Excited-state proton phototransfer in the (3-methyl-7-azaindole)-(7- azaindole) heterodimer

Article abstract of DOI:10.1016/j.cplett.2005.11.072

A molecule of 7-azaindole and another of 3-methyl-7-azaindole form a doubly hydrogen bonded heterodimer of C_s symmetry with a first electronic excited state a′ where the electronic excitation is highly localized on one of the molecular halves. Also, because the electronic excitation only increases the acidity of the pyrrole nucleus and the basicity of the pyridine nucleus in one of the two hydrogen bonds, the heterodimer undergoes single rather than double proton transfer.

Full text of DOI:10.1016/j.cplett.2005.11.072

Chemical Physics Letters 419 (2006) 164–167
www.elsevier.com/locate/cplett
Excited-state proton phototransfer in the
(
3-methyl-7-azaindole)-(7-azaindole) heterodimer
*
Javier Catal a´ n , Cristina D ´ı az, Jos e´ L.G. de Paz
Departamento de Qu ı´ mica F ı´ sica Aplicada, Facultad de Ciencias, C-II-203, Grupo de Quimica Cuantica Organic, Universidad Aut o´ noma
de Madrid, Cantoblanco, ES-28049 Madrid, Spain
Received 30 September 2005
Available online 15 December 2005
Abstract
A molecule of 7-azaindole and another of 3-methyl-7-azaindole form a doubly hydrogen bonded heterodimer of C
first electronic excited state a where the electronic excitation is highly localized on one of the molecular halves. Also, because the elec-
tronic excitation only increases the acidity of the pyrrole nucleus and the basicity of the pyridine nucleus in one of the two hydrogen
bonds, the heterodimer undergoes single rather than double proton transfer.
s
symmetry with a
0
ꢀ
2005 Elsevier B.V. All rights reserved.
1
. Introduction
proton and basicity in the pyridine nitrogen, the atoms
involved in the hydrogen bonds of the dimer [4,9]. As a
result, the C2h dimer can undergo concerted double proton
phototransfer.
One scarcely explored question is what happens upon
electronic excitation of the doubly hydrogen bonded dimer
of 7AI if it loses its high centrosymmetry in the ground
state. The symmetry loss can be induced by the presence
of isotopes in one of the 7AI molecules forming the dimer
(isotopomers), through interaction of the dimer with a
polar molecule (complexes) or by some substituent present
in one of the monomers (heterodimers).
There has been some recent research into these three
symmetry loss mechanisms. Thus, Sakota and Sekiya
[10,11] found asymmetric deuterated isotopomers of 7AI
dimer to exhibit localized electronic excitation and con-
certed double proton transfer under free-jet conditions.
Also, Catal a´ n and P e´ rez [12] found such isotopomers to
exhibit localized electronic excitation and single proton
transfers in the condensed phase. Complexes formed
between the doubly hydrogen bonded dimer of 7AI and a
polar molecule [13] have revealed that, on losing its C2h
symmetry, the dimer produces an excited electronic state
The two fundamental proposals of Kasha et al. [1] in
relation to the photophysics of the C2h dimer of 7-azain-
dole (7AI) which results from a double hydrogen bonding
interaction between the corresponding pyrrole and pyridine
units in two 7AI monomers, viz. that the proton transfer is
double and that it takes place in a concerted manner, have
been verified beyond doubt [2–7]. However, because
research in this field has focussed on these two points over
the past three decades, most of the basic spectroscopy of
7
AI dimer has been ignored and remains unknown.
The basic concepts supporting the hypotheses of Kasha
rely on the C2h symmetry of 7AI dimer; this entails com-
plete delocalization of the electronic excitation on its two
halves and the change in its acid/base properties to be
expected from electronic excitation which was used by Wel-
ler [8] to establish Excited State Intramolecular Proton
Transfer (ESIPT) mechanisms. Electronic excitation in
the C2h dimer of 7AI causes complete delocalization of
the excitation on its two halves; more important, it induces
a simultaneous marked increase in acidity in the pyrrole
*
of lower energy than the 1Bu and 2Ag states in the C2h
Corresponding author. Fax: +34 91 497 4785.
E-mail address: Javier.Catalan@uam.es (J. Catal a´ n).
dimer; this appears to red-shift the onset of the first
0
009-2614/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2005.11.072

J. Catal a´ n et al. / Chemical Physics Letters 419 (2006) 164–167
165
absorption band. Such complexes [14] have also allowed
the identification of the molecular structure responsible
for the phosphorescence of 7AI with its onset at 430 nm,
which was first detected by Kasha et al. [1]. Sekiya et al.
spectro fluorimeter, using a Suprasil quartz cell of 1 cm
light path. The sample temperature was controlled by an
Oxford DN1704 cryostat equipped with an ITC4 controller
interfaced to the spectrophotometer. The cryostat was
purged with dried nitrogen 99.99% pure.
[
7
15] studied heterodimers between 3-Me-7-azaindole and
-azaindole under free-jet conditions and interpreted their
7-Azaindole was obtained in 99% purity from Sigma
and recrystallized in spectroscopic-grade cyclohexane
twice. 3-methyl-7-azaindole was synthesized from 2-pyr-
idylhydrazine and propionaldehyde to obtain the corre-
sponding hydrazone. This hydrazone was refluxed with
diethylene glycol in an inert atmosphere [29]. The resulting
residue was purified by silica gel column chromatography
(eluent: hexane/ethyl acetate 6/4 and dichloromethane/
ethyl acetate 6/4) yielding 3-methyl-7-azaindole (23%) with
a purity over 99%. All samples used were obtained from
freshly made 2-methylbutane (2 MB) solutions containing
results as evidence for a double proton transfer in the gas
phase. On the other hand, Catal a´ n and de Paz [7] inter-
preted the results for the heterodimer of 4-Me-7-azaindole
and 7AI in the condensed phase as evidence of a single pro-
ton transfer.
This paper analyses in theoretical and experimental
terms the dimer symmetry loss mechanism involving the
formation of heterodimers and rationalizes reported infor-
mation about the topic. Specifically, it deals with the het-
erodimer formed by one molecule of 7-azaindole and
another of 3-methyl-7-azaindole, which was previously
studied under free-jet conditions by Sekiya et al. [15]. Their
conclusions raise three pivotal questions, namely:
ꢀ
4
a 10 M concentration of indole residue. The 2 MB sol-
vent was Uvasol-grade and supplied by Merck.
3. Results and discussion
(
a) If the spectroscopic behaviour of (3M7AI-7AI) dimer
is so similar to that of 7AI dimer, how can simply
This section initially examines the theoretical data avail-
2
substituting a proton at position 3, possibly shift the
able for the heterodimer (3M7AI-7AI) in order to confirm
whether the electronic excitation is localized and its impli-
cations on the proton transfer process in the excited hetero-
dimer. Then, it deals with its absorption and emission
spectra in relation to the type of proton transfer it
undergoes.
0
–0 component of the S ! S transition by as much
0
1
ꢀ1
as 1666 cm
?
(
b) If the electronic excitation is delocalized on the two
molecular halves of the C2h dimer of 7AI, but only
on one in (3M7AI-7AI), how can both dimers undergo
an identical double proton transfer process?
(
c) According to Sekiya et al., ‘‘the fluorescence intensity
of (3M7AI-7AI) was too weak to measure the dis-
persed fluorescence (DF) spectrum’’ Why?
3.1. The 0–0 components of the S ! S transition
0
1
Sekiya et al. [15] obtained fluorescence excitation (FE)
spectrum for (3M7AI-7AI) heterodimer near the 0–0
region of the S ! S transition (specifically, from 30560
0
1
ꢀ1
2
. Theoretical and experimental section
to 31100 cm ) by tuning only the emission visible through
a Toshiba Y45 glass filter. Based on such spectrum and, by
All computations were done within the framework of
analogy with those for 7AI , they assigned the 0–0 compo-
nent of the (3M7AI-7AI) to the peak at 30585 cm .
2
ꢀ
1
the Density Functional Theory (DFT) and the Time
Dependent Density Functional Theory (TDDFT), using
the software Turbomole v. 5.26, which was developed by
the Quantum Chemistry Group of the University of Kar-
lsruhe (Germany) [16]. Full geometry optimization of the
ground and excited electronic states was done by using
the hybrid functional B3LYP [17–19] as implemented in
Turbomole [20]. The TZVP basis set [21] was used. Excited
states were studied at the TDDFT level [22] as imple-
mented in Turbomole [23–27]. Previous studies had shown
this methodology to be accurate with photoexcited mole-
cules [7,9,28].
The theoretical value for the 0–0 component of the
S ! S transition in (3M7AI-7AI) as computed at the
0
1
TDDTF level and corrected for the zero-point energy is
ꢀ
1
28457 cm . This value is located at much lower energy,
ꢀ1
2100 cm , than the experimental value obtained under
ꢀ1
free-jet conditions [15], 30585 cm . This great disagree-
ment between the theoretical and experimental results,
for the heterodimer 3M7AI-7AI appears to be remarkable
ꢀ
1
if we bring about the small deviation of only 900 cm for
the C2h 7AI dimer, when comparing the experimental
ꢀ1
ꢀ1
(32252 cm ) and the theoretical values (31349 cm ).
The 0–0 components for the S ! S electronic transi-
As recently shown by the authors [7], the C dimer of 7-
0
1
s
0
tions were calculated from the fully optimized geometry
for each state, which was corrected for the zero point
energy as computed from the vibrational frequencies for
the compound.
azaindole produces a first excited electronic state a with its
ꢀ
1
0–0 component 484 cm below that of the 1Bu state in the
C2h dimer. Such a state is responsible for the above-
described red shift in the onset of the first absorption band
seemingly occurring in 7-azaindole dimer [12,13]. As shown
below, this is also the case for the (3M7AI-7AI) heterodi-
mer, and probably, what Sekiya et al. have reported might
UV–visible spectra were recorded on a Cary-5 spectro-
photometer and corrected fluorescence spectra were
obtained by using a calibrated Aminco-Bowman AB2

166
J. Catal a´ n et al. / Chemical Physics Letters 419 (2006) 164–167
be the excitation spectra of the sample to a higher excited
electronic state.
3.3. Absorption and emission spectra of (3M7AI-7AI) in
2 MB
3.2. Localization of the electronic excitation and proton
transfer
Fig. 2 shows the variation of the UV–visible absorption
ꢀ
5
spectrum for a solution containing a 5 · 10 M concentra-
tion of both 7AI and 3M7AI in 2 MB with temperature. As
can be clearly seen, the absorption spectrum expands above
315 nm, that is, beyond the 0–0 component of the C2h
dimer (at 315 nm)[2]. This onset at longer wavelengths
facilitates the analysis of the emission of the heterodimer
and its photoselection at 325 nm.
Fig. 1 shows the molecular orbitals (MO) involved in the
first electronic transition in the following systems: (a) the
dimer of 7AI corresponding to the energy minimum in
the ground electronic state of C2h symmetry; and (b) that
corresponding to the energy minimum in the ground elec-
tronic state of the (3M7AI-7AI) heterodimer of C symme-
Fig. 3 shows the fluorescence emission obtained by
excitation at 325 nm where only the (3M7AI-7AI) hetero-
dimer absorbs. As can be clearly seen, the emission spec-
s
try. As can be seen, electronic excitation in the C2h dimer is
completely, symmetrically delocalized on the two halves of
the dimer. Also, upon photoexcitation the MO suggest a
large simultaneous acidity increase in both pyrrole nuclei
and basicity in the two pyridine nuclei. The principal out-
come is that the dimer undergoes a concerted double pro-
ton transfer.
0
trum, which is structureless, for the a state in the
heterodimer peaks in the 430 nm region. It must be
reminded that the emission for the double proton transfer
of the C2h dimer peaks at 480 nm and exhibits structure at
low temperature.
The situation in the C heterodimer is rather different. In
s
fact, the electronic excitation is strongly localized (Fig. 1)
on one half of the initial MO (HOMO) and in another half
of the final MO (LUMO). In this case, the MO suggest an
increase of pyrrole acidity and pyridine basicity, but only at
one of the hydrogen bonds. As a result, electronic excita-
tion triggers a single proton transfer.
102 K
27
177
0.8
0.6
1
2
2
2
2
2
02
27
52
77
93 K
0.4
0.2
0.0
2
60
280
300
320
340
λ abs in nm
ꢀ5
Fig. 2. UV–visible spectra for a 5 · 10 M solution of both 7AI and
M7AI in 2 MB between 293 and 102 K.
3
0
0
0
.2
.1
.0
350
400
450
500
550
λ em in nm
Fig. 1. HOMO (below) and LUMO (above) orbitals involved in the
description of the S
M7AI -7AI heterodimer.
0
! S
1
transition for the 7AI homodimer and for the
Fig. 3. Emission spectrum for the heterodimer upon excitation at 325 nm
of a 5 · 10 M solution of both 7AI and 3M7AI in 2 MB at 102 K.
ꢀ
5
3

J. Catal a´ n et al. / Chemical Physics Letters 419 (2006) 164–167
167
5
4
3
2
1
00
00
00
00
00
0
4. Conclusions
The interesting experiments on the double proton trans-
fer in (3M7AI-7AI) heterodimer recently reported by
Sekiya et al. [15] should be revisited in the light of the fact
that the heterodimer undergoes a single proton transfer
only. This conclusion is supported by both the theoretical
and experimental data reported in this work, the latter of
which were obtained in a 2 MB solution.
Acknowledgements
260
280
300
320
340
We are greatly indebted to Direcci o´ n General de Inves-
tigaci o´ n Cientifico T e´ cnica (Spain) for the support project
No. BQU2002-02106.
λexc in nm
Fig. 4. Excitation spectrum for the heterodimer by monitoring light at
30 nm, for a 5 · 10 M solution of both 7AI and 3M7AI in 2 MB at
ꢀ5
4
1
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