Solvent influence on excited-state intramolecular proton transfer in 3-hydroxychromone derivatives studied by cryogenic high-resolution fluorescence spectroscopy

Article abstract of DOI:10.1016/S1386-1425(02)00361-X

High-resolution Shpol'skii spectra (recorded at 10 K in n-octane) of 3-hydroxychromone (3HC) substituted at the 2-position with a furan (3HC-F), a benzofuran (3HC-BF) or a naphthofuran group (3HC-NF) are presented. Being close analogues of 3-hydroxyflavone (3HF), these compounds can undergo excited-state intramolecular proton transfer (ESIPT). Luminescence can occur from the normal N* state (blue) or from the tautomeric T* state (green). Whether blue or green emission is observed is strongly dependent on hydrogen-bonding interactions with the environment. For all three chromones studied, high-resolution emission spectra in the green region (T*→T) were obtained in pure n-octane, showing four sites with distinct emission bands and detailed vibrational structures, whereas no blue emission was detected. Contrary to the spectra published for 3HF, the emission lines were very narrow (line-broadening effects beyond detection) which implies that the ESIPT rate constants are >10¹² s^-1, at least 25 times lower than for 3HF. In order to study the effects of hydrogen-bonding solvents, four isomers of octanol (1-, 2-, 3- and 4-octanol) were added, forming 1:1 complexes with the 3HC derivatives. For all the combinations considered both blue and additional green emission was observed and in some cases narrow-banded spectra were obtained, mostly in the green. Only for the 3HC-NF/2-octanol complex, narrow-banded emission was found both in the blue and in the green region. It is demonstrated that these emissions come from different configurations of the complex. Possible structures for the two complex species are proposed, supported by semi-empirical calculations on complex formation enthalpies.

Full text of DOI:10.1016/S1386-1425(02)00361-X

Spectrochimica Acta Part A 59 (2003) 1593ꢂ1603
/
www.elsevier.com/locate/saa
Solvent influence on excited-state intramolecular proton
transfer in 3-hydroxychromone derivatives studied by
cryogenic high-resolution fluorescence spectroscopy
Arjen N. Bader a, Vasyl Pivovarenko b, Alexander P. Demchenko c,
Freek Ariese a, Cees Gooijer a,*
a Department of Analytical Chemistry and Applied Spectroscopy, Laser Centre, Vrije Universiteit, De Boelelaan 1083, 1081 HV
Amsterdam, The Netherlands
b Department of Chemistry, Kiev Taras Shevchenko University, Kiev 01033, Ukraine
c TUBITAK Research Institute for Genetic Engineering and Biotechnology, Gebze-Kocaeli 41470, Turkey
Received 3 June 2002; received in revised form 16 October 2002; accepted 16 October 2002
Abstract
High-resolution Shpol’skii spectra (recorded at 10 K in n-octane) of 3-hydroxychromone (3HC) substituted at the 2-
position with a furan (3HC-F), a benzofuran (3HC-BF) or a naphthofuran group (3HC-NF) are presented. Being close
analogues of 3-hydroxyflavone (3HF), these compounds can undergo excited-state intramolecular proton transfer
(ESIPT). Luminescence can occur from the normal N* state (blue) or from the tautomeric T* state (green). Whether
blue or green emission is observed is strongly dependent on hydrogen-bonding interactions with the environment. For
all three chromones studied, high-resolution emission spectra in the green region (T*0
/
T) were obtained in pure n-
octane, showing four sites with distinct emission bands and detailed vibrational structures, whereas no blue emission
was detected. Contrary to the spectra published for 3HF, the emission lines were very narrow (line-broadening effects
beyond detection) which implies that the ESIPT rate constants are ꢀ
1012 sꢀ1, at least 25 times lower than for 3HF. In
/
order to study the effects of hydrogen-bonding solvents, four isomers of octanol (1-, 2-, 3- and 4-octanol) were added,
forming 1:1 complexes with the 3HC derivatives. For all the combinations considered both blue and additional green
emission was observed and in some cases narrow-banded spectra were obtained, mostly in the green. Only for the 3HC-
NF/2-octanol complex, narrow-banded emission was found both in the blue and in the green region. It is demonstrated
that these emissions come from different configurations of the complex. Possible structures for the two complex species
are proposed, supported by semi-empirical calculations on complex formation enthalpies.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Chromone derivatives; 3-Hydroxyflavone; Shpol’skii spectroscopy; ESIPT; Proton transfer; Ratiometric probe
1. Introduction
* Corresponding author. Tel.: ꢁ
20-444-7543.
E-mail address: gooijer@chem.vu.nl (C. Gooijer).
/
31-20-444-7540; fax: ꢁ31-
/
For more than 20 years it has been known that
3-hydroxyflavone (3HF, see Scheme 1) undergoes
1386-1425/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 1 3 8 6 - 1 4 2 5 ( 0 2 ) 0 0 3 6 1 - X

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A.N. Bader et al. / Spectrochimica Acta Part A 59 (2003) 1593ꢂ1603
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Scheme 1. Chemical structures of 3HF, 3HC-F, 3HC-BF, and 3HC-NF.
excited-state intramolecular proton transfer
(ESIPT) [1]. The interesting feature of this mole-
cule is that fluorescence can be observed originat-
ing from two different excited states, its normal
form (N*) and its tautomeric form (T*). Numer-
ous experiments have demonstrated that, in solu-
tion, the occurrence of these emissive forms
depends on both non-specific interactions with
the solvent (polarity effects) and on competition
from hydrogen-bonding solvent molecules for the
intramolecular hydrogen bond between the 3-
hydroxy group and the 4-carbonyl, which is the
deuterioxyflavone (3DF), the bandwidths ob-
served for the latter were much narrower. The
reason for this is that deuteron transfer is much
slower than proton transfer; apparently for 3HF
the additional (homogeneous) broadening is due
to lifetime effects, whereas for 3DF such effects are
rather small. Since the lifetime of N* is limited by
ESIPT, the additional broadening in the excitation
spectrum is a measure for the rate of that process.
In a similar way, the rate of back proton transfer
(BPT) from the tautomeric ground-state T to the
normal ground-state N can be obtained from the
site of the ESIPT reaction [2ꢂ
a hydrogen-bonding solvent, mainly blue emission
(380ꢂ460 nm) originating from N* is observed. In
a non-polar solvent, however, ESIPT takes place
and the tautomer T* is formed that emits in green
/
7]. In the presence of
additional broadening in the T*0
spectrum. Thus, it was estimated that ESIPT takes
place in 39910 fs, whereas BPT is five times
slower: 210930 fs. These results are in good
/
T emission
/
/
/
agreement with the data obtained in recent ultra-
fast pump/probe experiments, where an instru-
ment-limited 35 fs was reported for the rise time of
T* [10]. Furthermore, it should be noted that the
results point to a tunnelling mechanism of proton/
deuteron transfer that may be coupled with vibra-
tions along the intramolecular hydrogen bond.
In addition, it was shown that the Shpol’skii
approach can be used to study the effect of a
hydrogen-bonding impurity in detail [13]. It was
found that only a few hydrogen-bonding impu-
rities are able to form a complex with 3HF that fits
in the n-octane matrix and that their geometrical
structure is a key factor; only two of the four
octanol isomers yielded high-resolution Shpol’skii
spectra. It was observed that when a minor
amount of 2-octanol was added, an additional
site appeared in emission that was about 7 nm blue
shifted. Addition of a much larger amount of 3-
octanol had a similar effect, whereas addition of 1-
octanol or 4-octanol did not change the spectrum
(510ꢂ
rescence lifetime measurements [2ꢂ
lation in argon [8,9] and ultrafast pump/probe
experiments [10ꢂ12] have been used to determine
the rate of the transfer process and to study the
transfer mechanism both in the presence and
absence of a hydrogen-bonding solvent.
The application in these studies of high-resolu-
tion spectroscopy in cryogenic Shpol’skii matrices
is very attractive, since it allows one to exclude the
/
570 nm). Various methods, including fluo-
/
7], matrix iso-
/
universal soluteꢂsolvent interactions that generate
/
the inhomogeneous broadening of spectra and
produce solvent polarity effects that may influence
the ESIPT kinetics. In a recent paper [13], we
showed that detailed kinetic and mechanistic
information about the proton transfer in 3HF
can be obtained using steady-state Shpol’skii
spectroscopy in n-octane at 10 K. When compar-
ing the Shpol’skii excitation and emission spectra
of 3HF to those of its deuterated analogue 3-

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1595
at all. Apparently, the position of the hydroxy
group in octanol determines whether or not the
3HF/octanol complex fits in the matrix and gives
rise to the narrow-banded structure of the spec-
trum. The vibrational energies of the additional
sites observed in the presence of 2-octanol or 3-
octanol were different from the ones of uncom-
plexed 3HF. It was concluded that the additional
sites originate from 3HF/2-octanol (or 3HF/3-
octanol) complexes. Both ESIPT and BPT are
influenced by the presence of the OH group of
octanol and a model for the changed proton
transfer mechanism in these complexes was sug-
gested.
As mentioned earlier, 3HF and its derivatives
show very intense fluorescence spectra with a large
separation between the N* and T* emission bands.
They are highly sensitive to the polarity and the
hydrogen-bonding properties of the environment.
Because of these unique features, they are often
used as fluorescent probes for micelles, proteins
prohibited and one in which the proton transfer is
influenced to a minor extent. Furthermore, the
most likely structures of these complexes are
presented.
2. Experimental
The synthesis of 3HC-BF and 3HC-NF has
been described in earlier work [21]. Klymchenko
(TUBITAK RIGEB, Turkey) kindly provided
these samples. 3HC-F was prepared by alkaline
condensation of 2-furaldehyde with 2-hydroxyace-
tophenone (both obtained from Aldrich) and
subsequent oxidative heterocyclization of the re-
sulting chalcone with hydrogen peroxide [24].
Light greenish-yellow crystals of 3HC-F were
recrystallized twice from ethanol to give a pure
product according to TLC and 1H NMR with m.p.
171ꢂ172 8C. In the mass spectrum, the chromone
/
3HC-F showed the peak of the molecular ion Mꢁ
at 228 c.u. (the calculated molecular mass for
C13H8O4 is 228.2). The 1H NMR spectrum of
3HC-F was measured on a Varian Mercury-400
apparatus in DMSO-d6. Below are the chemical
and biological membranes [14ꢂ20]. Most impor-
/
tant in this respect is their application as two-
wavelength ratiometric probes for spectroscopy
and microscopy of living cells [14]. Attempts to
improve these properties have been made during
shifts, multiplicities (s*
/
singlet, d*
/
doublet, t*
/
recent years [14,21ꢂ/23]. Most successful in this
triplet, m*multiplet), and signal intensities (in
/
respect was the substitution of the 2-phenyl group
by furyl derivatives, which led to an increase in
fluorescence quantum yield and lifetime and a shift
in both absorption and fluorescence spectra to
longer wavelengths. These molecules show in-
creased sensitivity to solvent perturbations. In
this paper, we focus on three 2-furyl derivatives,
i.e., on 2-furyl-3-hydroxychromone (3HC-F), 2-
benzofuryl-3-hydroxychromone (3HC-BF) and 2-
naphthofuryl-3-hydroxychromone (3HC-NF; see
Scheme 1). The spectroscopic properties of 3HC-
BF and 3HC-NF have already been described in
previous papers [14,21]. In order to understand
their increased sensitivity to solvent perturbations,
detailed information about the structures of the
complexes between the 3HC derivatives and the
hydrogen-bonding molecule is needed. In this
paper, steady-state Shpol’skii spectroscopy is
used for this purpose. It will be shown that two
different types of monosolvate complexes can be
formed, one in which the proton transfer is fully
brackets): 9.80s (1H), 8.12d (1H), 7.90s (1H), 7.74t
(1H), 7.63d (1H), 7.43t (1H), 7.31d (1H), 6.70m
(1H). The 13C NMR spectrum of 3HC-F was
measured in acetone-d6: 147.7, 136.5, 128.5, 127.6,
121.2, 118.8, 115.7 (all-tertiary carbons), 175.4,
157.9, 147.3, 141.9, 139.4, 124.2 (quaternary
carbons).
Semi-empirical theoretical calculations were
performed to determine the possible geometrical
structures in the electronic ground state and the
formation energies of a number of 3HC-F/octanol
complexes. Energy minimization in vacuo was
done with the semi-empirical AM1 method using
the MOPAC 6.0 program.
10 mM solutions of 3HC-F, 3HC-BF or 3HC-
NF in puriss n-octane (Fluka) were studied. The
solvents 1-, 2-, 3- and 4-octanol (all ꢀ97% purity)
/
were obtained from Fluka, Sigma, Fluka and
Aldrich, respectively. The samples were trans-
ferred to a home-made sample holder containing
four sample cells. The sample holder was cooled to

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/
10 K by a Cryodyne Model 21 closed-cycle helium
refrigerator (CTI Cryogenics, Waltham, MA). The
sample was illuminated at an angle of 308 with a
XeCl excimer laser (Lambda Physik LPX 110i,
Gottingen, Germany) pumping
¨
a dye laser
(Lambda Physik, LPD 3002). The excimer laser
was operated at 10 Hz, producing 10 ns 50 mJ
pulses. Using QUI as a dye, a tunable output in the
370ꢂ400 nm range was obtained, while Exalite 411
/
was used to excite in the range from 403 to 420 nm;
excitation spectra were recorded with 0.05 nm
steps. The fluorescence emission was collected by a
3 cm F/l.2 quartz lens and focused on the entrance
slit of a Spex 1877 0.6 m triple monochromator
(Edison, NJ) by a 10 cm F/4 quartz lens. The
spectral resolution of this monochromator was 0.1
nm. For detection an Andor Technologies (Bel-
fast, Northern Ireland) intensified CCD camera
type iStar DH720-25U-03 was used in the gated
mode. The gate was opened at the end of the
(scattered) laser light pulse.
Fig. 1. Emission spectra of 3HC-F in n-octane, step-scanning
the excitation wavelength from 372 to 382 nm. 3HC-F is present
in four sites, emitting at 520.3, 522.3 (probably two overlapping
sites), 523.5 and 525.9 nm.
3. Results and discussion
3.1. Site structures
High-resolution Shpol’skii spectra of the tauto-
meric form of 3HC-F were obtained using n-
octane as a solvent, the same solvent used for 3HF.
In Fig. 1, an excitation-emission plot of 3HC-F in
an n-octane matrix at 10 K is shown. It can be seen
that 3HC-F is present in four sites in the matrix,
Fig. 2. Emission spectra of 3HC-BF in n-octane, step-scanning
the excitation wavelength from 383 to 389 nm. 3HC-BF is
present in four sites, emitting at 534.2, 537.4, 538.9 and 539.7
nm.
with T*0/T (0,0) emission transitions at 520.3,
522.3 (probably two overlapping sites), 523.5 and
525.9 nm, respectively. Three of these sites are
simultaneously excited at about 373 nm. Only the
lem
ꢃ
/
525.9 nm site has a distinctly red shifted
the third compound studied here, i.e., 3HC-NF,
high-resolution fluorescence spectra in a 10 K n-
octane matrix were recorded as well with analo-
gous results. The excitation-emission plot in Fig. 3
shows that 3HC-NF fits in the matrix in four
different ways with (0,0) emission transitions at
550.1, 552.9, 555.2 and 558.6 nm, all having
similar vibrational energies.
N0
/N* (0,0) excitation transition, i.e., at 381 nm.
Despite the relatively large energy difference
between the four sites dealt with, their vibrational
energies in emission are the same, i.e., unaffected
by the differences in surrounding. Also for 3HC-
BF, four sites are present in the high-resolution
Shpol’skii spectra obtained in n-octane (see Fig.
2). The T*0
found at 534.2, 537.4, 538.9 and 539.7 nm; their
vibrational energies are practically identical. For
/T (0,0) emission transitions can be
The three compounds studied here have in
common that they all fit in an n-octane matrix in

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1597
be different in a crystalline n-octane matrix and
cannot straightforwardly be used to calculate the
intensity distribution over the sites.
3.2. Bandwidths
Compared with 3HF, the bandwidths of 3HC-F,
3HC-BF and 3HC-NF in both emission and
excitation are very narrow, for the (0,0) line as
well as for the vibronic transitions. The band-
widths, obtained from the excitation and emission
spectra, are approximately 0.2 nm in all cases. In
comparison, for 3HF in excitation, the bandwidth
is 10 times larger (2 nm; 150 cmꢀ1) and in
emission three to four times larger (0.7 nm; 26
cmꢀ1) [13]. The bandwidths as observed for the
3HC derivatives are amongst the narrowest that
can be obtained with our approach. Line-broad-
Fig. 3. Emission spectra of 3HC-NF in n-octane, step-scanning
the excitation wavelength from 405 to 411 nm. 3HC-NF is
present in four sites, emitting at 550.1, 552.9, 555.2 and 558.6
nm.
four different ways, whereas 3HF shows only two
sites. This difference in site structure can be
understood as follows. Probably, the molecule
3HF embedded in the polycrystalline n-octane
matrix prefers to adopt a planar configuration,
although semi-empirical AM1 calculations for this
molecule in vacuo indicate that a twisted config-
ening effects in excitation*
excited-state proton transfer N*0
/
reflecting the rate of
/
T**are, there-
/
fore, beyond detection. The same holds for the
bandwidth in emission that reflects the BPT from
T to N in the electronic ground state. Also here,
the proton transfer rates in 3HC-F, 3HC-BF and
3HC-NF appear to be too low to affect the
bandwidths. Nonetheless, from the observed
widths the maximum values of the ESIPT and
BPT rates of furyl derivatives of 3-hydroxychro-
mone can be inferred; they are both lower than
uration (fꢃ308) is somewhat more favourable
/
(0.6 kcal/mol) [25]. In the Shpol’skii emission
spectrum, two substitutional sites are observed
[13] since for the planar 3HF molecule two
different orientations are possible in the n-octane
matrix, a phenomenon quite frequently observed
for planar hydrocarbons [26]. Contrary to 3HF,
for the three 3HC derivatives concerned the
molecule is present in two inequivalent planar
conformations, one in which the oxygen of the
furan ring is directed towards the oxygen at the 1-
1012 sꢀ1. This is much slower than ESIPT (25ꢄ
/
1012 sꢀ1) and BPT (5ꢄ1012 sꢀ1) in 3HF [13].
/
Apparently, the energy barrier heights and the
energy differences between N* and T* for ESIPT
and between N and T for BPT are significantly
different for both molecules, leading to very
different tunnelling rates. Theoretical calculations
to support and quantify this interpretation are
beyond the scope of the work presented here.
position of the chromone ring (fꢃ08 conformer)
/
and the other in which the oxygen of the furan ring
is pointing towards the 3-hydroxy group of the
chromone ring (fꢃ
/
1808 conformer). Because
3.3. Effects of hydrogen-bonding impurities
these two conformers will have different (0,0)
transitions, the total number of sites for 3HC-F
increases to 4. Semi-empirical AM1 calculations
for the isolated 3HC-F molecule support this
interpretation. Contrary to 3HF, for which the
As mentioned in Section 1, liquid-state experi-
ments have shown that in comparison to 3HF the
3HC derivatives have an increased sensitivity to
solvent effects, but in hydrogen-bonding solvents
one cannot distinguish the effects of polarity and
of hydrogen-bonding proper. Since in Shpol’skii
matrices the inhomogeneous broadening is largely
fꢃ08 and 1808 conformers are identical; there are
/
indeed two planar conformations, which differ 2.5
kcal/mol in energy. Obviously, these energies may

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/
eliminated, these spectra provide more detailed
information and one can study the effects of the
furyl moiety on hydrogen bonding under condi-
tions that are not influenced by solvent polarity.
This approach was successfully applied in the
studies of 3HF [13]. In this study, we have
systematically investigated the complexes formed
between the 3HC derivatives and a hydrogen-
bonding impurity by Shpol’skii spectroscopy.
Since the 3HC derivatives considered fit in an n-
octane matrix, four octanol isomers were used to
model the hydrogen-bonding impurity; these sol-
vent molecules have the highest probability of
forming a complex that fits in the matrix as well.
We observed that when any of the octanol
isomers was added to 3HC-F, 3HC-BF or 3HC-
NF, in the low-temperature emission spectra both
blue emission of the N* form and additional green
emission of the T* form could be seen. Interest-
ingly, not all the emission observed was narrow
banded. In the case of 3HC-F, only in the presence
of 2-octanol did we obtain narrow-banded green
emission, coming from an additional site with the
(0,0) emission transition at 510.1 nm (see Fig. 4); in
the blue emission no sharp lines could be observed.
For 3HC-BF, only addition of 2-octanol or
(although less favourable) 3-octanol gave rise to
narrow-banded additional emission in the green
(see Fig. 5) and also here not in the blue. The (0,0)
emission transitions of the 3HC-BF/2-octanol and
3HC-BF/3-octanol complexes were found at 522.2
and 526.4 nm, respectively. It should be empha-
sized that narrow bands in the blue region were
observed for 3HC-NF with 1-octanol or 2-octanol
added. The same molecule showed narrow-banded
green emission as well, i.e., upon addition of 2-
octanol or 3-octanol (see Fig. 6). The associated
sites have (0,0) emission transitions at 424.7 (1-
octanol added), 416.5, 535.0 (2-octanol added) and
544.3 nm (3-octanol added).
From the results shown here, the following can
be concluded. Considering the vibrational patterns
of the additional green sites, it should be noted
that these are different from the ones of the
uncomplexed compounds. This indicates that for
the complexed and the uncomplexed molecules the
tautomeric species formed are different. Of course,
Fig. 5. Shpol’skii emission spectra of 3HC-BF in n-octane at 10
K. The upper spectrum shows one of the sites in the absence of
octanol, the middle spectrum shows the additional site when
0.1% 2-octanol was added and the lower spectrum shows the
additional site in the presence of 0.1% 3-octanol. For ease of
comparison, the spectra are plotted as a function of the energy
difference in wavenumbers with respect to the (0,0) transition,
giving the ground-state vibrational energies. The wavelengths of
the (0,0) transitions are also given.
Fig. 4. Shpol’skii emission spectra of 3HC-F in n-octane at 10
K. The upper spectrum shows one of the sites in the absence of
octanol, the lower spectrum shows the additional site upon
addition of 0.5% 2-octanol. For ease of comparison, the spectra
are plotted as a function of the energy difference in wavenum-
bers with respect to the (0,0) transition, giving the ground-state
vibrational energies. The wavelengths of the (0,0) transitions are
also given.

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1599
The most striking feature of the results obtained
here is the simultaneous presence of both addi-
tional green and blue emission upon addition of a
hydrogen-bonding impurity, which indicates the
emission of both N* and T* forms. In principle,
this could be explained in two ways. Either the rate
of ESIPT is slowed down so heavily that it
becomes of the same order of magnitude as the
fluorescence decay rate of N*, i.e., with a factor as
large as 103ꢂ104, or, alternatively, two different
/
complex species are observed that behave quite
differently as far as ESIPT is concerned. In one of
these complexes the proton transfer is influenced,
but nonetheless it must be much faster than about
1010 sꢀ1 so that blue emission is obstructed and
green emission is observed exclusively. In the other
complex, the proton transfer is fully prohibited,
i.e., slower than 109 sꢀ1 so that blue emission of
N* is observed and T* is not formed at all.
If the former explanations were correct, both the
blue and the green emission would originate from
the same conformation of the 3HC-NF/octanol
complex, i.e., from a single species. If such a
complex fits in the matrix, then narrow bands are
expected both in the blue and in the green.
However, from the results shown above it is clear
that the presence of narrow bands in the green
region does not necessarily mean that the blue
emission is narrow banded as well. Therefore, the
latter explanation is more likely: the complexes
that give emission in the blue are different from the
ones that give green emission. Special attention
should be given to the case of 3HF-NF in the
presence of 2-octanol that showed both narrow-
banded blue and green emission (see Fig. 6). In
Fig. 7, the Shpol’skii excitation spectra recorded at
Fig. 6. Shpol’skii emission spectra of 3HC-NF in n-octane at
10 K. The upper spectrum shows the blue emission when 0.2%
of 1-octanol is added, the second spectrum shows the blue
emission upon addition of 0.2% of 2-octanol, the third spectrum
shows the additional green site when 0.2% 2-octanol is added
and the lower spectrum shows the additional green site in the
presence of 0.2% 3-octanol. For ease of comparison, the spectra
are plotted as a function of the energy difference in wavenum-
bers with respect to the (0,0) transition, giving the ground-state
vibrational energies. The wavelengths of the (0,0) transitions are
also given.
for 3HC-NF, the high-resolution emission spec-
trum in the blue reflects the ground-state vibra-
tions of the N-form of 3HC-NF in a complex with
1-octanol or 2-octanol. Unfortunately, the vibra-
tional bands in the presence of 1-octanol are very
low in intensity; nonetheless they appear to be
rather similar to those for the 2-octanol complex.
We only obtained more than marginal differences
in vibrational energies for the bands at 815/825,
1460/1486, 1550/1570 and 1619/1637 cmꢀ1 for the
N-form of 3HC-NF complexed to 1-octanol and 2-
octanol (see Fig. 6).
lem
ꢃ445.3 and 535.0 nm are compared. Because
/
both the band positions and the shapes of these
spectra are different, we can conclude that the blue
and the additional green fluorescence are emitted
from two different complexes indeed.
When considering the possible structures of the
‘‘blue’’ and the ‘‘green’’ complexes, the following
has to be taken into account. The complexes are
observed at a similar concentration of octanol and
show a similar intensity of emission. Furthermore,
since a complex composed of more than one
solvent molecule is not likely to fit in the matrix

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A.N. Bader et al. / Spectrochimica Acta Part A 59 (2003) 1593ꢂ1603
/
fewer degrees of freedom. According to the
calculations, the binding of a methanol molecule
causes a red shift (1ꢂ4 nm) of the absorption
/
maximum in all cases. This is in line with the
experimental results.
In the first possible structure, configuration 1 in
Table 1, there is a hydrogen bond between the
hydrogen of octanol and the oxygen of the
chromone’s 3-hydroxy group, while in configura-
tion 2 the oxygen of the carbonyl group of the
chromone is involved. It is very unlikely that one
of these two structures gives the extra green
spectra observed because the vibrational pattern
of configurations 1 and 2 are not expected to be
different from that of uncomplexed 3HC-F. A
Fig. 7. Shpol’skii excitation spectra of 3HC-NF in n-octane
upon addition of 0.2% of 2-octanol. The spectrum was obtained
by scanning the dye laser and recording a spectrum every 0.05
nm. The upper spectrum shows the excitation spectrum of the
structure in which*
/
instead of ESIPT*the pro-
/
additional green site at lem
ꢃ/535.0 nm. The lower spectrum
ton is transferred externally to the hydroxy group
of octanol (configuration 3 in Table 1) is unlikely
as well: the anion thus created will certainly not
emit fluorescence in the narrow green region
concerned. The hydrogen bond between the octa-
nol’s hydrogen and the oxygen at the 1-position in
the chromone ring (configuration 4) is presumably
weak so that this structure should also be left out
of consideration. This holds even more for the S1
state where the lone pair electrons of the chromo-
ne’s oxygen are involved in the p-electronic system
that makes it less basic, contrary to the oxygen of
the chromone’s carbonyl group. The hydrogen
bond to the latter group is, therefore, expected to
be much stronger, especially in the S1 state, where
charge transfer from the furan to the chromone
ring takes place. The most likely structure is,
therefore, configuration 5 in Table 1. It is char-
acterized by two hydrogen bonds in which the
octanol’s hydrogen as well as the octanol’s oxygen
atoms are involved. For this seven-ring complex,
the vibrational energies are expected to be differ-
ent from those of free 3HC-F. Interestingly,
configuration 5 has the most favourable enthalpy
of formation within the series of structures con-
sidered this far. Although one has to be careful in
using such quantitative gas-phase data, they may
serve to support the above conclusion. Last, but
not least, it should be recalled that a similar
complex has been proposed in the case of 3HF/
octanol [13].
shows the excitation spectrum of the blue emission at lem
ꢃ
/
445.2 (the vibrational band at 1570 cmꢀ1 was chosen as
emission wavelength to be able to excite in the (0,0) region as
well).
in a regular way, only 1:1 complexes composed of
one 3HC derivative and one octanol molecule need
to be considered. For the ‘‘green’’ 1:1 complexes,
ESIPT takes place while the interaction with the
octanol induces a change of the T-ground-state
vibrational pattern. For the ‘‘blue’’ 1:1 complexes,
the internal hydrogen bond between the carbonyl-
and the hydroxy-group is probably weakened or
even broken so that ESIPT is inhibited. Finally,
the complexes considered should be stable in the
N-form of the electronic ground state, otherwise
they would not have been formed in the n-octane
matrix at all. Of course, they should also fit in the
matrix in order to show Shpol’skii spectra; un-
fortunately, this property cannot be easily related
to the detailed molecular structure of the complex.
Possible structures for the ‘‘green’’ complexes
are given in configurations 1ꢂ5 of Table 1.
/
Enthalpies of formation based on minimization
in vacuum for the suggested complexes are in-
cluded in the table. 3HC-F/methanol complexes
were used as model systems, because their enthal-
pies of formation have the same dependence on the
structure as 3HC-F/octanol complexes, whereas
the calculations for 3HC-F/octanol are less precise
than for 3HC-F/methanol because the latter has

A.N. Bader et al. / Spectrochimica Acta Part A 59 (2003) 1593ꢂ
/1603
1601
Table 1
Possible configurations of complexes formed between the 3HC derivatives and a hydrogen-bonding impurity
For simplicity 3HC-F is shown. The drawn structures correspond to the 08 conformer, unless otherwise indicated.a Formation
enthalpy values were calculated for the 3HC-F/methanol 1:1 complexes.
Possible structures of the ‘‘blue’’ complex in
which ESIPT is weakened or even blocked are
configurations 6 and 7 in Table 1. In these
structures, the orientation of the 3-hydroxy group
of the chromone is no longer towards the carbonyl
group. Furthermore, since the furyl chromones are
much stronger influenced by the presence of a
hydrogen-bonding impurity than 3HF, it is feasi-
ble that the oxygen of the furan group plays an
important role in the complex formation, and so

1602
A.N. Bader et al. / Spectrochimica Acta Part A 59 (2003) 1593ꢂ1603
/
the furan chromone is probably present in the fꢃ
/
there is only a weak intramolecular hydrogen
bond, which is part of a five-membered ring [27].
1808 conformation. If the octanol molecule forms
a hydrogen bond with the carbonyl group of the
chromone, the 3-hydroxy group of the chromone
can form a hydrogen bond with the oxygen of the
furan ring (configuration 6). Another possibility is
the formation of a hydrogen bond between the
octanol and the 3-hydroxy group of the chromone,
on the one hand, and the furan oxygen, on the
other (see configuration 7). The latter structure is
less likely than complex 6 because the seven-ring
might cause a non-planarity that will not fit very
well in the matrix. Gas-phase AM1 calculations
indicate that configuration 7 has the energetically
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