Luminescence of N-Arylbenzamides in Low-Temperature Glasses

Article abstract of DOI:10.1021/jp9927294

The low-temperature luminescence of benzamide and six additional N-arylbenzamides has been investigated in a methyltetrahydrofuran glass. The luminescence of microcrystalline benzamide has also been studied. Assignments of the fluorescence and phosphoresc

Full text of DOI:10.1021/jp9927294

9678
J. Phys. Chem. A 1999, 103, 9678-9686
Luminescence of N-Arylbenzamides in Low-Temperature Glasses
Frederick D. Lewis* and Weizhong Liu
Department of Chemistry, Northwestern UniVersity, EVanston, Illinois 60208-3113
ReceiVed: August 2, 1999; In Final Form: October 1, 1999
The low-temperature luminescence of benzamide and six additional N-arylbenzamides has been investigated
in a methyltetrahydrofuran glass. The luminescence of microcrystalline benzamide has also been studied.
Assignments of the fluorescence and phosphorescence have been made on the basis of comparisons with the
spectra of benzamide and the six aminoarenes and with the results of semiempirical ZINDO calculations for
the amines and amides. This information was used to construct state-energy diagrams for the six amides. The
430 nm fluorescence of benzanilide and the amides derived from 4-aminobiphenyl and 4-aminodiphenyl-
acetylene is assigned to a n,π* state having a relaxed geometry and localized on the benzoyl group. The
structured fluorescence of the amides derived from 4-aminostilbene and 2-aminoanthracene is attributed to a
delocalized π,π* singlet state. The amide derived from 2-aminophenanthrene displays dual fluorescence. The
amides derived from 4-aminostilbene and 2-aminoanthracene are nonphosphorescent. The structured
phosphorescence of the amides derived from the other aminoarenes is assigned to a π,π* triplet state similar
to that for the aminoarene. These results serve to further elucidate the unusual luminescence properties of the
benzamides in solution as well as in low-temperature glasses.
Introduction
CHART 1
Benzanilide (1) displays broad, weak, Stokes-shifted fluo-
rescence in nonpolar solvents at room temperature.1-5 Its
absorption and emission maxima in methylcyclohexane solution
are at 265 and 475 nm, respectively, and its fluorescence
quantum yield is 0.6 × 10-3.5 This emission has been attributed
variously to an imidol tautomer, to a TICT state whose formation
requires twisting about the amide C-N bond, and to dual
emission from both a tautomer and TICT state.6 Stokes-shifted
fluorescence is not observed in model compounds that cannot
undergo C-N rotation.2 A second, weaker fluorescence has also
been observed in nonpolar solvents and variously attributed to
a Franck-Condon singlet state or to a fluorescent impurity.2,5
Intense luminescence with a maximum of ca. 430 nm is
observed in both nonpolar and polar glasses at 77 K.4,7
Luminescence with a maximum at 410 nm is also observed from
crystalline benzanilide.2 The luminescence in glasses is attributed
to overlapping fluorescence and phosphorescence, with the
fluorescence maximum occurring at a longer wavelength than
the phosphorescence maximum (430 vs 410 nm). The low-
temperature fluorescence has been attributed to an n,π* singlet
state localized on the benzoyl group;4 however, the nature of
the lowest triplet state has not been determined. Thus, no
explanation exists for the peculiar inversion of fluorescence and
phosphorescence maxima.
studies; namely, restricted molecular motion, more intense
fluorescence, and the observation of phosphorescence as well
as fluorescence. Comparative analysis of the fluorescence and
phosphorescence of the six N-arylbenzamides with those of
benzamide and the primary aminoarenes was used in conjunction
with semiempirical ZINDO calculations to construct state-energy
diagrams. The fluorescence of benzanilide and the amides
derived from the smaller aminoarenes is assigned to an n,π*
state having a relaxed geometry localized on the benzoyl group,
whereas the amides derived from the larger amines display
fluorescence from a delocalized π,π* singlet state. In all cases
where phosphorescence is observed, it arises from a π,π* triplet
state similar to that for the aminoarene. These results serve to
Our interest in the photophysics and photochemistry of
aromatic amides5,8,9 led us to investigate a family of six
N-arylbenzamides (Chart 1) in which the size of the N-aryl group
is increased from aniline to 2-aminoanthracene. By lowering
the singlet and triplet energy of the N-aryl group, we hoped to
determine whether the singlet and triplet states of benzamides
are delocalized or whether they are localized on the benzoyl
and aminoarene subunits. Studies of luminescence in low-
temperature glasses offers several advantages over solution
* Corresponding author. E-mail: lewis@chem.nwu.edu.
10.1021/jp9927294 CCC: $18.00 © 1999 American Chemical Society
Published on Web 11/05/1999

Luminescence of N-Arylbenzamides
J. Phys. Chem. A, Vol. 103, No. 48, 1999 9679
further elucidate the unusual luminescence properties of the
benzamides in solution as well as in low-temperature glasses.
was added 2 equiv of triethylamine, followed by 1 equiv of
benzoyl chloride in dichloromethane (5-20 mL). After the
mixture was stirred for 1 h, the solvent was removed using a
rotary evaporator. The solid residue was washed with water and
recrystallized from DMF/water (10:1). The resulting crystalline
amide was washed with water and dried under vacum.
Experimental Section
1
General Methods. H NMR spectra were measured on a
Varian Gemini 300 spectrometer. UV-vis spectra were mea-
sured on a Hewlett-Packard 8452A diode array spectrometer
using a 1 cm path length quartz cell. Total emission spectra
were measured on a SPEX Fluoromax spectrometer. Low-
temperature spectra were measured either in a Suprasil quartz
EPR tube (i.d. ) 3.3 mm) using a quartz liquid nitrogen
coldfinger dewar at 77 K or in an Oxford Cryogenics DN1704
cryostat fitted with a Oxford Instruments ITC4 temperature
controller. Total emission quantum yields were measured by
comparing the integrated area under the total emission curve at
an equal absorbency and the same excitation wavelength as an
external standard, 9,10-diphenylanthracene (Φf ∼ 1.0 at 77 K
in methyltetrahydrofuran (MTHF)).10 In the case of aniline (1a)
the fluorescence and phosphorescence are well resolved and their
quantum yields can be determined directly from the total
emission spectrum. The overlapping fluorescence and phos-
phorescence spectra of the N-arylbenzamides were resolved by
time dependent integration: fluorescence spectra were integrated
for a period of 1 ns beginning 1 ns after the excitation pulse,
whereas the phosphorescence spectra were integrated for 500
ns beginning 300 ns after the excitation pulse. Their fluorescence
and phosphorescence quantum yields were calculated by
comparing their intensities with those for aniline obtained under
the same conditions. All emission spectra are uncorrected, and
the estimated error for the quantum yields is (20%. Fluores-
cence decays were measured on a Photon Technologies Inter-
national (PTI) Timemaster stroboscopic detection instrument
with a gated hydrogen or nitrogen lamp using a scatter solution
to profile the instrument response function. Phosphorescence
decays were measured on a PTI Timemaster phosphorescence-
time-based detection instrument excited with a Xenon flash lamp
as the excitation source. Nonlinear least-squares fitting of the
decay curves employed the Levenburg-Marquardt algorithm
as described by James et al.11 and implemented by the Photon
Technologies International Timemaster (version 1.2) software.
Goodness of fit was determined by judging the ø2 (<1.3 in all
cases), the residuals, and the Durbin-Watson parameter (>1.6
in all cases). Solutions degassed under vacuum (<0.1 Torr)
through five freeze-pump-thaw cycles have similar quantum
yields and lifetimes at 77 K as solutions that were not
deoxygenated. ZINDO(S)/CI calculations were performed using
Cache Release 3.5. Ground-state structures were optimized with
the AM1 method using MOPAC (version 94.10) programs.
Orbital diagrams were calculated using Hyperchem 5.0 or a
Visualizer program implemented in the MOPAC program
package. The diagrams obtained by the two methods are
essentially the same.
N-Benzoyl-4-aminobiphenyl (2): mp 233-234 °C (lit.14
233-234 °C); 1H NMR (DMSO-d6, 300 MHz) δ 7.34 (t, 3J )
3
7.5 Hz, 1H), 7.46 (t, J ) 7.5 Hz, 2H), 7.51-7.64 (m, 3H),
3
3
7.68 (m, 4H), 7.90 (dm, J ) 8.7 Hz, 2H), 7.98 (dm, J ) 6.9
Hz, 2H), 10.36 (s, 1H, N-H).
N-Benzoyl-4-aminodiphenylacetylene (3): mp 208-209 °C;
1H NMR (DMSO-d6, 300 MHz) δ 7.40-7.45 (m, 3H), 7.56-
3
3
7.65 (m, 7H), 7.88 (dm, J ) 9 Hz, 2H), 7.96 (dm, J ) 7.5
Hz, 2H), 10.46 (s, 1H, N-H); MS m/e (%) 297 (M•+, 100),
105 (100).
N-Benzoyl-2-aminophenanthrene (4): 3-Aminostilbene (0.7
g, 3.5 mmol) was dissolved in 100 mL of ether, 500 mL of
hexane and I2 (ca. 0.1 mg) were added, and the solution was
irradiated at 300 nm in a Rayont RPR-100 photoreactor. The
reaction was monitored by GC until the starting material was
consumed (1-3 h). The irradiated solution was concentrated
to ca. 5 mL. The 2-aminophenantharene was separated by
column chromatography (1:1 hexane/ether as eluent) and directly
benzoylated with benzoyl chloride and triethylamine since the
amine is extremely air-sensitive. The crude benzamide was
recrystallized from DMF/water (10:1) to give 0.35 g of 4 (34%
1
overall yield): mp 223-225 °C (lit.15 216.5 °C); H NMR
(DMSO-d6, 300 MHz) 7.52-7.75 (m, 5H), 7.77-7.88 (m, 2H),
7.94-8.12 (m, 4H), 8.53 (s, 1H), 8.81 (t, 3J ) 9 Hz, 2H), 10.58
(s, 1H); MS m/e (%) 298 (MH+, 100), 105 (PhCO+, 50).
N-Benzoyl-4-aminostilbene (5): mp 245-246 °C (lit.16 244-
1
245 °C); H NMR (DMSO-d6, 300 MHz) δ 7.2-7.3 (m, 2H),
3
3
7.27 (d, J ) 7 Hz, 1H), 7.38 (t, J ) 7 Hz, 1H), 7.50-7.65
3
3
(m, 7H), 7.82 (d, J ) 8 Hz, 2H), 7.96 (d, J ) 7 Hz, 2H),
10.33 (s, 1H); MS m/e (%) 299 (M•+,18), 254 (18), 225 (26),-
105 (100).
1
N-Benzoyl-2-aminoanthracene (6): mp 285 °C; H NMR
(DMSO-d6, 300 MHz) δ 7.44-7.54 (m, 2H), 7.54-7.68 (m,
3
3H), 7.83 (d, J ) 9.6 Hz, 1H), 8.02-8.12 (m, 5H), 8.51 (s,
1H), 8.53 (s, 1H), 8.68 (s, 1H), 10.53 (s, 1H). MS m/e (%) 298
(MH+,100).
Results
Absorption and Fluorescence Spectra. The N-arylbenz-
amides 1-6 were prepared via the reaction of benzoyl chloride
with the corresponding amines 1a-6a and characterized by their
spectral data (see Experimental Section). The amides were
recrystallized from 10:1 dimethylformamide and water. In some
cases, repeated recrystallization was necessary to remove
luminescent impurities. AM1 calculations indicate that all of
the amides have planar geometries in which the N-aryl group
is trans to the benzamide phenyl group with respect to rotation
about the amide C-N bond.
The room-temperature absorption spectra of aniline (1a) and
benzamide (1b) in methyltetrahydrofuran (MTHF) solution are
shown in Figure 1. Also shown in Figure 1 are their lumines-
cence spectra obtained at 77 K in a MTHF glass. The absorption
and luminescence spectra of benzanilide (1) in MTHF and the
luminescence emission and excitation spectra of microcrystalline
1 are shown in Figure 2. The overlapping fluorescence and
phosphorescence spectra of 1 were resolved by time dependent
integration (see Experimental Section). The luminescence
excitation spectra for highly dilute glasses are similar in
Materials. Benzoyl chloride, triethylamine, aniline, biphen-
ylamine, 2-aminoanthracene, and iodomethane are commercially
available and used as received. Anhydrous MTHF containing
200 ppm BHT (Aldrich) was distilled over soldium under a
nitrogen atmosphere. 4-Aminodiphenylacteylene was prepared
from 4-iodoaniline and cuprous 4-phenylacetylide by the method
of Stephens et al.,12 mp 127-128 °C (lit.12 128-129 °C).
4-Aminostilbene and 3-aminostilbene were obtained from
reduction of 4-nitrostilbene and 3-nitrostilbene, respectively,
using SnCl2‚2H2O.13 Benzamide and benzanilide (Aldrich) were
recrystallized with DMF and water (10:1) two times.
General Procedure for Preparing Secondary Amides. To
a solution of amine (1-5 mmol) in dichloromethane (5-20 mL)

9680 J. Phys. Chem. A, Vol. 103, No. 48, 1999
Lewis and Liu
Figure 3. Absorption (solid lines) and low-temperature fluorescence
(dashed lines) and phosphorescence (dash-dotted lines) spectra in
MTHF: (a) biphenylamine (2a, 20 µM); (b) its benzamide (2, 15 µM).
Figure 1. Absorption (solid lines) and low-temperature fluorescence
(dashed lines) and phosphorescence (dash-dotted lines) spectra in
MTHF: (a) aniline (1a, 0.14 mM); (b) benzamide (1b, 0.75 mM).
Figure 4. Absorption (solid lines) and low-temperature fluorescence
(dashed lines) and phosphorescence (dash-dotted lines) spectra in
MTHF: (a) benzamides of 4-aminodiphenylacetylene (3, 10 µM); (b)
benzamide of 2-aminophenanthrene (4, 15 µM).
Figure 2. Absorption (solid lines) and low-temperature fluorescence
(dashed lines) and phosphorescence (dash-dotted lines) spectra of
benzanilide (1, 0.1 mM): (a) in MTHF; (b) in crystal.
appearance to their absorption spectra. The room-temperature
absorption and 77 K luminescence spectra of amine 2a and
amide 2 are shown in Figure 3 and the spectra of amides 3-6
are shown in Figures 4 and 5. Spectra of the amines 3a, 5a,
and 6a are available as Supporting Information.
Fluorescence quantum yields were determined using 9,10-
diphenylanthracene (Φf ) 1.0 at 77 K in MTHF) as an external
standard and phosphorescence quantum yields were estimated
from the ratio of the integrated phosphorescence/fluorescence
area. Weak fluorescence (Φf ∼ 10-3) was detected from 1b
and no phosphorescence was detected from 5 or 6. Single-
exponential fluorescence and phosphorescence decays were
obtained except in the case of the amides 4 and 6, for which
dual exponential fluorescence decay was observed. Absorption
(maximum and molar absorptivity for lowest energy allowed
transitions) and emission (fluorescence and phosphorescence
maxima, quantum yields, and lifetimes) spectral data for amines
1a-6a and amides 1b and 1-6 are summarized in Table 1.
ZINDO Calculations. The electronic structure and singlet
and triplet states of the amides 1-6 and 1b and amines 1a-3a
were investigated by semiempirical INDO/S-SCF-CI (ZINDO)
calculations using the algorithm developed by Zerner and co-
workers.17 Ground-state geometries were obtained from AM1
calculations. The nodal patterns for the frontier orbitals of 1a,
1b, 1, and 2 are shown in Figure 6. In each case the lowest
energy orbital shown is that of the amide-localized nonbonding

Luminescence of N-Arylbenzamides
J. Phys. Chem. A, Vol. 103, No. 48, 1999 9681
n f B* (carbonyl f benzoyl) transition, which is not observed
in the absorption or fluorescence excitation spectra, even at high
concentrations. Since the energies of highly forbidden transitions
are subject to computational uncertainty, it is possible that the
n f B* transition is buried under the tail of the allowed π f
π* transitions, as suggested previously.4,5
The low-temperature luminescence of 1 was first investigated
by O’Connell et al.,7 who resolved the total luminescence into
overlapping fluorescence and phosphorescence in EPA solution.
The absorption and fluorescence spectral data in MTHF solution
(Figure 2a and Table 1) are similar to their reported data, except
for the larger luminescence quantum yields obtained in MTHF.
O’Connell et al.7 noted that the fluorescence spectrum is
exceptionally broad and displays a large Stokes shift. The
fluorescence maximum actually lies at a lower energy than the
phosphorescence maximum (430 vs 410 nm). They suggested
that the emitting singlet state differed significantly in geometry
from the Franck-Condon singlet state. An even larger Stokes
shift is observed for the very weak fluorescence of 1 in fluid
solution and has been attributed to a TICT state formed via
twisting of the amide C-N bond.1,5,6 However, a large amplitude
geometry change such as twisting about the C-N bond would
seem unlikely in the glassy state. Furthermore, the fluorescence
of 1 is similar in polar and nonpolar glasses and in the
mircrocrystalline solid 1 (Figure 2b), suggesting that no
significant charge transfer or geometry change occurs upon
relaxation of the Franck-Condon state to the fluorescent state.
The broad 77 K fluorescence of 1 in an EPA or MC glass has
been attributed by Heldt et al.4 to overlapping fluorescence from
multiple monomer excited states and from an aggregate.
However, we find that the fluorescence decay time is single
exponential and invariant with emission wavelength or concen-
tration (0.4-80 mM), suggesting a single fluorescent state.18
Three of the four lowest energy singlet states of 1, as
determined by ZINDO calculations (Table 2), are derived from
transitions that are localized on either the aniline (A,A*) or
benzoyl (n,B* and B,B*) subunits of the benzanilide molecule
(Figure 6). This suggests that the spectroscopy of aniline (1a)
and benzamide (1b) might provide clues as to the origin of the
Stokes-shifted fluorescence of 1. The absorption spectra of 1a
and 1b are similar, consisting of a weak long-wavelength band
and stronger shorter-wavelength band (Figure 1a,b). On the basis
of ZINDO calculations (Table 2) these bands are assigned to π
f π* transitions with differing extents of configuration interac-
tion. Similar splitting of the π f π* transitions into a weak
lower energy band and a strong higher energy band is observed
for monosubstituted benzenes with mesomeric substituents,
including styrene.19 ZINDO calculations indicate the presence
of a low-energy-forbidden n,B* transition for 1b, similar to that
for 1.20
Figure 5. Absorption (solid lines) and low-temperature fluorescence
(dashed lines) spectra in MTHF: (a) benzamide of 4-aminostilbene
(5, 15 µM); (b) benzamide of 2-aminoanthracene (6, 12 µM).
orbital. All of the remaining frontier orbitals are π-type. In the
case of amides 1 and 2 the frontier π orbitals resemble those of
benzamide (1b) or the amines 1a or 2a, being localized either
on the benzoyl group (B-type) or N-aryl group (A-type) rather
than delocalized over the entire π system. Also shown in Figure
6 are the two singly occupied orbitals (SOMO) of the lowest
triplet states of 1a, 1b, 1, and 2. In all cases the lower energy
SOMO resembles the HOMO of the singlet ground state. The
higher energy SOMO of 1a and 1b resemble the LUMO of the
singlet; however, the higher energy SOMO of amide 1 is
delocalized over both the amide and N-aryl groups and the
SOMO of 2 resembles the SLUMO (MO53) of the singlet
(Figure 6).
The calculated absorption maxima, oscillator strengths,
important configurations, and character of the lowest singlet
states of the amine 1a and amides 1b and 1-6 are summarized
in Table 2. Data for the amines 2a-6a is available as Supporting
Information. The lowest energy transitions of the amines are
of π f π* character. The results of ZINDO calculations for
amide 1 are similar to those we recently published using a
slightly different ground-state geometry obtained from MM2
calculations.5 The lowest excited singlet of amides 1b and 1-6
is of n,B* character (carbonyl f benzoyl). The calculated
energies for the n f B* transitions are all similar, and their
calculated oscillator strengths are very small. The higher energy
singlets of the amides can be classified as either N-aryl-localized
(A,A*), benzoyl-localized (B,B*), or delocalized (A,B* or
A,AB*) π,π* states. The energies and oscillator strengths for
the localized transitions are similar to those for the amines 1a-
6a (A f A* transitions) or benzamide 1b (B f B* transition).
Well-resolved fluorescence and phosphorescence are observed
for 1a at 77 K in MTHF (Figure 1a), with quantum yields and
lifetimes similar to those reported by Sarkar and Kastha for a
methylcyclohexane glass (Table 1).21 The fluorescence of 1a
bears a mirror image relationship to the absorption and
fluorescence excitation spectra and thus is assumed to arise from
a lowest π,π* singlet state with a geometry similar to that of
the ground state.22 Thus, it seems unlikely that relaxation of an
aniline-localized A,A* state can account for the Stokes-shifted
fluorescence of 1. Heldt et al.4 report that 1b displays weak
fluorescence at ca. 320 nm in addition to a stronger phospho-
rescence at 395 nm.4 We find that the 320 nm emission present
in a commercial sample of 1b largely disappears upon recrys-
tallization (Φf < 10-3), suggesting that it is derived from an
Discussion
Benzanilide Singlets. The absorption spectrum of amide 1
in MTHF solution consists of a single broad band with a
maximum at 265 nm (Figure 2a). ZINDO calculations (Figure
6 and Table 2) suggest that three closely spaced π f π*
transitions, the strongest of which is an A,B* type (aniline f
benzoyl) transition, contribute to this band. The lowest energy
singlet state is predicted by ZINDO to arise from a forbidden

9682 J. Phys. Chem. A, Vol. 103, No. 48, 1999
Lewis and Liu
TABLE 1: Absorbance and Low-Temperature Emission Spectra Data for Benzamides in MTHF
absorbanceb
max (nm)
ꢀ (M-1
fluoresencec
phosphoresencec
compda
PhNH2
PhCONH2
PhCONHPh
BpNH2
PhCONHBp
DpaNH2
PhCONHDpa
PhCONHPn
StNH2
λ
)
λ
max (nm)
τ (ns)
QY
λ
max (nm)
τ (s)
QY
1a
1b
1
2a
2
3a
3
4
5a
5
6a
6
280
270
265
286
293
314
308
309
340
342
410
283
4400
930
325
275
430
365
420
360
430
375
410
393
480
401
1.92
4.8
0.23
0.003
0.25
0.61
0.45
0.33
0.44
0.13
0.87
0.29
0.94
0.74
405
380
410
470
460
480
470
459
4.42
1.00
0.40
3.52
2.00
1.57
0.75
4.14
0.45
0.08
0.37
0.07
0.07
0.02
0.02
0.07
13600
21500
25400
14900
38900
20500
30100
52200
3680
2.73
10.65
3.97
1.25
2.06
29.4d
1.05
1.09
22.60
15.1e
PhCONHSt
AnNH2
PhCONHAn
59500
a Bp ) biphenyl, Dpa ) diphenylacetylenyl, Pn ) phenanthryl, St ) stilbenyl, An ) anthracenyl. b Measured at room temperature. c Measured
at 77 K. d 4.8 ns at 450 nm. e Contains 30% of a second component with τ ) 7.3 ns.
impurity. The absence of fluorescence from 1b indicates that
the lowest n,B* state is nonfluorescent, in agreement with its
vanishingly small calculated oscillator strength (Table 2). Thus,
the n,B* state of 1b is presumed to decay via intersystem
crossing or internal conversion to the ground state. The
photophysical behavior of 1b in a MTHF glass is summarized
in Figure 7a.
The absence of 430 nm fluorescence from either 1a or 1b
suggests, by a process of elimination, that both benzamide and
aniline subunits are involved in formation of the fluorescent
state of amide 1. We previously suggested that barrierless torsion
of the A,B* state about the C-N bond gives rise to a TICT
state responsible for the 510 nm room-temperature fluorescence
of 1.5 While such large-amplitude motion seems unlikely to
occur in a glass or cyrstal, a smaller geometric change might
result in the 430 nm fluorescence observed in rigid media. The
nature of this change remains to be elucidated; however,
preliminary exploration of the singlet-state potential energy
surface indicates that a scissoring motion that compresses both
the Ph-C-O and C-N-Ph dihedral angles can result in a
decrease in the A,B* energy, accompanied by an increase in
the ground-state energy. A repulsive potential energy surface
for the ground state can help account for both the large Stokes
shift and the exceptionally broad fluorescence spectrum for 1
energy SOMO is delocalized over both rings, resembling the
LUMO of a fully conjugated molecule such as stilbene. Thus,
the lowest triplet of 1 is assigned an A,AB* configuration
(Figure 7b). Extended conjugation should require that the triplet
retain a planar geometry similar to that of the ground state, in
accord with the weakly structured appearance of the phospho-
rescence band (Figure 2a). Similar ground-state and triplet-state
potential energy surfaces can account for the overlapping
fluorescence and phosphorescence spectra and the narrower
phosphorescence band.
Microcrystalline Benzanilide. The luminescence excitation
and emission spectra of microcrystalline 1 are shown in Figure
2b. The emission spectrum is obtained using 325 nm excitation,
near the maximum in the excitation spectrum. The excitation
maximum is red-shifted from that for the MTHF solution or
glass due to the high optical density of the crystal. The emission
band shape is similar to that in the glass (Table 1); however,
the emission maximum is at somewhat shorter wavelength in
the crystal (410 nm) vs glass. This may reflect a difference in
molecular geometry. According to AM1 and MM2 calculations,
1 has a totally planar structure in the absence of solvent.
However, in the crystal there is a moderately large 62° dihedral
angle between the two phenyl groups, each group having a
dihedral angle of ca. 31° with respect to the amide plane.25
The observation of similar luminescence for 1 in the crystal
and dilute glass indicate that there is no strong electronic
interaction between adjacent molecules within the crystal.
Hydrogen-bonded amide molecules form one-dimensional tapes
within the crystal.25 There are no close phenyl-phenyl contacts
between adjacent molecules within a tape. The close contacts
between molecules in adjacent tapes are edge-to-face, which
result in only weak electronic face-to-face interactions. Face-
to-face π-stacking interactions are necessary for the observation
of excimer fluorescence.9
(Figure 2a). The relaxed state, which we will refer to as S430
,
might be reached either directly from the vertical A,B* state or
via a surface crossing with the lowest n,B* state. Intersystem
crossing from either the A,B* or the n,B* state can account for
the observation of phosphorescence from 1. This model for the
behavior of 1 in a MTHF glass is summarized in Figure 7b.
Benzanilide Triplets. The phosphorescence spectra of aniline
(1a) and amides 1b and 1 are shown in Figures 1 and 2a. The
spectrum of 1b is structured, whereas those of 1a and 1 are not
structured. The lowest energy vibronic band for 1b is at 360
nm, similar to the wavelength of the onset of emission of 1a
and 1. The calculated center of gravity for all three bands is
405 ( 5 nm.23 The phosphorescence decay time for 1 (0.4 s) is
somewhat shorter than those for 1a or 1b (4.4 and 1.0 s,
respectively). These decay times are typical of aromatic
hydrocarbons with lowest π,π* triplet states.24
The ZINDO calculations provide further support for the
assignment of the phosphorescence from 1a, 1b, and 1 to π,π*
triplet states (Table 2). The two singly occupied orbitals of the
triplets of 1a and 1b correspond to the HOMO and LUMO of
the closed shell singlets (Figure 6). Thus, their triplets are
assigned to A,A* and B,B* states, respectively (Figure 7a). In
the case of 1, the lower energy triplet-state SOMO resembles
the HOMO of 1a or 1 (N-aryl localized). However, the higher
The luminescence decay of microcrystalline 1 is dominated
by a short-lived component (1.3 ns) whose decay time is
somewhat shorter than that for 1 in a MTHF glass (Table 1).
More rapid decay might result from a difference in molecular
structure or from quenching of singlet molecules by trap sites
in the interior of the crystal. In addition to short-lived
fluorescence there are minor components of longer lived
luminescence with maxima near 440 nm and decay times of
ca. 1 µs. The longer lived emission could either be phospho-
rescence or delayed fluorescence.26
4-Aminobiphenyl and Its N-Arylbenzamide. The room-
temperature absorption and 77 K luminescence spectra of
4-aminobiphenyl 2a27 and its benzamide 2 in MTHF are shown
in Figure 3. The absorption spectra of 2a and 2 display long-

Luminescence of N-Arylbenzamides
J. Phys. Chem. A, Vol. 103, No. 48, 1999 9683
Figure 6. Diagram of frontier orbitals for 1a, 1b, 1, and 2.

9684 J. Phys. Chem. A, Vol. 103, No. 48, 1999
Lewis and Liu
TABLE 2: Excitation Wavelength, Oscillator Strength,
Orbital Localization, and Composition of First Four
Transitions, Calculated for Benzamides Using ZINDO/S/CI
Method
predicted for both 2a and 2, the former of A,A* character and
the latter of n,B* character. The calculated energy and oscillator
strength for the n,B* state of 2 are similar to those for 1.
The luminescence spectrum of 2a displays weakly structured
fluorescence with a maximum at 365 nm and structured
phosphorescence with a highest energy vibronic band at 425
nm. The fluorescence and phosphorescence of 2a are similar
in appearance to those of unsubstituted biphenyl and are as-
signed to the lowest energy A,A* singlet and triplet states,
respectively. The time-resolved luminescence spectra of 2
consists of a broad fluorescence band at 430 nm and a structured
phosphorescence band with a highest energy maximum at 460
nm. The fluorescence band is similar in band shape and decay
time to the fluorescence of amide 1 (Table 1) and thus is
assigned to the relaxed S430 singlet state. The phosphorescence
band is similar in band shape and decay time to the phospho-
rescence of amine 2a and thus is assigned to an amine-localized
A,A* triplet state, an assignment supported by the ZINDO
calculation (Table 2). The state-energy diagram for amide 2 is
similar to that of amide 1 (Figure 7b) except that the lowest
triplet is of A,A* character.
compd
∆Ea (kcal) λb (nm)
fc
TLPd
øe (%)
ø10(68), ø11(31)
ø10(90), ø01(8)
ø20(91)
1a
S1
S2
S3
S4
T1
S1
S2
101.4
123.3
142.2
147.4
68.0
282 0.0273 A,A*
232 0.3162 A,A*
202 0.041 A,A*
194 0.7657 A,A*
ø00(31), ø11(65)
420
A,A*
ø02(66), ø22(32)
ø01(39), ø10(28),
ø00(19), ø11(10)
ø00(68),ø01(22)
ø03(57), ø10(13),
ø01(12)
1b
82.9
105.1
345 0.0006 n,B*
272 0.0128 B,B*
S3
S4
124.3
141.5
230 0.418 B,B*
202 0.0027 B,B*
T1
S1
S2
67.9
79.9
102.1
421
B,B*
ø04(70), ø44(26)
ø20(16),ø20 (37),
ø21(22)
1
358 0.0002 n,B*
280 0.08
A,A*
ø00(67), ø20(13)
ø02(18),ø12(21),
ø03(33), ø13(10)
S3
S4
103.6
104.7
276 0.37
273 0.12
A,B*
B,B*
4-Aminodiphenylacetylene and Its Benzamide. The absorp-
tion spectra of 3a (not shown) and 3 (Figure 4a) display broad
long-wavelength absorption bands with maxima at 314 and 310
nm, respectively. These bands are assigned on the basis of
ZINDO calculations to allowed π f π* transitions having A,A*
and A,B* character for 3a and 3, respectively. As in the case
of 1 and 2, the ZINDO calculations predict the presence of a
forbidden n f B* transition at a lower energy than the A f
B* transition of 3 (Table 2). The fluorescence spectrum of 3
(Figure 4a) is similar in appearance and decay time to those of
1 and 2 and is assigned to the relaxed S430 state. The
phosphorescence of 3 has the same structured appearance and
a decay time similar to that of the amine 3a and thus is assigned
to a A,A* triplet state. Thus, the state-energy diagram for 3 is
identical to that for 1 and 2 (Figure 7b).
T1
S1
S2
S3
S4
68.1
78.5
96.9
97.9
103.2
420
A,A*
ø06(68), ø66(18)
ø00(63), ø10(13)
ø30(39), ø40(18)
ø30(11), ø40(18),
ø11(29)
2
3
4
364 0.0003 n,B*
295 0.76
292 0.09
A,B*
A,A*
277 0.004 A,A*
T1
S1
S2
60.8
79.2
88.5
470
A,A*
ø17(64), ø67(25)
ø00(44), ø10(42)
A,B* +
361 0.0004 n,B*
323 1.2
A,A*
A,A*
ø30(50), ø40(8)
ø03(14), ø13(57)
S3
S4
T1
S1
96.3
96.6
60.8
79.2
297 0.02
296 0.0001 A,A*
470 A,A*
361 0.0002 n,B*
ø06(68), ø56(15),
ø66(12)
ø10(47), ø21(32)
ø00(57), ø20(20)
ø00(56), ø20(16),
ø01(11)
S2
S3
S4
83.6
95.6
101.4
342 0.005 A,A*
2-Aminophenanthrene and Its Benzamide. 2-Aminophenan-
threne 4a was not isolated due to its sensitivity toward air
oxidation and was converted directly to the amide 4. The
absorption spectrum of 4 (Figure 4b) displays a maximum at
309 nm and shoulders at lower energy. The appearance of the
absorption spectrum is similar to that of phenanthrene, but
broadened. According to the ZINDO calculation (Table 2), the
maximum can be assigned to an allowed A f B* transition
predicted to occur at 299 nm and the shoulders to an A f A*
transition predicted to occur at 342 nm. As is the case for amides
1-3, a forbidden n f B* transition is predicted to lie below
these π f π* transitions.
The luminescence spectrum of 4 (Figure 4b) consists of a
structured fluorescence band with a long-wavelength tail and a
structured phosphorescence band. The structured fluorescence
and phosphorescence bands are similar in appearance to those
of the unsubstituted phenanthrene.24 The fluorescence decay of
4 determined at 375 nm is dual exponential, the major and minor
components having decay times or 29.4 and 4.8 ns. The decay
determined at 450 nm is monoexponential with a decay time
of 4.8 ns, similar to that of the S430 singlets of amides 1-3.
Thus, the fluorescence of 4 apparently consists of two overlap-
ping bands: a structured shorter-wavelength emission assigned
to a phenanthrene-localized A,A* singlet and the broad long-
wavelength emission assigned to a relaxed S430 state. The
structured phosphorescence is assigned to an A,A* triplet and
its decay time is 4.1 s, similar to that of phenanthrene in an
EPA glass (3.4 s).24
299 0.42
282 0.65
A,B*
A,B*
T1
S1
S2
S3
S4
62.2
78.5
89.9
95.9
101.0
460
A,A*
ø06(62), ø66(24)
ø00(60), ø10(30)
ø30(47), ø40(11)
ø40(31), ø41(17),
ø52(12)
5
6
364 0.0004 n,B*
318 1.4
298 0.02
A,AB*
A,A*
283 0.003 A,A*
T1
S1
47.9
78.5
597
A,A*
ø05(22), ø15(45),
ø55(24)
364 0.0007 n,B*
ø10(12), ø10(31),
ø01(26),
S2
78.8
363 0.03
A,AB*
ø00(79)
S3
S4
T1
82.6
94.4
37.5
346 0.17
303 0.66
762
A,A*
A,B*
A,A*
ø10(59), ø01(20)
b
a Transition-state energy relative to ground state (kcal/mol). State
energy in wavelength. cOscillator strength. dTransition localization
property: A ) π orbital localized on arylamine moiety, B ) π or-
bital localized on benzoyl moiety, n ) n orbital localized on CO
n
e
moiety, AB ) delocalized π orbital. Percentage of transitions: øm
symbolizes a transition from a (HOMO + m) orbital to (LUMO + n)
orbital.
wavelength bands with maxima at 286 and 292 nm, respectively.
These bands are assigned, based on ZINDO calculations, to
allowed π f π* transitions: of A,A* and A,B* character for
2a and 2, respectively. Weaker transitions at lower energy are

Luminescence of N-Arylbenzamides
J. Phys. Chem. A, Vol. 103, No. 48, 1999 9685
Figure 7. Proposed state-energy diagrams for (a) 1b, (b) 1-3, (c) 4, and (d) 5 and 6.
The occurrence of dual fluorescence for amide 4 can be
explained using the state-energy diagram in Figure 7c. Excitation
of one of the allowed π f π* transitions followed by internal
conversion can yield either the A,A* state or the relaxed S430
state. Even though the A,A * state is of higher energy than the
S430 state, the only likely pathway from the former to the latter
state would be via the planar n,B* state. Since the center of
gravity of the A,A* fluorescence lies below the calculated
energy of the planar n,B* state, the two singlets decay
independently. It would be interesting to determine which of
these singlet states is responsible for formation of the A,A*
triplet; however, a method for investigating this question is not
readily apparent.
4-Aminostilbene and Its Benzamide. The absorption spectra
of both 4-aminostilbene (5a, not shown) and its benzamide 5
(Figure 5a) have a single broad long-wavelength band. The
absorption maximum of 5a is at 316 nm in hexane solution
and is attributed by ZINDO to an allowed, predominantly HO
f LU, π f π* transition.28 The absorption maximum of 5 lies
at 342 nm, the only case in which a significant red shift is seen
for the amide vs amine (Table 1). This absorption band is
attributed by ZINDO predominantly to an allowed, A f AB*,
HO f LU transition in which the HO is localized on stilbene,
but the LU is delocalized over the entire molecule (Table 2).
Increased delocalization presumably is responsible for the red
shift in the absorption band in comparison to the amine 5a. As
is the case for amides 1-4, a forbidden n f B* transition is
predicted to lie below the π f π* transitions.
the amine 5a. However, the fluorescence decay time remains
constant and single exponential across the entire fluorescence
band. Since this decay time is shorter than that for the S430 state
of 1-3, we conclude there is only one emitting singlet state.
This raises the question as to why emission is observed from
the stilbene-localized A,AB* state rather than the lower energy
S430 state? The 0,0 energy of the A,AB*, estimated from the
crossing point of the absorption and fluorescence spectra (Figure
5), is 365 nm. This is similar to the calculated singlet n,B*
energy (Table 2), which is subject to computational uncertainty.
Thus, the vertical A,AB* singlet may be of lower energy than
the vertical n,B* singlet. If the latter is not populated by internal
conversion from a higher energy spectroscopic singlet state, then
relaxation to form the S430 state cannot occur. This situation is
shown schematically in Figure 7d.
No phosphorescence is detected for either the amine 5a or
the amide 5. Unsubstituted trans-stilbene and many of its
derivatives are also nonphosphorescent in low-temperature
glasses.29 This results from a combination of factors: a short
singlet lifetime, slow intersystem crossing due to the large
singlet-triplet energy gap, and rapid nonradiative decay of the
triplet due to its conformational flexibility. The intersystem
crossing rate constant for stilbene is estimated to be ca. 4 ×
107 s-1. Assuming a similar rate constant for intersystem
crossing of 5, its quantum yield for intersystem crossing would
be ca. 0.05. The large singlet-triplet energy gap and absence
of intersystem crossing in 5 is indicated in Figure 7d.
2-Aminoanthracene and Its Benzamide. The absorption
spectra of both 2-aminoanthracene 6a30 (not shown) and the
amide 6 (Figure 5b) display a weak long-wavelength band with
a maximum near 395 nm and a strong band near 260 nm;
however, the 395 band is stronger and broader for the amine.
The amide has an additional band at 293 nm. On the basis of
ZINDO calculations (Table 2) the 395 nm band of 6 can be
assigned to an A f AB* delocalized transition similar to that
The fluorescence spectrum of amide 5 (Figure 5a) resembles
that of amine 5a but, like the absorption spectrum, is red-shifted
(from 380 nm for 5a to 393 nm for 5 in a MTHF glass). The
fluorescence decay times for 5a and 5 are 1.1 and 1.7 ns,
respectively. Thus, the structured fluorescence of 5 is assigned
to a delocalized A,AB* singlet state. The fluorescence spectrum
of 5 possesses a long-wavelength tail that is not observed for

9686 J. Phys. Chem. A, Vol. 103, No. 48, 1999
Lewis and Liu
for 5. The low-temperature luminescence spectra of 6a and 6
consist of structured fluorescence with a highest energy maxi-
mum at 470 nm for 6a and 405 nm for 6. Both the absorption
and fluorescence of 6 are blue-shifted with respect to those of
6a. The absence of S430 fluorescence from 6 is consistent with
a lower energy for the Franck-Condon A,AB* vs n,B* state,
as in the case of 5 (Figure 7d).
The fluorescence decay of 6 is dual exponential, with decay
times of 15.1 and 7.3 ns. Neither the decay times nor the
preexponentials are dependent upon the emission wavelength.
The two decays are tentatively assigned to the syn and anti
rotamers with respect to the N-anthracene bond, both of which
retain trans configurations with respect to the amide C-N bond.
The syn and anti rotamers of some 2-vinylanthracenes are
known to have similar spectra but display different photophysi-
cal behavior.31 The observation of emission from two rotamers
of 6 but not from amides 1-5 most likely reflects the presence
of an axis of symmetry (or quasi-axis) for N-aryl rotation in
amines 1a-5a.
invert the ordering of n,B* and π,π* states in solution. The
spectroscopy of secondary and tertiary N-arylbenzamides in
solution and the solid state is the subject of continuing inves-
tigation.
Acknowledgment. We thank Prof. Siegfried Schneider for
helpful discussions. Financial support for this research has been
provided by the National Science Foundation.
Supporting Information Available: Absorption and low-
temperature fluorescence and phosphorescence spectra for
N-arylamines 3a, 5a, and 6a in MTHF. Excitation wavelength,
oscillator strength, and composition of first four transitions
calculated for N-arylamines (2a, 3a, 5a, 6a) using ZINDO/S/
CI method. This information is available free of charge via the
Internet at http://pubs.acs.org.
References and Notes
(1) (a) Tang, G.-Q.; MacInnis, J.; Kasha, M. J. Am. Chem. Soc. 1987,
109, 2531. (b) Heldt, J.; Gormin, D.; Kasha, M. Chem. Phys. Lett. 1988,
150, 433. (c) Heldt, J.; Gormin, D.; Kasha, M. J. Am. Chem. Soc. 1988,
110, 8255. (d) Heldt, J.; Gormin, D.; Kasha, M. Chem. Phys. 1989, 136,
321.
(2) Lucht, S.; Stumpe, J.; Rutloh, M. J. Fluorescence 1998, 8, 153.
(3) Azumaya, I.; Kagechika, H.; Fujiwara, Y.; Itoh, A.; Yamaguchi,
K.; Shudo, K. J. Am. Chem. Soc. 1991, 113, 2833.
No phosphorescence is observed in a MTHF glass for either
amine 6a or amide 6. The absence of phosphorescence no doubt
reflects very slow intersystem crossing resulting from a large
singlet-triplet gap (Figure 7d). Anthracene derivatives display
phosphorescence only when the second triplet lies beneath the
lowest singlet.24
(4) Heldt, J.; Heldt, J. R.; Szatan, E. J. Photochem. Photobiol. A. 1999,
Concluding Remarks. The photophysical behavior of the
six N-arylbenzamides is highly dependent upon the electronic
interactions between benzamide and aminoarene portions of the
molecules. Assignment of the absorption, fluorescence, and
phosphoresce spectra of these amides has made use of com-
parisons with the spectra of benzamide and the aminoarenes
and with semiempirical ZINDO calculations. For all of the
amides the lowest energy bands in the absorption spectra can
be assigned to π f π* transitions that are either arene-localized
(A f A*) or of arene f amide character (A f B* or A f
AB*). In the case of 1-4 the amines and amides have similar
singlet energies; however, the singlet energy of 5 is lower than
that of 5a, whereas the energy of 6 is higher than that of 6a.
Amides 1-3 display broad Stokes-shifted emission with a
maximum at 430 nm, rather than the higher energy structured
emission expected from the lowest π,π* singlet states. This
anomalous Stokes-shifted fluorescence is attributed to a singlet
state, S430, formed upon relaxation of a π,π* or n,B* state, which
is of lower energy than the lowest π,π* singlet states. The n,B*
state is not observed spectroscopically but is predicted by
ZINDO calculations to lie at 360 nm for benzamide and all six
N-arylbenzamides. The amides 5 and 6 have lowest energy π,π*
states and thus show structured emission at a higher energy than
the S430 emission. The phenanthrene carboxamide 4 shows dual
fluorescence from both π,π* and S430 singlet states. The absence
of fluorescence from benzamide is attributed to rapid intersystem
crossing from a lowest n,B* singlet to the π,π* triplet. Amides
1-4 display structured phosphorescence assigned to delocalized
π,π* delocalized triplet states. No phosphorescence is observed
for amides 5 and 6. Inefficient intersystem crossing results pri-
marily from the large singlet-triplet splitting for these amides.
The results of this investigation are of relevance to the
ongoing controversy surrounding the Stokes-shifted room-
temperature fluorescence of benzanilide in nonpolar solvents.2-5
The nonspectroscopic vertical n,B* state is expected to lie below
the π,π* singlet states in solution as well as in low-temperature
glasses. A larger amplitude geometry change in solution vs the
glass could account for the larger Stokes shift observed in
solution. Extended conjugation of the N-aryl subunit might also
121, 91.
(5) Lewis, F. D.; Long, T. M. J. Phys. Chem. A 1999, 102, 5327.
(6) The Stokes-shifted solution fluorescence was initially attributed to
Kasha and co-workers1 to an imidol tautomer formed via a double proton
transfer in a hydrogen-bonded dimer. This assignment has been adopted in
some more recent publications.2,4 Evidence against imidol formation has
been presented by Azumaya et al.3 and by Lewis and Long.5.
(7) O’Connell, E. J., Jr.; Delmauro, M.; Irwin, J. J. Photochem.
Photobiol. 1971, 14, 189.
(8) (a) Lewis, F. D.; Barancyk, S. V.; Burch, E. L. J. Phys. Chem.
1992, 96, 4, 3866. (b) Lewis, F. D.; Burch, E. L. J. Phys. Chem. 1996,
100, 4055.
(9) Lewis, F. D.; Yang, J.-S. J. Phys. Chem. 1997, 101, 1775.
(10) Li, R.; Lim, E. C. J. Chem. Phys. 1972, 57, 605.
(11) James, D. R.; Siemiarczuk, A.; Ware, W. R. ReV. Sci. Instrum.
1992, 63, 1710.
(12) Stephens, R. D.; Castro, C. E. J. Org. Chem. 1963, 28, 3313.
(13) Bellamy, F. D.; Ou, K. Tetrahedron Lett. 1984, 839.
(14) Shine, H. J.; Cheng, J. D. J. Org. Chem. 1971, 36, 2787.
(15) Kunz, J. Justus Liebigs Ann. Chem. 1902, 321, 319.
(16) Pfeiffer, P.; Sergiewskaja, S. Chem. Ber. 1911, 44, 1112.
(17) (a) Bacon, A. D.; Zerner, M. C. Theor. Chim. Acta 1970, 53, 21.
(b) Zerner, M. C.; Loew, G. H.; Kircher, R. R.; Mueller-Westerhoff, U. T.
J. Am. Chem. Soc. 1980, 102, 589.
(18) The weak 320 nm emission observed by Heldt et al.4 in low-
temperature glasses was not observed by O’Connell et al.7 or in the present
investigation. We suspect that it arises from aniline, present as an impurity
or formed as a photoproduct of 1a.2,3.
(19) Lewis, F. D.; Bassani, D. M.; Caldwell, R. A.; Unett, D. J. J. Am.
Chem. Soc. 1994, 116, 10477.
(20) The lower energy band of 1b has been assigned by Heldt to an n
f π* transition.4.
(21) Sarkar, S. K.; Kastha, G. S. Spectrochim. Acta 1992, 48A, 1611.
(22) Aniline is pyramidally distorted at the NH2 group in the ground
state but quasi-planar in the lowest singlet state: Sinclair, W. E.; Pratt, D.
W. J. Chem. Phys. 1996, 105, 7942.
(23) Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic
Molecules, 2nd ed.; Academic Press: New York, 1971; p 27.
(24) Birks, J. B. Photophysics or Aromatic Molecules; Wiley-Inter-
science: London, 1970; p 193.
(25) Kashino, S.; Ito, K.; Haisa, M. Bull. Chem. Soc. Jpn. 1979, 52,
365.
(26) Birks, J. B. Photophysics or Aromatic Molecules; Wiley-Inter-
science: London, 1970; p 523.
(27) Hashimoto, S.; Thomas, J. K. J. Phys. Chem. 1984, 88, 4044.
(28) Lewis, F. D.; Kalgutkar, R. Unpublished results.
(29) Go¨rner, H.; Schulte-Frohlinde, D. J. Phys. Chem. 1979, 83, 3107.
(30) Rosenberg, H. M.; Eimutis, L. E. Spectrochim. Acta 1966, 22, 1751.
(31) Mazzucato, U.; Momicchioli, F. Chem. ReV. 1991, 91, 1679.