Spectroscopy and photophysics of bifunctional proton donor-acceptor indole derivatives

Article abstract of DOI:10.1021/jp402767x

We report spectroscopic and photophysical studies of a series of selected indole derivatives in solutions and under supersonic jet isolation conditions. All the compounds can assume two rotameric forms, syn and anti. The bifunctional molecules containing

Full text of DOI:10.1021/jp402767x

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Article
Spectroscopy and Photophysics of Bifunctional
Proton Donor-Acceptor Indole Derivatives
Volha Vetokhina, Michal Kijak, Teodozja Marianna Lipinska, Randolph
Paul Thummel, Jerzy Sepiol, Jacek Waluk, and Jerzy Herbich
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp402767x • Publication Date (Web): 19 May 2013
Downloaded from http://pubs.acs.org on May 29, 2013
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Spectroscopy and Photophysics of Bifunctional Proton Donor-
Acceptor Indole Derivatives
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Volha Vetokhina,a Michał Kijak,a Teodozja M. Lipinska,b Randolph P. Thummel,c Jerzy
Sepiol,a Jacek Waluk,a,d and Jerzy Herbich*a,d
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a
b
Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01ꢀ224 Warsaw,
Poland
Institute of Chemistry, Siedlce University of Natural Sciences and Humanities, 3 Maja 54, 08ꢀ110
Siedlce, Poland
Department of Chemistry, University of Houston, Houston, TX, 77204ꢀ5003, USA
Faculty of Mathematics and Science, Cardinal Stefan Wyszyński University, Dewajtis 5, 01ꢀ815
Warsaw, Poland
c
d
*
Correspondence author:
Prof. Jerzy Herbich, eꢀmail: herbich@ichf.edu.pl, Fax: + 48 (22) 3433333
Abstract
We report spectroscopic and photophysical studies of a series of selected indole derivatives in
solutions and under supersonic jet isolation conditions. All the compounds can assume two
rotameric forms, syn and anti. The bifunctional molecules containing both the hydrogen bond
donor (indole NH group) and acceptor centers (oxygen, nitrogen, or sulfur atoms) located in
separate moieties covalently linked by a single bond are compared with the compound which
does not have any acceptor center, 2ꢀ(1Hꢀpyrrolꢀ2'ꢀyl)ꢀ1Hꢀindole. The former compounds
(containing furan, thiazole, or thiophene moieties) were anticipated to show solventꢀdependent
photophysics. Contrary to the expectations, all the compounds reveal very efficient fluorescence,
independent of solvent polarity and hydrogen bond donor and acceptor abilities. Laser
spectroscopic studies combined with supersonic jet techniques and quantum chemical
computations have been performed in order to identify the rotameric forms and to gain insight
into the changes in the molecular structure accompanying electronic excitation.
Keywords: UV/VIS spectroscopy, luminescence, bifunctional proton donor–acceptor indole derivatives, supersonic
jet spectroscopy.
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1.
Introduction
A special class of bifunctional molecules consists of compounds for which the hydrogen
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bond donor and acceptor centers are located in separate moieties, connected by a single bond (for
example, 2ꢀ(2'ꢀpyridyl)indole (2-PI) and 2ꢀ(2'ꢀpyridyl)pyrrole (PP)). Such structure often leads
to an appearance of two rotameric forms, syn and anti, with both centers on the same or on the
opposite sides of the molecule, respectively. The two conformers, of which the relative fractions
are often solventꢀdependent, may reveal drastically different excited state properties.1ꢀ5 Detailed
understanding of the mechanisms responsible for syn ꢀ anti equilibria may, therefore, lead to
applications of bifunctional molecules, e.g., as probes of solvent polarity and/or proticity.
A general scheme showing the solventꢀinduced rotamerization and excitedꢀstate
deactivation channels in bifunctional heteroazaromatic molecules such as 2-PI2,5 and PP5 has
been proposed previously. In nonpolar solvents only the syn species should be present (this
rotamer predominantly exist in the gas phase in supersonic jets6ꢀ7). This form can undergo
excitedꢀstate intramolecular proton transfer (ESIPT) process depending on the mutual
arrangement of the proton donor and acceptor centers. A small fraction of the anti form can be
expected in polar aprotic solvents, while in alcohols it becomes significant. The stabilization of
the anti structure is due to a larger ground state dipole moment and stronger and more linear
hydrogen bonds with hydroxylic partners than in the syn species.2,5 In some cases (as for 2ꢀ(2'ꢀ
pyridyl)indoles) the anti conformer can be more stable in alcohol solutions than the syn one
because of a stabilization of the anti structure by two separate alcohol molecules connected to
the opposite sides of a chromophore. In alcohols and water the excited state of the syn conformer
is rapidly depopulated due to a formation of cyclic hydrogenꢀbonded complexes with a protic
partner. The deactivation may involve S1→S0 enhanced internal conversion and/or fast excitedꢀ
state double proton transfer (ESDPT). The latter process occurs in 1:1 complexes on a timeꢀscale
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of single picoseconds or faster, whereas the former involves complexes with two (or more)
molecules of alcohol (1:n complex), and takes much longer (tens to hundreds of picoseconds).8
The lack of tautomeric fluorescence in alcohols combined with quenching of the primary
fluorescence indicates a domination of complexes other than those of cyclic structure and 1:1
stoichiometry.
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In this work we describe studies of selected bifunctional indole derivatives for which the
hydrogen bond donor (the indole NH group) and the acceptor centers are located in separate
moieties, covalently linked by a single bond (Scheme 1). The aim of the work is to compare
spectroscopy and photophysics of the compounds which can form syn (with the donor and the
acceptor on the same side) and anti (with the two groups on the opposite sides) conformers via
internal rotation around a single bond (2ꢀ(thiophenꢀ2'ꢀyl)ꢀ1Hꢀindole (THOIN), 2ꢀ(thiazolꢀ2'ꢀyl)ꢀ
1Hꢀindole (THAIN), and 2ꢀ(furanꢀ2'ꢀyl)ꢀ1Hꢀindole, FURIN), with the compound which does
not have any acceptor center, 2ꢀ(1Hꢀpyrrolꢀ2'ꢀyl)ꢀ1Hꢀindole (PYRIN). Due to the fact that only
the syn rotamers of bifunctional molecules are able to form cyclic, doubly hydrogen bonded
dimers or complexes with protic partners, it was expected that the former three molecules, but
not the latter compound, should exhibit solventꢀdependent photophysics. This group of
compounds was especially synthesized to study the effects of electronegativity of the hydrogen
bond acceptor centers (oxygen, nitrogen, or sulfur atoms) on their photophysical properties.
H
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3
1'
S
4
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3
3'
S
2
2'
N
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5
6
2
1
2'
5'
4'
5'
N
N
O
N
1
N
N
4'
1'
3'
H
H
H
H
FURIN
THAIN
THOIN
PYRIN
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Scheme 1. Structures and acronyms of the studied indole derivatives. The compounds are
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shown in the most stable ground state conformation.
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2.
Experimental
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We attempted to prepare 2ꢀ(thiophenꢀ2'ꢀyl)ꢀ1Hꢀindole and 2ꢀ(furanꢀ2'ꢀyl)ꢀ1Hꢀindole using
the Fischer indole synthesis under conditions elaborated earlier by us for 2ꢀ(pyridꢀ2'ꢀyl)ꢀ
indoles.9ꢀ10 Starting materials, 2ꢀacetylthiophene or 2ꢀacetylfurane, were converted to appropriate
phenylhydrazones, and, in the next step, to the indole moiety. The syntheses were carried out in
0.16 M zinc chloride solution in triethylene glycol (TEG) under controlled microwave heating.
However, this method was unsuccessful for the preparation of 2ꢀ(1Hꢀpyrrolꢀ2'ꢀyl)ꢀ1Hꢀindole,
which we obtained by the excellent method of Bocchi and Palla11 from 3ꢀbromoindole and
pyrrole. The details of the syntheses will be published elsewhere.
The 2ꢀ(thiazolꢀ2'ꢀyl)ꢀ1Hꢀindole was prepared as follows: a mixture of 2ꢀacetylthiazole
(1.27 g, 10.0 mmol), phenylhydrazine (1.30 g, 12.0 mmol), and glacial HOAc (4 drops) in
absolute EtOH (10 mL) was refluxed for 5 h. Evaporation of the solvent and chromatography on
alumina, eluting with CH2Cl2/petroleum ether (1:1), provided a mixture of E and Z isomers (2.16
g, 99%). This hydrazone mixture (1.67 g, 7.7 mmol), ZnCl2 (5.44 g) and glacial HOAc (110 mL)
was refluxed under Ar. After 24 h, additional ZnCl2 (2.72 g) was added, and reflux continued for
48 h. After cooling, the reaction mixture was concentrated and the residue was dissolved in
CH2Cl2 (50 mL) and 10% NaOH was added until pH 14 was obtained. The mixture was
sonicated for 15 min and the white residue filtered. The two layers were separated and the
aqueous phase was extracted with CH2Cl2 (3 x 50 mL). The combined organic phase was washed
with brine (50 mL), dried (MgSO4) and concentrated. Chromatography on silica gel, eluting with
CH2Cl2, provided 2ꢀ(2'ꢀthiazolyl)indole (0.92 g, 56%) as a white solid: mp 125ꢀ126 ˚C; 1HꢀNMR
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(CDCl3) δ 9.65 (bs, 1H), 7.81 (d, 1H, J = 3.3 Hz), 7.65 (d, 1H, J = 7.8 Hz), 7.40 (dd, 1H, J = 8.1,
1.2 Hz), 7.32 (d, 1H, J = 3.3 Hz), 7.26 (td, 1H, J = 8.1, 1.2 Hz), 7.13 (td, 1H, J = 7.8, 1.5 Hz),
7.04 (dd, 1H, J = 1.8, 0.9 Hz); 13CꢀNMR (CDCl3) δ 161.6, 142.7, 137.2, 131.5, 128.5, 124.0,
121.4, 120.5, 118.5, 111.7, 103.6 ppm.
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The solvents used for our studies: nꢀhexane (HEX), ethyl acetate (EA), acetonitrile
(ACN), 1ꢀpropanol, and 1ꢀbutanol were of spectroscopic or fluorescence grade (Aldrich or
Merck). All solvents were checked for the presence of fluorescing impurities.
Electronic absorption spectra were recorded on a Shimadzu UV 3100 spectrophotometer.
Stationary fluorescence and fluorescence excitation spectra were obtained using an Edinburgh
FS 900 CDT spectrofluorometer. The spectra were corrected using the spectral sensitivity curves
of the instrument. Fluorescence spectra were recorded as a function of wavelength (λ) and
subsequently multiplied by a factor of λ2 in order to convert counts per wavelength into counts
per wavenumber. For the quantum yield determinations, the solutions had identical optical
densities at the excitation wavelength. For the determination of quantum yields, quinine sulfate
in 0.05 mol dmꢀ3 H2SO4 was used as a standard ( = 0.51).12
ϕ
Fluorescence decay lifetimes were measured using timeꢀresolved single photon
counting on an Edinburgh FL 900 CDT spectrofluorometer with an estimated time resolution
of about 500 ps.
A homeꢀmade spectrometer used for obtaining LIF and dispersed fluorescence (DF)
spectra has been described previously.13ꢀ14 In our case the samples were heated up to 370 K.
They were diluted by 3 atm of helium and injected via a modified high temperature pulsed
nozzle (General Valve 9) through an orifice (0,5 or 0,2 mm in diameter). LIF and SVLF
experiments were carried out using optical parametric oscillator (OPO, Continuum, Sunlite LX)
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pumped by the 3rd harmonic of a Nd/YAG laser (Continuum, Powerlite 8010). The tunable
excitation pulses in the UV range were produced by doubling red radiation generated from the
OPO in KDP crystal. Fluorescence from the sample was collected by a toroidal mirror and
directed by a flipꢀflop plane mirror to a Hamamatsu R2949 photomultiplier connected to a
Yokogawa DL9140 oscilloscope to measure LIF spectra or to a slit of a Sciencetech, model 9040
spectrograph with an Andor iDUS CCD camera to accumulate DF spectra.
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All calculations were performed using Gaussian 0915 suite of programs. Ground state
geometries were optimized using density functional theory (DFT) with B3LYP (Becke, Lee,
Yang and Parr threeꢀparameter) hybrid functional.16ꢀ17 The electronic transition energies were
obtained by timeꢀdependent DFT (TDꢀDFT) formalism. For the excited state geometry
optimizations TDꢀDFT and configuration interaction singles (CIS) procedures were used. In all
cases the doubleꢀzeta 6ꢀ31+G(d,p) basis set was applied. For all the stationary points frequencies
have been calculated to check if the geometry corresponds to the energy minimum. The Franckꢀ
Condon analysis, with HerzbergꢀTeller approximation (FCHT) for absorption, implemented in
the Gaussian 09 package was used to compute vibrationallyꢀresolved absorption and emission
spectra. The frequency scaling factors of 0.964 (DFT/TDꢀDFT) and 0.92 (CIS) were applied
throughout the paper.
3.
Results and discussion
In order to estimate the relative ground state energies of the rotameric forms, theoretical
calculations at the B3LYP/6ꢀ31+G(d,p) level were performed (Table 1). The predicted low
energy ground state species of THAIN and FURIN under vacuum isolation conditions
correspond to the planar syn forms due to their stabilization by intramolecular hydrogen bond.
For PYRIN and THOIN the predicted lowest energy species are anti rotameric forms, with the
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dihedral angles of 22.4° and 23.2º, correspondingly. Their syn conformers are also nonplanar.
The nonplanar structure of PYRIN can be explained by strong repulsive interactions of the NH
hydrogen atoms that do not allow the compound to become planar. The computed energy gaps
between syn and anti conformers, however, are small; the values change from 0.1 kcal/mol for
THOIN to ꢀ3.1 kcal/mol for THAIN.
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Table 1. Ground and excited state data: relative energies (computed at the DFT/B3LYP/3ꢀ
61+G(d,p) level) of the various rotameric and tautomeric forms (ꢁE), dipole moments (ꢀg and
ꢀe, respectively) and dihedral angles between proton donating and accepting moieties.
FURIN
THAIN
THOIN
PYRIN
syn
anti
taut
syn
anti
taut
syn
0.0
1.1
ꢀ
2.0
2.5
ꢀ
0.0°
0.0°
ꢀ
0.0
3.1
23.9
1.4
2.5
6.6
0.0°
0.0°
0.0°
0.1
0.0
ꢀ
1.8
2.2
ꢀ
27.9°
23.2°
ꢀ
1.6
0.0
ꢀ
2.6
1.7
ꢀ
33.3°
22.4°
ꢀ
ꢁE,
kcal/mol
S0
ꢀ
g, D
dihedral
angle
anti
taut
TDꢀDFT
TDꢀDFT
0.0
TDꢀDFT
TDꢀDFT
syn
anti
taut
syn
0.0
2.0
ꢀ
0.8
1.9
ꢀ
0.3
0.0
ꢀ
2.0
3.0
ꢀ
1.3
0.0
ꢀ
3.1
1.8
ꢀ
ꢁE,
kcal/mol
4.0
ꢀ7.9
3.4
3.6
1.8
S1
anti
taut
syn
anti
taut
ꢀ
e, D
0.0°
0.0°
ꢀ
0.0°
0.0°
1.6°
0.0°
0.0°
ꢀ
7.3°
4.0°
ꢀ
dihedral
angle
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Additional calculations were performed for the tautomeric form of THAIN which show
relatively large destabilization energy of 23.9 kcal/mol of this form with respect to the syn
rotamer (noteworthy, FURIN and THOIN are not able to form the tautomeric species, in which
the proton is shifted from the indole nitrogen atom onto the furan oxygen atom or onto the
thiophene sulphur atom, respectively). The computed ground state dipole moments values ꢀg of
the syn and anti forms are in the range from 1.4 D to 2.6 D.
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Computations at the TDꢀDFT/6ꢀ31+G(d,p) level predict similar situation for the lowest
energy species, syn for FURIN and THAIN, and anti for THOIN and PYRIN. The stabilization
energy spans the range from 0.3 kcal/mol (THOIN) to ꢀ4.0 kcal/mol (THAIN). Both compounds
(THOIN and PYRIN), nonplanar in the ground state, have a tendency to flatten upon excitation
(THOIN becomes totally planar). Similarly to 2-PI and PP, the tautomeric form of THAIN is
predicted to be significantly stabilized in the excited state. The ab initio computations at the
CIS/6ꢀ31+G(d,p) level predict that in the excited state the syn rotamer is relatively stabilized for
all the compounds under study. The stabilization energy, however, which is significant (ca. 3.1
kcal/mol) for FURIN and THAIN, seems to be negligibly small for PYRIN and THOIN. As
was the case of DFT computations, THOIN and PYRIN have a tendency to flatten upon
excitation.
The absorption and fluorescence spectra of the indole derivatives recorded in three
solvents: nꢀhexane, acetonitrile, and 1ꢀpropanol, i.e. representing nonpolar, polar aprotic and
protic media, respectively, are presented in Figure 1. The corresponding spectroscopic and
photophysical data are collected in Tables 2 and 3.
The shape and spectral positions of the broad absorption and emission spectra of PYRIN
and FURIN are very similar: two lowest absorption bands and the single emission spectra in
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various solvents are located at about 32000 cmꢀ1, 41000 cmꢀ1, and 28500 cmꢀ1, respectively. The
spectra of sulfurꢀcontaining compounds, THAIN and THOIN, are considerably redꢀshifted. The
Stokes shift values vary from ca. 3700 (±100) cmꢀ1 for FURIN and PYRIN to about 5400
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(±300) cmꢀ1 for THOIN.
Figure 1. Room temperature absorption and corrected and normalized fluorescence spectra of
FURIN, THAIN, THOIN, and PYRIN (from top to bottom). Full lines, nꢀhexane; dashed lines,
acetonitrile; dotted lines, 1ꢀpropanol.
The fluorescence excitation spectra of FURIN, THAIN, THOIN, and PYRIN in nꢀhexane
and 1ꢀpropanol, as well as in acetonitrile match well the corresponding absorption spectra. This
finding indicates that the fluorescence originates from the same species which exist in the ground
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state. The absorption and emission spectra of all the compounds under study, in agreement with
the small values of the computed ground and excited state dipole moments (0.8ꢀ3.6 D), do not
show any significant solvent dependence.
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Table 2. Solvent effects on the spectral positions of the absorption and fluorescence maxima
and fl , correspondingly, cmꢀ1), quantum yields (
Φf), decay times (τ, ns) and the resulting
%
ν
(
%
ν
abs
radiationless (knr, sꢀ1) and radiative (kf, sꢀ1) rate constants, and electronic transition dipole
moments in absorption (Mabs, D) and fluorescence (Mfl, D).
b
b
a
kr ×108 knr ×108 Mfl Mabs
%
%
νfl
νabs
Φf
τ a
Compound
Solvent
HEX
ACN
32300
32250
32000
29900
30300
29800
31550
31400
31100
32200
31800
31500
28600
28600
28500
26400
25400
25200
26400
25700
25700
28700
28000
27800
0.78
0.87
0.89
0.23
0.24
0.18
0.22
0.25
0.27
0.72
0.84
0.89
1.4
1.6
5.6
5.4
1.6
0.8
0.6
5.4
5.5
5.2
4.3
4.4
4.7
FURIN
1ꢀPrOH
HEX
1.7
5.2
~0.6
0.9
~3.8
2.7
~12.8 ~5.0 4.9
8.4 4.6 5.0
~13.7 ~4.7 5.0
THAIN
THOIN
PYRIN
ACN
1ꢀPrOH
HEX
~0.6
~0.8
0.9
~3.0
~2.8
2.8
~9.8
8.3
7.3
2.3
1.0
0.7
~4.3 4.7
ACN
4.6
4.4
5.6
5.6
5.6
4.8
4.7
4.4
4.5
4.2
1ꢀPrOH
HEX
1.0
2.7
1.2
6.0
ACN
1.6
5.3
1ꢀPrOH
1.6
5.6
a
Estimated error: 10%. Thus, the maximum error is about 20% for the rate constants knr and kf
and about 10% for the transition moment Mfl. b Scatter of results: ± 150 cmꢀ1.
Contrary to our expectations, the photophysical data of FURIN, PYRIN, THAIN, and
THOIN do not show any solvent dependence (Table 2). The fluorescence quantum yields
Φ
f are
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high (ca. 20% for the latter two compounds and ca. 80% for FURIN and PYRIN). The four
5
6
7
times smaller
Φf values for the sulfur containing compounds most probably manifest the increase
8
9
of the efficiency of the intersystem crossing processes (leading to the increase of population of
the triplet manifold) due to a “heavy atom effect”.18 Fluorescence decay times obtained on the
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nanosecond time scale can be well fitted to a monoexponential decay; the lifetime values τ are of
about 0.5ꢀ1.0 ns for THAIN and THOIN, and about 1.2ꢀ1.7 ns for FURIN and PYRIN.
The obtained quantum yields (Φf) and lifetimes (τ) lead to large values of the radiative kf
rate constants (eq. 1) as well as those of the electronic transition dipole moments in fluorescence
Mfl (obtained in the electric dipole approximation, eq. 3)19ꢀ20
kf = Φf /
τ
,
(1)
(2)
knr = 1−Φf
(
)
/
τ
,
64π 4
3h
r
2
3
%
kf =
n
νfl Mfl
.
(3)
(
)
where knr is the radiationless rate constant, h is the Planck constant, n is the refractive index of a
%
ν
solvent, and
is the wavenumber of fluorescence maximum.
fl
This finding indicates that the radiative transitions in emission are fully allowed. Moreover, the
Mfl values are similar to those of the electronic transition dipole moments Mabs related to the
1
lowest (π,π*)←S0 absorption band (eq. 4)20
r
2
3ln10hc
~ ~
Mabs
=
ε(ν )dν
,
(4)
~
8
π 3NAnνabs
∫
~
~
%
ν , νabs is the wavenumber of
where
ε
(
ν
) is the molar absorption coefficient at wavenumber
absorption maximum,
N
A is the Avogadro number and is the speed of light.
c
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This observation indicates a similar nature of the FranckꢀCondon excited states initially reached
in absorption and the solventꢀequilibrated fluorescent states.
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9
It should be stressed that FURIN, THAIN, and THOIN, contrary to the other bifunctional
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molecules for which the proton donor and acceptor groups are located in separate moieties linked
by a single bond (such as 2ꢀ(2’ꢀpyridyl)indole,1ꢀ2 2ꢀ(2’ꢀpyridyl)pyrroles,4,7,21ꢀ22 and 2ꢀ
pyridylpyrazoles23ꢀ24), do not reveal any spectral and kinetic evidence of rotamerization (for all
the compounds) and/or photoinduced tautomerization processes. The lack of the efficient
deactivation of the lowest excited singlet states S1 in protic media via internal conversion (IC)
and/or ESDPT processes suggests that the studied compounds do not form cyclic, doubly
hydrogenꢀbonded complexes. In order to gain more insight into the formation of the complexes,
titration experiments have been performed. Upon adding 1ꢀbutanol, at concentrations low
enough to prevent the formation of alcohol oligomers,1 to
nꢀhexane solution of THAIN and
PYRIN an isosbestic point in the absorption spectra is observed. This fact demonstrates an
equilibrium between two individua in the ground state, most probably between the uncomplexed
and complexed forms.1 The absorption data yield the 1:1 stoichiometry of the complex of
PYRIN and the equilibrium constant of 30 (±
5) dm3 molꢀ1. On the other hand, the analysis of the
absorption of THAIN complexes does not lead to the unequivocal results. The analysis
performed in the lowꢀenergy range of the absorption spectra (at ~27300 cmꢀ1) results in the 1:1
stoichiometry, the determination of stoichiometry from the absorption data at ~30500 cmꢀ1 and
~28570 cmꢀ1 lead to the values of 1:1.4 and 1:1.6, respectively. The latter findings can hardly be
explained in terms of the model assuming a reaction involving one chromophore molecule and
n
alcohol molecules,1 but this result is most likely related to the errors caused by small absorption
changes upon adding alcohol. For THOIN and FURIN no measurable changes in absorption
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were observed in a wide range of alcohol concentrations. The fluorescence spectra of THAIN
,
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THOIN, and PYRIN show a small shift to lower energies with the increase of 1ꢀBuOH
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concentration; the emission intensities, however, do not change significantly.
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60
ꢀ1
~
ν
Table 3. Comparison between TDꢀDFT/B3LYP/6ꢀ31+G(d,p) calculated energies ( , cm ) and
oscillator strengths (f π,π*) states* of the
) corresponding to the vertical transitions to lowꢀlying 1(
syn and anti forms of the studied compounds, and locations of the experimental absorption band
maxima ( exp , cmꢀ1) and molar absorption coefficients (
ε
, dm3molꢀ1cmꢀ1) in
n
ꢀhexane.
~
ν
~
a
~
~
ε
ν
ν
νexp
syn
anti
Compound
State
f syn
State
f anti
S1
S2
S4
S6
31650 0.676
34270 0.093
38970 0.052
42060 0.129
S1
S2
S3
S6
31940
34390
39150
41590
0.688
0.087
0.059
0.119
32300
23000
FURIN
41000
9200
S1
S2
S6
29130 0.392
31150 0.285
40780 0.064
S1
S2
S7
29420
31400
36650
0.504
0.208
0.035
29900
39500
31550
41000
24100
6400
THAIN
THOIN
PYRIN
S1
S2
S6
30500 0.512
33360 0.115
41030 0.056
S1
S2
S6
30280
33200
40700
0.507
0.126
0.091
20800
10500
S1
S4
S5
33170 0.591
39400 0.101
39860 0.088
S1
S5
S8
32800
39440
41200
0.665
0.137
0.061
32200
40500
26000
12700
a Scatter of results:
±
150 cmꢀ1. * Only transitions of significant intensity are shown (f > 0.03).
*) states
The TDꢀDFT computed spectral positions of transitions to the lowest excited 1(
π,π
of the syn and anti rotameric forms of all the compounds under vacuum isolation conditions are
in excellent agreement with the experimentally measured absorption maxima (Table 3). It should
be stressed, however, that the computed energies and oscillator strengths of the lowꢀlying
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vertical transitions of both rotameric forms of each compound are very similar. Therefore, the
position and intensities of the nearꢀUV experimental absorption bands do not allow us to
distinguish which forms of the compounds exist in the ground state. The lowest computed
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transitions S1
electron jump from the highest occupied molecular orbital (HOMO) to the lowest unoccupied
molecular orbital (LUMO). These La S0 type transitions carry high oscillator strength.
←S0 for all the compounds are dominated by a configuration corresponding to an
←
In an attempt to identify rotameric forms and to gain more insight into the changes
accompanying electronic excitation of FURIN and THAIN (as representatives of the studied
indole derivatives) high resolution spectroscopic investigations under supersonic jet isolation
conditions have been performed. The experimental laserꢀinduced fluorescence (LIF) excitation
(which provides information about vibrational frequencies in the lowest excited singlet state S1)
and dispersed fluorescence (DF) spectra (showing vibrational frequencies in the ground state S0),
obtained for excitation into specific vibronic level (SVLF) of bare molecules, are compared with
the DFT/TDꢀDFT calculated absorption and emission spectra obtained by the FranckꢀCondon
analysis. The 0ꢀ0 regions of the LIF and DF spectra of FURIN and THAIN show a rich pattern
of lowꢀfrequency vibrations (Figure 2). The origins of the S1
←S0 transitions are assigned to the
lowest energy bands of the LIF spectra which are observed at 31612 (
±
1) cmꢀ1 and 29350 (
± 1)
cmꢀ1, respectively. The 0ꢀ0 bands of the LIF spectra of both compounds are found to be relatively
strong, which suggests that no dramatic structural changes in FURIN and THAIN take place
upon 0ꢀ0 excitation to the S1 state. TDꢀDFT calculations predict the positions of the S1←S0
vertical transitions of FURIN at 31645 cmꢀ1 for the syn rotameric form and at 31940 cmꢀ1 for the
anti one; the computed values are very similar and both are in a very good agreement with the
experimental value. The same situation is observed for THAIN: the computed values of the
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S0 vertical transitions at 29130 cmꢀ1 for the syn and at 29415 cmꢀ1 for the anti rotameric
3
4
S1
←
5
6
forms do not allow us to distinguish between both rotameric forms.
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8
9
The comparison of the experimental LIF spectra with the DFT/TDꢀDFT computed
absorption spectra suggest that the LIF spectra of FURIN and THAIN monomer correspond to
the syn forms. For example, the spectral positions of the FURIN bands observed at 89 cmꢀ1, 179
cmꢀ1, and 270 cmꢀ1 are in a very good agreement with the bands at 89 cmꢀ1, 178 cmꢀ1, and 266
cmꢀ1 theoretically predicted for the syn form. For THAIN, theoretically predicted spectral
positions of the vibronic bands at 80 cmꢀ1, 159 cmꢀ1, 238 cmꢀ1 for the syn rotamer correspond to
the ones experimentally observed at 78 cmꢀ1, 156 cmꢀ1, and 236 cmꢀ1, respectively. The computed
absorption spectra of the anti forms of both compounds do not show these bands.
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The spectral positions of the other bands observed in the computed absorption of the syn
form fit satisfactorily to the LIF spectra. On the basis of theoretical computations tentative
vibrational assignments are proposed in Table 4. The calculations predict that the compounds
should be planar in both S0 and S1 states. FURIN exhibits 63 fundamental transitions, 43 of
those are due to inꢀplane polarized (a’) ones, whereas 20 correspond to outꢀofꢀplane (a”)
vibrations. THAIN has 60 normal modes, 41 of a’ and 19 of a” symmetry. Due to the fact that
the lowest excited singlet states of the compounds are of 1( *) character (the 1(n,
π,π π*) states are
predicted to lie higher than 40000 cmꢀ1) only totally symmetric, inꢀplane a’ vibrations are
expected to appear in the LIF and SVLF spectra. The transitions involving odd quanta of outꢀofꢀ
plane vibrations are nonꢀtotally symmetric and thus symmetry forbidden. Thus, only even
overtones of the latter vibrations are expected to be observed in the experimental spectra.
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Figure 2. LIF excitation (bottom) and the absorption spectra (middle, syn, top, anti) of FURIN
(left) and THAIN (right) calculated at the DFT/TDꢀDFT/B3LYP/6ꢀ31+G(d,p) level. Scaling
factor: 0.964.25ꢀ26 The inset presents the enlarged spectrum (intensity×100).
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ꢀ1
~
ꢁ
νcalc
Table 4. Transition energies (cm ) and tentative assignments (
computed at the TDꢀDFT
~
ꢁν
(in brackets) and CIS/B3LYP/6ꢀ31+G(d,p) levels) of the vibronic transitions (
exp ) in the lowꢀ
7
8
9
energy part of the LIF excitation spectraa of FURIN and THAIN
.
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FURIN
THAIN
~
~
b
b
~
~
assign. *
assign. *
ꢁν exp
ꢁν exp
ꢁ
νcalc
ꢁ
νcalc
syn
31645
94 (89)
50 (57) ×2 56 (58) ×2
anti
syn
29130
88 (80)
50 (48) ×2 46 (49) ×2
anti
31612
89
108
179
0ꢀ0
A
2×B
31940
98 (96)
29350
78
93
0ꢀ0
A
2×B
29415
93 (90)
2×
A
156
2×
+2×B?
+2×B
or 2×C
3×
A
A+2×B
202
171
A
or 2×C
98 (89) ×2 83 (77) ×2
A
216
269
293
4×B
178
236
255
87 (72) ×2 68 (43) ×2
3×
A
A
2×A
+2×B
+2×C
2×
or
A
A
+2×B
+2×C
D
ꢀ
or
A
305
321
325
263
277
307
D
312 (299) 315 (306)
333 (322) 337 (319)
270 (259) 270 (254)
318 (308) 319 (308)
A+4×B?
6×B
E
3×A+2×B
330
361
381
335
342
355
E
or 2×
A+2×C
4×
A
A
+D
3×
A+2×B
2×B+
D
or 2×A
+2×C
395
413
419
439
A+D
386
396
421
465
A+E
2×B+
2×B+D
E
D
E
A+E
2×
2×
A
A
+
+
2×B+
4× +2×B
+2×C
E
A
472
485
F
488 (470) 489 (465)
or 3×A
485
492
~508
512
2×A+D
522
545
571
596
ꢀ
ꢀ
G
H
I
560 (536) 558 (538)
595 (580) 592 (580)
623 (605) 622 (596)
2×
A+E
F
522 (502) 523 (505)
estimated error: 17 cmꢀ1.7 b Scaling coefficients: 0.964 (DFT) and 0.92 (CIS). 25ꢀ26 * Bold, inꢀ
a
plane vibrations; normal, outꢀofꢀplane ones.
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The transitions of FURIN (Table 4) observed at 89 cmꢀ1 (A), 305 cmꢀ1 (D), 330 cmꢀ1 (E),
and 512 cmꢀ1 (F) from the origin most probably correspond to the inꢀplane modes which are
visualized in Figure 3. The spectrum shows also their overtones at 179 cmꢀ1, 269 cmꢀ1, and 361
cmꢀ1. A most prominent is the progression built on the 0ꢀ0 transition by the inꢀplane C2ꢀC2'
rocking vibration. Combinations (the bands at 395 cmꢀ1, 485 cmꢀ1, as well as 419 cmꢀ1 and ~508
cmꢀ1) are assigned to the progressions of the 89 cmꢀ1 vibration built on the 305 cmꢀ1 and 330 cmꢀ1
fundamental transitions, respectively). Under assumption that even overtones of outꢀofꢀplane
vibrations are optically active, the band observed at 108 cmꢀ1 (2×B) is assigned to the active
overtone of the vibration predicted at 50 cmꢀ1 (CIS method) and at 57 cmꢀ1 (TDꢀDFT method) for
the syn form. Noticeably, the origin of the band recorded at 202 cmꢀ1 is not clear. On one hand, it
can be assigned to the combination of the 89 cmꢀ1 and 108 cmꢀ1 vibrations (A+2×B). On the
other hand, it corresponds well to the even overtone (2×C) of the vibration computed by CIS
method at 98 cmꢀ1 for the syn rotamer (the TDꢀDFT computed frequency at 89 cmꢀ1 does not fit
perfectly to this assignment, Figure 3). The even overtone of a similar outꢀofꢀplane mode is
observed for 2-PI at 163 cmꢀ1.6 Several observed transitions (e.g., those at 293 cmꢀ1, 321 cmꢀ1,
381 cmꢀ1, 413 cmꢀ1, 439 cmꢀ1, and 472 cmꢀ1) can be satisfactorily described by combinations of
the 2×B and/or 2×C overtones with inꢀplane modes. It is interesting to note that the most active
in the LIF spectrum are the vibrations related to the C2ꢀC2' single bond which links the indole
ring with furan or thiazole moieties, i.e. the inꢀplane rocking, as well as outꢀofꢀplane rocking
and/or torsion (Figure 3).
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S0
S1
ass.
syn
anti exp.
syn
anti exp.
6
7
8
9
outꢀofꢀ
plane
torsion
CIS 2×98 2×83
TDꢀ 2×89 2×77
DFT
2×46 2×31
84
202
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outꢀofꢀ
plane
rocking
CIS 2×50 2×56
TDꢀ 2×57 2×58
DFT
68
68
ꢀ
108
CIS
TDꢀ
DFT
94
89
98
96
inꢀplane
rocking
101 103
309 306
327 327
105
312
332
89
A
D
E
CIS
TDꢀ
DFT
312
299
315
302
inꢀplane
stretching
305
330
inꢀplane
skeletal
deformation
CIS
TDꢀ
DFT
333
322
337
319
inꢀplane
skeletal
deformation
CIS
TDꢀ
DFT
522
502
523
505
506 505
-
512
F
Figure 3. Forms of several lowꢀfrequency normal modes of FURIN with DFT/B3LYP/6ꢀ
31+G(d,p) (S0) or TDꢀDFT and CIS/B3LYP/6ꢀ31+G(d,p) (S1) computed (for the syn and anti
forms) and experimental (in bold) frequency values (cmꢀ1) for the ground (S0) and fluorescent
(S1) state. The experimental frequency values of the outꢀofꢀplane vibrations correspond to the
first even overtones.
Similarly to FURIN, the intense bands of THAIN observed at 78 cmꢀ1 (A), 263 cmꢀ1 (D),
307 cmꢀ1 (E), 485 cmꢀ1 (F), 545 cmꢀ1 (G), 571 cmꢀ1 (H), and 596 cmꢀ1 (I) are assigned to
fundamentals of various inꢀplane vibrations (Figure 2, Table 4). The spectrum shows a
progression (observed at 156 cmꢀ1 and 236 cmꢀ1) of the lowest vibrational mode corresponding to
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the inꢀplane C2ꢀC2' rocking vibration and its combination with the other inꢀplane modes (the
bands at 342 cmꢀ1 and 421 cmꢀ1, as well as at 386 cmꢀ1 and 465 cmꢀ1 are assigned to the
progressions of the 78 cmꢀ1 vibration built on the 263 cmꢀ1 and 307 cmꢀ1 fundamental transitions,
respectively). The band of lower intensity recorded at 93 cmꢀ1 is assigned to the even overtone of
the outꢀofꢀplane C2ꢀC2' rocking vibrations (2×B) computed at 50 cmꢀ1 (CIS) and 48 cmꢀ1 (TDꢀ
DFT) for the syn form. The band at 178 cmꢀ1, similarly to FURIN, can be assigned either to the
combination of the 78 cmꢀ1 and 93 cmꢀ1 vibrations (A+2×B) or to the even overtone (2×C) of the
C2ꢀC2' torsion (which is theoretically predicted at 87 cmꢀ1 (CIS) or 72 cmꢀ1 (TDꢀDFT) for the
syn rotameric form). Several optically active bands represent combinations of these transitions
(2×B and A+2×B/or 2×C) with various inꢀplane vibrations (e.g., observed at 255 cmꢀ1, 335 cmꢀ1,
355 cmꢀ1, and 396 cmꢀ1).
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Vibrationallyꢀresolved DF spectra of FURIN and THAIN, obtained for excitation into
specific vibronic levels (SVLF) are shown in Figure 4. These spectra give information about
corresponding vibrational modes in the ground and S1 electronic states. Excitation into the origin
of FURIN and into the vibronic bands corresponding to the fundamental (89 cmꢀ1) and overtones
(179 cmꢀ1 and 269 cmꢀ1) of the C2ꢀC2' inꢀplane vibration (Table 5) leads to the SVLF spectra
showing the prominent progression of this vibration at 105 cmꢀ1, ~208 cmꢀ1, and ~312 cmꢀ1 (the
last one can also originate from a longꢀaxis inꢀplane stretching vibration). Thus, most probably
the vibrations 89 cmꢀ1 and 105 cmꢀ1 represent the same mode in the S1 and S0 states, respectively.
On the other hand, the SVLF spectrum recorded upon excitation to the overtone of the outꢀofꢀ
plane vibrational band at 202 cmꢀ1 shows a different pattern of the low frequency vibrations. The
spectrum shows clear overtones of the 84 cmꢀ1 vibrational mode (e.g., observed at 165 cmꢀ1) and
combinations with the 105 cmꢀ1 mode and its overtones (observed at 185 cmꢀ1, 268 cmꢀ1, 283 cmꢀ
1
and ~380 cmꢀ1). It indicates that this mode at 84 cmꢀ1, which probably corresponds to the even
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overtone of the outꢀofꢀplane C2ꢀC2' torsion, increases its energy from about 42 cmꢀ1 to about 101
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cmꢀ1 upon S1
←S0 excitation. This hypothesis is supported by the analysis of charges on the furan
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nitrogen and indole oxygen atoms. In the ground state both of them are negatively charged. Upon
electronic excitation the charge on the nitrogen atom becomes positive. This effect forces them
to attract each other, which, in turn, leads to the increase of frequency of the respective outꢀofꢀ
plane C2ꢀC2' torsional mode.
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ꢀ1
~
ꢁ
νcalc
Table 5. Transition energies (cm ) and tentative assignments (
computed at the
~
ꢁ
ν
DFT/B3LYP/6ꢀ31+G(d,p) level) of the vibronic transitions ( exp ) in the lowꢀenergy part of the
SVLF spectra a of FURIN and THAIN
.
FURIN
THAIN
excitation into
the origin (0ꢀ0)
excitation into
the origin (0ꢀ0)
ν exp
excitation into
2×C ? (202 cmꢀ1)
~
~
b
~
b
~
~
assign. *
assign.
assign. *
ꢁ
ꢁ
ν exp
ꢁν exp
ꢁ
νcalc
ꢁνcalc
syn
anti
syn
anti
0
0ꢀ0
0
0ꢀ0
84
105
2×C'
83
2×C'
4×C'
'+2×C'
A'+4×C'
46×2 31×2
A'
94
A'
101
103
96
96
165
185
268
190
2×A'
186
208
~285
312
332
~380
A
'+2×C'
2×
'+2×C'
' or
A
A
'
2×
3×
A
283 2×
A
'+2×C'
'+4×C'
A
D
'
270
319
363
D'
ꢀ
309
327
306
327
262
267
E
'
2×
A
2×C'+
'+4×C'
' or
~380 2×
411
A
A
'+D
'
E
414
2×C'+E
'
407
ꢀ
A'+D'
433
517
537
607
A
'+
'+
'+
'
E
D
E
'
461
564
609
642
2×A'+D'
2×A
'
'
G
H
ꢀ
'
'
560
595
560
594
2×A
H
596
596
a
estimated error: 3ꢀ6 cmꢀ1.7 b The scaling coefficient is 0.964.25ꢀ26 * Bold, inꢀplane vibrations;
normal, outꢀofꢀplane ones.
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The presented assignment of the corresponding vibrational modes in the ground and S1
electronic states is supported by the vibrational structure of the SVLF spectrum obtained for the
simultaneous excitation of the inꢀplane and outꢀofꢀplane modes at 293 cmꢀ1 (both 84 cmꢀ1 and
105 cmꢀ1 bands of similar intensity are observed in the spectrum). The computed ground and
excited state vibrations of FURIN (Figure 3, all the data of calculations are presented in Ref. 27)
are in agreement with the proposed correlations. The analysis of the vibrational structure of the
LIF and SVLF spectra indicates that the most considerable geometry change following electronic
excitation to the fluorescent state takes place along the inꢀplane and outꢀofꢀplane low frequency
modes connected with the C2ꢀC2' single bond.
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SVLF spectra of THAIN recorded for excitation into the origin and into two other vibronic
levels (corresponding to the inꢀplane C2ꢀC2' rocking vibration and its overtone) are presented in
Figure 4. The bands observed at 94 cmꢀ1, 270 cmꢀ1, 564 cmꢀ1, and 609 cmꢀ1 are assigned to
fundamentals of the various inꢀplane modes (Table 5). The spectra show an overtone of the inꢀ
plane C2ꢀC2' rocking vibration (at 190 cmꢀ1) and several combinations (e g., at 363 and 461
cmꢀ1). Contrary to FURIN, the spectra do not show any sign of the optical activity of the outꢀofꢀ
plane C2ꢀC2' vibrations, which are predicted to be observed at about 96 cmꢀ1 (the first even
overtone of the torsional vibration) and at about 132 cmꢀ1 (the overtone of the rocking vibration).
The analysis of the LIF and SVLF spectra of THAIN suggests that the change of its geometry
takes place along the inꢀplane C2ꢀC2' rocking vibration. The frequency value of this particular
mode decreases from 94 cmꢀ1 to 78 cmꢀ1 upon electronic excitation.
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Figure 4. SVLF (top) and calculated emission (syn form, at the DFT/TDꢀDFT/B3LYP/6ꢀ
31+G(d,p) level) spectra of FURIN (left) and THAIN (right). Experimental data was obtained
for excitation to the origin (0ꢀ0) and to different vibronic bands corresponding to the
fundamental (A) and overtone (2×A). Additionally, SVLF FURIN spectra obtained for
excitation to 3×A overtone of the inꢀplane C2ꢀC2' rocking vibration as well as to the even
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overtone of the outꢀplane C2ꢀC2' torsional vibration (2×C) and its combination with the inꢀplane
A mode (293 cmꢀ1) or, depending on the interpretation, to the combinations of A and 108 cmꢀ1
vibrations are presented.
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4.
Summary
This work presents spectroscopic and photophysical investigations and quantum chemical
calculations for a series of bifunctional indole derivatives which can form syn and anti
conformers via internal rotation around a single bond (THOIN THAIN, and FURIN) and their
comparison with the compound which does not have a hydrogen bond acceptor center (PYRIN).
The study of solvent effects on the photophysical properties of FURIN THAIN, and THOIN
,
,
does not reveal any spectral or kinetic evidence of rotamerization and/or photoinduced
tautomerization processes. The compounds show strongly allowed and efficient fluorescence.
The lack of efficient deactivation of the lowest excited singlet states S1 in protic media, either via
internal conversion (IC) or ESDPT processes can be explained by assuming that the studied
compounds do not form actively quenched cyclic, doubly hydrogenꢀbonded complexes. The use
of supersonic jet techniques combined with laser spectroscopy and TDꢀDFT calculation leads to
the results which suggest that the syn rotamer is a dominant form of THAIN and FURIN. The
analysis of the LIF and SVLF spectra indicates that the changes following electronic excitation
are connected with the outꢀofꢀplane rocking and torsion motions along C2ꢀC2' single bond which
links the indole ring with the furan or thiazole moiety.
Finally, it should be stressed that the preliminary investigations of complexes of FURIN
and THAIN with acetonitrile, methanol, and water under supersonic jet conditions have also
been performed. The obtained LIF excitation spectra (not shown), however, are difficult to
analyze without parallel investigations by resonant twoꢀphoton ionization (R2PI) and IR/R2PI
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ion depletion spectroscopy, combining the structurally sensitive infrared (IR) vibrational
predissociation spectroscopy with the mass selective R2PI spectrometry. These laser
spectroscopy/ supersonic jet experiments are in progress in our laboratory.
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Acknowledgments
This work was supported by grant NN 204 264 238 from the Ministry of Science and Higher
Education. The technical help and assistance of Dr. Paweł Borowicz and Mrs. Anna Zielińska is
greatly appreciated. RPT thanks the National Science Foundation (CHEꢀ0911354) and the
Robert A. Welch Foundation (Eꢀ621) for financial support. We acknowledge the computing
grant G17ꢀ14 from the Interdisciplinary Centre for Mathematical and Computational Modeling
of the Warsaw University.
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