
ν˜ (flu) (Table 2). From a solvatochromic plot of these data
-
max
1
2
1
2 25,30,42,44
for ν˜ (ICT) against f(ꢀ) - /2f(n ) in Figure 6, a dipole moment
2
µe(ICT) of 17.6 ( 0.4 D is determined, see Table 3.
is reduced by mutually compensating the specific solute/solvent
3
0
The increase in the slope of the solvatochromic plot for NTC6
for solvents more polar than diethyl ether as compared with
the less polar solvents (Figure 3b) hence means that for the
more polar solvents, the major contribution to the overall
fluorescence spectrum comes from the ICT state. For the
solvents between n-hexane and diethyl ether, however, the data
interactions.
The slope of the plot in Figure 7a is equal to the expression
µe(ICT)(µe(ICT) - µg(FC))/F (see eq 1) for NTC6 divided by
that for DMABN. From this slope (0.88 ( 0.02), an ICT dipole
moment µe(ICT) of 18.9 ( 0.2 D is calculated for NTC6 (Table
3), based on the dipole moment of 17 D for the ICT state of
3
J. AM. CHEM. SOC.
9
VOL. 126, NO. 6, 2004 1711

A R T I C L E S
Zachariasse et al.
Figure 5. Fluorescence spectra of 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6) at 25 °C. HEX, n-hexane; DBE, di(n-butyl) ether; DEE, diethyl
ether; THF, tetrahydrofuran; PrCN, n-propyl cyanide; MeCN, acetonitrile. The separate emissions from the LE and ICT states are shown. In the spectral
subtraction, 1-isopropyl-6-cyano-1,2,3,4-tetrahydroquinoline (NIC6) is used as a model compound for the LE state.
DMABN,43 which has been determined by the TRMC method.
max
From the ν˜ (ICT) plot of NTC6 versus DIABN (Figure 7b),
a value of 19.2 ( 0.2 D results for µe(ICT) of NTC6. Similar
max
values are obtained from plots of the ν˜ (ICT) data of NTC6
against those of PP4C and PP3C, see Table 3. In this method
(see Figure 7), the impact of the Onsager radius F (eq 1)
obviously is much smaller than when the ratio µe(ICT)(µe(ICT)
3
-
µg)/F is used directly in the dipole moment determination
(Figures 3b and 6).
The value of 19 D for µe(ICT) of NTC6 indicates that the
strongly Stokes-shifted main fluorescence band of NTC6 in
solvents more polar than diethyl ether indeed originates from
an ICT state, with a similar dipole moment and hence intramo-
lecular charge separation as that observed with the not-planarized
molecules DIABN, DMABN, and PP4C.
Fluorescence Quantum Yield Ratio Φ′(ICT)/Φ(LE) of
NTC6 in Dibutyl Ether as a Function of Temperature. The
fluorescence spectra of NTC6 in a number of solvents were
measured as a function of temperature. As an example, the ICT/
LE fluorescence quantum yield ratio Φ′(ICT)/Φ(LE), see eq 4,
was determined for NTC6 in di(n-butyl) ether (DBE) from 140
Figure 6. Fluorescence emission maxima ν˜ max(ICT) of 1-tert-butyl-6-cyano-
,2,3,4-tetrahydroquinoline (NTC6) at 25 °C against the solvent polarity
parameter f(ꢀ) - /2f(n ), see eqs 1-3. The numbers refer to the solvents in
Table 2.
1
°
C down to -90 °C (Figure 8). NIC6, only showing LE
1
2
emission over the entire temperature range, was again used as
1712 J. AM. CHEM. SOC.
9
VOL. 126, NO. 6, 2004

1-tert-Butyl-6-cyano-1,2,3,4-tetrahydroquinoline
A R T I C L E S
Figure 8. Plot of the natural logarithm of the LE/ICT fluorescence quantum
yield ratio Φ′(ICT)/Φ(LE) versus the reciprocal absolute temperature for
1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6) in di(n-butyl) ether
(DBE). The activation energies of the LE f ICT (Ea) and ICT f LE (Ed)
reactions and the enthalpy difference -∆H between the LE and ICT states
are indicated in the Figure, see Schemes 1 and 2. The line through the data
points represents the fitting of the data with eq 4.
Scheme 1
Scheme 2
Figure 7. ICT fluorescence emission maxima ν˜ max(ICT) of 1-tert-butyl-
-cyano-1,2,3,4-tetrahydroquinoline (NTC6) at 25 °C against the corre-
sponding data of (a) 4-(dimethylamino)benzonitrile (DMABN) and (b)
6
4
-(diisopropylamino)benzonitrile (DIABN), see eqs 1-3. The numbers refer
to the solvents in Table 2.
max
Table 6. Solvent Parameters and Fluorescence Maxima ν˜
ICT) (in 1000 cm- ) for DIABN, DMABN, and PP4C
1
(
solvent
f(ꢀ) − 1/
2
f(n2)
DIABN
DMABN
PP4C
n-hexane (1)
n-hexadecane (3)
0.092
0.102
0.177
0.192
0.207
0.253
0.266
0.281
0.292
0.308
0.307
0.319
0.326
0.366
0.375
0.383
0.393
0.393
25.73
25.57
24.40
24.34
24.17
23.53
22.68
22.38
22.26
21.95
22.37
21.78
21.68
21.30
21.10
20.88
20.49
a
a
29.30
29.09
26.83
26.64
26.24
25.17
23.49
23.26
23.15
22.66
23.30
23.40
23.05
di(n-pentyl) ether (6)
di(n-butyl) ether (7)
di(n-propyl) ether (8)
diethyl ether (9)
n-butyl acetate (10)
n-propyl acetate (11)
ethyl acetate (12)
methyl acetate (13)
tetrahydrofuran (14)
dichloromethane (15)
(25.20)
(24.50)
23.70
22.40
22.11
21.96
21.70
22.15
22.77
22.44
20.84
21.06
20.84
20.36
19.61
It is seen from the Stevens-Ban plot for NTC6 in DBE in
Figure 8 that Φ′(ICT)/Φ(LE) becomes larger upon lowering the
temperature, without reaching a maximum, where kd ) 1/τo′.21
This means that NTC6 in DBE is in the high-temperature limit
48-50
(HTL), for which kd . 1/τo′.
In the case of DMABN, the
HTL condition is encountered in solvents such as toluene. For
1
,2-dichloroethane (16)
n-butyl cyanide (17))
n-propyl cyanide (18)
ethyl cyanide (19)
acetonitrile (20)
this system with a relatively small -∆H (8 kJ/mol), kd ) 45 ×
9
-1
9
-1
11,21
21.60
21.32
20.98
10 s and 1/τo′ ) 0.4 × 10 s at 20 °C.
The potential energy surfaces and energies describing the
LE h ICT reaction are depicted in Scheme 2, where E(S1) is
the energy of the S1 state, Ea and Ed are the activation energies
of the ICT reaction rate constants ka and kd (Scheme 1), -∆H
methanol (21)
a
No ICT emission.
(
) Ed - Ea) is the difference in enthalpy between the LE and
the model compound for LE in the spectral subtraction
procedure.
max
ICT states, ν˜ (ICT) is the energy of the maximum of the ICT
fluorescence band, and δEFC is the energy, relative to that of
S0, of the Franck-Condon state reached by fluorescence from
the ICT state. The reaction coordinate ê involves changes in
Φ′(ICT)/Φ(LE) ) k ′/k {k /(k + 1/τ ′)}
(4)
f
f
a
d
o
In eq 4 and Scheme 1, ka and kd are the rate constants of the
forward and backward ICT reaction, τo(LE) and τo′(ICT) are
the fluorescence lifetimes, and kf(LE) and kf′(ICT) are the
radiative rate constants.
(48) Birks, J. B. Photophysics of Aromatic Molecules; Wiley: London, 1970.
(49) Zachariasse, K. A.; Busse, R.; Duveneck, G.; K u¨ hnle, W. J. Photochem.
1985, 28, 237.
(50) Zachariasse, K. A. Trends Photochem. Photobiol. 1994, 4, 211.
J. AM. CHEM. SOC.
9
VOL. 126, NO. 6, 2004 1713

A R T I C L E S
Zachariasse et al.
fluorescence spectra, -∆H ) 13 kJ/mol and ν˜ max(ICT) ) 26 700
Table 7. The Energy E(S1) of the S1 State, the Activation
Energies Ea and Ed of the Forward and Backward ICT Reactions
-
1
cm (Table 2) in eq 5, it follows that δEFC ) 31 kJ/mol, see
Table 7. For the more flexible DMABN, δEFC is considerably
(
Schemes 1 and 2), the Difference in Enthalpy -∆H between the
max
LE and ICT States, the ICT Emission Maximum ν˜ (ICT) at 25
°
NTC6 in Di(n-butyl) Ether
E(S
63
C, and the Energy δEFC of the Franck-Condon Ground State for
11
larger (55 kJ/mol), which is the cause of the appearance of
a
dual fluorescence with two clearly separated LE and ICT
maxima, see Figure 1. The relatively small δEFC value of NTC6
therefore is the reason for its strongly overlapping LE and ICT
emission bands in, for example, n-hexane and DBE, see Figures
ν˜ max(ICT)
δEFC
1
)
E
a
E
d
−∆H
3
1
14
13
319
31
a
All data are in kJ/mol. See Figure 8 and Scheme 2.
2
and 5.
LE and ICT Fluorescence Decays in Diethyl Ether at -70
°
C. The LE and ICT fluorescence decays if(LE) and if(ICT) of
NTC6 were measured in diethyl ether at -70 °C. The experi-
ments were carried out at wavelengths at which only LE (350
nm) or only ICT (470 nm) fluorescence occurs, see the
fluorescence and absorption spectra in Figure 9. A global
5
1
analysis of the LE and ICT fluorescence decays is presented
in Figure 10. The decays are double exponential, as is to be
expected with two excited species (Scheme 1). The two LE
decay times are 7 ps and 8.54 ns, with relative contributions of
2
% and 98%. The ICT fluorescence grows in with the same
time of 7 ps, and the amplitude ratio -A22/A21 (eq 7) has a
value close to unity (0.96), which means that the ICT state of
NTC6 is formed from the primarily excited LE state and not
Figure 9. Fluorescence and absorption spectra at -70 °C in diethyl ether
DEE). The fluorescence spectrum consists of emissions from an LE and
an ICT state. The arrows indicate the excitation (276 nm) and emission
350 and 470 nm) wavelengths used in Figure 10.
(
21
by direct excitation of the ground state.
(
From the decay times τ2 and τ1 together with the amplitude
ratio A (eqs 6-8) and the lifetime τ0 of a model compound, the
ICT rate constants ka and kd and the lifetime τ0′(ICT) can be
bond lengths and bond angles as well as in the orientation of
solvent molecules occurring during the ICT reaction. It follows
from Scheme 2 that
11,21
determined,
see Table 8. The fluorescence lifetime of NIC6
in diethyl ether at -70 °C (3.09 ns) is taken for τ0, as dual
emission is not observed with this system (Figures 2 and 5).
max
E(S ) ) -∆H + ν˜ (ICT) + δEFC
(5)
1
By fitting the results for Φ′(ICT)/Φ(LE) in Figure 8 with eq
, we obtained an enthalpy difference -∆H of 13 kJ ( 1 kJ/
i
f
(LE) ) A11 exp(-t/τ
1
) + A12 exp(-t/τ
2
)
(6)
(7)
(8)
4
i (ICT) ) A exp(-t/τ )+ A exp(-t/τ )
f
21
1
22
2
mol, see Table 7. Inserting the data (Table 7) for E(S1) ) 30 380
-
1
A ) A /A
12 11
cm , determined from the crossing of the absorption and
Figure 10. LE and ICT fluorescence response functions of 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6) in diethyl ether (DEE) at -70 °C. The
LE and ICT decays are analyzed simultaneously (global analysis). The decay times (τ2,τ1) and their preexponential factors A1i and A2i are given (eqs 6-8).
2
The weighted deviations, expressed in σ (expected deviations), the autocorrelation functions A-C, and the values for ø are also indicated. Time resolution:
2
ps/chnl. Excitation wavelength: 276 nm. Emission wavelength: 350 nm (LE) and 470 nm (ICT), see Figure 9.
1714 J. AM. CHEM. SOC.
9
VOL. 126, NO. 6, 2004

1-tert-Butyl-6-cyano-1,2,3,4-tetrahydroquinoline
A R T I C L E S
Table 8. Data for NTC6 in Diethyl Ether at -70 °C; Fluorescence Quantum Yields Φ(LE) and Φ′(ICT), Decay Times τ2 and τ1 (eqs 6 and
) and τ0, Amplitude Ratio A (eq 8), Rate Constants of the Forward (ka) and Backward (kd) ICT Reactions, ICT Fluorescence Lifetime
τ0′(ICT), and the LE and ICT Radiative Rate Constants kf(LE) and kf′(ICT), See Scheme 2 and Figure 10
7
Φ′(ICT)/
Φ(LE)
τ
[ns]
2
τ
[ns]
1
τ
[ns]
0
k
a
k
d
τ
0
′(ICT)
[ns]
τ(LE/ICT)d
[ns]
k
[10
f
(LE)
k
[10
f
′(ICT)
k
f
′(ICT)/
Φ(LE)
Φ′(ICT)
A
[1011 s-1
]
[109 s-1
]
8
s-1
]
8
s-1
]
k′(LE)
f
0
.043
0.55
12.8
0.007
8.54
22
3.09a
1.3
5.9
9.29
0.152
1.3b
0.6c
0.53
a
Fluorescence decay time τ0 of NIC6 in diethyl ether at -70 °C. b From Φ(LE) and eq 9, see Scheme 2. c From Φ′(ICT) and eq 10, see Scheme 2.
d
From a deconvolution of the ICT fluorescence decay with that of LE, see text.
With τ2 ) 7 ps, τ1 ) 8.54 ns, A ) 21.9, and τ0 ) 3.09 ns
Conclusions
1
1
(
s
NIC6, Table 8), values for the rate constant ka of 1.3 × 10
With the planarized tetrahydroquinoline NTC6, ICT and dual
fluorescence is observed in all solvents investigated, from the
nonpolar n-hexane to the polar acetonitrile and methanol over
extended temperature ranges. This is in contrast to the observa-
tion of a single LE emission with two other tetrahydroquinolines
having a methyl (NMC6) or an isopropyl (NIC6) amino
substituent, as well as with the three indolines NMC5, NIC5,
and NTC5. The appearance of an ICT state with a large dipole
moment (19 D) in the case of NTC6 is attributed to a decrease
of the energy gap ∆E(S1,S2), as compared with the other
molecules NXC6 and NXC5. A similar reduction of ∆E(S1,S2)
and a corresponding increase in ICT efficiency caused by the
introduction of a tert-butyl substituent was found for tBMABN
as compared with IMABN and DMABN. The observation of
dual fluorescence with NTC6 shows that fast and efficient ICT
is possible with aminobenzonitriles that cannot undergo a
perpendicular twist and electronic decoupling of their amino
group.
The results obtained here with NTC6 make clear that the
absence of ICT in model compounds such as NMC6 does not
mean that the ICT state of aminobenzonitriles has a TICT
structure. In the aminobenzonitriles with close-lying S1(Lb) and
S2(La,ICT) states, the magnitude of ∆E(S1,S2) determines
whether ICT does occur or not. Therefore, seemingly small
modifications of the molecular structure, such as the introduction
of an alicyclic ring (NMC6) or the replacement of an amino
methyl substituent by hydrogen (MABN), can prevent ICT by
increasing the energy gap ∆E(S1,S2).
-
1
9 -1
and kd of 5.9 × 10 s are obtained. The lifetime τ0′(ICT)
equals 9.29 ns. From this analysis, kd + 1/τ0′(ICT) ) 164 ps is
obtained, in good agreement with the result (153 ps) of a
deconvolution of the ICT fluorescence decay with that of LE,
44,52
see Table 8. The fact that this decay time is single exponential
is in accordance with the presence of two excited-state species,
LE and ICT, see Scheme 1.
Radiative Rate Constants kf(LE) and kf′(ICT). The radia-
tive rate constants kf(LE) and kf′(ICT) of NTC6 can be calculated
(eqs 9 and 10) from the fluorescence quantum yields Φ(LE)
and Φ′(ICT), determined in diethyl ether at -70 °C, see Table
8
.
k (LE) ) Φ(LE){1/τ + 1/τ ′(k /(k + 1/τ ′)}
(9)
f
0
0
a
d
0
k ′(ICT) ) Φ′(ICT){1/τ ′ + 1/τ ((k + 1/τ ′)/k )} (10)
f
0
0
d
0
a
From these data and ka, kd, τ0′(ICT), and τ0(NIC6) (Table 8
and previous section), the following radiative rates of NTC6 in
8
diethyl ether at -70 °C are obtained: kf′(ICT) ) 0.59 × 10
-1
8
-1
s
and kf(LE) ) 1.3 × 10 s . These values are similar to
those of other dual fluorescent 4-aminobenzonitriles such as
11
DMABN. Our finding that kf′(ICT) is smaller than kf(LE) also
in the case of the planarized NTC6 indicates that the decrease
in the value of the ICT radiative rate constant is not necessarily
connected with an electronic decoupling of the amino and
6-10
benzonitrile moieties, as postulated in the TICT model.
This
decrease is attributed to the larger increase in CT character
relative to the ground state of the ICT than of the LE state.
This explanation is similar to that presented for the relatively
small value of the radiative rate of intermolecular exciplexes
Acknowledgment. The support of the Volkswagen Founda-
tion (Project Intra- and Intermolecular Electron Transfer) is
gratefully acknowledged. Dr. Attila Demeter is thanked for
measuring fluorescence spectra of DIABN. We also thank Mr.
J u¨ rgen Bienert and Mr. Helmut Lesche for technical support.
1
-
+ 7,53,54
(
A D ).
(
51) Striker, G. In DeconVolution and ReconVolution of Analytical Signals;
Bouchy, M., Ed.; University Press: Nancy, France, 1982; p 329.
52) Conte, J. C.; Martinho, J. M. G. Chem. Phys. Lett. 1987, 134, 350.
53) Beens, H.; Weller, A. In Organic Molecular Photophysics; Birks, J. B.,
Ed.; Wiley: London, 1975; Vol. 2, Chapter 4.
Supporting Information Available: Synthetic procedures,
table of solvent properties (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.
(
(
(54) Gould, I. R.; Young, R. H.; Mueler, L. J.; Albrecht, A. C.; Farid, S. J. Am.
Chem. Soc. 1994, 116, 3147.
JA037544W
J. AM. CHEM. SOC.
9
VOL. 126, NO. 6, 2004 1715