Intramolecular NH/π complexes of 2-allylaniline derivatives in the ground and excited states

Article abstract of DOI:10.1021/jp046031o

The photochemical and photophysical properties of three 8-allyl-l,2,3,4-tetrahydroquinolines (1a-c) have been studied. These compounds exhibit a 2-allylaniline-like photochemical behavior, undergoing photocyclization to lilolidines (3a-c). The absorption,

Full text of DOI:10.1021/jp046031o

1758
J. Phys. Chem. A 2005, 109, 1758-1763
Intramolecular NH/π Complexes of 2-Allylaniline Derivatives in the Ground and Excited
States
Edgar A. Leo,† Rosa Tormos,† Sandra Monti,‡ Luis R. Domingo,§ and Miguel A. Miranda*,†
Departamento de Qu´ımica-Instituto de Tecnolog´ıa Qu´ımica UPV-CSIC, UniVersidad Polite´cnica de Valencia,
Camino de Vera s/n, 46022 Valencia, Spain, Istituto per la Sintesi Organica e la FotoreattiVita` (ISOF), CNR,
Via Piero Gobetti 101, I-40129 Bologna, Italy, and Departamento de Qu´ımica Orga´nica, Instituto de Ciencia
Molecular, UniVersidad de Valencia, Edificio Jeromi Mun˜oz, C/ Dr. Moliner, 50, 46100 Burjassot,
Valencia, Spain
ReceiVed: September 2, 2004; In Final Form: December 27, 2004
The photochemical and photophysical properties of three 8-allyl-1,2,3,4-tetrahydroquinolines (1a-c) have
been studied. These compounds exhibit a 2-allylaniline-like photochemical behavior, undergoing photocy-
clization to lilolidines (3a-c). The absorption, emission, and excitation spectra of 1a-c, employing convenient
model compounds for comparison, demonstrate the formation of a NH/π intramolecular ground-state complex
(AB). This species can absorb light at long wavelengths (330-340 nm), giving rise to the corresponding
excited complex AB*. Emission from AB* is red-shifted (420 nm) with respect to that observed when the
monomer A is excited (λexc ) 300 nm). These experimental results have been rationalized by means of density-
funtional theory calculations.
Introduction
SCHEME 1
SCHEME 2
The photochemistry of 2-allylanilines has attracted consider-
able attention for the last three decades.1-9 The main photore-
action reported is photocyclization to five- and/or six-membered
ring compounds (Scheme 1), depending on the allyl substitution.
It was initially suggested that the photoprocess occurs via
intramolecular electron transfer, with involvement of the first
excited singlet state. This hypothesis has recently been con-
firmed by photochemical and photophysical studies on N-acyl
derivatives.8 The primary charge-separated intermediates un-
dergo intramolecular proton transfer, leading to biradicals which
immediately collapse to the final cyclic products (Scheme 2).
As the amino group and the allyl system are the two
substructures participating in photocyclization, their intramo-
lecular interaction (both in the ground and in the excited states)
should be a key factor influencing the photophysical and
photochemical properties of this group of compounds. Such
interaction has been evidenced by IR spectroscopy and theoreti-
cally analyzed by semiempirical calculations.10,11 Similar studies
have been performed on the OH/π interaction in 2-allyl-
phenols.11-13
To our knowledge, the photophysical characterization of the
XH/π-associated species constitutes a matter of further inves-
tigation. Thus, it would be interesting to know whether this type
of species exhibit characteristic absorption bands allowing their
selective excitation. If so, their emission properties could be
different from those of the isolated aniline-like chromophore,
in the case that some XH/π association remains in the excited
state.
cence measurements looking at both the emission and the
excitation spectra. To limit the conformational freedom of the
amino group, 8-allyl-1,2,3,4-tetrahydroquinolines 1a-c (Chart
1) have been selected as substrates.
As a result of this effort, we have found evidence for a
ground-state complex (AB in Scheme 3) that upon light
absorption gives rise to an excited species (AB*). The spectra
of these species (excitation of AB and emission of AB*) are
considerably red-shifted with respect to those of model com-
pounds unable to participate in intramolecular NH/π interactions.
The existence of such a ground-state complex and its lower
excitation energy are supported by density-functional theory
(DFT) theoretical calculations.
The aim of the present work has been to undertake a
systematic investigation of this problem by means of fluores-
* To whom correspondence should be addressed. E-mail:
mmiranda@qim.upv.es.
† Universidad Politecnica de Valencia.
‡ Istituto per la Sintesi Organica e la Fotoreattivita.
§ Universidad de Valencia.
10.1021/jp046031o CCC: $30.25 © 2005 American Chemical Society
Published on Web 02/10/2005

Intramolecular NH/π Complexes
J. Phys. Chem. A, Vol. 109, No. 9, 2005 1759
CHART 1
Computational Methods. DFT calculations were carried out
using the B3LYP18 exchange-correlation functional, together
with the standard 6-31G** and 6-31+G** basis set.19 In
addition, HF/6-31G**, HF/6-31+G**, and MP2/6-31G** op-
timizations were performed.19 The optimizations were carried
out using the Berny analytical gradient optimization method.20
All calculations were carried out with the Gaussian 98 suite of
programs.21 Vertical energies of the singlet-excited state were
calculated using the time-dependent (TD) DFT method.22
8-Allyl-2-methyl-1,2,3,4-tetrahydroquinoline (1b). 1H NMR
(δ, ppm): 1.17 (d, J 6.0 Hz, 3H), 1.45-1.61 (m, 1H), 1.82-
1.93 (m, 1H), 2.65-2.90 (m, 2H), 3.20 (d, J 6.0 Hz, 2H), 3.40
(m, 1H), 3.70 (m, 1H), 5.03-5.17 (m, 2H), 5.83-6.00 (m, 1H),
6.56 (t, J 7.0 Hz, 1H), 6.79-6.89 (m, 2H). 13C NMR (δ, ppm):
23.1 (CH3), 27.4 (CH2), 30.3 (CH2), 36.7 (CH2), 47.7 (CH),
116.4 (CH2), 116.9 (CH), 121.4 (C), 122.8 (C), 127.9 (CH),
128.1 (CH), 136.7 (CH), 143.3 (C). MS (m/z, (%)): 187 (65),
172 (100). Exact mass (EI): Calcd for C13H17N: 187.1361
(M+). Found: 187.1366.
SCHEME 3
1-Allyl-2-methyl-1,2,3,4-tetrahydroquinoline (2b). 1H NMR
(δ, ppm): 1.14 (d, J 6.5 Hz, 3H), 1.70-1.95 (m, 2H), 2.60-
2.90 (m, 2H), 3.48 (m, 1H), 3.75-3.98 (m, 2H), 5.07-5.24
(m, 2H), 5.84 (m, 1H), 6.47-6.58 (m, 2H), 6.92-7.06 (m, 2H).
13C NMR (δ, ppm): 19.7 (CH3), 24.5 (CH2), 28.6 (CH2), 52.9
+ 52.5 (CH + CH2), 111.5 (CH), 115.9 + 115.6 (CH2 + CH),
122.1 (C), 127.4 (CH), 129.2 (CH), 135.2 (CH), 144.9 (C). MS
(m/z, (%)): 187 (32), 172 (100), 130 (32). Exact mass (EI):
Calcd for C13H17N: 187.1361 (M+). Found: 187.1357.
cis-2-Methyl-4-methyl-1,2,3,4-tetrahydro-4H-pyrrolo[3,2,1-
Experimental Section
Chemicals. Compounds 1a-c (see structures in Chart 1) were
obtained through amino-Claisen rearrangement of 2a-c with
ZnCl2 in p-xylene at 130 °C, following a previously described
procedure.14 1-Allyl-2-alkyl-1,2,3,4-tetrahydroquinolines 2a-c
were prepared by reaction between 2-alkyl-1,2,3,4-tetrahydro-
quinolines and allyl bromide, employing the standard method.3
Compounds 3a-c were obtained as described in the general
irradiation procedure.
1
ij]quinoline (cis-3b). H NMR (δ, ppm): 1.24 (d, J 6.5 Hz,
3H), 1.44 (d, J 6.0 Hz, 3H), 1.65-1.85 (m, 2H), 2.56-2.68
(m, 3H), 2.98-3.12 (m, 2H), 3.53-3.65 (m, 1H), 6.57 (t, J 7.5
Hz, 1H), 6.81 (d, J 7.5 Hz, 1H), 6.87 (d, J 7.5 Hz, 1H). 13C
NMR (δ, ppm): 22.7 (CH3), 23.5 (CH3), 30.8 (CH2), 38.8
(CH2), 43.0 (CH2), 52.5 (CH), 63.4 (CH), 117.8 (CH), 119.9
(C), 121.8 (CH), 126.2 (CH), 127.6 (C), 150.6 (C). MS (m/z,
(%)): 187 (10), 172 (28), 83 (24), 71 (35), 69 (44), 57 (100).
Exact mass (EI): Calcd for C13H17N: 187.1361 (M+). Found:
187.1339.
Isolation and purification were done by conventional column
chromatography on silica gel, using hexane/dichloromethane as
eluent, or by means of isocratic high-performance liquid
chromatography (HPLC) with a semipreparative column, using
hexane/ethyl acetate as eluent.
trans-2-Methyl-4-methyl-1,2,3,4-tetrahydro-4H-pyrrolo-
Compounds 1a,15 1c,9 2a,15 2c,9 3a,16 cis-3c,9 and trans-3c9
were known, and their structures were confirmed by comparison
with the spectroscopic data reported in the literature. The
1
[3,2,1-ij]quinoline (trans-3b). H NMR (δ, ppm): 1.11 (d, J
6.5 Hz, 3H), 1.15 (d, J 6.5 Hz, 3H), 1.76-1.89 (m, 1H), 1.96-
2.08 (m, 1H), 2.54 (dd, J1 8.5 Hz, J2 6.5 Hz, 1H), 2.67 (t, J 6.5
Hz, 2H), 3.07 (dd, J1 8.5 Hz, J2 7.0 Hz, 1H), 3.40 (m, 1H),
3.86 (m, 1H), 6.54 (t, J 7.5, 1H), 6.80 (d, J 7.5 Hz, 1H), 6.88
(d, J 7.5 Hz, 1H). 13C NMR (δ, ppm): 16.9 (CH3), 17.2 (CH3),
22.9 (CH2), 31.0 (CH2), 37.5 (CH2), 46.5 (CH), 57.3 (CH), 117.5
(CH), 118.7 (C), 122.5 (CH), 126.5 (CH), 127.9 (C), 148.4 (C).
MS (m/z, (%)): 187 (7), 172 (26), 125 (22), 123 (38), 119 (31),
97 (66), 83 (67), 71 (43), 69 (91), 57 (100). Exact mass (EI):
Calcd for C13H17N: 187.1361 (M+). Found: 187.1339.
unknown compounds were fully characterized by H and 13C
1
NMR spectra, which were recorded at 300 and 75 MHz,
respectively. The relative arrangement of the substituents for
3b (trans or cis) was unambiguously determined by means of
nuclear Overhauser effect (NOE) measurements (see Supporting
Information).
General Irradiation Procedure. Thoroughly deoxygenated
solutions of 100 mg of the substrate in 100 mL of HPLC-grade
acetonitrile were irradiated at room temperature through quartz
with a multilamp photoreactor, equipped with eight lamps with
a maximum emission at 254 nm (monochromatic) or 300 nm
(Gaussian distribution). The reaction course was followed by
means of gas chromatography mass spectroscopy (GC/MS) and
1H NMR. Photoreaction quantum yields were determined using
phenylglyoxylic acid as actinometer.17
Fluorescence Measurements. Emission and excitation spec-
tra were recorded on a standard spectrofluorimeter provided with
a monochromator in the 200-900-nm wavelength range. The
samples were placed into 10 × 10 mm2 Suprasil quartz cells
with a septum cap. The solutions were purged with argon for
15 min before measurements. The absorbance of the samples
at the excitation wavelength was kept below 0.3.
Results and Discussion
To ascertain whether 1a-c exhibit a photochemical behavior
similar to that of the more simple 2-allyanilines, they were
irradiated using 254- or 300-nm light, in acetonitrile solution,
under argon atmosphere. The photoproducts were isolated by
column chromatography. They were found to be the cis- and
trans-lilolidines 3 (Chart 1); their stereochemical assignment
was achieved by means of NOE experiments.
The photoproducts distribution after 30 min irradiation is
summarized in Table 1. In the cases of 1b and 1c, a mixture of
cis- and trans-lilolidines was obtained; the diastereomeric
excesses (de) were low. Figure 1 shows the photoreaction

1760 J. Phys. Chem. A, Vol. 109, No. 9, 2005
Leo et al.
Figure 2. (a) Absorption spectra of 1a in acetonitrile at 0.3 and 0.03
mM concentrations; (b) subtracted UV spectra (1a-4a) in the long-
wavelength region.
CHART 2
Figure 1. Photoreaction kinetics of 1a (×), 1b (O), 1c (b), and
2-allylaniline ([) using (a) 254 nm and (b) 300 nm.
TABLE 1: Photoproducts Distribution Found for the
Irradiation of 1a-c
concentrated solution. In principle, this long-wavelength absorp-
tion could be attributed to a ground-state NH/π complex. To
check this hypothesis, the absorption spectra of 1a were recorded
at different concentrations and compared with those of 1,2,3,4-
tetrahydroquinoline (4a in Chart 2), which lacks the allyl
substituent, at the same concentrations. The subtracted spectra
(1a-4a) are given in Figure 2b. They clearly show a well-
defined emerging band with maximum at 340-350 nm.
The fluorescence spectra of 1a were obtained both by
excitation at the maximum (300 nm) and at the long-wavelength
band (330 nm). Remarkably, the emission λmax was strongly
dependent on the excitation wavelength (Figure 3). The ap-
pearance of the 420-nm band, strongly red-shifted with respect
to the usual emission of anilines (320-350 nm), could in
principle be attributed to the excited complex AB*.23
As the emission exhibited a marked dependence on the
excitation wavelength, full excitation spectra were recorded; they
are shown in Figure 4. Interestingly, while the excitation
maximum for the species emitting at 360 nm was essentially
coincident with the main absorption maximum shown in Figure
2a, the excitation spectrum obtained for the emission at 420
nm closely matched the long-wavelength absorption observed
in Figure 2b. This confirms the formation of a ground-state
products (%)
compd
λ (nm)
conv. (%)
cis-3
trans-3
de (%)
1a
1b
1c
1a
1b
1c
254
254
254
300
300
300
46
26
36
50
47
58
100
52
61
48
39
4
22
100
50
51
50
49
0
2
kinetics of compounds 1a-c. The corresponding data for the
related compound 2-allylaniline are included for comparison.
In general, the photoreaction quantum yields were low (0.15 (
0.05 at 254 and 300 nm). This was specially the case at
wavelengths longer than 300 nm (φ < 0.05 at 335 nm).
After the realization that 1a-c exhibit a 2-allylaniline-like
photochemical behavior, the photophysical properties of these
compounds were studied, in the search for experimental
evidence in favor of an intramolecular NH/π interaction in the
ground and/or the excited state.
Figure 2a shows the ultraviolet absorption spectra of 1a at
two different concentrations (0.3 and 0.03 mM), which present
two maxima at around 250 and 300 nm. Besides, a tail extending
until ca. 380 nm was distinguishable in the case of the more

Intramolecular NH/π Complexes
J. Phys. Chem. A, Vol. 109, No. 9, 2005 1761
Figure 3. Normalized emission spectra of 1a in acetonitrile, exciting
at λ ) 300 and 330 nm.
Figure 4. Normalized excitation spectra of 1a in acetonitrile, for
emissions at λmax ) 360 and 420 nm.
Figure 5. Normalized emission spectra of 1b (top) and 1c (bottom)
in acetonitrile, exciting at λ ) 300 and 330 nm.
complex, present in minor amounts, that upon light absorption
generates the corresponding excited species.
Analogous fluorescence studies were performed for 1b and
1c. Figure 5 shows the emission spectra of both compounds,
using 300 and 330 nm as the excitation wavelength. Again, two
distinct bands were observed. Their excitation spectra were
different (Figure 6), as expected for the existence of two
different species in the ground state. Thus, the behavior of 1b
and 1c was similar to that stated above for 1a. On the basis of
the existing background, our assumption was that the intramo-
lecular interaction in AB and AB* was of the NH/π type. To
verify this assumption, parallel studies were performed on two
model substrates, namely, 4a and 5a (Chart 2). Thus, the parent
1,2,3,4-tetrahydroquinoline (4a) would allow us to determine
the role of the allyl group. On the other hand, 8-allyl-N-methyl-
1,2,3,4-tetrahydroquinoline (5a) would be an adequate substrate
to ascertain whether the free NH group is required for complex
formation.
The UV spectra of 4a and 5a (Figure 7) were similar to that
of 1a, with maxima around 250 and 300 nm, but with a
markedly lower absorption at longer wavelengths. The fluores-
cence spectra were meassured by exciting at 300 and 330 nm.
As expected for the lack of NH/π interaction, no emission
resulting from excitation of a ground-state complex was detected
for these model compounds (Figure 8).
These experimental results with 4a and 5a confirm that both
the NH group and the olefin are necessary for formation of the
ground-state complex observed for 1a-c.
The intramolecular NH/π interaction was theoretically studied
in 1a using quantum-chemical calculations19 at the B3LYP/6-
31G** computational level.18 Two structures (I and II in Figure
10) were optimized: in I, the allyl substituent displays an
extended arrangement relative to the benzene ring; in II, the
sigma bonds of the allyl substituent are twisted in order to
approach the π system to the amine hydrogen atom. The
calculated energies are summarized in Table 2. According to
the B3LYP/6-31G** method, structure II is 2.3 kcal/mol more
stable than I. To confirm the DFT result, further optimizations
were performed at HF and MP2 computational levels.19 The
results were close to those obtained at the B3LYP level, with
II between 2.0 and 2.5 kcal/mol lower in energy than I, in
agreement with a favorable NH/π interaction (see Table 2). The
same trend was observed when diffuse functions were used to
optimize structures I and II at the HF/6-31+G** and B3LYP/
6-31+G** levels, where the energy differences were somewhat
smaller (between 1.7 and 1.9 kcal/mol), although still larger
than those estimated using the semiempirical perturbed config-
uration interaction with localized orbitals (PCILO) model for
related 2-allylanilines (between 1.0 and 1.7 kcal/ mol).11
A detailed analysis of the geometries of I and II was
performed at the three computational levels; for each structure,
To rule out that the observed band at 340-360 nm could be
the result of two overlapping emissions, excitation spectra were
recorded at different λem for 4a and 5a. As shown in Figure 9,
no evidence for a ground-state complex was obtained.

1762 J. Phys. Chem. A, Vol. 109, No. 9, 2005
Leo et al.
Figure 8. Emission spectra of 4a (top) and 5a (bottom) in acetonitrile,
exciting at λ ) 300 and 330 nm.
Figure 6. Normalized excitation spectra in acetonitrile of 1b (top),
for emissions at λ ) 350 and 420 nm, and of 1c (bottom), for emissions
at λ ) 330 and 420 nm.
TABLE 2: Total (au) and Relative (kcal/mol, in
parentheses) Energies of Structures I and II
I
II
HF/6-31G**
-517.610065
-517.622489
-521.068650
-521.086983
-519.403955
-517.613309
-517.625136
-521.072260
-521.089951
-519.407946
(-2.0)
(-1.7)
(-2.3)
(-1.9)
(-2.5)
HF/6-31+G**
B3LYP/6-31G**
B3LYP/6-31+G**
MP2/6-31G**
TD22 approach at the B3LYP/6-31G** level. The resulting
vertical excitation energies were 105.3 kcal/mol (for I) and 100.6
kcal/mol (for II). Without overemphasizing the quantitative
value of these energies that do not take into account the
temperature and solvent effects, nor the geometry changes in
the excited state, it is interesting to note that they are in
qualitative agreement with the experimental results: the NH/π
interaction existing in II produces a decrease of the energy
required for vertical excitation.
Figure 7. Absorption spectra of 4a and 5a in acetonitrile (0.03 mM).
Conclusion
the differences associated with the employed methodology were
very minor. The allyl substituent in I is positioned perpendicular
to the aromatic plane (see Figure 10), with the CdC double
bond in an eclipsed arrangement relative to the benzylic
hydrogens, positioned far from the amine. In II, the CH2-CHd
single bond is twisted ca. 120° (relative to that of I), and the
distances between the amine hydrogen atom and the CH2d and
CHd carbon atoms are 2.62 and 2.47 Å, respectively. In I, the
shortest distance between the amine hydrogen and the olefinic
carbons is 2.70 Å. For the relevant dihedral angles of both
structures, see Figure 10. Inclusion of diffuse functions at the
B3LYP/6-31+G** level increases slightly the distances between
the amine hydrogen atom and the CH2d and CHd carbon atoms
of II to 2.76 and 2.58 Å, respectively.
Photophysical studies performed on 8-allyl-1,2,3,4-tetrahy-
droquinolines 1a-c demonstrate the formation of a NH/π
intramolecular ground-state complex (AB). Excitation of this
complex, which can be achieved at long wavelengths (330-
340 nm), gives rise to the corresponding excited species AB*.
Emission from AB* is red-shifted (420 nm) with respect to that
observed when the monomer A is excited (λexc ) 300 nm). This
is supported by theoretical DFT studies that reinforce the
conclusions drawn from previous semiempirical PCILO calcula-
tions and provide estimation on the vertical singlet-singlet
excitation that are in reasonable agreement with the experimental
data.
The spectroscopic properties of the two structures were
evaluated computing the first singlet excited state, using the
Acknowledgment. Financial support from the Spanish
MCYT (Grant No. BQU2001-2725), the Generalitat Valenciana

Intramolecular NH/π Complexes
J. Phys. Chem. A, Vol. 109, No. 9, 2005 1763
Supporting Information Available: 1H and 13C NMR
spectra of compounds 1b, 2b, cis-3b, and trans-3b; relevant
NOE interactions for assignment of cis-3b and trans-3b. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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Figure 9. Excitation spectra in acetonitrile of 4a, for emissions at λ
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Figure 10. B3LYP/6-31G**-optimized geometries for the structures
I and II of 1a.
(Grupos 03/82), and the Universidad Polite´cnica de Valencia
(fellowship to E.A.L.) is gratefully acknowledged.