Anomalous dual fluorescence of benzanilide

Article abstract of DOI:10.1021/jp972259i

The absorption and fluorescence spectra of benzanilide and N-methylbenzanilide have been investigated in solution and low-temperature glasses and assigned with the aid of ZINDO calculations. The anomalous dual fluorescence observed by previous workers has been assigned to the two lowest singlet states of benzanilide. The lower energy n,π^* state is populated by excitation in the long-wavelength tail of the absorption band. Its fluorescence is readily detected in low-temperature glasses and at room temperature in aromatic solvents. Large NMR solvent-induced shifts provide evidence for ground-state complex formation of benzanilide with aromatic solvents. The higher energy π,π^* state is populated by excitation of an allowed transition. It undergoes twisting about the amide C-N bond to form a fluorescent twisted intramolecular charge-transfer state with a maximum of 520 nm in benzene solution. In rigid glasses twisting to form the twisted charge-transfer state cannot occur, and the π,π^* state undergoes internal conversion to the lower energy n,π^* state.

Full text of DOI:10.1021/jp972259i

J. Phys. Chem. A 1998, 102, 5327-5332
5327
ARTICLES
Anomalous Dual Fluorescence of Benzanilide†
Frederick D. Lewis* and Timothy M. Long
Department of Chemistry, Northwestern UniVersity, EVanston, Illinois 60208-3113
ReceiVed: July 11, 1997; In Final Form: NoVember 3, 1997
The absorption and fluorescence spectra of benzanilide and N-methylbenzanilide have been investigated in
solution and low-temperature glasses and assigned with the aid of ZINDO calculations. The anomalous dual
fluorescence observed by previous workers has been assigned to the two lowest singlet states of benzanilide.
The lower energy n,π* state is populated by excitation in the long-wavelength tail of the absorption band.
Its fluorescence is readily detected in low-temperature glasses and at room temperature in aromatic solvents.
Large NMR solvent-induced shifts provide evidence for ground-state complex formation of benzanilide with
aromatic solvents. The higher energy π,π* state is populated by excitation of an allowed transition. It
undergoes twisting about the amide C-N bond to form a fluorescent twisted intramolecular charge-transfer
state with a maximum of 520 nm in benzene solution. In rigid glasses twisting to form the twisted charge-
transfer state cannot occur, and the π,π* state undergoes internal conversion to the lower energy n,π* state.
Assignment of the weak room-temperature luminescence of
benzanilide (BA) has proven to be a formidable challenge.
O’Connell et al.1 initially reported that BA displays strongly
overlapping fluorescence and phosphorescence in an EPA glass
at 77 K but is nonfluorescent in ethanol solution. Kasha and
co-workers2 subsequently reported that BA displays dual
fluorescence at room temperature in methylcyclohexane (MC)
solution. The weaker fluorescence (F1) has an approximate
mirror-image relationship to the absorption spectrum and was
attributed to a state of mixed n,π* and π,π* character.2b The
stronger fluorescence (F2) has a large Stokes shift and initially
was assigned to the imidol tautomer, supposedly formed in a
double proton transfer from a hydrogen-bonded dimer.2a Sub-
sequent observation of similar Stokes-shifted emission for
N-methylbenzanilide (MBA), which cannot undergo tautomer-
ization, led to the suggestion that the F2 fluorescence of MBA
originated from a twisted intramolecular charge-transfer (TICT)
state.2b The assignment of F2 fluorescence of BA was then
suggested to result from overlapping emission from a TICT state
and the imidol tautomer. The TICT assignment for MBA and
BA was supported by the observation of moderate solvent-
induced shifts in alkane and ether solvents.2b The proposal of
dual TICT and imidol F2 fluorescence for BA was based on
the bandwidth and the observation of dual-exponential F2
fluorescence decay.2c,d
consistent with the absence of Stokes-shifted fluorescence in a
77 K EPA glass.1 Azumaya et al.3 did not investigate the origin
of the weak F1 fluorescence of BA and MBA but noted that its
intensity increased with time, possibly due to the formation of
a fluorescent photoproduct.
Our interest in the structure and spectroscopy of aromatic
carboxamides4 prompted us to reinvestigate the nature of the
singlet states of BA and MBA using a combination of
spectroscopic and computational techniques. We find that both
the F1 and F2 fluorescence of BA and MBA can be readily
observed in aromatic solvents. The higher energy F1 fluores-
cence is proposed to originate from the lowest energy singlet
state, S1, which is assigned to an amide-localized forbidden n
f π* transition. The lower energy F2 fluorescence is assigned
to a TICT state generated via twisting of a higher energy vertical
excited state, S2, which is populated via an allowed π f π*
transition. Internal conversion from S2 to S1 competes with
twisting at low temperature but not at room temperature.
Experimental Section
Materials. BA (Aldrich) was recrystallized from ethanol
prior to use. MBA was prepared by the reaction of benzoyl
chloride with N-methylaniline in the presence of triethylamine.
No impurities were detected by gas chromatography or 1H NMR
spectroscopy. Methylcyclohexane (Aldrich, anhydrous) and
toluene (Fischer, ACS grade) were used as received. Benzene
was distilled over sodium metal prior to use.
Methods. 1H NMR spectra were measured using a Varian
Gemini 300 spectrometer. UV-vis absorption spectra were
obtained using a Hewlett-Packard 8542A diode array spectro-
photometer using a 1 cm path quartz cell. Fluorescence spectra
of degassed solutions (N2) were recorded with a SPEX Fluo-
romax spectrometer. Low-temperature emission spectra were
obtained using a liquid nitrogen cooled fluorescence Dewar with
An investigation of the fluorescence of BA and MBA in MC
solution by Azumaya et al.3 led to the conclusion that the F2
fluorescence of both molecules could be assigned to a single
TICT state. Their study of various derivatives of BA and MBA
with restricted geometries indicated that twisting about the amide
C-N bond was a prerequisite for TICT formation. The
requirement of geometric relaxation for TICT formation is
† Dedicated to Nicholas J. Turro, postdoctoral mentor extraordinary, on
the occasion of his 60th birthday.
S1089-5639(97)02259-7 CCC: $15.00 © 1998 American Chemical Society
Published on Web 01/30/1998

5328 J. Phys. Chem. A, Vol. 102, No. 28, 1998
Lewis and Long
Figure 2. Fluorescence spectra of 2.0 × 10-5 M BA (s) and 2.6 ×
Figure 1. Absorption spectra of 2.0 × 10-5 M BA (s) and 2.6 ×
10-5 M MBA (- - -) in methylcyclohexane solution. The baseline for
MBA is displaced upward for clarity.
10-5 M MBA (- - -) in methylcyclohexane solution at 293 K (λex
290 nm, X ) scatter).
)
in MC solution were determined to be 80 and 20 M-1 cm-1 for
BA and MBA, respectively. The appearance and intensity of
the long wavelength tail are similar in more polar solvents and
in the aromatic solvents benzene and toluene.
a hanging finger window or a Oxford Instruments variable
temperature cryostat model DN1704 controlled with a model
ITC4 temperature controller. Fluorescence quantum yields were
determined using phenanthrene (Φf ) 0.14) as a reference
standard.5a The integrated fluorescence intensity of the ben-
zanilide and standard were compared for solutions with equal
absorbance at the excitation wavelength. The estimated error
in the measured quantum yields is (20%. Fluorescence decays
were measured on a Photon Technology International LS-1
single-photon-counting apparatus with a gated hydrogen arc
lamp using a scatter solution to profile the lamp decay. Decays
were analyzed using deconvolution and single-exponential or
multiexponential least-squares fitting as described by James et
al.5b Goodness of fit was judged by reduced ø2 values (<1.2
in all cases), the randomness of residuals, and the autocorrelation
function.
The fluorescence spectra of 2 × 10-5 M BA and MBA in
MC solution are shown in Figure 2. The spectra display maxima
at 475 and 515 nm, respectively, similar to those assigned to
the F2 band by Kasha and co-workers.2 The position of this
broad fluorescence band is independent of excitation wavelength
(280-330 nm), and the fluorescence excitation spectrum is
similar to the absorption spectrum (λmax ∼ 270 nm for BA), as
observed by Azumaya et al.3 The fluorescence at ca. 330 nm
assigned to the F1 band by Kasha and co-workers2 is absent in
freshly prepared solutions but increases in intensity upon
exposure to light, as observed by Azumaya et al.3 The 330 nm
band most likely is due to a photoproduct (see Discussion
section).
Minimum-energy conformations were calculated using a
MM2 type force field as supplied in the Chem 3D software
package (Chem 3D Plus, Cambridge Scientific Computing, Inc.,
Version 3.2) for a Power Macintosh 7600/120 computer or in
the CACHe program (Cache Scientific, version 3.6). The
calculations of electronic structure and absorption spectra were
performed using the semiempirical INDO/S-SCF-CI (ZINDO)
algorithm developed by Zerner and co-workers.6 The ZINDO
calculations were performed using CACHe 3.6. The calculation
was carried out in two steps. First, the ground state was
calculated using MM2 minimized initial geometries to acquire
molecular orbital coefficients and eigenvalues. This was
followed by configuration interaction (CI) calculations, in which
9-12 filled and 9-12 unfilled orbitals were used. For the
calculation of vertical transitions as a function of amide C-N
dihedral angle, the ground-state structure was rotated about the
appropriate bond and the CI calculation performed without
reminimization of the complete structure.
The F2 fluorescence maxima of BA and MBA are red-shifted
in benzene solution to 510 and 540 nm, respectively. The
excitation maximum for F2 fluorescence is ∼310 nm due to
competitive absorption by solvent at shorter wavelengths.
Excitation of a 3 × 10-3 M solution of BA in the long-
wavelength tail of its absorption spectrum results in fluorescence
emission with a maximum at 433 nm which we assign as F1
fluorescence (Figure 3). The excitation maximum for F1
fluorescence is 340 nm. Excitation at progressively shorter
wavelengths results in a decrease in the F1 fluorescence and an
increase in the F2 fluorescence intensity. Similar excitation-
wavelength-dependent dual fluorescence was observed for MBA
in benzene solution and for both BA and MBA in toluene
solution. Exposure to light results in an increase in 330 nm
fluorescence but not the 433 nm fluorescence. Attempts to
observe F1 fluorescence in several other solvents, including MC,
dibutyl ether, dioxane, tetrahydrofuran, and acetonitrile, were
unsuccessful even when high concentrations of BA (>10-3 M)
were used. Competitive absorption between 300 and 340 nm
precludes the use of more polar aromatic solvents such as
chlorobenzene and benzonitrile.
Results
Electronic Spectra. The absorption spectra of BA and MBA
in MC solution are shown in Figure 1. As previously reported
by Kasha and co-workers,2 both spectra consist of a single broad
band with maxima at 268 and 252 nm for BA and MBA,
respectively, and with tailing at long wavelengths. Only minor
changes in the absorption maxima or extinction coefficients are
observed in more polar solvents such as tetrahydrofuran or
acetonitrile. Since the long-wavelength tail assumes importance
in our fluorescence studies, extinction coefficients at 330 nm
The emission spectra of BA at several temperatures in 1:1
MC-toluene are shown in Figure 4. Only long wavelength
505 nm emission is observed at room temperature in MC-
toluene solution (Figure 4). At 193 K, which is above the glass
transition temperature (Tg), only shorter wavelength 434 nm
emission is observed. At 77 K (below Tg) the spectrum is
slightly broader and more intense than at 193 K. As previously
reported by O’Connell et al.,1 no emission is observed at room

Anomalous Dual Fluorescence of Benzanilide
J. Phys. Chem. A, Vol. 102, No. 28, 1998 5329
TABLE 1: Fluorescence Emission Maxima, Quantum
Yields, and Lifetimes
amide,
emission
λex,
λmax,
solvent
nma
nmb
103Φfc
τ, ns (A)d
BA, F2
BA, F2
BA, F1
MBA, F2
MBA, F2
MBA, F1
MC
290
310
340
290
310
340
475
510
433
515
540
435
0.6
0.4
7.0
0.4
0.3
1.0
0.86e
benzene
benzene
MC
benzene
benzene
1.0 (0.99)
2.1 (0.83)f
1.7 (0.96)e
2.1 (0.98)
1.0 (0.98)
a Excitation wavelength used for fluorescence quatum yield and
lifetime measurements. b Maximum intensity of fluorescence band.
c Fluorescence quantum yield. Estimated error (20%. d Preexponential
for major decay component. e Data from ref 3. f Second component has
a lifetime of 9.5 ns and a preexponential of 0.17.
Figure 3. Fluorescence spectra of 3 × 10-3 M BA in benzene solution
with λex ) 310 (×2.5), 325, 330, and 340 nm (X ) scatter). Baseline
indicated by a dashed line.
Figure 5. Calculated dihedral angles for Z-BA and E-MBA.
TABLE 2: 1H NMR Aromatic Solvent-Induced Shifts for
BA
1H
δCDCl3 (ppm)a
δC D6 (ppm)
∆δ (δC D - δCDCl )
3
6
6
6
2
3
4
2′
3′
4′
7.88
7.49
7.56
7.64
7.38
7.16
7.61
7.08
7.11
7.58
6.97
6.92
-0.27
-0.41
-0.45
-0.06
-0.41
-0.24
a Data from ref 7a.
Figure 4. (a) Fluorescence spectra of BA in MC-toluene solution at
293 (×32), 193, and 77 K (λex ) 320 nm). Baselines indicated by
dashed lines.
combination low fluorescence intensity and low efficiency of
the optical Dewar when used with the single-photon-counting
apparatus.
temperature in EPA solution. Long-wavelength 501 nm emis-
sion is observed at 110 K (above Tg) and short-wavelength 429
nm emission at 77K (below Tg) in EPA.
Ground-State Structure. The ground-state structures of BA
and MBA in solution and in the solid state have been
1
Fluorescence quantum yields (Φf) and decay times (τ) were
measured for BA and MBA in MC and benzene solution (Table
1). Values of Φf for F2 fluorescence are in the range (3-6) ×
10-4 M in both solvents. A larger value is obtained for the F1
fluorescence in 3 × 10-3 M benzene solution. Fluorescence
decays for BA and MBA were determined in benzene solution
using a single-photon-counting apparatus with a time resolution
of approximately 0.2 ns. The decays were deconvoluted using
a single or multiple least-squares analysis, as described by James
et al.5b Decays were best fit by a biexponential function with
short- and long-lived decay components. No rising components
were detected for either F1 or F2 fluorescence. The F2
fluorescence of both BA and MBA was dominated by the short-
lived components reported in Table 1, along with the values
reported by Azumaya et al.5 for MC solution. The F1
fluorescence of MBA was also dominated by the short-lived
component; however, the long-lived component has a significant
preexponential in the case of BA. Attempts to determine decay
times in low-temperature glasses were unsuccessful due to a
investigated by Shudo and co-workers.7 According to their H
NMR assignments, BA exists predominantly as the Z conformer
and MBA as the E conformer in CDCl3 solution (Figure 5).
We have assigned the NMR spectra of BA and MBA in C6D6
solution by analogy to the assignments of Itai et al.7a Chemical
shift values for BA in CDCl3 and C6D6 solution are summarized
in Table 2, along with the aromatic solvent-induced shift (ASIS)8
for each aromatic proton. Similar negative (upfield) shifts are
seen for the aromatic protons of MBA.
Dihedral angles obtained from MM2 calculations for Z-BA
and E-MBA (unsolvated molecules) are summarized in Figure
5. The calculated ground-state amide dihedral angles for Z-BA
and E-MBA are similar to those reported for their crystal
structures.7 The calculated energy of E-BA is ∼5 kcal/mol
higher than that of Z-BA, and the calculated barrier to rotation
is 18 kcal/mol. This barrier is similar to that determined from
temperature-dependent NMR data for N,N-dimethylbenzamide.9
The calculated energy of Z-MBA is ∼1 kcal/mol lower than
that of E-MBA, and the calculated barrier to rotation is 9 kcal/

5330 J. Phys. Chem. A, Vol. 102, No. 28, 1998
Lewis and Long
Figure 7. Calculated potential energy surfaces for amide C-N rotation
in BA (0° is Z isomer and 180° is E isomer).
ZINDO calculations have also been performed for MBA. The
frontier π orbitals of MBA are delocalized over both the benzoyl
and aniline groups, and the singlet states display more extensive
configuration interaction than is the case for BA. The first
singlet of MBA is largely n,π* in character. The lowest energy
state of MBA with largely π,π* character is S4. It is calculated
to lie at higher energy (279 nm) and have a lower oscillator
strength (0.17) than S2 for BA (Table 3).
Figure 6. Frontier molecular orbitals for BA (HOMO-4 is lowest in
energy and LUMO+4 highest in energy).
Since rotation about the amide C-N bond has been implicated
in the formation of the singlet state responsible for F2 fluores-
cence, we have investigated the effect of this bond rotation on
the energies of the lowest singlet states of BA. Vertical
transitions were calculated at 15° increments of rotation about
this bond, using the MM2 minimized geometries. The calcu-
lated ground-state energies were added to the vertical transitions,
and potential energy surfaces were generated using Microsoft
Excel 5.0 to interpolate and produce smooth fits to the data. As
shown in Figure 7, the calculated barrier for rotation of S1 (16
kcal/mol) is similar to that calculated by MM2 for ground-state
rotation. In contrast, there is no barrier for rotation of S2.
Analogous calculations for MBA produced similar potential
energy surfaces with a barrier of 14.7 kcal/mol for rotation of
S1 and no barrier for rotation of S4.
TABLE 3: Calculated Dipole Moments, Absorption
Maxima, Oscillator Strengths, and Character of the Singlet
States of Benzanilide
singlet µ, D
λ
max, nm
f
CI (%) and charactera
S0
S1
5.3
5.5
375.1
289.7
0.0005 73.7 HOMO-4 f LUMO
21.2 HOMO-4 f LUMO+4
S2
17.0
0.43
84.9 HOMO f LUMO
3.5 HOMO f LUMO+2
2.9 HOMO f LUMO+3
2.6 HOMO f LUMO+4
a Benzanilide frontier orbitals shown in Figure 6.
mol. Preferential solvation of the E conformer could reverse
the relative energies in solution. The lower barrier for MBA
vs BA may result from ground-state repulsion between the two
benzene rings in both the E and Z conformers (Figure 5).
Discussion
ZINDO Calculations. The electronic structure and spectra
of BA and MBA were investigated by semiempirical INDO/
S-SCF-CI (ZINDO) calculations using the algorithm developed
by Zerner and co-workers.6 The molecular structures were those
obtained from MM2 calculations. The nodal patterns for the
five highest occupied and lowest unoccupied molecular orbitals
of BA are shown in Figure 6. The HOMO-4 and LUMO+4
orbitals are amide-localized, whereas the other frontier orbitals
are largely localized on either the benzoyl or aniline portions
of the molecule. The calculated absorption maxima, oscillator
strengths, dipole moments, and character of the three lowest
energy singlet states of BA are summarized in Table 3. The
lowest energy singlet state is largely n,π* (amide f benzoyl)
in character and possesses a very low oscillator strength. The
second singlet state is largely π,π* (aniline f benzoyl) in
character and possesses a large oscillator strength. The
calculated dipole moment of S2 is significantly larger than that
of S0 or S1. The next three lowest energy singlet states are
also π,π* in character (S3 is largely aniline f aniline and S4
and S5 are largely benzoyl f benzoyl).
Previous investigations of the photophysics of BA and MBA
have left unresolved questions concerning the identity of the
excited states responsible for both the Stokes-shifted fluores-
cence and the shorter wavelength fluorescence, labeled F2 and
F1, respectively, by Kasha and co-workers.2 F2 fluorescence
was originally attributed to adiabatic formation of the tautomeric
imidol2a but later reattributed to a TICT state.2b,3 F1 fluorescence
was attributed by Kasha to a Franck-Condon (FC) singet state
that rapidly relaxes to the TICT state.2d Azumaya et al.3 noted
that the intensity of the very weak F1 emission increased upon
exposure to light and might arise from a photoproduct. Aniline,
which is formed with a quantum yield of 1 × 10-3 upon
irradiation of BA in ethanol solution,10a appears to be a likely
candidate for the F1 fluorescence previously observed in MC
solution. BA also undergoes photo-Fries rearrangement10a and
photocyclization to phenanthridones; however, these products
do not fluoresce in the region of F1 fluorescence.10b
An impediment to the investigation of the excited states of
BA and MBA is their very weak fluorescence. We find that
the F2 fluorescence quantum yields are ∼5 × 10-4 in the

Anomalous Dual Fluorescence of Benzanilide
J. Phys. Chem. A, Vol. 102, No. 28, 1998 5331
nonpolar solvents MC and benzene (Table 1). Increasing
solvent polarity results in a decrease in fluorescence intensity,
and fluorescence has not been detected in solvents more polar
than tetrahydrofuran. We are unable to detect F1 fluorescence
from freshly prepared solutions of recrystallized BA in MC
(Figure 2) and other nonaromatic solvents. However, dual
fluorescence can readily be observed in aromatic solvents
(Figure 3). In benzene solution the F2 fluorescence has an
emission maximum similar to that reported by Heldt et al.2b for
ether solvents. Benzene and diethyl ether have very similar
values of ET(30), the Dimroth-Reichardt solvent polarity
parameter.11
The most striking result of the present investigation is the
observation of significantly enhanced F1 fluorescence upon
excitation of BA and MBA at long wavelengths (>310 nm) in
aromatic solvents (Figure 3). Because of the low intensity of
the long-wavelength absorption (Figure 1), the use of higher
concentrations facilitates the observation of F1 fluorescence in
aromatic solvents, but not in nonaromatic solvents such as MC.
The quantum yield and lifetime for F1 fluorescence in benzene
solution are larger than those for F2 fluorescence (Table 1). Heldt
et al.2d reported a lifetime of 12 ps for the Franck-Condon
singlet state of BA in MC solution, as determined by transient
absorption spectroscopy. Assuming that the same state is
responsible for F1 fluorescence, the longer lifetime in benzene
vs MC solution could account for its much stronger fluorescence
in benzene.
Figure 8. State diagram for BA.
transition is lower for BA, in accord with the appearance of
their absorption spectra (Figure 1).
A singlet-state diagram that can account for the anomalous
fluorescence behavior of BA is shown in Figure 8. The F1
fluorescence observed upon excitation in the long-wavelength
tail of the absorption band in aromatic solvents (Figure 3) is
attributed to an amide-localized lowest singlet (S1) n,π* state.
The forbidden nature of this transition is consistent with its weak
absorption and fluorescence. The absence of F2 fluorescence
upon long-wavelength excitation indicates that S1 is not a
precursor of F2 fluorescence. Excitation of the allowed π f
π* transition results in the observation of F2 fluorescence.
However, several lines of evidence indicate that the FC S2 state
is not fluorescent. As noted Kasha and co-workers,2 the large
Stokes shift for F2 fluorescence is indicative of a large change
in geometry prior to fluorescence. In addition, the absence of
F2 fluorescence both in rigid glasses and for geometrically
constrained analogues of BA3 indicates that a substantial
geometric change is necessary for the observation of F2
fluorescence. The solvent-induced shift in the fluorescence
maxima2b supports the assignment of F2 to a TICT state.
Azumaya et al.3 have proposed that twisting about the amide
C-N bond in BA and MBA is required for the formation of
the TICT state responsible for F2 fluorescence. Rigid analogues
of BA in which this torsion is inhibited are either nonfluorescent
or very weakly fluorescent. Twisting about the amide C-N
bond is also responsible for the E,Z photoisomerization of simple
aliphatic amides.17 Our investigation of the potential energy
surface for amide C-N twisting in BA and MBA indicates that
barriers exist in the ground state and S1, but not for S2 (Figure
7). A low barrier for amide rotation of the S2 FC singlet state
to form the TICT state could explain the absence of internal
conversion of S2 to S1 in solution at room temperature.
Lowering the temperature of a MC-toluene solution of BA from
273 to 193 K results in a change from F2 to F1 fluorescence
(Figure 4). Further cooling to 77 K results in only modest
changes in emission intensity and band shape, which is similar
to that for F1 fluorescence in benzene solution.
The enhanced intensity of F1 fluorescence in aromatic solvents
presumably results from solvent perturbation of the radiative
or nonradiative decay rates. Solvent perturbation of radiative
decay is noticeable only when the transition moment of the
unsolvated molecule is very small,12 as is the case for the n,π*
state of BA in nonaromatic solvents. Evidence for ground-state
interaction of BA with aromatic solvents is provided the large
differences in 1H NMR chemical shifts in aromatic vs nonaro-
matic solvents (Table 2). Aromatic solvent-induced shifts
(ASIS) for amides were first reported by Hatton and Richards,13
who attributed them to formation of a π complex in which
protons near the amide functional group lie in the shielding
region of the aromatic solvent. Amide ASIS data have been
used to assign the conformation of tertiary amides.9,14 Ground-
state interaction between aromatic solvents and porphyrins
results in selective enhancement of solvent Raman bands via
resonance vibrational coupling.15 We suspect that similar
coupling is responsible for the enhanced radiative decay of the
n,π* state of BA in aromatic solvents which results in the
observation of F1 fluorescence.
The observation of F1 fluorescence upon long-wavelength
excitation and F2 fluorescence upon short-wavelength excitation
of BA (Figure 3) requires that emission occurs from two
different singlet states that do not interconvert. Heldt et al.2b
proposed that a forbidden transition of mixed n f π* and π f
π* character is buried under the allowed π f π* transition,
which dominates the absorption spectra of BA and MBA (Figure
1). In acetanilide the n f π* transition is resolved from the
higher energy π f π* transition.16 The results of our ZINDO
calculations (Table 3) are consistent with the presence of an
amide-localized forbidden n f π* transition at lower energy
than the first allowed π f π* transition for both BA and MBA.
In the case of BA the allowed π f π* transition is predomi-
nantly aniline HOMOfbenzoyl LUMO in character, whereas
for MBA there is more extensive configuration interaction. The
calculated oscillator strength for the π f π* transition of BA
is larger than that for MBA and the calculated energy of this
Observations of dual fluorescence have frequently been
attributed to emission from two excited states: short-wavelength
fluorescence originating from a state of geometry similar to that
of the FC excited state and long-wavelength emission to a TICT
state with relaxed geometry.18-20 In the case of 4-(dimeth-

5332 J. Phys. Chem. A, Vol. 102, No. 28, 1998
Lewis and Long
(4) (a) Lewis, F. D.; Burch, E. L. J. Phys. Chem. 1996, 100, 4055. (b)
Lewis, F. D.; Yang, J.-S. J. Phys. Chem. B 1997, 101, 1775.
(5) (a) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-
Interscience: London, 1970; p 128. (b) James, D. R.; Siemiarczuk, A.; Ware,
W. R. ReV. Sci. Instrum. 1992, 63, 1710.
(6) (a) Bacon, A. D.; Zerner, M. C. Theor. Chim. Acta 1970, 53, 21.
(b) Zerner, M. C.; Loew, G. H.; Kirchner, R. R., Mueller-Westerhoff, U.
T. J. Am. Chem. Soc. 1980, 102, 589.
(7) (a) Itai, A.; Toriumi, Y.; Tomioka, N.; Kagechika, H.; Azumaya,
I.; Shudo, K. Tetrahedron Lett. 1989, 30, 6177. (b) Itai, A.; Toriumi, Y.;
Saito, S.; Kagechika, H.; Shudo, K. J. Am. Chem. Soc. 1992, 114, 10649.
(c) Azumaya, I.; Kagechika, H.; Yamaguchi, K.; Shudo, K. Tetrahedron
1995, 51, 5277.
(8) (a) Jauquet, M.; Laszlo, P. In Techniques of Chemistry; Dack, M.
R. J., Ed.; Wiley: New York, 1975; Vol. VIII, p 233. (b) Reichardt, C.
SolVents and SolVent Effects in Organic Chemistry; VCH: Weinheim,
Germany, 1990; p 311.
ylamino)benzonitrile formation of the TICT state is proposed
to occur via the relaxed FC state.18 Wintgens et al.20 have
recently suggested that the dual fluorescence of N-phenyl-2,3-
naphthalimides results from the formation of two singlet states
that are not kinetically connected from a single FC excited state.
The dual fluorescence of BA is unique in that the lowest singlet
S1 yields only the higher energy F1 fluorescence, whereas the
higher energy FC S2 state is the precursor of the lower energy
F2 TICT fluorescence (Figure 8). This seeming anomaly is
attributed to a decrease in the S2 energy and increase in S0
energy upon twisting of the amide C-N bond, resulting in a
lower energy for the TICT state than for the planar S1. S2 f
S1 internal conversion does not occur in fluid solution at room
temperature but does occur at low temperature in MC/toluene
solution and in several rigid glasses. Formation of the TICT
state from S1 is not observed under any conditions, and the
observation of S1 fluorescence requires the use of either aromatic
solvents or low temperatures.
(9) Stewart, W. E.; Siddall, III, T. H. Chem. ReV. 1970, 70, 517.
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(11) Reference 8b, p 363.
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Acknowledgment. Financial support for this research has
been provided by the National Science Foundation. T.M.L.
thanks Northwestern University for an undergraduate scholarship
and the Marple-Schwitzer Award. We thank Jye-Shane Yang
for helpful advice.
References and Notes
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