Structures and excited states of extended and folded mono-, di-, and tri-N-arylbenzamides

Article abstract of DOI:10.1021/jp0264937

The relationship between molecular structure and electronic properties of 18 N-arylbenzamides has been investigated. The secondary amides adopt extended trans-amide conformations, whereas the tertiary amides adopt folded cis-amide conformations in which the N-aryl group is approximately perpendicular to the benzoyl group. The N-aryl groups of the tertiary di- and triamides occupy the same face of the benzoyl group and display edge-to-face aryl-aryl interactions. The long-wavelength absorption maxima of the secondary amides are red-shifted as the number of N-aryl groups increases, whereas the tertiary amides display are blue-shifted. ZINDO calculations aid in the assignment of the absorption and luminescence spectra. In all cases the lowest energy singlet state is assigned to a forbidden n,B* (carbonyl lone-pair to benzoyl) transition. The structureless fluorescence of the secondary and tertiary amides is assigned to planar and twisted n.B* singlets, respectively. The allowed absorption bands are assigned to A,B* (arene-to-benzoyl) or A,A* (arene-to-arene) transitions which are localized on a single arene, and the structured phosphorescence is assigned to A,A* triplet states.

Full text of DOI:10.1021/jp0264937

3254
J. Phys. Chem. A 2003, 107, 3254-3262
Structures and Excited States of Extended and Folded Mono-, Di-, and
Tri-N-Arylbenzamides†
Frederick D. Lewis,* Timothy M. Long, Charlotte L. Stern, and Weizhong Liu
Department of Chemistry, Northwestern UniVersity, EVanston, Illinois 60208-3113
ReceiVed: July 10, 2002; In Final Form: August 21, 2002
The relationship between molecular structure and electronic properties of 18 N-arylbenzamides has been
investigated. The secondary amides adopt extended trans-amide conformations, whereas the tertiary amides
adopt folded cis-amide conformations in which the N-aryl group is approximately perpendicular to the benzoyl
group. The N-aryl groups of the tertiary di- and triamides occupy the same face of the benzoyl group and
display edge-to-face aryl-aryl interactions. The long-wavelength absorption maxima of the secondary amides
are red-shifted as the number of N-aryl groups increases, whereas the tertiary amides display are blue-shifted.
ZINDO calculations aid in the assignment of the absorption and luminescence spectra. In all cases the lowest
energy singlet state is assigned to a forbidden n,B* (carbonyl lone-pair to benzoyl) transition. The structureless
fluorescence of the secondary and tertiary amides is assigned to planar and twisted n,B* singlets, respectively.
The allowed absorption bands are assigned to A,B* (arene-to-benzoyl) or A,A* (arene-to-arene) transitions
which are localized on a single arene, and the structured phosphorescence is assigned to A,A* triplet states.
Introduction
dimensional chemical structures are shown in Chart 1.12 These
amides are identified using a code in which the first number
designates the order of the amide (2 ) secondary; 3) tertiary),
the letter R designates the identity of the aryl group (P ) phenyl,
BP ) biphenyl, and DPA ) diphenylacetylene), and the second
number designates the number of amide substituents. All three
aryl groups are derived from axially symmetric amines, thus
limiting the conformational heterogeneity of the di- and tri-
amides.
Benzanilide adopts extended trans conformations both in
solution and in the solid state, whereas N-methylbenzanilide
adopts folded cis conformations.1,2 We recently investigated the
spectroscopic properties of these molecules and several other
secondary and tertiary benzanilides.3-5 In rigid media, confor-
mational differences result in different fluorescence maxima for
the secondary and tertiary benzanilide; however both form
relaxed twisted singlet states with similar fluorescence maxima
in fluid solution. Azumaya and co-workers6-8 reported that the
conformational preference observed for the benzanilides is
conserved in the corresponding secondary and tertiary 1,3-
dicarboxamides and 1,3,5-tricarboxamides. Moreover, in the
solid state the tertiary di- and tri-carboxamides adopt syn
conformations in which the anilide groups are located on the
same side of the central ring with an edge-to-face arrangement
of their phenyl groups. The syn conformation of the diamide
also predominates in solution, however, the all-syn conformation
of the triamide is in equilibrium with the syn-anti conformation
Experimental Section
1
General Methods. H NMR spectra were measured on a
Varian INOVA-500 MHz NMR spectrometer. UV-vis spectra
were measured 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 (3.3 mm i.d.) using a quartz liquid nitrogen
coldfinger dewar at 77K or in an Oxford Cryogenics DN1704
cryostat fitted with an Oxford Instruments ITC4 temperature
controller. Total emission quantum yields were measured by
comparison to an external standard, 9,10-diphenyl anthracene
(Φf ∼ 1.0 at 77K in MTHF),13 using the integrated area under
the total emission curves at equal absorbency and the same
excitation wavelength. The overlapping fluorescence and phos-
phorescence spectra were resolved by time dependent integration
as described previously.5 All emission spectra are uncorrected,
and the estimated error for the quantum yields is (20%.
Fluorescence decays were measured on a Photon Technolo-
gies International (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 using 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.14 and implemented by
the PTI Timemaster (version 1.2) software. Goodness of fit was
determined by judging the ø2 (<1.3 in all cases), the residuals,
1
in solution. From the temperature dependence of the H NMR
spectrum of the triamide, the all-syn conformation was found
to be favored enthalpicly but disfavored entropicly (∆H° ) 2.1
kcal/mol; ∆S° ) 8.4 cal/mol K).8
The conformational preferences of the secondary and tertiary
di- and triamides present a unique opportunity to investigate
electronic interactions in complex molecules with well-defined
extended and folded geometries. To our knowledge, the only
extended 1,3- and 1,3,5-trisubstituted benzene derivatives whose
electronic spectra have been studied to date are the parent meta-
terphenyl and 1,3,5-triphenylbenzene and several of their
derivatives.9-11 The electronic interactions between syn aryl
groups in folded 1,3- and 1,3,5-trisubstituted benzene derivatives
have not been investigated. We report here the synthesis,
molecular conformation, and electronic spectra of the 18
secondary and tertiary mono-, di-, and triamides whose two-
† Part of the special issue “George S. Hammond & Michael Kasha
Festschrift”.
* Address correspondence to this author. E-mail: lewis@chem.
northwestern.edu.
10.1021/jp0264937 CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/31/2002

Structures and Excited States of N-Arylbenzamides
J. Phys. Chem. A, Vol. 107, No. 18, 2003 3255
CHART 1: Structures of Secondary and Tertiary
Mono-, Di-, and Tribenzamides. R ) -H (P), R ) -Ph
(BP), and R ) -CtCPh (DPA).
Figure 1. ORTEP representation of 3BP3.
N,N′-Diphenyl-1,3-benzenedicarboxamide (2P2). mp 279-
281 °C (lit.15 mp 280 °C).
N,N′,N′′-Triphenyl-1,3,5-benzenetricarboxamide (2P3). mp
326-328 °C (lit.16 mp 327-329 °C).
and the Durbin-Watson parameter (>1.6 in all cases). Solutions
were degassed under vacuum (<0.1 Torr) through five freeze-
pump-thaw cycles.
N-[1,1′-Biphenyl]-4-yl-benzamide (2BP1). mp 232-234 °C
(lit.4 mp 233-234 °C). 1H NMR (DMSO-d6, 300 MHz): δ 7.34
(t, 3J ) 7.5 Hz, 1H), 7.46 (t, 3J ) 7.8 Hz, 2H), 7.48-7.63 (m,
3H), 7.68 (dm, 3J ) 8.7 Hz, 4H), 7.89 (dm, 3J ) 8.7 Hz, 2H),
A single crystal of 3BP3 (C48H39N3O3, FW 705.86) was
grown from toluene. A colorless prismatic crystal having
dimensions 0.34 × 0.33 × 0.19 mm3 was mounted on a glass
fiber, and data for its X-ray structure collected at -120 °C on
an Enraf-Nonius CAD4 diffractometer with graphite mono-
chromated Mo KR radiation (λ ) 0.71069 Å). The crystal
system is trigonal, the space group P3(no. 143), lattice
parameters a ) 16.425(4) Å, c ) 12.086(2) Å: Z ) 3, R )
0.104, Rw ) 0.072, and GOF ) 3.06.
3
7.98 (dm, J ) 7.2 Hz, 2H), 10.46 (s, 1H).
N,N′-Bis[1,1′-biphenyl]-4-yl-1,3-benzenedicarboxamide (2BP2).
1
3
mp > 300 °C. H NMR (DMSO-d6, 300 MHz): δ 7.35(dd, J
) 7.5 Hz, 2H), 7.46 (t, 3J ) 7.5 Hz, 4H), 7.55 (d, 3J ) 7.2 Hz,
1H), 7.70 (t, 3J ) 6.6 Hz, 8H), 7.91 (t, 3J ) 8.4 Hz, 4H), 8.18
3
3
(dd, J ) 8.1 Hz, 2H), 8.58 (dd, J ) 8.4 Hz, 1H), 10.58 (s,
2H). MS m/e 468 (M+•).
N,N′,N′′-Tris[1,1′-biphenyl]-4-yl-1,3,5-benzenetricarboxam-
ZINDO (INDO/S SCF-CI) calculations were performed using
Cache Release 4.4 with ground-state structures optimized with
AM1 method using MOPAC programs.
1
ide (2BP3). mp >360 °C. H NMR (DMSO-d6, 300 MHz): δ
3
3
7.35 (tt, J ) 7.5 Hz, 3H), 7.47 (t, J ) 7.5 Hz, 6H), 7.71 (d,
3J ) 9.3 Hz, 6H), 7.74 (d, 3J ) 8.1 Hz, 6H), 7.97 (d, 3J ) 8.7
Hz, 6H), 8.78 (s, 3H), 10.75 (s, 3H). MS m/e (%) 664 (MH+,
95), 495(12), 307(100), 289(50).
Materials. Benzoyl chloride, isophthaloyl dichloride, 1,3,5-
benzenetricarbonyl trichloride, aniline, 4-biphenylamine, and
iodomethane are commercially available and used as received.
4-Aminodiphenylacetylene was prepared from 4-iodoaniline and
cuprous 4-phenylactylide as previously reported.4 Benzanilide
(2P1) was recrystallized from DMF and water (1:1) two times.
N-Methyl benzanilide (3P1, Aldrich) was recrystallized from
acetone and hexane (1:1) two times. Anhydrous MTHF contain-
ing 200 ppm BHT (Aldrich) was distilled over sodium under a
nitrogen atmosphere.
General Procedure for Preparing Secondary Benzamides.
To a solution of arylamine (5 mmol) in dichloromethane (20
mL) was added 2.2 equiv of triethylamine, followed by 1 equiv
of benzoyl chloride, or 0.5 equiv of isophthaloyl dichloride, or
0.33 equiv of 1,3,5-benzenetricarbonyl trichloride in dichlo-
romethane (10 mL). After the mixture was stirred for 10 min,
the solvent was removed using a rotary evaporator. The solid
residue was washed with water and recrystallized from DMF:
water. The resulting crystalline amide was washed with water
and dried under vacuum.
N-[4-(Phenylethynyl)phenyl]-benzamide (2DPA1). mp 207-
209 °C (lit.4 mp 208-209 °C). 1H NMR (DMSO-d6, 300
3
MHz): δ 7.4-7.45 (m, 3H), 7.50-7.65 (m, 7H), 7.88 (dm, J
3
) 9 Hz, 2H), 7.96 (dm, J ) 7.5 Hz, 2H), 10.46 (s, 1H). MS
m/e 297 (M+•).
N,N′-Bis-[4-(phenylethynyl)phenyl]-1,3-benzenedicarboxam-
1
ide (2DPA2). mp >360 °C. H NMR (DMSO-d6, 300 MHz):
δ 7.4-7.46 (m, 6H), 7.52-7.61 (m, 8H), 7.73 (t, 3J ) 7.2 Hz,
3
4
1H), 7.88-7.91 (m, 4H), 8.17 (dd, J ) 7.8 Hz, J ) 1.5 Hz,
2H), 8.54 (s, 1H), 10.63 (s, 2H). MS m/e 516 (M+•).
N,N′,N′′-Tris-[4-(phenylethynyl)phenyl]-1,3,5-benzenetricar-
1
boxamide (2DPA3). mp > 360 °C. H NMR (DMSO-d6, 300
3
MHz): δ 7.42-7.44 (m, 9H), 7.54-7.58 (m, 6H), 7.60 (d, J
3
) 8.7 Hz, 6H), 7.92 (d, J ) 8.7 Hz, 6H), 8.74 (s, 3H), 10.80
(s, 3H). MS m/e 735 (M+•).
General Procedure for Preparing Tertiary Benzamides.
A solution of NaH (10 mmol) in DMF (10 mL) was added

3256 J. Phys. Chem. A, Vol. 107, No. 18, 2003
Lewis et al.
Figure 3. Absorption spectra of 10 mM benzamides in acetonitrile
solution. (a) 2P1 (solid), 2P2 (dash), 2P3 (dash dot). (b) 3P1 (solid),
3P2 (dash), 3P3 (dash dot). (c) 2BP1 (solid), 2BP2 (dash), 2BP3 (dash
dot). (d) 3BP1 (solid), 3BP2 (dash), 3BP3 (dash dot). (e) 2DPA1
(solid), 2DPA2 (dash), 2DPA3 (dash dot). (f) 3DPA1 (solid), 3DPA2
(dash), 3DPA3 (dash dot).
TABLE 2: UV Spectral Data for Benzamides in Acetonitrile
Solution
Figure 2. Optimized ground-state geometries for mono-, di-, and
tribenzamides.
absorbancea
intensity
amide
λ
max, nm
ꢀ, M-1
ratiob
2P1
2P2
2P3
3P1
3P2
3P3
2BP1
2BP2
2BP3
3BP1
3BP2
3BP3
2DPA1
2DPA2
2DPA3
3DPA1
3DPA2
3DPA3
262
268
274
250
250
250
290
294
298
266
264
262
308
312
314
294
290
288
14500
20700
29500
5500
(1.0)
1.43
2.0
TABLE 1: Calculated Heats of Formation for Low-Energy
Benzamide Conformations
monosubtituted amide
Eo, kcal/mola
transb
cisb
(1.0)
2.16
3.25
(1.0)
1.49
2.35
(1.0)
1.87
2.78
(1.0)
1.56
2.26
(1.0)
1.78
2.42
2P1
-2948.1
-4156.3
-4447.6
-3212.9
-4421.2
-4712.8
(0.0)
(0.0)
(0.0)
+1.9
+1.9
+3.0
+2.7
+3.0
+2.5
(0.0)
(0.0)
(0.0)
11400
16600
28400
42300
66900
19100
35700
53300
43700
68500
99100
30800
54800
74600
2BP1
2DPA1
3P1
3BP1
3DPA1
syn
anti
disubstituted amide Eo, kcal/mola trans,trans (cis,cis) (cis,cis)
2P2
-4578.0
-6994.7
-5109.5
-7525.8
(0.0)
(0.0)
+4.1
+5.1
+5.0
+5.0
(0.0)
(0.0)
+4.5
+6.1
+0.1
+0.5
2BP2
3P2
3BP2
syn
(all-cis)
anti
(all-cis)
trisubstituted amide
Eo, kcal/mola
all-trans
a Measured at room temperature. b Ratio of intensities of di- or tri-
amides to monoamide (1.0).
2P3
-6207.0
-9834.0
-7005.4
-10630.3
(0.0)
(0.0)
+6.2
+6.0
+6.5
+8.8
(0.0)
(0.0)
6.3
+9.4
-0.2
+0.7
2BP3
3P3
3BP3
temperature and followed by TLC until the starting material
was consumed (ca. 2-3 h), mixed with water (20 mL), and
extracted with 20 mL CH2Cl2. The organic layer was washed
with water (20 mL) three times and dried with anhydrous
potassium carbonate (3 g). After the solvent was removed, the
residue was purified by column chromatography using mixed
solvents (acetone and hexane).
a Heats of formation for low-energy conformers were obtained from
AM1 calculations using Hyperchem, with geometry optimization by
AM1 to RMS gradient of 0.1 kcal. b Energy of higher energy conform-
ers relative to that of the lowest energy conformer (0.0).
dropwise to a solution of the secondary benzamides (0.6 equiv
for monoamide, 0.3 equiv for diamides, and 0.2 equiv for
triamides) in DMF (20 mL) in 20 min, during which 1.2 equiv
of iodomethane was added. The mixture was stirred at room
N,N′-Dimethyl-N,N′-diphenyl-1,3-benzenedicarboxamide (3P2).
mp 165-166 °C (lit.15 mp 164-166 °C)

Structures and Excited States of N-Arylbenzamides
J. Phys. Chem. A, Vol. 107, No. 18, 2003 3257
Figure 4. Energies, oscillator strengths, and dominant configurations for the lowest singlet-singlet transitions of (a) 2P1, (b) 2P2, and (c) 2P3.
Occupied orbitals shown in green and blue, unoccupied orbitals shown in red and yellow.
N,N′,N′′-Trimethyl-N,N′,N′′-triphenyl-1,3,5-benzenetricarbox-
amide (3P3). mp 262-263 °C (lit.15 mp 262-263 °C).
N-Methyl-N-[1,1′-biphenyl]-4-yl-benzamide (3BP1). mp 117-
N-Methyl-N-[4-(Phenylethynyl)phenyl]benzamide (3DPA1).
mp 118-119 °C. 1H NMR (CDCl3, 300 MHz): δ 3.51 (s, 3H),
3
7.01 (d, J ) 8.7 Hz, 2H), 7.16-7.23 (m, 2H), 7.24-7.28 (m,
1
1H), 7.30-7.32 (m, 1H), 7.33-7.36 (m, 4H), 7.38 (d, 3J ) 8.7
119 °C. H NMR (CDCl3, 300 MHz): δ 3.55 (s, 3H), 7.09-
3
7.12 (m, 2H), 7.17-7.27 (m, 3H), 7.34-7.38 (m, 3H), 7.39-
7.48 (m, 4H), 7.51-7.55 (m, 2H). MS m/e (%) 288 (MH+, 100),
221(8), 182(5).
Hz, 2H), 7.51 (d, J ) 10.5 Hz, 2H), 7.48-7.52 (m, 2H). MS
m/e 309 (M+•).
N,N′-Dimethyl-N,N′-bis-[4-(Phenylethynyl)phenyl]-1,3-ben-
zenedicarboxamide (3DPA2). mp 220-222 °C. 1H NMR
N,N′-Dimethyl-N,N′-bis[1,1′-biphenyl]-4-yl-1,3-benzenedicar-
1
boxamide (3BP2). mp 216-218 °C. H NMR (CDCl3, 300
3
(CDCl3, 300 MHz): δ 3.45 (s, 6H), 6.86 (dm, J ) 8.4 Hz,
MHz): δ 3.46 (s, 6H), 6.93 (d, 3J ) 8.4 Hz, 4H), 6.96 (t, 3J )
4H), 7.0 (tm, 3J ) 8.4 Hz, 1H), 7.13-7.17 (m, 2H), 7.26-7.34
3
8.4 Hz, 1H), 7.16 (d, J ) 7.2 Hz, 2H), 7.34-7.48 (m, 15H).
4
3
(m, 6H), 7.39 (t, J ) 1.5 Hz, 1H), 7.44 (d, J ) 8.7 Hz, 4H),
MS m/e (%) 497 (MH+, 100), 314(20), 307(5), 289(5), 260-
(10).
7.48-7.51 (m, 4H). MS m/e 544 (M+•).
N,N′,N′′-Trimethyl-N,N′,N′′-tris-[4-(Phenylethynyl)phenyl]-
N,N′,N′′-Trimethyl-N,N′,N′′-tris[1,1′-biphenyl]-4-yl-1,3,5-ben-
1
1
1,3,5-benzene-tricarboxamide (3DPA3). mp 293-294 °C. H
zenetricarboxamide (3BP3). mp > 300 °C. H NMR (CDCl3,
3
3
NMR (CDCl3, 300 MHz): δ 3.38 (s, 9H), 6.58 (d, J ) 8.4
300 MHz): δ 3.38 (s, 9H), 6.46 (d, J ) 7.8 Hz, 6H), 6.98
(dm, 3J ) 7.5 Hz, 12H), 7.16 (s, 3H), 7.36-7.44 (m, 9H). MS
m/e (%) 706 (M+, 100), 555(20), 523(20).
Hz, 6H), 7.14 (s, 3H), 7.16-7.34 (m, 9H), 7.48-7.52 (m, 6H),
3
7.54 (d, J ) 8.7 Hz, 6H). MS m/e 777 (M+•).

3258 J. Phys. Chem. A, Vol. 107, No. 18, 2003
Lewis et al.
Figure 5. Energies, oscillator strengths, and dominant configurations for the lowest singlet-singlet transitions of (a) 3P1, (b) 3P2, and (c) 3P3.
See Figure 4 for color code.
Results and Discussion
The calculated heats of formation for the AM1-minimized
conformations of most of the amides are summarized in Table
1. For the secondary amides, the trans amide conformations are
more stable than the cis by a minimum of 2.5 kcal/mol.
Conversely, for the tertiary amides the cis conformations are
more stable than the trans by a minimum of 1.9 kcal/mol. For
the tertiary di- and triamides the calculated heats of formation
for the syn and anti conformations of 3P2 and 3P3 are similar,
in accord with the observation of fast NMR exchange of the
syn and anti rotamer of 3P3.8 However, the syn conformations
of 3BP2 and 3BP3 are calculated to be more stable than the
anti conformers by ca. 0.5 kcal/mol.
Molecular Structure. The crystal structures of the secondary
amides 2P1, 2P2, and 2P3 and those of the tertiary amides 3P1,
3P2, and 3P3 have been reported.8 The secondary amides
possess all-trans amide conformations, whereas the tertiary
amides possess all-cis conformations. The crystal structure of
3BP3 has been determined and found to belong to the space
group P3(no. 143). Each of the three molecules in the unit cell
has an all-cis conformation in which all three biphenyl groups
reside on the same side of the benzenetricarboxamide ring, but
have slightly different geometries (Figure 1). The three biphenyls
are nonplanar, with phenyl-phenyl torsion angles of 22, 31,
and 32°. The parent biphenyl molecule has a torsion angle of
35° in solution.17 Both twisted and planar structures have been
reported for biphenyl in solid phases, the torsion angle being
determined by crystal packing forces. The three biphenyls in
3BP3 adopt an edge-to-face arrangement, similar to that
observed for adjacent molecules in a one-dimensional tape of
N,N-dimethyl-1,4′-biphenyldicarboxamide.18
The AM1-minimized structures of the secondary amides 2P1,
2P2, and 2P3 and those of the tertiary amides 3P1, 3P2, and
3P3 are shown in Figure 2 along with the all-syn structures of
3BP3 and 3DPA3. The calculated structure for 3BP3 is similar
to the crystal structure. The crystal structure and AM1 mini-
mized structures of 2P1 and 3P1 have previously been
compared.5 In the crystal structure, the dihedral angle between
phenyl rings is ca. 30° as a consequence of twisting about the
phenyl-CO and phenyl-NH bonds. The calculated potential
energy surface for twisting about these bonds is very shallow,
the energy of the fully planar structure differing from that of
the nonplanar structure by <1 kcal/mol. The experimental and
calculated twist angles for 3P1 are larger than those for 2P1.
Experimental evidence for a syn-conformer preference for
the tertiary di- and triamides of BP and DPA is provided by
analysis of their NMR spectra. The protons adjacent to the
nitrogens appear as a readily identified doublet, which moves
upfield with the introduction of successive amide substituents
as a consequence of aryl-aryl shielding. For example, the shifts
of these protons for 3BP1, 3BP2, and 3BP3 are at 7.10, 6.93,
and 6.46, respectively. Similar results are obtained for the
tertiary DPA-containing amides. The larger upfield shift for the
tri- vs diamide is consistent with an all-syn conformation rather
than a syn-anti conformation for both 3BP3 and 3DPA3. The
larger preference for the all-syn conformation of these amides
vs 3P3 can be attributed to more extensive edge-to-face
interactions in the larger arenes.19
Electronic Absorption Spectra. The absorption spectra of
the 18 benzamides are shown in Figure 3. All of the spectra
display a short wavelength band near 200 nm and a single long-
wavelength band. The position and molar absorbance of the
long-wavelength maxima are reported in Table 2. In the case
of the tertiary 3P(n) series, the long-wavelength band is not

Structures and Excited States of N-Arylbenzamides
J. Phys. Chem. A, Vol. 107, No. 18, 2003 3259
resolved and the absorbance at 250 nm is reported. In the case
of the secondary amides, increasing the number of amide groups
results in a red shift in the absorption maxima (Figure 3a,c,e).
The absorbance ratios for the secondary mono-, di-, and
triamides are approximately 1:1.5:2.2. In the case of the tertiary
amides of BP and DPA, increasing the number of amide groups
results in a blue shift in the absorption maximum (Figure 3d,f).
The absorbance ratios for the tertiary mono-, di-, and triamides
are approximately 1:2:3. In the case of the 2BP(n), 2DPA(n),
and 3BP(n) families there is also distinct change in band shape
with increasing substitution, the bands appearing to “lean” in
the direction of the band maximum.
The modest red shifts for the families of secondary amides
are similar to those reported for biphenyl, m-terphenyl, and
1,3,5-triphenylbenzene.9 However, this series displays a more
pronounced increase in oscillator strength for the long-
wavelength absorption band (1:2.9:3.6) than is observed for the
secondary amides. The fluorescence maxima undergo a pro-
nounced red shift accompanied by an increase in singlet lifetime.
Evidently the singlet state is delocalized, unlike the long-
wavelength transitions in the absorption spectra, which are
proposed to be localized on the individual biphenyl units.9
ZINDO Calculations. We have previously employed semiem-
perical INDO/S SCF-CI (ZINDO) calculations using the algo-
rithm developed by Zerner and co-workers20 to investigate the
electronic transitions of 2P1, 2BP1, 2DPA1, and 3P1.4,5 ZINDO
calculations have been conducted for all 18 amides using their
AM1-minimized geometries. The results of these calculations
are shown in Figure 4 for the 2P(n) family, in Figure 5 for the
3P(n) family, and in Figure 6 for 3BP3. The use of color coding
(blue and green for occupied orbitals and yellow and red for
unoccupied orbitals) facilitates visualization of the dominant
configuration for each of the transitions. The calculated
wavelength and oscillator strength for each transition are also
reported in the figures.
The lowest energy transition for all 18 amides is a forbidden
n,π* benzoyl-localized (n,B*) state, which accounts for the
fluorescence behavior of these molecules (vide infra). The lowest
energy allowed transitions for the 2P(n) family are E4, E6,7
,
and E8,9, for n ) 1, 2, and 3, respectively (Figure 3). All of
these are π,π* transitions of aniline,benzoyl (A,B*) character.
Their maxima are slightly red-shifted with increasing substitu-
tion, as are the observed spectra (Figure 4a). The lowest energy
allowed transitions for the 3P(n) family are E4, E6,7,8, and E10,11
,
Figure 6. Energies, oscillator strengths, and dominant configurations
for the lowest singlet-singlet transitions of 3BP3. See Figure 4 for
color code.
for n ) 1, 2, and 3, respectively. Most of these transitions can
also be described as A,B* transitions in which the occupied
orbital is localized on a single aniline ring. The increase in
calculated oscillator strength near 340 nm with increasing
substitution is consistent with the appearance of the spectra
(Figure 3b).
parallel biphenyl chromophores (H-aggretate structure) in 3BP2
and 3BP3 should result in a weak long wavelength band and a
strong short wavelength band.21 In contrast, exciton splitting
between the biphenyls in 2BP2 and 2BP3, which have aligned
vector components (J-aggregate structure), should result in a
strong long wavelength band and a weak short wavelength band.
The strength of the exciton splitting shows a 1/R3 dependence
upon the distance between chromophores and hence should be
larger for the tertiary versus secondary amides.21 This may
account for the larger gain in molar absorbance for the tertiary
versus secondary amides with increasing substitution (Table 2).
The transitions for one of the more complex structures, 3BP3,
are shown in Figure 6. The lowest energy states correspond to
three forbidden n,B* transitions originating in the three different
carbonyl n-orbitals. The lowest energy allowed transitions are
E9,13,14 (A,B*) and E15 (A,A*). The A,A* transitions E5,6,7 are
localized on a singlet biphenyl and have low oscillator strengths,
as does the lowest energy transition of the parent biphenyl
molecule.17 Transitions E10,11,12 display coupling between two
biphenyls, but also have low oscillator strengths.
The ability of the ZINDO calculations to replicate the
observed absorption spectra is illustrated in Figures 7 and 8.
Figure 7 shows the calculated spectrum of 2P1 obtained with
no band broadening (vectors) and using three different peak
widths. The maxima at 265 and 240 nm correspond to transitions
E4 and E5 in Figure 4, while the maximum at 200 nm
Excitonic interaction between biphenyl-localized transitions
provides a possible explanation for the blue-shifted absorption
maximum for the 3BP(n) and 3DPA(n) families (Figure 3d,f)
and the red-shifted absorption for the 2BP(n) and 2DPA(n)
families with increasing n (Figure 3b). Exciton splitting between

3260 J. Phys. Chem. A, Vol. 107, No. 18, 2003
Lewis et al.
Figure 8. Comparison of experimental (solid) and calculated (dash)
absorption spectra of (a) 2P1, (b) 3P3, and (c) 3BP3.
2P1, 2BP1, and 2DPA1.4 The two singly occupied molecular
orbitals (SOMO1 and SOMO2) for the 2P(n) and 3P(n) families
and those for 2BP1 are shown in Figure 9. In the case of 2P1
both SOMOs are largely localized on the anilide portion of the
molecule. Delocalization into the benzoyl portion is observed
to increase with increasing amide substitution for 2P2 and 2P3.
Both SOMOs are delocalized for all three members of the 3P-
(n) family. For all of the other secondary and tertiary amines,
both SOMOs are largely localized on a single N-aryl substituent,
as shown for 2BP1 in Figure 9. The character of the triplet state
will be seen to influence the phosphorescence behavior (vide
infra).
Luminescence Spectra. All 18 amides display overlapping
fluorescence and phosphorescence at 77 K in MTHF glass.
Wavelengths, decay times, and quantum yields are summarized
in Table 2 along with data for the parent arylamines. Repre-
sentative time-resolved fluorescence and phosphorescence spec-
tra are shown in Figure 10. Broad structureless fluorescence is
observed in all cases. The phosphorescence of the benzanilides
is also structureless; however, the phosphorescence of the amides
possessing BP or DPA is structured and similar in appearance
to that of the parent arylamines, as previously reported for 2BP1
and 2DPA1.4
The fluorescence of 2P1 and 3P1 has been the subject of
several investigations and some controversy.3,7,23 We recently
reported the results of an experimental and computational study
of the luminescence and excited-state potential energy surfaces
for these benzanilides.5 Both display very weak room-temper-
ature fluorescence with large Stokes shifts (λmax ) 475 nm) in
nonpolar solvents. This emission is assigned to an intramolecular
charge transfer state which is twisted about the amide C-N
bond, in agreement with the assignment of Azumaya et al.7 The
emission maximum of 2P1 in MTHF glass at 77 K occurs at
shorter wavelength (Table 3), as a consequence of restricted
Figure 7. Calculated absorption spectra for 2P1 with bandwidths of
(a) 1600 cm-1, (b) 3200 cm-1, and (c) 4800 cm-1. Vectors indicate
calculated energies and oscillator strengths of singlet transitions. Tailless
vectors have very low oscillator strengths.
corresponds to higher energy transitions. Figure 8 compares the
observed spectra for 2P1, 3P3, and 3BP3 with spectra calculated
using peak widths of 4800 cm-1. The agreement is excellent
for all three amides. Whereas we have used all of the ZINDO-
calculated transitions to simulate the absorption spectrum, the
fits are dominated by a small number of individual or degenerate
transitions and thus can be reproduced using three arbitrary
Gaussian functions. Substantial band broadening is necessary
in order to replicate the observed spectra. The band broadening
presumably is a consequence of several factors, including
shallow ground-state potential energy minima with underlying
vibrational progressions.
The ZINDO calculations also appear to replicate the excitonic
interactions between arene groups. For example, transitions E6
and E7 of 2P2 and transitions E8 and E11 of 2P3 (Figure 4)
have the same symmetries but different energies. The lower
energy transition possesses the greater oscillator strength, as
expected for an J-aggregate. In the case of the tertiary amide
3BP3, transitions E4 and E9 have the same symmetries but
different energies. The higher energy transition possesses the
greater oscillator, as expected for an H-aggregate. There is
precedent for the ability of ZINDO calculations to replicate
aggregate spectra. ZINDO/S calculations for clusters of four
quaterthiophene molecules in an H-aggregate structure reproduce
the absorption spectral shifts observed in thin films, which
presumably exist as larger aggregates, when compared to the
isolated molecules in solution.22
The lowest triplet states of all 18 amides were also calculated
using ZINDO, as previously reported for the secondary amides

Structures and Excited States of N-Arylbenzamides
J. Phys. Chem. A, Vol. 107, No. 18, 2003 3261
Figure 10. Absorption (solid), fluorescence (dash), and phosphores-
cence (dash dot) spectra of (a) 3P1, (b) 3BP1, and (c) 3DPA1.
TABLE 3: Low-Temperature Emission Spectra Data for
Benzamides in MTHFat 77 K
fluorescence
phosphorescence
amine or amidea
λmax, nm ts, ns
Φf
λ
max, nm
tt, s
Φp
PhNH2
2P1
2P2
2P3
3P1
3P2
3P3
BpNH2
2BP1
2BP2
2BP3
3BP1
3BP2
3BP3
DPANH2
2DPA1
2DPA2
2DPA3
3DPA1
3DPA2
3DPA3
325
430
430
430
510
490
470
365
420
430
440
510
510
510
360
420
425
430
500
500
500
1.92 0.23
3.63 0.25
3.50 0.24
3.02 0.16
2.52 0.01
405
410
420
430
460
430
430
470
460
460
460
460
455
450
480
470
475
480
470
470
470
4.42
0.4
0.17
0.08
0.0017 0.01
0.0009 0.01
0.0033 0.04
3.52
2.0
1.78
1.45
1.76
1.8
1.98
1.57
0.747
0.655
0.644
0.675
0.634
0.609
0.45
0.37
0.10
0.03
b
0.01
3.00 0.04
10.65 0.61
3.97 0.45
3.34 0.27
3.04 0.26
3.30 0.05
3.91 0.03
3.32 0.02
1.25 0.33
2.06 0.44
3.15 0.39
2.67 0.29
3.43 0.06
4.73 0.05
4.35 0.05
0.07
0.07
0.03
0.02
0.20
0.14
0.10
0.02
0.02
0.01
0.07
0.07
0.05
0.05
Figure 9. Singly occupied orbitals for the triplet states of 2P(n), 3P-
(n), and 2BP1.
C-N rotation. The emission maxima of the highly nonplanar
amide 3P1 are similar in solution and at 77 K and are attributed
to the twisted n,B* singlet state.
a Data for amines from ref 4. b Lifetime not measured.
The 77 K fluorescence spectra of all of the secondary amides
are similar in appearance and are attributed to the planar n,B*
lowest singlet state. This assignment is consistent with the results
of ZINDO calculations showing that the n,B* states for all of
the secondary amides have similar energies (λmax ) 430 ( 10
nm) and lie well below the allowed π,π* transitions. They also
have similar singlet lifetimes (2-4 ns) and moderately large
fluorescence quantum yields (Table 3). The fluorescence spectra
of all of the tertiary amides resemble that of 3P1 and are
attributed to the nonplanar n,B* singlet state. Their fluorescence
maxima are similar (500 ( 10 nm, except in the case of 3P3).
Their singlet decay times are also similar (2.5-4.7 ns), and their
fluorescence quantum yields are distinctly smaller than those
of the secondary amides.
The phosphorescence spectra of all of the secondary and
tertiary BP and DPA amides display structure and decay times
similar to those of the parent arylamines.4 This observation is
consistent with the results of ZINDO calculations (Figure 9)
which indicate that both SOMOs of the triplet state are localized
on the arylamine. This is not the case for the secondary and
tertiary anilides, all of which have red-shifted emission maxima
and shorter fluorescence decay times than aniline. The ZINDO
calculations for the triplet states indicate that the SOMOs of
all of the 3P(n) amides are extensively delocalized. Mixing of
n,π* character in the case of these amides should reduce their
triplet lifetimes compared to that of aniline, as is observed. In
the case of the 2P(n) family, delocalization increases as n

3262 J. Phys. Chem. A, Vol. 107, No. 18, 2003
Lewis et al.
increases, providing a plausible explanation for the decrease in
triplet lifetime and quantum yield.
References and Notes
(1) Itai, A.; Toriumi, Y.; Tomioka, N.; Kagechika, H.; Azumaya, I.;
Shudo, K. Tetrahedron Lett. 1989, 30, 6177-6180.
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(3) Lewis, F. D.; Long, T. M. J. Phys. Chem. A 1999, 102, 5327-
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Concluding Remarks. The conformational preferences of
the secondary and tertiary benzamides possessing BP or DPA
groups are similar to those previously reported for the corre-
sponding secondary and tertiary anilides. All of the secondary
benzamides have trans-amide conformations, resulting in ex-
tended geometries. In contrast, the tertiary benzamides have cis-
amide conformations, resulting in folded geometries. The aryl
groups of the di- and trisubstituted tertiary benzamides of BP
and DPA occupy the same face of the benzamide ring. This
syn preference is attributed to attractive edge-to-face interactions
between the aryl groups.19
The absorption spectra of the secondary amides display a red
shift with increasing substitution, whereas the spectra of the
tertiary amides display a blue shift. Exciton splitting between
transitions localized on the individual BP or DPA groups may
be responsible for these shifts. Analysis of the electronic spectra
using ZINDO calculations indicates that the lowest singlet state
for all of the amides arises from a forbidden n,B* transition
and that the lowest energy allowed transitions are of A,B* or
A,A* character. All of the amides are very weakly fluorescent
at room temperature. At 77 K in a MTHF glass the secondary
and tertiary amides display structureless fluorescence attributed
to planar and nonplanar n,B* states. The structured phospho-
rescence of both secondary and tertiary amides possessing BP
and DPA groups is attributed to arene-localized A,A* transi-
tions.
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Acknowledgment. Financial support for this research has
been provided by the National Science Foundation.
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H. Freeman: New York, 1980; p 395.
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Supporting Information Available: X-ray crystallographic
data for the tertiary amide 3BP3. This material is available free
of charge via the Internet at http://pubs.acs.org.