Conformational equilibria and photoinduced tautomerization in 2-(2′-pyridyl)pyrrole

Article abstract of DOI:10.1016/j.cplett.2004.10.121

Depending on the polarity and protic abilities of the solvent, 2-(2′-pyridyl)pyrrole can exist in either syn or anti rotameric forms. In nonpolar solvents, intramolecular excited state single proton transfer is observed, manifested by the appearance of low-energy tautomeric emission. The solvent-assisted excited state double proton transfer reaction is also detected. DFT calculations confirm low barriers for both single and double proton transfer processes in the lowest excited singlet state and show different character of the tautomerization in both cases: in the intramolecular reaction, mutual approach of two nitrogen atoms plays an important role.

Full text of DOI:10.1016/j.cplett.2004.10.121

Chemical Physics Letters 400 (2004) 279–285
www.elsevier.com/locate/cplett
Conformational equilibria and photoinduced tautomerization
in 2-(20-pyridyl)pyrrole
a,b
Michał Kijak , Anna Zielinska , Charles Chamchoumis , Jerzy Herbich
a
c
a,b
´
Randolph P. Thummel , Jacek Waluk
,
c
a,b,
*
a
Institute of Physical Chemistry, Polish Academy of Sciences, Photochemistry and Spectroscopy, Kasprzaka 44/52, 01-224 Warsaw, Poland
b
´
Faculty of Mathematics and Science, Cardinal Stefan Wyszynski University, Dewajtis 5, 01-815 Warsaw, Poland
c
Department of Chemistry, University of Houston, Houston, TX 77204-5003, USA
Received 6 July 2004; in final form 26 October 2004
Available online 13 November 2004
Abstract
Depending on the polarity and protic abilities of the solvent, 2-(20-pyridyl)pyrrole can exist in either syn or anti rotameric forms.
In nonpolar solvents, intramolecular excited state single proton transfer is observed, manifested by the appearance of low-energy
tautomeric emission. The solvent-assisted excited state double proton transfer reaction is also detected. DFT calculations confirm
low barriers for both single and double proton transfer processes in the lowest excited singlet state and show different character of
the tautomerization in both cases: in the intramolecular reaction, mutual approach of two nitrogen atoms plays an important role.
Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction
however, the photoreaction is much slower (several
100 ps, depending on the alcohol) [3].
Bifunctional molecules, capable of acting simultane-
ously as hydrogen bond acceptors and donors, have at-
tracted much interest due to the role they play in
biological systems, e.g., as DNA base pairs. Studies of
proton transfer occurring along hydrogen bonds in such
structures are of crucial importance, since tautomeriza-
tion in DNA pairs may lead to mutations. A model
system for the investigation of photoinduced tautomeri-
zation that has received much attention as well as con-
troversy is the cyclic, doubly hydrogen-bonded dimer
of 7-azaindole (7AI) [1–4]. Upon excitation, two protons
are rapidly (several picoseconds) translocated from the
pyrrole to pyridine-type nitrogens. The same process
has been observed for alcohol complexes of 7AI. Here,
We have been studying similar systems, differing from
7AI by the number of bonds separating the donor and
acceptor groups – three instead of two [5–16]. This seem-
ingly minor structural variation has large consequences
for the photophysics. In alcohol complexes of 1H-pyr-
rolo[3,2-h]quinoline (PQ) [7], dipyrido[2,3-a; 30,20-i]car-
bazole (DPC) [8], and similar compounds, alcohol or
water-assisted excited state double proton transfer
(ESDPT) occurs at a rate about two orders of magni-
tude faster than in 7AI [9]. This has been attributed to
stronger, more linear hydrogen bonds in alcohol/water
solvates. Contrary to 7AI, cyclic, 1:1 complexes can be
formed while the molecule is still in the ground state
and are, therefore, well prepared for phototautomeriza-
tion. On the other hand, no photoreaction has been de-
tected in dimers, which, in turn, cannot achieve linear
arrangement of hydrogen bond [10]. Thus, these com-
pounds are, in a sense, complementary to 7AI (and its
*
Corresponding author.
E-mail address: waluk@ichf.edu.pl (J. Waluk).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2004.10.121

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M. Kijak et al. / Chemical Physics Letters 400 (2004) 279–285
derivatives such as 1-azacarbazole) with regard to pho-
totautomerization capabilities in solvates and dimers.
Interestingly, photoautomerization could not be
detected in alcohols for structurally similar
2-(20-pyridyl)indole and several of its derivatives [11].
In a 2-carbon-bridged pyridylindole, 3,30-dimethylene-
2-(20-pyridyl)indole, only an extremely weak tautomeric
fluorescence could be observed. In protic solvents, the
major excited state depopulation channel was found to
correspond to S1–S0 internal conversion, strongly
dependent on the viscosity, and disappearing in rigid
environment. The origin of this channel was tentatively
attributed to Ôaborted double proton transferÕ [17], most
probably involving complexes of 1:2 stoichiometry.
Protic solvents were also found to induce syn–anti
rotamerization in 2-(20-pyridyl)indole, due to the fact
that the formation of two independent hydrogen bonds
on opposite sides of the molecule was favorable.
Rotamerization results in a complete change of photo-
physical properties, since neither ESDPT nor enhanced
internal conversion can occur in the anti form [12].
In this work, we study the parent and simplest mole-
cule of the series, 2-(20-pyridyl)pyrrole (PP, Scheme 1).
Somewhat to our surprise, this molecule reveals not only
protic solvent-assisted ESDPT, but also an intramolecu-
lar excited state single proton transfer reaction. This dif-
ference in behavior with regard to other molecules of the
group (e.g., PQ) is in agreement with the results of cal-
culations of the barrier for the excited state tautomeriza-
tion. The proton transfer coordinate contains a large
contribution from the in-plane bending motion, which
brings the two nitrogens involved in the transfer close
to each other, thus facilitating the photoreaction.
which is a convenient immediate precursor to PP [18,19].
We prepare the ketoaldehyde by a Grignard approach
[20] involving the addition of the protected aldehyde,
3,3-trimethylenedioxy-1-propylmagnesium bromide, to
2-cyanopyridine to provide 1-(20-pyridyl)-4,4-trimethy-
lenedioxy-1-butanone which is then readily hydrolyzed
to the ketoaldehyde precursor. Treatment of this mate-
rial with ammonium acetate provides the 2-(20-pyr-
idyl)pyrrole in 88% yield. The compound was purified
by vacuum sublimation.
The absorption spectra were recorded on a Shimadzu
UV3100 spectrophotometer. Stationary fluorescence
and fluorescence excitation spectra were measured on
an Edinburgh FS 900 CDT spectrofluorimeter. Fluores-
cence lifetimes were obtained using an Edinburgh FL
900 CDT time-resolved fluorometer, with estimated time
resolution of 300 ps. Spectral grade solvents were used,
checked for the presence of fluorescing impurities. Spe-
cial care was taken to remove hydroxylic impurities
from n-hexane: it was refluxed over LiAlH4 for 6 h,
and then distilled under reduced pressure directly to
the cell that contained the freshly sublimed PP.
Ground state geometry optimizations were per-
formed using either HF, DFT or MP2 models, with 6-
31+G(d,p) basis set. Electronic transition energies were
calculated using time-dependent DFT. Excited state
geometries were optimized using CIS/6-31+G(d,p). The
STQN method, implemented in the GAUSSIAN package,
was applied to optimize transition state geometries. In
the case of complexes with alcohol, basis set superposi-
tion error was corrected using the counterpoise tech-
nique. All the calculations were performed using the
GAUSSIAN 98 [21] or GAUSSIAN 03 [22] suites of
programs.
2. Experimental and computational details
3. Results and discussion
Synthetic approaches to PP either involve coupling of
the two components or utilize a pre-formed 2-pyridyl
ring and chemistry which requires formation of the 2-
pyrrole ring. We have adopted the latter approach
involving the ketoaldehyde, 4-(20-pyridyl)-4-oxobutanal,
Fig. 1 presents the absorption and fluorescence spec-
tra of PP recorded at room temperature in various sol-
vents. Both fluorescence quantum yields and lifetimes
strongly depend on the solvent. Interestingly, biexpo-
nential fluorescence decay is detected in acetonitrile.
The shorter decay component, 0.6 ns, is close to that
observed in n-hexane (<0.4 ns), whereas the longer
one (4.3 ns) is, within error limits, identical to the
the value of the monoexponential decay time in n-pro-
panol (4.1 ns).
By analogy with the results reported earlier for 2-(20-
pyridyl)indole [11], we attribute the two lifetimes to two
rotameric forms of PP, syn and anti. In 2-(20-pyr-
idyl)indole the anti form could only be observed in polar
and protic alcohols. The calculated energy differences
between syn and anti forms are similar for the two mol-
ecules, the anti rotamer being higher by slightly less than
5 kcal/mol. Two factors contribute to lowering the
Scheme 1. Pyrrolo[3,2-h]quinoline (PQ) and syn and anti forms of 2-
(20-pyridyl)pyrrole (PP).

M. Kijak et al. / Chemical Physics Letters 400 (2004) 279–285
281
1.0
1.0
0.8
0.6
0.4
0.8
0.6
0.2
0.0
26
28
30
32
34
36
38
Wavenumber [103cm-1
]
0.4
0.2
0.0
12
16
20
24
28
32
38
40
Wavenumber [103cm-1]
Fig. 1. Absorption and fluorescence of PP at 293 K in n-hexane (a); acetonitrile (b); propanol (c); Inset: excitation spectra of ÔnormalÕ (d); and low-
energy fluorescence (e) in n-hexane (monitored at the maxima of both emissions).
energy of the anti form in polar and protic solvents with
respect to the syn rotamer. The first is the solvent polar-
ity, which stabilizes the anti form more, due to its larger
dipole moment (calculated values are 1.06 and 3.22 D
for the syn and anti forms, respectively). The second is
the ability to form hydrogen bonds with protic solvents,
such as alcohols or water. Two linear, independent
hydrogen bonds with the solvent in the anti form are
more energetically favorable than the corresponding sol-
vates in the syn conformer. This has been checked for PP
by B3LYP/6-31+G(d,p) calculations for the four possi-
ble complexes with methanol (Fig. 2). The anti 1:2 struc-
ture turned out to be more stable by 1.6 kcal/mol than
the corresponding syn form. The entropy term is also ex-
pected to favor the anti structure.
The fact that the anti form of PP is detectable already
in acetonitrile, and thus, that, contrary to the case of 2-
(20-pyridyl)indole, it can be sufficiently stabilized by
nonspecific effects, can be ascribed to the smaller size
of PP. A smaller value of the Onsager cavity radius
leads to a larger magnitude of the dipolar solvation
term.
The fluorescence decay observed in n-hexane, as well
as one of the components found for acetonitrile is about
an order of magnitude shorter than that measured in
alcohols. The short lifetime in the syn form must be
Fig. 2. Possible structures of the complexes with methanol with complexation energies (kcal/mol) obtained by B3LYP/6-31+G(d,p) or MP2//B3LYP/
6-31+G(d,p) calculations (upper and lower numbers, respectively).

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M. Kijak et al. / Chemical Physics Letters 400 (2004) 279–285
due to a depopulation channel not available in the anti
structure. Indeed, in n-hexane, a long-wavelength fluo-
rescence band was observed in addition to ÔnormalÕ
emission (Fig. 1). The excitation spectra of both bands
coincide, showing that both emissions originate from
the same precursor. We assign the low-energy fluores-
cence to the product of excited state proton transfer
from the NH group onto the pyridinic nitrogen atom.
Using Fo¨rster cycle, huge basicity increase of the latter
was found in S1 (DpKa = +11.0), as well as the acidity in-
crease of the NH group (DpKa = ꢀ9.4). However, the
intramolecular hydrogen bond should be rather weak,
due to its nonlinearity. Therefore, care has been taken
to ensure that the tautomeric fluorescence is really due
to the intramolecular process, but not to solvent-assisted
double proton transfer caused by the presence of
hydroxylic impurities (water) in the solvent. First, titra-
tion of n-hexane solutions of PP with methanol yielded
ꢁ4 Mꢀ1as the value of the equilibrium constant of 1:1
complex formation; it is much smaller than that previ-
ously reported for PQ and its derivatives [7,8]. Thus, gi-
ven the low solubility of water in n-hexane, the fraction
of solvates should be negligible. Second, experiments
were performed in carefully dried n-hexane distilled
from LiAlH4 directly into the cell containing freshly
sublimed PP. Finally, our assignment parallels that re-
cently published for structurally similar pyrazoles [23],
for which the dual fluorescence pattern nearly identical
to that of PP was observed in carefully dried
cyclohexane.
cyclic hydrogen-bonded complexes, no evidence was
found for an intramolecular single proton transfer
process. In order to elucidate the difference between
these molecules and PP, S0 and S1 energy profiles have
been calculated. Two methods were used. First, the
ground state reaction profile was obtained, followed
by TD-DFT calculations for the S0–S1 transition en-
ergy at each point along the reaction path. In the sec-
ond approach, the reaction path used corresponded to
that obtained for S1 using CIS/6-31+G(d,p). Both ap-
proaches gave similar reaction paths. The tautomeric
form is less stable (by 16.9 kcal/mol) in S0, but be-
comes energetically more favorable in S1 (by 17.8
kcal/mol) (Fig. 3). The barrier for tautomerization in
S1 is calculated to be very low, 1.2 kcal/mol. This is
in contrast to PQ, for which the corresponding value
is very high (18.8 kcal/mol using B3LYP/6-31G(d,p)).
The reason for this difference becomes obvious upon
analysing the reaction path coordinate (Fig. 4). A very
significant shortening of the N–N distance is obtained
in the transition state (231 pm, compared to 272 pm
in the initial form and 264 pm in the phototautomer).
Such a close approach of the heavy atoms participating
in proton transfer is not possible in structures based on
PQ, for which the donor and acceptor groups are
linked via a rigid bridge, not just a single bond, as in
PP. The large contribution of a coordinate not involv-
ing proton motion may explain the observed lack of
deuterium effect in the phototautomerization rate ob-
served in pyrazoles [23].
For molecules related to PQ that readily display sol-
vent-assisted excited state double proton transfer in 1:1
The calculations performed for the 1:1 cyclic com-
plex of PP with methanol also yielded a very low
Fig. 3. Calculated barriers for intramolecular tautomerization in S0 and in S1 (zero-point energies included).

M. Kijak et al. / Chemical Physics Letters 400 (2004) 279–285
283
Fig. 4. Consecutive steps of ESIPT calculated by CIS/6-31+G(d,p).
barrier for the solvent-assisted double proton transfer
in S1. For this process, the values obtained for PP
and PQ are comparable. Experimental verification of
the possibility of ESDPT in alcohol complexes of PP
is not straightforward, since, as shown above, only anti
forms are present in bulk alcohol. We have therefore
monitored fluorescence changes upon adding propa-
nol-1 to an n-hexane solution. The decrease of the pri-
mary fluorescence intensity was observed, indicating a
smaller quantum yield of the primary emission in alco-
hol solvates. At the same time, however, the intensity
of tautomeric emission remained unchanged, although

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M. Kijak et al. / Chemical Physics Letters 400 (2004) 279–285
ture, intramolecular excited state single proton transfer
occurs, facilitated by the flexibility of the molecular skel-
eton, which enables close approach of the nitrogen
atoms participating in the process. Solvent-assisted ex-
cited state double proton transfer can also occur, mak-
ing PP a rare example of a molecule that exhibits both
types of photoreactions (Fig. 5). Given that the small
size of PP also makes it attractive for calculations, we
plan thorough investigations of this interesting mole-
cule, involving detailed structural, spectral and kinetic
studies.
Fig. 5. ESIPT and solvent-assisted ESDPT processes in the S1 state of
PP.
Acknowledgements
This work was sponsored, in part, by the Polish Com-
mittee for Scientific Research Grant 7T09A 044 21.
R.P.T. and C.C. thank Robert A. Welch Foundation
(E-621) and the National Science Foundation (CHE-
0352617).
a small change in the shape could be detected. Thus,
the fraction of emitting molecules that disappeared in
the ÔnormalÕ emission was recovered in the tautomeric
fluorescence. This important result implies that the
phototautomerization occurs also in alcohol solvates,
and, moreover, that the solvent-assisted reaction
involving two protons must be faster than the single
proton intramolecular process, as evidenced by the rel-
ative increase in the intensity of the tautomeric vs. nor-
mal emission.
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shows the additive effects of specific and nonspecific sol-
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