High resolution electronic spectra of 7-azaindole and its Ar atom
van der Waals complex
Cheolhwa Kang, John T. Yi, and David W. Pratt
a͒
Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
͑Received 15 February 2005; accepted 8 June 2005; published online 7 September 2005͒
Rotationally resolved fluorescence excitation spectra of the S
1
←S
0
origin band of 7-azaindole
͓1H-pyrrolo͑2,3-b͒pyridine͔ and its argon atom van der Waals complex have been recorded and
assigned. The derived rotational constants give information about the geometries of the two
molecules in both electronic states. The equilibrium position of the argon atom in the azaindole
complex is considerably different from its position in the corresponding indole complex.
Furthermore, the argon atom moves when the UV photon is absorbed. There are significant
differences in the intermolecular potential energy surfaces in the two electronic states. A large,
vibration-state-dependent rotation of the S
1
←S
0
electronic transition moment vector of 7-azaindole
relative to that of indole suggests that these differences have their origin in S
1
/S
2
electronic state
mixing in the isolated molecule, a mixing that is enhanced by nitrogen substitution in the
six-membered ring. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1990119͔
INTRODUCTION
The doubly hydrogen bonded dimer of 7-azaindole ͑7AI͒
has been extensively studied both in the gas phase and in the
condensed phase.
1–5
This is because ͑7AI͒
2
undergoes a
double proton transfer reaction on excitation with light. Pro-
ton transfer ͑PT͒ reactions in electronically excited states are
fundamentally important chemical reactions. They also play
a crucial role in a large variety of photochemical and biologi-
cal processes, such as DNA base-pair tautomerization.
The driving force for an excited state PT in ͑7AI͒
2
is the
electronic rearrangement that occurs on excitation of its
ground state ͑S
0
͒ to the first
*
excited state ͑S
1
͒. Studies
of these two states of isolated 7AI and related molecules
should aid in the elucidation of this dynamics and the tau-
tomerization process. In the gas phase, the
1
L
b
state is gen-
erally the lowest excited state in indole, with the
1
L
a
-
1
L
b
energy gap depending on the attached substituents. In 7AI, a
relatively small
1
L
a
-
1
L
b
gap has been reported.
1
Because the
1
L
a
state is believed to be more polar than the
1
L
b
state, it is
preferentially stabilized by interaction with the environment,
the result being that
1
L
a
emission dominates in the polar
solvent and possibly also in ͑7AI͒
2
.
Kim and Bernstein
6
analyzed the nature of the first ex-
cited singlet states of 7AI and several of its complexes with
rare gas atoms and other small molecules. Differences in
their behavior compared with those of indole were attributed
to strong
*
-n
*
mixing and to the hydrogen atom attached
to the pyrrole nitrogen of 7AI being out of the molecular
plane in the S
1
state. Huang et al.
7
also studied jet-cooled
7AI and 7AI clusters with polar solvent molecules. Their
data suggest mixing with the
1
L
a
͑S
2
͒ state rather than with an
n
*
state.
The character of an electronically excited state is often
revealed by the orientation of its electronic transition mo-
ment ͑TM͒, in transitions from the ground state. The orien-
tations of the TM’s of the S
1
and S
2
states of 7AI have been
determined both theoretically and experimentally. Catalan
and Perez
8
predicted TM angles of +7.5°͑S
1
͒ and −13°͑S
2
͒
with respect to the a inertial axis. Ilich
9
predicted TM angles
of −0.8°͑S
1
͒ and −23°͑S
2
͒ using semiempirical intermediate
neglect of differential overlap/S1-configuration-interaction
͑INDO/S1-CI͒ methods. From a complete active space self-
consistent-field ͑CASSCF͒ study, Borin and
Serrano-Andrés
10
suggested a value of +27° for the S
1
state.
Experimentally, an evaluation of the S
1
-S
0
rotational contour
by Hassan and Hollas
11
gave an ab-hybrid band with 93% a-
and 7% b-type character, resulting in an angle of ±15°. Na-
kajima et al.
12
obtained a value of −16±5° from studies of
several azaindole–͑H
2
O͒
n
͑n=1,2,3͒ complexes in the gas
phase. More recently, Schmitt et al.
13
obtained the value
−21° based on their analyses of the high resolution spectra of
four different isotopomers of 7-azaindole. This study also
provided accurate values of the rotational constants of both
electronic states of the isolated molecule.
High resolution electronic spectroscopy is an extremely
powerful tool for addressing such issues. Previously, we have
used this method in a detailed study of indole and indole–Ar
in their S
0
and S
1
electronic states.
14
Here, a comparable
study of 7AI and its Ar complex is described. This study
yields unique information about the position of the attach-
ment of the Ar atom to the 7AI frame, its large amplitude
motions, and how these change when the photon is absorbed.
These properties of 7AI are quite different from those of
indole. The two molecules also have significantly differently
oriented S
1
-S
0
TM’s. Thus, their quite different properties
appear to have their origin in the differences in the electronic
distributions of the two species, which may be traced to the
single substitution of the nitrogen atom for the C7 carbon in
the six-membered ring.
a͒
Author to whom correspondence should be addressed. Electronic mail:
pratt@pitt.edu
THE JOURNAL OF CHEMICAL PHYSICS 123, 094306 ͑2005͒
0021-9606/2005/123͑9͒/094306/7/$22.50 © 2005 American Institute of Physics123, 094306-1