Effect of molecular structure and hydrogen bonding on the fluorescence of hydroxy-substituted naphthalimides

Article abstract of DOI:10.1039/a904520a

Fluorescence properties of hydroxy-naphthalimides were studied in methylene chloride in the absence and the presence of hydrogen-bonding additives. The position of the HO-substituent only slightly affects the radiative rate, however, the triplet yield and the rate of the radiationless processes are considerably higher for the 3-hydroxy derivative. Addition of nitrogen-heterocyclic compounds leads not only to hydrogen-bonding in the ground state but also fluorescence quenching. The parallel change through of the series of the hydrogen-bond acceptors between the proton affinity and the rate constants of dynamic quenching indicates that proton displacement plays a crucial role in the excited hydrogen-bonded complexes. Interaction of hydroxy-naphthalimides with pyridine arid benzoxazole results in rapid radiationless deactivation from the singlet excited state, whereas intense emission as well as long fluorescence lifetime characterize imidazole and pyrazole complexes. The dual emission of the imidazole complexes observed in solvents of medium polarity is assigned to two conformers which differ in the extent of the proton shift along the hydrogen-bond.

Full text of DOI:10.1039/a904520a

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E†ect of molecular structure and hydrogen bonding on the
Ñuorescence of hydroxy-substituted naphthalimides
Laszlo Biczok,*a Pierre Valatb and Veronique Wintgensb
a Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 17, 1525 Budapest,
Hungary
b L aboratoire des Materiaux Moleculaires, C.N.R.S., E.R. 241, 2, 8 rue H. Dunant, 94320 T hiais,
France
Received 7th June 1999, Accepted 23rd August 1999
Fluorescence properties of hydroxy-naphthalimides were studied in methylene chloride in the absence and the
presence of hydrogen-bonding additives. The position of the HO-substituent only slightly a†ects the radiative
rate, however, the triplet yield and the rate of the radiationless processes are considerably higher for the
3-hydroxy derivative. Addition of nitrogen-heterocyclic compounds leads not only to hydrogen-bonding in the
ground state but also Ñuorescence quenching. The parallel change throughout the series of the hydrogen-bond
acceptors between the proton affinity and the rate constants of dynamic quenching indicates that proton
displacement plays a crucial role in the excited hydrogen-bonded complexes. Interaction of
hydroxy-naphthalimides with pyridine and benzoxazole results in rapid radiationless deactivation from the
singlet excited state, whereas intense emission as well as long Ñuorescence lifetime characterize imidazole and
pyrazole complexes. The dual emission of the imidazole complexes observed in solvents of medium polarity is
assigned to two conformers which di†er in the extent of the proton shift along the hydrogen-bond.
Hydroxy-substituted naphthalimides were chosen as model
Introduction
compounds in these studies because they have both hydrogen-
bond donor and acceptor moieties, and, on the basis of our
previous studies,12 they are expected to be strongly Ñuores-
cent. In addition, the electron withdrawing character of the
imide group probably enhances the acidity in the excited state
and light absorption may serve as an ultrafast trigger for the
proton transfer reaction. Recent studies of related compounds
demonstrated that substitution of naphthols with cyano or
methanesulfonyl groups markedly increases the photoaci-
dity.13h15
Another main goal of this work was to examine how the
introduction of a hydroxy substituent into the 1,8-naphthali-
mide moiety alters the dominant energy dissipating pathways
occurring from the singlet excited state. The investigated com-
pounds are given in Scheme 1.
The molecular mechanism of the excited state relaxation
induced by intermolecular hydrogen-bond formation is of
great interest because it belongs to the most fundamental pro-
cesses of photochemistry. Most of the studies in this Ðeld have
dealt with the e†ect of the hydrogen-bond donors on the Ñuo-
rescent properties of aromatic heterocyclic1 and carbonyl
compounds.2h6 These molecules form hydrogen-bonded com-
plexes with alcohols and hydroperoxides in the excited state
and the hydrogen-bond acts as an efficient vibronic dissipative
mode in the nonradiative transition.2h6 On the other hand,
coupled electronÈproton movement was found to play a dom-
inant role in the deactivation of excited hydrogen-bonded
porphyrins7,8 and Ru(II)polypyridyl complexes8 as well as in
the interaction of excited ketones with phenols.4,9
Picosecond laser photolysis studies established that excited
hydrogen-bond donors, such as aromatic TNÈH or ÈOÈH
compounds, are efficiently quenched by pyridine derivatives
via non-Ñuorescent hydrogen-bonded complex, in which
associated electron and proton displacement facilitates the
charge transfer interaction between the two conjugate p-
electronic systems.10 However, when phenols form hydrogen-
bonded complexes with aliphatic amines no charge
delocalization is possible along the hydrogen-bond and
photoexcitation induces proton transfer.11
The present paper focuses on the question of how the varia-
tion of the molecular structure of the hydrogen-bonding addi-
tive inÑuences the Ñuorescent properties and the deactivation
mechanism of the excited molecules. In order to reveal the
role of aromaticity and proton affinity in the hydrogen-
bonding induced quenching process, aromatic heterocyclic
compounds were used as hydrogen-bond acceptors. We show
examples where the excited hydrogen-bonded complexes emit
dual Ñuorescence, which is assigned to two conformers di†er-
ing in the extent of the proton shift along the hydrogen-bond.
Experimental
Acetonitrile, methylene chloride (Prolabo, HPLC grade),
dimethylsulfoxide (Merck, spectroscopic grade) and tri-
Ñuoroethanol (Aldrich) were used as received. Benzoxazole,
imidazole, isoxazole, pyrazole and pyridine were purchased
from Aldrich (highest quality available).
N-Methyl-1,8-naphthalimide (NI), also called 2-methyl-1H-
benz[d,e]isoquinoline-1,3(2H)-dione, was available from our
previous study.16 4-Hydroxy-N-methyl-1,8-naphthalimide (4-
HONI),
also
called
6-hydroxy-2-methyl-1H-benz[d,e]
Scheme 1
Phys. Chem. Chem. Phys., 1999, 1, 4759È4766
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isoquinoline-1,3(2H)-dione, was prepared by demethylation of
4-methoxy-N-methyl-1,8-naphthalimide,17 following a pro-
cedure similar to one described previously.18 One equivalent
of the methoxy derivative mixed with 40 equivalents of pyri-
dine hydrochloride was heated at 190 ¡C under argon for 35
min. After cooling, the solid reaction mixture was added to an
aqueous HCl solution (1 M). The solid was Ðltered, washed
with aqueous HCl solution (1 M) and water. The crude
product was puriÐed by dissolution in an aqueous Na CO
2
3
solution, followed by extraction with ether and precipitation
by the addition of perchloric acid in the aqueous phase (45%
yield), m.p. 298È302 ¡C (lit.18 303È305 ¡C).
3-Hydroxy-N-methyl-1,8-naphthalimide (3-HONI), also
called 5-hydroxy-2-methyl-1H-benz[d,e]isoquinoline-1,3(2H)-
dione, was synthesized via four reaction steps. First, 3-nitro-
N-methyl-1,8-naphthalimide was prepared by condensation of
3-nitro-1,8-naphthalic anhydride (Aldrich) with methylamine
hydrochloride in acetic acid. Then 1 equivalent of 3-nitro-N-
methyl-1,8-naphthalimide was reduced by 4 equivalents of
tin(II)chloride in hot hydrochloric acid.19 The obtained amino
derivative was diazotized and the diazonium salt was hydro-
lyzed by aqueous HCl solution. The Ðnal product was puriÐed
by the method described for 4-HONI (15% yield, m.p. 249È
251 ¡C).
The UVÈvisible absorption spectra were obtained with a
VarianÈCary model 50 Bio apparatus. Fluorescence spectra
were recorded with an SLM-Aminco model 8100 device. Fluo-
rescence quantum yields of 3-HONI and 4-HONI were deter-
mined by comparison with that of 4-methoxy-N-methyl-1,8-
naphthalimide in acetonitrile solution, for which a reference
Fig. 1 Absorption and Ñuorescence spectra of 3-HONI (ÈÈ) and
4-HONI (É É É) in CH Cl .
2
2
cates the larger extent of conjugation between the electron
donating OH and the electron withdrawing imide moiety in
4-HONI. The Stokes-shift of the Ñuorescence spectrum is
more pronounced (78 nm) for 4-HONI compared with that of
3-HONI (35 nm). Changing the solvent from methylene chlo-
ride to acetonitrile leads to a 6 nm and 2 nm Ñuorescence
maximum displacement for the former and the latter com-
pounds, respectively. These small solvatochromic shifts
together with the small decrease of the S state energy with
1
increasing solvent polarity (vide infra) suggest that the lowest
excited singlet states have limited charge transfer character for
both HONI isomers.
yield of U \ 0.88 was taken.12 Singlet lifetimes were mea-
Fig. 2 shows the excitation and the Ñuorescence spectra of
the hydroxy-naphthalimides and their conjugated bases in
acetonitrile. In this solvent we used 1.5 ] 10~4 M perchloric
acid to prevent the dissociation of the OH-moiety and to
record solely the spectra corresponding to the phenolic form.
In order to deprotonate the OH group, 2 ll of 1 M KOH in
methanol was added into 2 ml of hydroxy-naphthalimide
solution. Under these conditions, the spectra are assigned to
the naphtholate anion and the bands are located at lower
energies. Based on the intersection of the normalized excita-
tion and Ñuorescence spectra, 305 kJ mol~1 and 210 kJ mol~1
were calculated for the energies of the Ðrst excited singlet state
of 3-HONI and its conjugated base, respectively. A much
F
sured by excitation with a B.M. Industries frequency-tripled
NdÈYAG laser (pulse duration 30 ps FWHM), using the
experimental set-up already described.20 Laser Ñash photoly-
sis experiments were carried out with 8 ns FWHM pulse of a
NdÈYAG laser and the monitoring light from Applied Photo-
physics xenon lamp passed through the sample perpendicular
to the excitation. Intersystem crossing (ISC) quantum yields
were determined in oxygen-free solutions relative to triplet
benzophenone. We compared the initial tripletÈtriplet absorb-
ances of the investigated compound (A) at 480 nm with that of
the benzophenone reference (A ) at 530 nm. The solutions
ref
had matched absorbances at the excitation wavelength (355
nm). The triplet yields (U ) were obtained based on the equa-
ISC
tion
U
\ 3U (A/A )(e /e)
ref ref ref
(1)
ISC
using the well-established yield (3U \ 1.00)21 and molar
ref
absorption coefficient (e \ 7200 M~1 cm~1 at 530 nm)22 for
ref
triplet benzophenone, whereas e \ 10 000 M~1 cm~1 was
taken for the triplet molar absorption coefficient of naphthali-
mides at 480 nm.16
Results and discussion
I. Photophysical properties of hydroxy-naphthalimides
Absorption and Ñuorescence spectra. Fig. 1 presents the
absorption and Ñuorescence spectra of hydroxy-naph-
thalimides in methylene chloride. The absorption spectra
resemble those of the corresponding naphthols, however, the
introduction of the imide moiety results in a remarkable
bathochromic shift of the maxima, which becomes most
apparent for the low energy bands. For the energy of the
lowest excited singlet states, 308 kJ mol~1 and 300 kJ mol~1
were obtained from the locations of the intersections of the
normalized absorption and Ñuorescence spectra in the case of
3-HONI and 4-HONI, respectively. The energy di†erence
between the Ðrst absorption band of the hydroxy-naph-
thalimide and the corresponding naphthol was found to be
more considerable for the 4-HO derivative. This clearly indi-
Fig. 2 Fluorescence and excitation spectra of (A) 3-HONI and (B)
4-HONI in acetonitrile. Phenolic form in the presence of 1.5 ] 10~4
M perchloric acid (É É É) and deprotonated anion form in the presence
of 1 ] 10~3 M KOH (ÈÈ).
4760
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smaller di†erence was observed between the S energies of
gap, thermal activation is not able to initiate the endothermic
1
4-HONI and its deprotonated form (297 kJ mol~1 and 229 kJ
S ] T transition, consequently, triplet formation can occur
1
n
mol~1, respectively). According to the Forster cycle,23 these
results suggest that 4-HONI undergoes a smaller acidity
enhancement upon electronic excitation. The lack of the long
wavelength emission in CH Cl clearly indicates that none of
only by the slower transitions to lower triplet levels. Similar
e†ects were observed for 4-methoxy-N-methyl-1,8-naphthali-
mide, which also has an electron donating moiety.12
The signiÐcant di†erence in the triplet formation rate con-
stant between 3-HONI and 4-HONI (Table 1) can be ration-
alized based on the relative position of the singlet and the
triplet energy levels. Semi-empirical calculation24 showed that
the S ÈT energy gap is larger for 4-HONI compared with
2
2
the singlet excited hydroxy-naphthalimides are sufficiently
acidic to transfer proton to this solvent.
Photophysical properties. Table 1 demonstrates that substi-
tution of N-methyl-1,8-naphthalimide (NI) with a hydroxy
moiety leads to a considerable change in Ñuorescence yield
(U ), Ñuorescence lifetime (q ) and triplet yield (U ). Fluores-
1
1
that of 3-HONI (144 kJ mol~1 and 132 kJ mol~1 were
obtained, respectively). This larger energy gap may lessen the
magnitude of the spinÈorbit coupling between the S and T
F
F
ISC
1
1
cence yields and lifetimes increase more than one order of
magnitude through the series of NI, 3-HONI, 4-HONI,
whereas the variation of the triplet yields exhibits the opposite
tendency; they decrease parallel with the energy of the lowest
states leading to a slower intersystem crossing process for
4-HONI.
It is evident from the data summarized in Table 1 that the
introduction of a hydroxy group into the 1,8-naphthalimide
moiety in position 3 hardly inÑuences the rate constant for
internal conversion (k ), however, the S ] S radiationless
excited singlet (E(S )). In order to get a deeper insight into the
1
factors controlling the rate of the energy dissipating pathways,
IC
1
0
the rate constants for Ñuorescence (k ), intersystem crossing
transition is markedly decelerated for 4-HONI. It seems to be
a general tendency that the 4-substitution of the 1,8-naphtha-
limide skeleton with an electron donating group leads to a
reduced k . This e†ect can be observed for HOÈ, CH OÈ and
F
(k ) and internal conversion (k ) were derived using the
expressions given below:
ISC
IC
k \ U /q
(2)
(3)
(4)
IC
3
F
F F
NH È derivatives alike. Prado et al. suggested25 that the mol-
ecule has a quinoid resonance form if an electron donating
2
k
\ U /q
ISC
ISC F
group is attached to position 4. This kind of electron displace-
ment, which can occur both in the S and the S states, may
k
\ (1 [ U [ U )/q
IC
ISC
F
F
1
0
It is seen from the rate constants presented in Table 1 that
Ñuorescence emission is the dominant process from the singlet
excited state of 4-HONI, however, radiationless transitions
prevail for 3-HONI. In contrast with the very fast intersystem
crossing observed for unsubstituted NI,16 triplet formation is
fairly slow for both hydroxy-naphthalimides. This is especially
a†ect the vibronic coupling between these states and conse-
quently, may lead to slower internal conversion.
II. E†ect of hydrogen-bonding additives
In order to reveal how hydrogen-bonding inÑuences the rate
and the mechanism of the deactivation processes originating
from the singlet excited state, we systematically varied the
proton affinity, the hydrogen-bonding ability and the aromati-
city of the additives using both hydrogen-bond donors and
acceptors.
true of the 4-HO derivative, where almost negligible k was
ISC
found and the phosphorescence is extremely weak at 77 K in
organic glass. In the case of 3-HONI the more efficient inter-
system crossing permits the determination of the tripletÈtriplet
absorption and the phosphorescence spectra. The tripletÈ
triplet absorption maxima detected by a laser Ñash photolysis
technique appear at 450 and 480 nm at ambient temperature
in CH Cl , whereas the phosphorescence peaks can be found
TriÑuorethanol (TFE). We have previously shown that the
Ñuorescence lifetime and quantum yield of N-methyl-1,8-
naphthalimide considerably increase16 in TFE, a solvent
which has high hydrogen-bonding power. In the present work,
we extend these studies to hydroxy-naphthalimides, In con-
trast with that found for the unsubstituted N-methyl-1,8-
naphthalimide, addition of 0.035 M TFE does not inÑuence
the Ñuorescent behavior of its hydroxy-derivatives in CH Cl .
2
2
at 590 and 640 nm at 77 K in 95 : 5 butyronitrile: butyl
acetate mixture. From the 0È0 transition of the phosphor-
escence the energy of the Ðrst triplet excited state is calculated
to be 213 kJ mol~1. This value is lower than the one found for
N-methyl-1,8-naphthalimide (221 kJ mol~1) indicating that
substitution with an OH group not only decreases the energy
of the S state but also that of the T state.
2
2
A ten times higher amount of TFE caused a ca. 9 nm batho-
chromic shift of the Ñuorescence maxima for both the 3- and
the 4-substituted derivatives, but neither Ñuorescence quen-
ching nor appearance of a new emission band was observed.
The excitation and ground state absorption spectra exhibited
a red-shift as a function of the TFE concentration. The clear
isosbestic points in the absorption spectra demonstrated that
a 1 : 1 hydrogen-bonded complex formed with TFE. The equi-
librium constants for hydrogen-bonding (K) were determined
using the following relationship.26
1
1
The efficient triplet formation for N-methyl-1,8-naphthali-
mide (U \ 0.94) was rationalized in terms of the transition
ISC
from the lowest singlet excited state to a close-lying higher
triplet state (T ).16 As we established in a previous paper,16
n
this is a thermally enhanced process in moderately and
strongly polar solvents, which results in a temperature depen-
dent Ñuorescent behavior. However, the Ñuorescence lifetimes
of 3-HONI and 4-HONI were found to be temperature inde-
pendent in the 296È198 K range. Based on these results, we
can exclude the thermally activated intersystem crossing
pathway for hydroxy-naphthalimides. The electron donating
[1 [ (A /A) ]/[additive] \ [K ] K(e /e ) (A /A) (5)
0
j
C A j
0
j
where (e /e ) is the ratio of the molar absorption coefficients
C A j
hydroxy group decreases the S state energy and thereby,
for the complexed and free hydroxy-naphthalimides at a par-
1
increases the S ÈT energy gap. With such an increased energy
ticular wavelength (j), A and A denotes the absorbances in
1
n
0
Table 1 Photophysical properties of hydroxy-naphthalimides in CH Cl
2
2
jmax/nm
jmax/nm
E(S )/kJ mol~1
U
q /ns
U
k /107 s~1
k /107 s~1
k /107 s~1
abs
F
I
F
F
ISC
F
IC
ISC
NIa
3-HONI
4-HONI
348
376
362
378
411
440
334
308
300
0.031
0.13
0.85
0.14
1.9
9.0
0.94b
0.50
0.03
22
6.8
9.4
21b
19
1.3
670b
26
0.3
a Ref. 16. b Based on triplet yield in acetonitrile.
Phys. Chem. Chem. Phys., 1999, 1, 4759È4766
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the presence and the absence of TFE, respectively. Plotting
not bring about any signiÐcant change in the spectra of
methoxy-naphthalimides, where no hydrogen-bonding is feas-
ible. In addition, we can exclude proton transfer from the OH-
group to DMSO because no Ñuorescence was detected in the
500È800 nm range, where emission from the conjugate base
(anion) of the hydroxy-naphthalimides is expected (Fig. 2).
The excitation and the absorption spectra showed similar
changes indicating that hydrogen-bonding with DMSO in the
ground state is the dominant process. The variation of the
Ñuorescence intensity (I) as the function of [DMSO] was
analyzed using a relationship analogous to eqn. (5).
the left-hand side of the function against (A /A) gives good
0
j
linear correlation. From the intercepts, 1.5 M~1 and 1.1 M~1
hydrogen-bonding equilibrium constants were obtained for
3-HONI and 4-HONI, respectively. These values are compa-
rable with that reported for ÑuorenoneÈTFE complex (0.7
M~1 in CH Cl )4 where TFE is also attached to a carbonyl
2
2
moiety.
Dimethylsulfoxide (DMSO). In contrast to the small e†ect
of TFE hydrogen-bond acceptors, which interact with the
HO-substituent, DMSO considerably alters the spectral
behavior of hydroxy-naphthalimides. When weakly basic but
strongly hydrogen-bonding DMSO is added to the hydroxy-
naphthalimide solutions, characteristic changes are observed,
which are demonstrated for 3-HONI in Fig. 3. The red-shift of
the long-wavelength absorption band and the clear isosbestic
points at 337, 346.5, 380 nm for the 3-hydroxy and at 366.5
nm for the 4-hydroxy derivatives indicate 1 : 1 hydrogen-
bonded complex formation. From the e†ect of DMSO on the
absorption spectra, the hydrogen-bonding equilibrium con-
stants (K) were derived using eqn. (5). The average values of K
calculated from the data measured at di†erent wavelengths
are 101 ^ 15 M~1 and 191 ^ 13 M~1 for the 3-HONI and
4-HONI complexes, respectively.
[1 [ (I /I) ]/[additive] \ [K ] K(e /e ) (U /U ) (I /I) (6)
0
F
C A j
C
A F 0
F
where (I /I) represents the ratio of Ñuorescence intensities in
0
F
the presence and the absence of DMSO, and (U /U ) is the
C
A F
ratio of Ñuorescence efficiencies for the complex and the free
Ñuorophore at the detection wavelength.26 Fitting this func-
tion to the experimental data leads to K values of 85 ^ 9 M~1
and 195 ^ 23 M~1 for the equilibrium constants for
hydrogen-bonding of DMSO with 3-HONI and 4-HONI,
respectively. The good agreement of these results with the cor-
responding values derived from absorption spectroscopic
studies indicates that excitation of the complex does not cause
signiÐcant change in the hydrogen-bonding equilibrium con-
stant, i.e., K is not signiÐcantly di†erent for the excited and
the ground state complex. Time-resolved Ñuorescence mea-
surements proved that the dynamic quenching by DMSO is
negligible. The lifetimes of the singlet excited hydrogen-
bonded complexes of 3-HONI and 4-HONI are 2.2 ns and 9.3
ns, respectively. Since the lifetimes of the free and the com-
plexed molecules agree very closely, we conclude that no
energy dissipation takes place via the hydrogen-bond with
DMSO.
The Ñuorescence maximum gradually shifts to lower ener-
gies with increasing DMSO concentration and an isoemissive
point appears but the quantum yield of Ñuorescence does not
change signiÐcantly. As the red-shift is only ca. 11 nm, we
assign the new Ñuorescence band to the hydrogen-bonded
complex. This view is supported by the fact that DMSO does
Pyridine. Addition of pyridine to the hydroxy-
naphthalimide solutions in CH Cl leads not only to
2
2
hydrogen-bonding in the ground state but also to Ñuorescence
quenching. The change in the absorption spectrum closely
resembles that found in the case of DMSO. Plotting the
absorbances according to eqn. (5), we determined the equi-
librium constant of hydrogen-bonding (K). The average values
of K derived from measurements at various wavelengths are
summarized in Table 2.
The increase of the pyridine concentration in the 0È0.06 M
range results in considerable Ñuorescence quenching but
neither the appearance of a new band nor a shift of the
maximum can be seen in the Ñuorescence spectra. Based on
these observations, we conclude that the hydroxy-
naphthalimideÈpyridine hydrogen-bonded complexes have
negligible Ñuorescence yield. The SternÈVolmer plot of the
Fig. 3 Absorption and Ñuorescence spectra of 3-HONI in CH Cl at
2
2
di†erent DMSO concentrations (0, 0.0014, 0.0035, 0.007, 0.014, 0.021,
0.035 M).
Table 2 Hydrogen-bonding equilibrium constants and rate constants of Ñuorescence quenching
Proton
afÐnity/kJ mol~1
Aromaticity
Ka
Kb
k c
q
Quencher
index, I
absorption/M~1
Ñuorescence/M~1
lifetime/ 109 M~1 s~1
x
64.0
85.7
73.0
38.0
È
240
46
21
d
101
d
183
47
17
d
85
d
13
8.8
9.3
0.75
e
47.0
e
4-Hydroxy-naphthalimideÈ
Imidazole
Pyridine
Pyrazole
Benzoxazol
DMSO
942.8
930.0
894.1
891.6
884.4
848.6
64.0
85.7
73.0
38.0
900
138
44
d
191
d
615
137
26
d
195
d
9.6
8.9
7.6
1.8
e
Isoxazole
47.0
e
a From absorption spectra. b From Ñuorescence spectra. c From Ñuorescence lifetime measurements. d No hydrogen-bonding can be detected.
e No quenching.
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steady-state Ñuorescence intensities in the absence (I ) and the
presence of pyridine (I) shows an upward curvature (Fig. 4).
This suggests that the Ñuorophore can be quenched both in
dynamic and static processes. If the static quenching is attrib-
uted entirely to ground state hydrogen-bonding, the modiÐed
form of the SternÈVolmer equation describes the variation of
shifts and the isosbestic points in the absorption spectra
suggest 1 : 1 hydrogen-bonding. The experimental data were
analyzed as described above for the other additives and the
results are included in Table 2.
The most interesting feature of the spectra in Fig. 5 is the
appearance of the long-wavelength (LW) emissions, which are
attributed to the singlet excited complexes of hydroxy-
naphthalimide with pyrazole and imidazole. The extent of
proton transfer within these complexes is probably very sensi-
tive to the acidÈbase properties of the constituents and the
local polarity of the solvate shell.
In the case of 3-HONI, the SW and the LW bands are well-
separated, therefore, we could readily see the formation and
the decay of the species emitting in these spectral ranges. The
variation of the Ñuorescence intensity as the function of time is
presented in Fig. 7 for the solution containing 3-HONI and
0.024 M imidazole. The dotted line in Fig. 7A exhibits the
Ñuorescence decays detected at 405 and 640 nm, whereas the
continuous lines represent the Ðtted curves (vide infra), which
were calculated by a non-linear least-squares deconvolution
method. The parameter describing the rise of the signal at 640
nm (1.3 ns) perfectly agrees with the decay parameter obtained
at 405 nm. The excitation pulse proÐle (dotted line) and the
growing in of the Ñuorescence at 640 nm are shown in Fig. 7B
using a better time resolution. The calculated curves match
the measured data so well that they are hardly distinguishable
in the Ðgure. The Ñuorescence decay at 405 nm can be well
described with a single exponential function. (The signal
shown in Fig. 7A has 1.3 ns lifetime.) The time-resolved Ñuo-
rescence of the 3-HONIÈimidazole complex exhibits more
complex kinetics. The data were analyzed with a double expo-
nential function:
0
I /I vs. quencher concentration.27
0
I /I \ (1 ] K[quencher])(1 ] k q [quencher])
(7)
0
q 0
where q refers to the lifetime of singlet excited hydroxy-
naphthalimide, k is the rate constant of the dynamic quen-
ching and K denotes the equilibrium constant of complex
0
q
formation in the ground state. The quenching rate constants
(k ) were determined by time-resolved Ñuorescence technique
q
(vide infra) and the K values were calculated by the nonlinear
least-square Ðt of eqn. (7) to the experimental data. It is appar-
ent in Fig. 4 that the calculated curves describe very well the
experimental results. Table 2 demonstrates that the hydrogen-
bonding equilibrium constants derived from both absorption
and Ñuorescence measurements closely agree.
Addition of pyridine to the solutions of hydroxy-
naphthalimides shortens the lifetime of the lowest singlet
excited state. The Ñuorescence decays are well described by a
single exponential function. The Ñuorescence lifetimes in the
absence (q ) and the presence (q) of pyridine are plotted in Fig.
0
4 based on the following equation:
q /q \ 1 ] k q [quencher]
(8)
0
q 0
Since the Ñuorescence decay times are inÑuenced only by
dynamic quenching, linear correlation is found between q /q
0
and the quencher concentration. The quenching rate constants
(k ) derived from the slopes are given in the last column of
q
Table 2.
C exp([t/q ) [ C exp([t/q )
(9)
2
2
1
1
Benzoxazole and isoxazole. In order to reveal the major
factors controlling the rate of the hydrogen-bonding induced
Ñuorescence quenching, we extended our studies to hetero-
cyclic compounds containing a Ðve-membered ring. Isoxazole
a†ects neither the Ñuorescence decay nor the spectral charac-
teristics of hydroxy-naphthalimides indicating that no inter-
action occurs between these compounds either in the ground
or the excited state. However, the e†ect of benzoxazole resem-
bles that observed for pyridine. Time-resolved Ñuorescence
measurements proved that dynamic quenching takes place but
the reaction rate is much lower than that of pyridine. No clear
indication was found for ground state hydrogen-bonding
because of the overlap between the benzoxazole and the
hydroxy-naphthalimide absorption.
where t denotes time, C and C are constants. Calculation
1
2
resulted in q \ 1.3 ns and q \ 4.9 ns for the decay constants
1
2
when 0.024 M imidazole concentration was used. The C /C
1
2
ratio is expected to be 1 if the species emitting at long wave-
lengths is produced only in the quenching reaction.28 We
found C /C \ 1.57, which clearly indicates that both the
2
1
direct excitation of the ground state hydrogen-bonded
complex and the dynamic quenching of the singlet excited
3-HONI result in LW emission. As it is expected for a pseudo-
Ðrst-order process, the growing in of the LW Ñuorescence and
the decay of the SW Ñuorescence strongly depend on the con-
centration of the quencher. However, the decay time of the
LW emission only slightly decreases with the quencher con-
centration.
Pyrazole and imidazole. Compounds containing two hetero-
cyclic nitrogens in a Ðve-membered ring induce a di†erent
type of Ñuorescent behavior. They not only quench the Ñuo-
rescence of hydroxy-naphthalimides but also cause a new Ñuo-
rescence in the 500È800 nm spectral range whose intensity
increases with the quencher concentration. It is apparent in
Fig. 5 that the new emission consists of two bands for 3-
HONI ] imidazole and 3-HONI ] N-methylimidazole solu-
tions, whereas in the other cases no such clear evidence can be
observed for dual luminescence in the long wavelength band.
The short wavelength (SW) emission around 370È480 nm
originates from the excited hydroxy-naphthalimides. The Ñuo-
rescence lifetimes measured in this band decrease with increas-
ing concentration of pyrazole and imidazole. This proves that
dynamic quenching occurs. Fig. 6 gives the SternÈVolmer
plots of the data obtained by steady-state and time-resolved
Ñuorescence techniques for HONIÈimidazole systems. In con-
trast with the linear dependence obtained from lifetime mea-
surements, the SternÈVolmer plots of the steady-state
Ñuorescence intensities are concave indicating that static
quenching has an important contribution as well. The red-
It is evident from the spectra displayed in Fig. 5 that the
excited hydrogen-bonded complexes of pyrazole have di†erent
characteristics compared with that of imidazole and N-
methylimidazole. In the former case, the LW emissions have a
Gaussian shape with maxima at 600 nm and 520 nm for
3-HONI and 4-HONI, respectively. Since these maxima are at
higher energies than the Ñuorescence peak of the deprotonat-
ed hydroxy-naphthalimides (Fig. 2), we suggest that only a
partial proton shift takes place along the hydrogen-bond in
these excited complexes.
It is especially noteworthy that the addition of imidazole to
the 3-HONI solution leads to structured LW Ñuorescence,
which can be resolved to two components (Fig. 5A). In order
to exclude the possibility that one of the LW Ñuorescence
components originates from the interaction of 3-HONI with
imidazole dimer29 we studied the reactions of N-
methylimidazole as well. This compound is not able to form a
hydrogen-bonded dimer because it does not contain an NÈH
moiety. Fig. 5A demonstrates that the structured LW emission
appearing in the presence of N-methylimidazole resembles
that obtained with imidazole. Thus, we can rule out that
Phys. Chem. Chem. Phys., 1999, 1, 4759È4766
4763

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Fig. 4 SternÈVolmer plots of the results obtained by time-resolved
and steady-state Ñuorescence technique for HONIÈpyridine systems
in CH Cl . (A) 3-HONI: >, steady-state measurements, …, time-
Fig. 6 SternÈVolmer plots of the data obtained by steady-state and
time-resolved Ñuorescence techniques for the HONIÈimidazole
systems in CH Cl . (A) 3-HONI: >, steady-state measurements; …,
2
2
2 2
resolved measurements. (B) 4-HONI: |, steady-state measurements,
time-resolved measurements. (B) 4-HONI: |, steady-state measure-
L, time-resolved measurements.
ments; L, time-resolved measurements.
tures which di†er in the extent of their proton shift. However,
the complex that Ñuoresces at higher energies is bound with a
hydrogen-bond and possesses only a limited proton transfer
character when the proton is removed from the HO group
toward the heterocyclic nitrogen in the species emitting at
long wavelengths. The identical Ñuorescence decay times
throughout the LW bands of the 3-HONIÈimidazole complex
in CH Cl suggest that a fast equilibrium is established
between the two types of complex. The polar solvents weaken
the hydrogen-bond and promote proton transfer, therefore, no
dual emission can be seen in acetonitrile. The ion-pair charac-
ter of the complex in acetonitrile is supported by the fact that
the Ñuorescence maximum of both the 3-HONIÈimidazole
complex (Fig. 8) and the 3-HONI anion (Fig. 2A) are located
around 630 nm in this polar solvent.
dimerization of the additive causes the dual emission at long
wavelengths.
The relative intensity of the two Ñuorescence bands in the
500È800 nm spectral domain is temperature dependent both
in ethyl acetate and in CH Cl . The substantial increase of
2
2
the higher energy component is particularly discernible in the
295È181 K temperature range in ethyl acetate where the two
bands are better separated than in CH Cl .
2
2
2
2
It is readily seen in Fig. 8 that the intensity ratio of the two
emissions in the 500È800 nm region strongly depends on the
media. In acetonitrile, the band with a maximum around 550
nm disappears, and, likewise, addition of ethanol in the
CH Cl solution signiÐcantly weakens this emission. Our
2
2
results indicate that in solvents of medium polarity the 3-
HONIÈimidazole excited complexes have two dominant struc-
Fig. 5 Fluorescence spectra in CH Cl . (A) 3-HONI in the presence
Fig. 7 Fluorescence decays in 3-HONI ] 0.024 M imidazole solu-
2
2
of 0.156 M pyrazole (heavy line), 0.024 M imidazole (dotted line) and
0.024 M N-methylimidazole (thin line); (B) 4-HONI in the presence of
0.057 M pyrazole (heavy line), 0.018 M imidazole (dotted line) and
0.018 M N-methylimidazole (thin line).
tion in CH Cl . (A) Fluorescence decay (dotted line) and Ðtted curve
2
2
(continuous line) at 405 nm and 640 nm. (B) Excitation pulse proÐle
(dotted line), Ñuorescence growing in and Ðtted curve (continuous line)
at 640 nm.
4764
Phys. Chem. Chem. Phys., 1999, 1, 4759È4766

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than that of the corresponding amine for equal values of
hydrogen-bonding equilibrium constant.36 Moreover, the data
reported by Abraham et al. demonstrate that DMSO forms
stronger hydrogen-bonds than the much more basic pyridine
derivatives.37
Coupled electronÈproton movement was suggested to
promote the radiationless deactivation when a heterocyclic
molecule containing an aromatic p-electronic system is con-
nected to excited hydroxyarenes directly via a hydrogen-
bond.10 The extremely weak Ñuorescence for the
hydrogen-bonded complexes of benzoxazole and pyridine
with hydroxy-naphthalimides is probably due to a rapid inter-
nal conversion via a similar process. The proton shift toward
the hydrogen-bond acceptor induces efficient nonradiative
energy dissipation. However, the intensive emission as well as
the long lifetime (ca. 4È9 ns) of the excited hydrogen-bonded
complexes containing imidazole and pyrazole obviously indi-
Fig. 8 Fluorescence spectra of 3-HONI in the presence of 0.024 M
imidazole: (A) in CH Cl , (B) in CH Cl ] 0.17 M EtOH, (C) in the
2
2
2 2
presence of 0.052 M imidazole in acetonitrile and (D) in ethyl acetate.
cate slow internal conversion (k value of ca. 108 s~1 can be
IC
deduced from the experimental data). We did not Ðnd a corre-
lation between the aromaticity index of the hydrogen-bond
acceptor and the radiationless deactivation rate of the excited
hydrogen-bonded complex. This seems to indicate that the
energy dissipation mechanism suggested for the excited
hydroxyareneÈpyridine species does not play a dominant role
if other types of nitrogen heterocyclics serve as the hydrogen-
bond acceptor.
Comparison of the e†ects of the various hydrogen-bond
acceptors. It was demonstrated that the intermolecular
hydrogen-bonding with alcohols in the singlet excited state
acts as an e†ective accepting mode of radiationless deactiva-
tion for aromatic carbonyl compounds.2h6 We should
comment on the question of why hydroxy-naphthalimides,
which also contain carbonyl groups, are not quenched by the
strong hydrogen-bond donor TFE. For the example of 2-
substituted Ñuorenones we demonstrated that efficient
hydrogen-bonding induced internal conversion can occur only
if the carbonyl oxygen has high electron density in the excited
state.30 As stated above, the absorption and the Ñuorescence
spectra of hydroxy-naphthalimides exhibit small solvatochro-
mic shifts because excitation leads to minor change in the
dipole moment. Theoretical calculations also corroborated
that there is no signiÐcant di†erence in the electron density of
the carbonyl oxygen for the S and the S states of naphthali-
Acknowledgements
We very much appreciate the support of this work by the
Hungarian Science Foundation (OTKA, Grant T 023428) and
the scientiÐc exchange program between the French Ministry
of Foreign A†airs and the Hungarian Committee for Techno-
logical Development (Balaton Project F-10/97).
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0
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Paper 9/04520A
4766
Phys. Chem. Chem. Phys., 1999, 1, 4759È4766