Photophysical and dynamic NMR studies on 4-amino-7-nitrobenz-2-oxa-1, 3-diazole derivatives: Elucidation of the nonradiative deactivation pathway

Article abstract of DOI:10.1021/jp982154v

A series of 4-aminonitrobenzoxadiazole (NBD) derivatives in which the amino nitrogen is part of a four- to seven-membered heterocyclic ring and a second series of 4-dialkylamino NBD derivatives with different alkyl chain lengths have been prepared and fully characterized and their photophysical properties investigated in an attempt to find out the nonradiative deactivation pathway of the fluorescent state in these systems. It is observed that the fluorescence properties of the NBD derivatives are highly sensitive to the structure of the amino moiety and the polarity of the surrounding media. It is shown that the nonradiative rate constants for the derivatives with large alkyl groups and large ring systems are considerably higher than those for systems with smaller alkyl groups and smaller ring systems. Dynamic NMR experiments have been carried out to probe the internal motion in the systems. The internal rotation around the bond connecting the amino nitrogen and the NBD ring is found to be rather slow in the ground state. The rate for the internal rotation is found to be the highest for the six-membered ring, and this has been interpreted in terms of the partial double bond character of the C-N bond. The results of the experiments seem to indicate that inversion of the amino nitrogen plays the most important role in determining the fluorescence efficiency of the systems.

Full text of DOI:10.1021/jp982154v

J. Phys. Chem. A 1998, 102, 7903-7912
7903
Photophysical and Dynamic NMR Studies on 4-Amino-7-nitrobenz-2-oxa-1, 3-diazole
Derivatives: Elucidation of the Nonradiative Deactivation Pathway
Satyen Saha and Anunay Samanta*
School of Chemistry, UniVersity of Hyderabad, Hyderabad 500 046, India
ReceiVed: May 7, 1998; In Final Form: July 10, 1998
A series of 4-aminonitrobenzoxadiazole (NBD) derivatives in which the amino nitrogen is part of a four- to
seven-membered heterocyclic ring and a second series of 4-dialkylamino NBD derivatives with different
alkyl chain lengths have been prepared and fully characterized and their photophysical properties investigated
in an attempt to find out the nonradiative deactivation pathway of the fluorescent state in these systems. It
is observed that the fluorescence properties of the NBD derivatives are highly sensitive to the structure of the
amino moiety and the polarity of the surrounding media. It is shown that the nonradiative rate constants for
the derivatives with large alkyl groups and large ring systems are considerably higher than those for systems
with smaller alkyl groups and smaller ring systems. Dynamic NMR experiments have been carried out to
probe the internal motion in the systems. The internal rotation around the bond connecting the amino nitrogen
and the NBD ring is found to be rather slow in the ground state. The rate for the internal rotation is found
to be the highest for the six-membered ring, and this has been interpreted in terms of the partial double bond
character of the C-N bond. The results of the experiments seem to indicate that inversion of the amino
nitrogen plays the most important role in determining the fluorescence efficiency of the systems.
1
. Introduction
extensive investigations carried out over the past decade it was
realized that the TICT state need not be fluorescent.1
9-22
It is
The electron donor-acceptor molecules have become the
important to note in this context that whether fluorescent or
not, a low-lying nonfluorescent TICT state can be responsible
for the nonradiative decay of the fluorescent LE state in donor-
acceptor systems exhibiting only ‘normal’ fluorescence. Very
recently, Zachariasse and co-workers have pointed out some of
the deficiencies of the TICT model in explaining the photo-
physical behavior of the aminobenzonitrile derivatives.1
Notable among them are (i) the absence of a linear correlation
between the energy of the anomalous fluorescence band of the
compounds and the redox potentials of the donor and the
acceptor subgroups and (ii) observation of the anomalous
fluorescence in systems where 90° twist of the amino group is
not possible. A new model, termed as planar intramolecular
charge transfer (PICT), has been proposed by Zachariasse et
al. to interpret the photophysical behavior of aminobenzoni-
focus of extensive investigations in recent years primarily due
to the fact that separation of charge is the basis of numerous
transformations in chemistry and biology and an understanding
of the photoinduced charge separation helps in understanding
the mechanism of long-range separation of charge in plants
during photosynthesis and designing efficient systems for solar
energy conversion.1 The donor-acceptor systems are also a
favorite testing ground for the theories of electron transfer and
solvation.2 The organic conductors, superconductors, nonlinear
optical materials, and a good majority of the fluorescence probes
5-18
3
for the organized media are also systems of this class.
A large number of systems in which the donor and acceptor
moieties are connected by a single bond with the possibility of
conjugation have been investigated in recent years with a view
to understand the mechanism of dual fluorescence exhibited by
1
5-18
triles.
According to this model, inversion of the amino
them.4
-18
4-Dimethylaminobenzonitrile (DMABN) is the most
nitrogen is believed to be the promoting mode for the occurrence
of the long-wavelength anomalous fluorescence band of DMABN.
A second condition that is invoked in this model is that the
energy gap between the two lowest excited states of aminoben-
zonitriles should be sufficiently small so as to allow vibronic
coupling of the two states.
4-18
extensively studied system of this class.
It was believed
that electronic excitation of DMABN leads to population of an
electronic state in which one unit of charge is transferred from
the dimethylamino group to the benzonitrile moiety and the
donor and the acceptor orbitals are completely decoupled in a
-8
9
0° twisted conformation.6 While the ‘normal’ fluorescence
The present paper is concerned with a study of the fluores-
cence behavior of a series of donor-acceptor molecules (NBD
derivatives) in which an amino group serves as the donor and
a nitro group as the acceptor. NBD derivatives are known to
be extremely popular candidates as fluorescent probes in
of DMABN originates from a locally excited (LE) state, the
‘
anomalous’ one originates from the highly dipolar twisted
6-8
intramolecular charge transfer (TICT) state.
Even though
the TICT mechanism was originally invoked by Grabowski and
co-workers to account for the dual fluorescence of benzoni-
2
3
24
triles,6-8 it was soon realized that the mechanism was helpful
biological applications. NBD chloride has often been used
for labeling proteins with fluorescent groups and in studies of
in understanding the fluorescence properties of a wide number
2
5,26
of other donor-acceptor systems.9-14 During the course of
protein structure and conformational changes.
A large
number of phospholipids and cholesterol analogues with NBD
groups as fluorophores find application in the study of biological
*
Corresponding author. E-mail: assc@uohyd.ernet.in. Fax: +91-40-
27-29
3
010120.
and model membranes.
The fluorescence quenching of the
S1089-5639(98)02154-9 CCC: $15.00 © 1998 American Chemical Society
Published on Web 09/23/1998

7904 J. Phys. Chem. A, Vol. 102, No. 41, 1998
Saha and Samanta
CHART 1
SCHEME 1
were received from Fluka and were used as received. Pyrro-
lidine and piperidine from Aldrich, dimethylamine (40 wt %
solution in water), diethylamine, and n-dibutylamine were
received from Acros and were distilled before use. Since
extremely pure and dry solvents are essential for spectroscopic
measurements involving dipolar species, all the solvents used
in this investigation for spectroscopic measurements were
37
rigorously purified following standard procedures. The extent
of dryness of each solvent was checked by monitoring the
wavelength of the absorption maximum of a polarity-sensitive
betaine dye in the given solvent and comparing the same with
3
8
the literature value of the solvent.
Most of the compounds synthesized are new and are listed
in Chart 1 with the structures and the abbreviated names. All
the compounds were prepared following a general procedure
outlined in Scheme 1. In this method, NBD-chloride (1 mmol)
was dissolved in 3 mL of ethyl acetate. The amine (1.2 mmol)
was diluted in 2 mL of ethyl acetate and was added slowly
dropwise to an NBD-chloride solution with stirring in an ice
bath. After stirring for 30 min in the ice bath, the reaction
mixture was then stirred for another 2 h at room temperature.
The product, which appeared as a red precipitate, was filtered
out and purified by column chromatography using silica gel
column. Hexane and ethyl acetate were used as eluent in
different proportion for the purification of different compounds.
The purified compounds were recrystallized from absolute
ethanol. The products were confirmed by IR and NMR
spectroscopy. A few compounds for which single crystals were
obtained were also characterized by X-ray crystallography.
2.2. Apparatus and Methods. The fluorescence quantum
yields of the NBD derivatives were measured using fluorescein
NBD moiety by transition-metal ions has also been used in
determining the location of the fluorophore in complex media.
30
The fluorescence response of a few NBD derivatives toward
different guests has been studied.31-34 We have recently studied
the fluorescence sensory behavior of some NBD fluorophore-
labeled supramolecular systems toward the quenching transition-
3
5
metal ions. The solvent polarity dependence of a few NBD
derivatives has recently been reported, where the involvement
of a nonfluorescent TICT state in the nonradiative decay of the
36
fluorescent state has been speculated. We have undertaken
this work on two series of the NBD derivatives (Chart 1) with
a view to find out the nature of the nonradiative pathway in
this class of systems. In particular, we have examined in this
as the reference compound (φf ) 0.90 in 0.1 N NaOH).39
A
solution of the NBD derivative in any given solvent was
prepared with an absorbance (OD ≈0.15) the same as that of
the reference compound at the excitation wavelength. The
fluorescence spectra were measured under the same operating
conditions, and the settings and quantum yields were determined
by comparing the areas underneath the fluorescence spectra by
the ‘cut and weigh’ method.
The absorption and the fluorescence spectra were recorded
on a Jasco UV-vis spectrophotometer (model 7800) and a
Hitachi spectrofluorimeter (model 4010), respectively. The
fluorescence decay curves were recorded using an IBH single-
photon-counting spectrofluorimeter (model 5000U). The instru-
ment was operated with a thyratron-gated flash lamp filled with
hydrogen at a pressure of 0.5 atm. The lamp was operated at
a frequency of 40 kHz, and the pulse width of the lamp under
6
-8
paper whether the TICT model proposed by Grabowski
or
the recent PICT model proposed by Zachariasse1
5-18
is best
suited for the description of the photophysical behavior of the
NBD derivatives. The photophysical measurements have been
supplemented by dynamic NMR spectroscopic measurements.
To the best of our knowledge, this is the first study where
measurements of this kind have been performed specifically to
understand the photophysical responses of the donor-acceptor
systems.
2
. Experimental Section
.1. Materials. 4-Chloro-7-nitrobenzofurazan (NBD-chlo-
2
ride), trimethyleneimine(azetidine), and hexamethyleneimine

Photophysical and Dynamic NMR
J. Phys. Chem. A, Vol. 102, No. 41, 1998 7905
the operating condition was ∼1.2 ns. The lifetimes were
estimated from the measured fluorescence decay curves and the
lamp profile using a nonlinear least-squares iterative fitting
4
0
procedure. The goodness of the fit was evaluated from the
2
ø values and the plot of the residuals.
1
The IR and H NMR spectra were recorded on a JASCO
FT-IR (model 5300) and a Bruker ACF-200 spectrometer (200
MHz), respectively. The NMR measurements have been carried
out in CDCl3.
The crystal structures of a few compounds were determined
using an Enraf-Nonius single-crystal diffractometer (MACH 3),
employing CAD-4 Software (version 5.0). Structure solution
and refinement were done by SHELXS-97 and SHELXL-97
software, respectively.
The theoretical calculations based on the semiempirical AM1
Austin Model 1) method4
1,42
were carried out using the
(
Hyperchem package (release 5.0) obtained from Hypercube, Inc.
The calculations were performed on a personal computer. The
initial optimization of the structures was achieved using the
MM+ method. Subsequently, unrestricted geometry optimiza-
tions at the semiempirical level were performed to obtain the
ground-state dipole moments and the structural parameters of
the molecules. The optimization of the ground-state geometry
of the molecules was achieved using a conjugate gradient
(
Polak-Ribiere) type of algorithm with a root-mean-square
(rms) gradient as the convergence criterion. The rms gradient
was kept below 0.001 kcal/(Å mol). The excited-state dipole
moments were obtained for the ground-state-optimized structures
of the molecules using configuration interaction (singly excited,
orbital criteria: HOMO ) 8, LUMO ) 8).
Figure 1. Absorption spectra of 5NBD in toluene (‚‚‚), 1,4-dioxane
(
s), tetrahydrofuran (- - -) and acetonitrile (- ‚ -) at room temperature.
solvatochromic response of the fluorescence band. In aprotic
media, the fluorescence maximum displays a Stokes shift with
3
. Results and Discussion
.1. Photophysical Behavior. 3.1.1. Absorption Spectra.
an increase in the polarity of the solvent (Figure 2).
A
3
comparatively larger bathochromic shift for the fluorescence
band maximum (Table 1) in comparison with that for the
absorption is indicative of an excited state that is relatively more
polar than the ground state. As far as the influence of the
structure of the molecules on the wavelength of the fluorescence
maximum is concerned, in any given solvent, no significant
variation could be noticed among the alkylated or ring systems.
However, ANBD displays its maximum at a wavelength much
lower than those observed for the other derivatives. This is
clearly due to the absence of the inductive influence of the alkyl
groups in the system.
The absorption spectra of the NBD derivatives are characterized
by broad bands. The lowest energy absorption maximum
displays solvatochromic behavior typical of a charge-transfer
transition in which the amino group acts as a donor and the
nitro group as an acceptor. The intramolecular charge transfer
in the NBD chromophore is in agreement with that observed in
structurally similar but relatively more extensively investigated
p-nitroaniline.4
3-49
A few representative absorption spectra of
5NBD are illustrated in Figure 1. As seen from the figure, with
an increase in the polarity of the medium, the lowest energy
absorption band maximum exhibits a gradual bathochromic shift.
A typical shift of the absorption maximum on change of the
solvent ranges between 16 and 22 nm in aprotic media. In protic
media, the bathochromic shift of the absorption maximum is
considerably larger, presumably due to the specific hydrogen-
bonding interaction between the NBD derivatives and the solvent
molecules. The absorption and emission spectral data of the
NBD derivatives in a series of solvents are collected in Table
3
.1.3. Dipole Moment Change on Electronic Excitation.
Attempts were made to quantify the extent of charge separation
on electronic excitation of the various NBD derivatives, in
particular, to evaluate the change in the dipole moment (∆µ)
from an analysis of the Stokes shift between the wavenumbers
of the absorption and the fluorescence maxima (∆ν ) νj a - νj f)
as a function of the solvent polarity parameter, ∆f
1
. It can be seen from the table that for ANBD, the position of
2
ꢀ
- 1
n - 1
2
the absorption maximum in any given solvent is drastically
different from that of the other derivatives. Among the ring
systems, the wavelength of the absorption maxima does not
show any appreciable variation with change in the heterocyclic
ring size, and for the alkyl derivatives, the band maximum shifts
gradually toward red with increasing chain length of the alkyl
groups. These observations can be rationalized in terms of the
inductive influence of the various groups.
)
-
(
2
)
2ꢀ + 1
n + 1
50-52
based on the Lippert-Mataga equation,
2
2
2
(µ - µ )
e
g
ꢀ - 1
ꢀ + 1
n - 1
2n + 1
νj - νj )
-
+ constant (1)
a
f
3
[
2
]
2
hca
3
.1.2. Fluorescence Spectra. The NBD derivatives are
known to display broad emission bands devoid of any fine
structure due to the intramolecular charge-transfer (ICT) nature
of the fluorescence.36 The ICT is also evident from the
where, νj a and νj f are the wavemnumbers of the absorption and
fluorescence maxima, µg and µe are the ground- and excited-
state dipole moments, ꢀ and n are the dielectric constant and

7906 J. Phys. Chem. A, Vol. 102, No. 41, 1998
Saha and Samanta
Figure 2. Fluorescence spectra of 5NBD in toluene (‚‚‚), 1,4-dioxane
(
s), tetrahydrofuran (- - -) and acetonitrile (- ‚ -) at room temperature.
The solutions were excited at 470 nm.
g
TABLE 2: AM1-Calculated Ground-State (µ ) and
e
Excited-State (µ ) Dipole Moments of the NBD Derivatives
compound
µ
g
(D)
µ
e
(D)
∆µ (D)
4
5
6
7
NBD
NBD
NBD
NBD
9.64
11.97
12.67
12.61
12.60
12.11
12.24
12.54
2.33
2.13
3.31
2.62
2.15
2.38
2.23
10.54
9.30
9.98
9.96
9.86
10.31
MNBD
ENBD
BNBD
refractive index of the medium, c is the velocity of light, and a
is the Onsager cavity radius.
However, since the correlation between and ∆ νj , and ∆f was
found to be rather poor (correlation coefficient, r ) 0.2-0.3),
no estimate of ∆µ could be obtained from this analysis. In this
context, it may be noted that a similar observation was made
earlier by Forgues et al. in the case of diethylamino derivative
3
6
of NBD. Since it is generally observed that ∆ νj values are
correlated to the microscopic solvent polarity parameter (ET-
N
53-55
(
30) or ET ) better than to the polarity function (∆f),
involving bulk properties such as the dielectric constant and
refractive index, we made another attempt to evaluate the ∆µ
5
3
values for the systems based on a recently suggested procedure.
According to this procedure, the ∆ νj is related to ETN as
53
follows:
2
3
N
νj - νj ) 11307.6{(∆µ/∆µ ) (a /a) }E + constant (2)
a
f
D
D
T
Where, ∆µD and aD represent the dipole moment change and
Onsager cavity radius, respectively, for the betaine dye.
However, for the NBD derivatives, the plots based on this
equation, although superior to those obtained using eq 1, were
still far from satisfactory (r between 0.7 and 0.8) to allow
estimation of reliable ∆µ values. Having not succeeded in
obtaining the ∆µ values of the compounds from the solvato-
chromic data, we have calculated these values for the AM1-
optimized structures of the compounds. The ∆µ values for the
ground and first excited singlet state were found to lie between
2
.1 and 3.3 D (Table 2). These values are in agreement with
56
the literature values of structurally similar compounds.

Photophysical and Dynamic NMR
J. Phys. Chem. A, Vol. 102, No. 41, 1998 7907
Figure 3. Fluorescence decay curve of 5NBD in 1,4-dioxane at room temperature. The decay was measured at 520 nm. Shown also are the
excitation lamp profile and the best fit to the fluorescence decay (single-exponential fit with a lifetime of 7.32 ns).
f
TABLE 3: Fluorescence Quantum Yields (O ) of the Cyclic and Linear NBD Derivatives in Different Solvents at Room
Temperature
solvent
E
T
(30)
4NBD
5NBD
6NBD
7NBD
ANBD
MNBD
ENBD
BNBD
toluene
dioxane
THF
DCM
acetone
ACN
ethanol
methanol
water
33.9
36.0
37.4
40.7
42.2
45.6
51.9
55.4
63.1
0.49
0.57
0.59
0.51
0.52
0.44
0.22
0.19
0.02
0.49
0.41
0.22
0.27
0.12
0.11
0.06
0.06
0.01
0.48
0.12
0.03
0.03
0.01
0.01
0.01
0.01
0.002
0.28
0.07
0.02
0.02
0.01
0.01
0.01
0.01
0.004
0.13
0.46
0.79
0.45
0.51
0.57
0.52
0.15
0.14
0.16
0.02
0.02
0.01
0.02
0.01
0.48
0.08
0.03
0.03
0.01
0.01
0.02
0.01
0.01
0.48
0.08
0.02
0.02
0.01
0.01
0.01
0.01
0.01
3
.1.4. Fluorescence Quantum Yield and Lifetime. The fluor-
The fluorescence lifetimes (τf) of the NBD derivatives have
been estimated in several solvents from the measured fluores-
cence decay curves. A typical fluorescence decay curve along
with the single-exponential fit to the data is shown in Figure 3.
The τf values of the systems in different solvents are collected
in Table 4. The τf values of ANBD and 4NBD are found to be
least sensitive to the polarity of the medium. On the other
hand, for higher ring systems and alkyl derivatives, even
though τf is fairly high in nonpolar toluene, it drastically
decreases as the polarity of the surrounding environment is
increased. Both the φf and τf values indicate clearly that a
nonradiative relaxation pathway is operational for the alkyl
derivatives and higher membered ring systems, particularly in
relatively polar media.
escence quantum yields (φf) of the various NBD derivatives in
a series of solvents of different polarities are collected in Table
. These data can be summarized as follows. First, the φf
3
values of the systems are significantly higher in aprotic media
compared to those in the protic media because of specific
hydrogen-bonding interactions in the latter solvents. Second,
φf of ANBD, the system with a free amino group, is the highest
among all the compounds in any given solvent except in toluene.
Third, in a given solvent, with an increase in the length of the
alkyl group or the ring size, the quantum yield decreases
steadily. Fourth, for a given system, as the polarity of the
medium is increased, the fluorescence quantum yield is de-
creased, and this solvent polarity induced decrease in the
fluorescence yield is most pronounced for systems with higher
ring size or larger alkyl groups.
One point that perhaps deserves some mention is that both
the φf and τf values of ANBD are surprisingly low in the least

7908 J. Phys. Chem. A, Vol. 102, No. 41, 1998
Saha and Samanta
TABLE 4: Fluorescence Lifetimes (ns) of the NBD Derivatives in Different Solventsa
compound
toluene
dioxane
THF
DCM
acetone
ACN
ethanol
methanol
water
4
5
6
7
NBD
NBD
NBD
NBD
9.81
9.08
7.74
4.54
3.59
9.74
7.64
7.50
10.70
7.32
1.98
0.98
10.62
3.06
1.53
1.48
10.85
4.29
0.58
0.42
11.70
2.87
0.51
0.48
9.84
5.09
0.57
0.32
6.49
2.72
0.52
0.41
10.80
2.41
0.32
0.27
9.12
0.43
0.27
0.21
9.96
2.24
0.16
0.12
11.45
0.44
0.27
0.18
5.73
1.57
0.14
0.22
5.32
1.78
0.15
0.20
1.00
0.42
0.10
ANBD
MNBD
ENBD
BNBD
0.30
0.44
0.27
0.45
0.29
0.23
0.13
0.07
0.21
a
The solutions were excited at the respective absorption maxima of the compounds. The fluorescence decays were found to be single exponential
in most of the cases. In some cases, particularly in hydrogen-bonding solvents, the decays consisted of a minor second component, which was
neglected. The measurement error is (10% for lifetimes above 1 ns. The lifetimes below 1 ns indicate only the trend.
TABLE 5: Nonradiative Rate Constants (10 s-1) of the NBD Derivatives in Aprotic Solvents at Room Temperature
8
solvent
4NBD
5NBD
6NBD
7NBD
ANBD
MNBD
ENBD
BNBD
toluene
dioxane
THF
DCM
acetone
ACN
0.52
0.40
0.38
0.50
0.45
0.57
0.56
0.80
1.82
1.43
3.63
3.96
0.62
4.42
16.74
17.03
30.87
62.14
1.59
9.48
23.38
30.66
36.62
82.58
2.44
0.50
0.18
0.84
0.53
0.37
0.49
2.77
2.99
3.07
22.56
22.12
0.68
6.04
18.89
18.39
36.46
36.49
0.70
6.20
20.48
23.89
46.60
54.15
polar solvent, toluene. One can rationalize this behavior by
considering the existence of two close-lying states (CT and nπ*)
in the systems. In the least polar solvent, toluene, low φf and
τf values for ANBD are presumably due to the fact that the
emission originates from the nπ* state, which is the lowest
excited state. In relatively higher polarity solvents, the CT state
is stabilized below the nπ* states, accounting for the enhance-
ment of the φf and τf values. For all other systems, because of
the inductive influence, the CT state is lower than nπ* state
even in toluene.
studied as a function of the temperature. Figure 5 illustrates
the variation of the NMR signals of interest for the cyclic com-
pounds as a function of the temperature. Two clearly resolved
triplets could be observed on lowering of the temperature in
all cases.
Several different internal motions in the systems, such as
twisting around the C-N bond (internal rotation), inversion of
the amino nitrogen, and ring inversion (Scheme 2), may give
rise to broad NMR peaks. The fact that ring inversion is not
responsible for the broadening of the NMR signal is evident
from the fact that (i) the linear alkyl derivatives of NBD also
give rise to similar broad peaks that can be resolved at lower
temperatures and (ii) the ring inversion process is known to be
too rapid for the four- and five-membered ring systems to be
3.1.5. The NonradiatiVe Rate Constants. We have evaluated
the nonradiative rate constants (knr) of all the derivatives in
several aprotic media from the measured fluorescence yield (φf)
and lifetime (τf) using knr ) (1 - φf)/τf, and the same are
collected in Table 5. As seen from the table, while the knr values
of ANBD and 4NBD do not vary appreciably in different media,
those of the remaining derivatives are severalfold higher in polar
media compared to the nonpolar media. As mentioned in
section 1, the dependence of knr values on the solvent polarity
was noted in the case of ENBD, and it was speculated that
twisting of the dialkylamino group about the C-N bond that
connects this group with the NBD chromophore (henceforth
simply referred to as C-N bond) is the nonradiative deactivation
5
7
observed on the NMR time scale. It should be noted in this
context that since the axial and equatorial proton signals are
not resolvable in the present experimental condition, nitrogen
inversion that leads to an exchange of the nuclei bearing the
hydrogen atoms (Scheme 2) cannot be responsible for the broad
NMR signal. Therefore, the nuclear exchange process observed
here has to be assigned to the internal rotation (or twisting, as
it is more commonly called) about the C-N bond. The peaks
observed between 3 and 5 δ (integration corresponding to four
protons) should be assigned to the two sets of methylene protons
adjacent to the amino nitrogen atom. The chemical shifts of
these two sets of protons are different because of the difference
in their environments due to the presence of the diazaoxazole
ring on one side. At lower temperature, when the internal
rotation is slow, the methylene groups display two signals (each
appearing as triplet) typical of an AB system. On the other
hand, at higher temperature, when the internal rotation is fast,
a sharp signal is observed. Between the slow and the fast
exchange limits, changes in the line shape of the NMR signals
are observed. The separation between the two sets of peaks
and the temperature at which they coalesce (Tc) have been
measured for all the systems, and the values are tabulated in
Table 6. The rate constant of the internal rotation (kc) at the
coalescence temperature has been obtained using the following
3
6
pathway. As will be shown in the following sections, when
the photophysical data of all the derivatives are taken into
consideration, it becomes difficult to interpret them in terms of
the TICT mechanism.
3.2. Dynamic NMR Spectroscopy. The room-temperature
proton magnetic resonance spectra of the NBD derivatives,
which were primarily obtained to establish the structural identity
of the compounds, were found to be unusually broad for some
systems, which suggests an exchange of the nuclear sites. Since
the nuclear motion, whether internal rotation or any other, is
likely to affect the decay of the emitting state, the exchange
57
process has been investigated by dynamic NMR spectroscopy.
Figure 4 illustrates the room-temperature NMR spectra of
the cyclic derivatives. It can be seen that the NMR signal
that appears at around 3-5 δ is unusually broad for 7NBD.
In the case of 4NBD and 5NBD, the signal appears as two
broad doublets, whereas one sharp peak is observed for 6NBD
at room temperature. Since this behavior is a reflection of a
dynamic process in which the nuclear sites are exchanged in
the molecule, the NMR spectra of all the systems have been
5
7
equation:
x
kc ) π∆ν/ 2
(3)
where ∆ν is the difference in the chemical shifts of the two

Photophysical and Dynamic NMR
J. Phys. Chem. A, Vol. 102, No. 41, 1998 7909
3
Figure 4. NMR spectra of the cyclic NBD derivatives in CDCl at 25 °C.
q
sites. The free energy of activation (∆G c) for the exchange
in the size of the ring. However, since the C-N bond is
expected to have considerable double bond character because
of intramolecular charge separation between the amino and the
nitro group in the fluorophore, the rotational barrier is expected
to be governed mainly by the extent of the double bond character
of the C-N bond rather than by the size of the group. Since
both the C-N bond length and the ground-state dipole moments
of the compounds are reasonably good measures of the double
bond character of the concerned bond, one expects, on the basis
of the NMR results, the six-membered ring system to have the
lowest ground-state dipole moment and highest C-N bond
length among the ring systems. The data presented in Table 7
show that that is indeed the case. The results of the X-ray
crystallographic measurements on a few systems also indicate
the C-N bond length to be shorter than the single bond
has been obtained using57 the following equation:
Tc
q
c
∆
G ) 4.57T 9.97 + log
(4)
c
(
)
∆
ν
q
It can be seen from the ∆G c values, collected in Table 6,
that for the alkyl derivatives, the barrier to internal rotation
increases marginally with an increase in the size of the alkyl
group. On the other hand, for the ring systems, even though
the barrier height was expected to increase with an increase in
q
the size of the ring, the results indicate the ∆G c value to be
the lowest for the six-membered ring system. We have found
out that this apparently unusual behavior is linked to the partial
double bond character of the C-N bond. It may be noted that
had there been no double bond character of the C-N bond, the
barrier to internal rotation would have been considerably lower
and perhaps the barrier would have increased with an increase
5
8
q
distance. A plot of the measured ∆G c values versus the
ground-state dipole moments of the ring systems (Figure 6)
clearly shows that the barrier to the twisting or internal rotation

7910 J. Phys. Chem. A, Vol. 102, No. 41, 1998
Saha and Samanta
Figure 5. NMR spectra of the cyclic NBD derivatives in CDCl
3
as a function of temperature (K). Only the NMR peaks of interest are shown here.
around the C-N bond is governed by the bond order of the
derivatives are considerably higher, particularly in the polar
media. Since the ground-state structure of the donor-acceptor
molecules often dictate the photophysical properties of these
derivatives, we have probed the ground-state structure of the
systems by AM1 calculations and investigated the internal
motion in the systems using dynamic NMR spectroscopic tech-
nique.). These studies reveal that the amino nitrogen atom in
the systems is slightly pyramidal in the ground state,59 with the
C-N bond having some partial double bond character. The
internal rotation about the C-N bond is measured to be rather
slow because of the double bond character. The other internal
motions such as nitrogen inversion in all the systems and ring
inversion in the cyclic systems could not be observed as these
motions either are too rapid compared to the NMR time scale
system rather than by the ring size.
4
. Discussion and Conclusion
The NBD derivatives, as found in this investigation and
36
elsewhere, display only one emission band. The fluorescence
originates from an LE state with charge-transfer character
(except for ANBD in highly nonpolar solvents), and the dipole
moment of this state is slightly higher than that of the ground
state. The fluorescence yield, lifetime, and the nonradiative rate
constants of the NBD derivatives are strongly dependent on the
structure of the amino moieties. While for ANBD and 4NBD,
the knr values are low and do not depend significantly on the
polarity of the surrounding media, the knr values for the other

Photophysical and Dynamic NMR
J. Phys. Chem. A, Vol. 102, No. 41, 1998 7911
SCHEME 2
q
Figure 6. Plot of ∆G
c
values versus the ground-state dipole moments
g
(µ ) of the cyclic NBD derivatives. The straight line represents the best
linear fit to the data (r ) 0.98).
TABLE 7: AM1-Calculated Bond Length and Ground-State
Dipole Moment (µ ) of the Ring Systems
g
compound
µ
g
(D)
C-N bond length (Å)
c
TABLE 6: Coalescence Temperature (T ), Difference in
Chemical Shifts of the Two Sites (∆ν), Rate Constant of the
4
5
6
7
NBD
NBD
NBD
NBD
9.64
1.3678
1.3601
1.3841
1.3783
Exchange (k ) at the Coalescence Temperature, and the
c
10.54
9.30
9.98
q
Barrier to Rotation (∆G
c
) about the C-N Bond in the NBD
Derivatives
q
compounds
T
c
(K)
∆ν (Hz)
k
c
(s-1)
∆G
c
(kJ/M)
membered rings57 and hence according to this mechanism,
the fluorescence efficiencies of 4NBD and 5NBD should have
been the lowest (which is not the case). Finally, the open
dialkylamino systems, where this motion is not possible, also
display nonradiative decay similar to that exhibited by the ring
system.
4
5
6
7
NBD
NBD
NBD
NBD
302
312
250
293
275
274
282
88.2
127.1
151.6
120.0
105.8
100.9
105.7
197.5
275.5
336.7
266.6
235.0
224.1
234.8
59.9
62.0
48.1
57.1
54.8
54.7
55.4
MNBD
ENBD
BNBD
On the basis of the above considerations, we are led to
conclude that nitrogen inversion, which is also known to be a
fast process, is responsible for the nonradiative decay of the
excited state of the NBD derivatives. Even though this motion
has not been observed by NMR, it is known in the literature
that the barrier to nitrogen inversion in trialkylamines is much
or do not lead to an exchange of the hydrogen nuclei. Since
the fluorescent state of the NBD derivatives is shown to be more
polar than the ground state, one can expect the double bond
character of the C-N bond to be higher and the amino nitrogen
to be more planar in the excited state as compared to the ground
state. Consideration of these factors led us to suggest that the
barrier to the internal rotation around the C-N bond is higher
in the excited state. Consequently, the internal rotation is
expected to be slower in the excited state and hence can not be
expected to be an important factor in determining relatively
much faster nonradiative decay of the systems. We can discount
the possibility of internal rotation affecting the nonradiative rates
in the systems using a different line of argument. One can
conclude based on the measured data on the rotational rates of
the systems that if internal rotation in the excited state is
responsible for the observed variation of the nonradiative rates
in different systems, then in any given solvent, the nonradiative
rate should have been the highest for 6NBD among the ring
systems and for MNBD and ENBD among the alkyl derivatives.
However, it is clear from the data shown in Table 5 that knr
values are not in accordance with the trend one expects based
on consideration of internal rotation or twisting as the promoting
mode for the nonradiative decay process.
-1
lower in the range of 6-8 kcal mol , and this barrier is lowered
6
0
as the size of the alkyl group is increased.
For the ring
systems, it is reported that nitrogen inversion is the slowest in
the case of three-membered nitrogen-containing heterocycles
5
7
(aziridines).
As the ring size is increased, the barrier to
nitrogen inversion is known to decrease steadily. This behavior
has been rationalized in terms of the strain in the planar
conformation of the molecule, which is close to the transition
state for the inversion process. It is therefore evident that the
knr values of the NBD derivatives agree rather well with the
rates of the nitrogen inversion motion.
In summary, the photophysical behavior of several new NBD
derivatives as a function of their structure and the polarity of
the media has been studied. To rationalize the fluorescence
response of the systems, dynamic NMR spectroscopic measure-
ments, AM1 calculations, and X-ray crystallographic measure-
ments have been performed. The results of these measurements
indicate the inadequacy of the TICT model in explaining the
photophysical behavior of the systems. The inversion of the
amino nitrogen seems to be responsible for the nonradiative
decay in the systems.
Among the other possible modes of internal motion, inversion
of the ring, which is known to be fast enough to compete with
the radiative process, could not be considered as a dominant
mechanism in these systems for the following reasons. First,
this motion does not lead to any change in the configuration of
the amino nitrogen and hence is not expected to affect the
fluorescence efficiency of the systems. Second, this motion is
known to be extremely rapid in the case of four- and five-
Acknowledgment. The research described herein is sup-
ported by grants received from Department of Science and
Technology, Government of India. S.S. thanks the University
Grants Commission for a fellowship.

7912 J. Phys. Chem. A, Vol. 102, No. 41, 1998
Saha and Samanta
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