Full text of DOI:10.1021/jp012013k

1976
J. Phys. Chem. A 2002, 106, 1976-1984
Temperature- and Conformation-Dependent Luminescence of Benzanilides†
Frederick D. Lewis* and Weizhong Liu
Department of Chemistry, Northwestern UniVersity, EVanston, Illinois 60208-3113
ReceiVed: May 24, 2001; In Final Form: August 14, 2001
The luminescence of benzanilide, N-methylbenzanilide, and two cyclic analogues has been investigated as a
function of temperature both in microcrystals and in the glass-forming solvent methyltetrahydrofuran. Potential
energy surfaces for small-amplitude geometric relaxation of the excited states and large amplitude twisting
about the amide bond in the ground and excited states have been explored using AM1 and RCIS calculations,
respectively. ZINDO calculations have been used to probe the electronic configuration of the planar and
twisted singlet and triplet states. Correlation of the spectroscopic and computational results indicates that at
77 K benzanilide phosphorescence occurs from a planar π,π* triplet state whereas fluorescence occurs from
a singlet state with partially relaxed geometry. At higher temperatures, large-amplitude geometric relaxation
results in the formation of an intramolecular charge-transfer (TICT) state that is twisted about the amide
C-N bond. This TICT state is responsible for the weak Stokes-shifted fluorescence observed from benzanilide
and N-methylbenzanilide in fluid solution. Inhibition of amide twisting in cyclic derivatives precludes
observation of TICT fluorescence.
Introduction
geometry and the phosphorescence to a planar π(aniline)-π*-
(delocalized) triplet state. The effects of phase and molecular
The luminescence of benzanilide (BA) and its derivatives
continues to be the source of interest and conflicting interpre-
structure on the luminescence behavior of BA and MBA
suggested that this behavior is strongly dependent upon mo-
lecular conformation. To better define the relationships among
molecular conformation, excited-state configuration, and lumi-
nescence, we have investigated the luminescence behavior of
BA and MBA in MTHF over a wide range of temperatures and
in the microcrystalline state at 77 and 298 K. The luminescence
of two cyclic derivatives of MBA, N-phenylisoindole (PI) and
the N-methyldibenzoazocinone (MDBA), in MTHF have also
been investigated. The phenyl groups in PI and MDBA are
constrained to E and Z configurations, analogous to those of
1-8
tations. At room temperature, BA is very weakly fluorescent
-
3
(
Φf < 10 ) in nonpolar solvents and nonfluorescent in polar
solvents. No fewer than three fluorescent states have been
identified by various workers: a weak band at shorter wave-
length (320 nm in methylcyclohexane, MC) attributed to a
vertical (Franck-Condon, FC) singlet state and a broad, stronger
band at longer wavelength (475 nm in MC) that has been
attributed either to an imidol tautomer or a twisted intramo-
lecular charge-transfer (TICT) state or to the overlap of the
tautomer and TICT fluorescence. Similar Stokes-shifted fluo-
rescence is observed for N-methylbenzanilide (MBA), which
is incapable of tautomer formation. An investigation of the
room-temperature luminescence of BA, MBA, and several rigid
9
the BA and MBA ground states (Scheme 1). The experimental
results are correlated with the ground- and excited-state potential
energy surfaces, which have been constructed on the basis of
10
RCIS calculations, and with excited-state configurations
4
derivatives by Azumaya et al. led to the conclusions that (a)
11
obtained from ZINDO calculations.
the weak short-wavelength emission is an artifact arising from
an impurity and (b) the longer wavelength emission, which is
absent in some rigid derivatives, arises from a TICT state, which
is twisted about the amide C-N bond. Recent publications have
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 (i.d. ) 3.3 mm) using a quartz liquid nitrogen
coldfinger dewar at 77 K or in an Oxford Cryogenics DN1704
cryostat fitted with an 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-diphenyl anthracene (Φf ≈ 1.0 at 77K
reiterated assignments of the short-wavelength FC fluorescence
and imidol tautomer fluorescence.5,6
The luminescence of BA in low-temperature glasses consists
of a broad band attributed to overlapping fluorescence and
phosphorescence. As initially noted by O’Connell et al., the
1
fluorescence maximum occurs at longer wavelength than the
phosphorescence maximum but at shorter wavelength than the
room temperature fluorescence. We recently investigated the
luminescence of a family of N-arylbenzamides both as micro-
crystalline solids and in methyltetrahydrofuran (MTHF) glasses
8
at 77 K. The fluorescence (λmax ) 430 nm) was assigned to a
singlet state of undetermined configuration having a relaxed
12
in MTHF). The overlapping fluorescence and phosphorescence
spectra were resolved by time-dependent integration as described
*
To whom correspondence should be addressed. E-mail: lewis@chem.
8
previously. All emission spectra are uncorrected, and the
northwestern.edu.
†
Part of the special issue “Noboru Mataga Festschrift”.
estimated error for the quantum yields is (20%.
1
0.1021/jp012013k CCC: $22.00 © 2002 American Chemical Society
Published on Web 10/23/2001

Luminescence of Benzanilides
J. Phys. Chem. A, Vol. 106, No. 10, 2002 1977
SCHEME 1
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 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.13 and implemented by
the PTI Timemaster (version 1.2) software. Goodness of fit was
Figure 1. Absorption (solid line, λ ) 262 nm, log ꢀ ) 4.16),
fluorescence (short dash, 77 K; dotted, 298 K), and phosphorescence
(dash, 77 K) spectra of benzanilide: (a) 0.1 mM in MTHF; (b) in
microcrystals.
2
determined by judging the ø (<1.3 in all cases), the residuals,
TABLE 1: Fluorescence Parameters for the Benzanilidesa
and the Durbin-Watson parameter (>1.6 in all cases). Solutions
were degassed under vacuum (<0.1 Torr) through five freeze-
pump-thaw cycles. Solid samples were measured in a Suprasil
quartz EPR tube and similarly degassed without freezing.
All ab initio calculations were performed using Gaussian 98,
MTHF
solid
-8
10-8
λ
fl
,
τ
s
,
ns
10
k
fl
,
k
nr
,
λ
fl
,
s
τ ,
s-1
s-1
T, K nm
Φ
fl
nm ns
BA
PI
77 430
98 520
77 400
298 425 <0.1 0.007
77 500
98 550
2.7 0.25
0.4 0.0005 0.012
4.6 0.60
0.93
2.8
25
0.9
200
3.6
25
420 3.8
430 1.3
405 4.1
405 1.8
460 5.1
480 2.3
1
0
revision A.7 on a 500 MHz Pentium III personal computer.
The level of calculation is limited by the size of the molecules
studied and the limit of computing power. The S1 state geom-
etries were first optimized with RCIS method using a STO-3G
basis set and then with a 3-21G basis set because direct
optimization with a 3-21G basis set fails to converge. The T1-
state geometries were optimized with the UHF method first
using a STO-3G basis set and then using a 3-21G basis set.
The S1 and T1 transition energies were calculated using the
RCIS method.
2
1.3
1.4
0.037
MBA
2.7 0.01
0.4 0.0002 0.005
2
a
Structures shown in Scheme 1.
tetrahydrodibenz[b,f]azocine-6-one (1.1 g, 5 mmol) and MeI
0.63 mL, 10 mmol) in 20 mL of DMF. The resulting mixture
(
was stirred for 2 h and then mixed with 50 mL water and
extracted with 30 mL ethyl acetate. The organic layer was
washed with brine and dried with anhydrous MgSO4. After the
solvent was removed on a rotavap, the residue was recrystallized
from 1:1 ethyl acetate-hexane to give 0.7 g of MDBA (65%),
ZINDO(S)/CI calculations were performed using Cache
release 3.5 with ground-state structures optimized using the AM1
11
method using MOPAC (version 94.10) programs. The 3D
molecular structures were generated using Hyperchem version
1
5
1
mp 96-97 °C (lit. 95-97 °C). H NMR (CDCl3, 500 MHz):
δ 2.87-2.98 (m, 2H), 3.16-3.28 (m, 1H), 3.40 (s, 3H), 3.16-
5
.0.
Materials. Benzanilide (BA) and N-methylbenzanilide (MBA)
3
.28 (m, 1H), 6.8-7.2 (m, 8H).
Results
Luminescence Spectra. The absorption and fluorescence
were puchased from Aldrich and recrystallized twice from DMF
and acetone-hexane (1:1), respectively. Aniline, o-phthalde-
hyde, and 5,6,11,12-tetrahydrodibenz[b,f]azocine-6-one are com-
ercially available and were used as received. Anhydrous MTHF
containing 200 ppm BHT (Aldrich) was distilled over sodium
under a nitrogen atmosphere.
spectra of BA in MTHF solution at 298 K are shown in Figure
1a. A single broad fluorescence band is observed with a
maximum at 520 nm and a fluorescence quantum yield of 5 ×
4
-N-Phenylisoindolinone (PI). The procedure of Grigg et al.14
-
4
was modified. A mixture of o-phthaldehyde (2.7 g, 20 mmol)
and aniline (1.9 mL, 20 mmol) in 20 mL of ethyl acetate and
10 . The fluorescence decay is best fit by a single exponential
with a decay time of 0.40 ns. Also shown in Figure 1a are the
fluorescence and phosphorescence spectra of BA in a MTHF
glass at 77 K. The overlapping spectra were resolved by time-
5
mL of acetic acid was stirred under a nitrogen atomsphere at
room temperature for 3 h. The resulting white precipitate was
collected, washed with ether, and recrystallized with ethyl
acetate and hexane (1:1) to give 1.9 g of PI (50%), mp 162-
8
dependent integration as previously described. The lumines-
cence spectra of microcrystalline BA obtained at 77 and 298 K
are shown in Figure 1b. The fluorescence maxima at both
temperatures are similar to that observed in the MTHF glass.
The 77 K phosphorescence is strongly structured and its maxi-
mum occurs at longer wavelength than that in the glass. The
luminescence parameters are summarized in Tables 1 and 2.
The temperature dependence of the fluorescence of BA in
MTHF over the range of 77-343 K is shown in Figure 2.
1
4
1
1
4
2
63 °C (lit. 162-163 °C). H NMR (CDCl3, 500 MHz): δ
3
3
.88 (s, 2H), 7.21 (t, J ) 7.0 Hz, 1H), 7.45 (t, J ) 8.0 Hz,
3
3
H), 7.53 (t, J ) 7.5 Hz, 2H), 7.62 (t, J ) 7.0 Hz, 1H), 7.90
3
3
(
d, J ) 8.0 Hz, 2H), 7.95 (d, J ) 7.5 Hz, 1H).
N-Methyl-5,6,11,12-tetrahydrodibenz[b,f]azocine-6-one
MDBA).A solution of NaH (0.24 g, 10 mmol) in 10 mL of
(
DMF was added dropwise into a solution of 5,6,11,12-

1978 J. Phys. Chem. A, Vol. 106, No. 10, 2002
Lewis and Liu
TABLE 2: Phosphorescence Parameters for the
Benzanilidesa
MTHF
solid
T, K
λph, nm
τ
T
Φ
ph
λ
ph, nm
τ
T
BA
PI
MBA
MDBA
77
77
77
77
410
420
485
398
0.4 s
1.0 s
1.7 ms
0.9 s
0.37
0.03
0.02
0.01
460
430
450
430
5.2 ms
1.1 s
4.1 ms
4.0 ms
a
Structures shown in Scheme 1.
Figure 3. Plot of fluorescence maxima (ν, cm-1) vs temperature (T,
K) for BA (9), PI ([), and MBA (b), and phosphorescence maximum
of BA (0).
Figure 2. Fluorescence spectra of benzanilide (0.1 mM) in MTHF
over the temperature range 77-343 K.
Between 77 and 150 K, the fluorescence intensity continuously
decreases with increasing temperature and the emission maxi-
mum shifts from 430 to 550 nm. Above 150 K, the emission
intensity increases as the maximum shifts to shorter wavelength
reaching a value of 525 nm at 343 K. The temperature
dependence of νmax is shown in Figure 3. The phosphorescence
intensity of BA also decreases with increasing temperature and
is too weak to be observed when T g 140 K. The phosphores-
cence emission maximum shifts continuously from 416 nm at
Figure 4. Absorption (solid line, λ ) 283 nm, log ꢀ ) 4.25),
7
7 K to 430 nm at 130 K (Figure 3).
fluorescence (short dash, 77 K; dotted, 298 K), and phosphorescence
The absorption and fluorescence spectra of PI in MTHF at
98 K are shown in Figure 4a along with its 77 K fluorescence
(dash, 77 K) spectra of N-phenylisoindolinone: (a) 25 µM in MTHF;
2
(b) in microcrystals.
and phosphorescence spectra. The fluorescence and phospho-
rescence spectra of microcrystalline PI are shown in Figure 4b.
Emission maxima, quantum yields, and decay times are sum-
marized in Tables 1 and 2. The fluorescence of PI undergoes
only a small red shift on warming from 77 to 298 K in MTHF
and no shift in the microcrystalline state. The emission maxima
are at shorter wavelength than those of BA under all conditions.
The fluorescence intensity of PI in MTHF decreases continu-
ously with warming from 77 to 298 K; however, the total
decrease in intensity is smaller than that observed for BA. The
fluorescence maximum shifts to longer wavelength upon warm-
ing from 77 to 120 K but is essentially constant at higher
temperatures (Figure 3).
times are summarized in Tables 1 and 2. Emission maxima are
at longer wavelength for MBA than for BA, the difference being
most pronounced at 77 K in MTHF, and the emission quantum
yields are smaller for MBA than for BA. The temperature
dependence of the fluorescence of MBA in MTHF (Figure 3)
is qualitatively similar to that of BA. As is the case for BA, no
phosphorescence is observed above 110 K.
The absorption spectrum of MDBA in MTHF at 298 K and
its 77 K phosphorescence spectrum are shown in Figure 6. No
fluorescence is observed at 77 or 298 K in either MTHF or the
microcrystalline solid. The phosphorescence parameters are
summarized in Table 2.
The absorption and fluorescence spectra of MBA in MTHF
at 298 K are shown in Figure 5a along with the 77 K fluor-
escence and phosphorescence spectra. The fluorescence and
phosphorescence spectra of microcrystalline MBA are shown
in Figure 5b. Emission maxima, quantum yields, and decay
Calculated Ground- and Excited-State Energy Surfaces.
The ground-state conformation of BA calculated by AM1 (gas
phase) is fully planar (Figure 7a); the calculated amide and
phenyl-amide dihedral angles are all <1°. Selected bond
lengths and dihedral angles (Scheme 2) obtained from the AM1

Luminescence of Benzanilides
J. Phys. Chem. A, Vol. 106, No. 10, 2002 1979
SCHEME 2
TABLE 3: Geometric Parameters for Benzanilide Ground
and Excited Singlet Statesa
dihedral anglesb
θ1° θ2° θ3° θ4° CO-N Ph-CO Ph-N C-O
180 180 31 32 1.35
bond lengths, Å
Figure 5. Absorption (solid line, λ ) 285 (sh) nm, log ꢀ ) 3.64),
c
BA, S0 crystal
1.48
1.50
1.47
1.44
1.49
1.46
1.41 1.28
1.41 1.25
1.40 1.32
1.40 1.30
1.40 1.24
1.39 1.31
fluorescence (short dash, 77 K; dotted, 298 K), and phosphorescence
BA, S0 calcd
BA, S1 relaxed
BA, S1 opt
PI, S0 calcd
PI, S1 opt
180 180
(180) 119
95 77
0
0
7
2
0
1.39
1.42
1.41
1.42
(dash, 77 K) spectra of N-methylbenzanilide: (a) 0.1 mM in MTHF;
0.3
0.7
0
b) in microcrystals.
180 180
168 150 30 13 1.42
c
MBA, S0 crystal 14
48 60 1.36
44 65 1.40
MBA, S0 calcd
MBA, S1 opt
MDBA, S0 calcd
MDBA, S1 opt
19
101 79.0
16
62 44
2
1.49
1.43
1.49
1.48
1.42 1.25
1.41 1.30
1.42 1.25
1.47 1.32
2
6
1.41
62 59 1.40
51 1.462
7
3
a
Structures shown in Scheme 1. b Dihedral angles defined in Scheme
2
. c Crystal data from ref 9a.
remain planar (θ3 and θ4 e 7°). Less pronounced changes are
obtained for PI, values of ∆θ e 30° being observed for all of
the dihedral angles. Intermediate amide dihedral angles are
obtained for S1 in MDBA (θ1 ) 62° and θ2 ) 43°), along with
a small phenyl-carbonyl dihedral angle (θ3 ) 3°) and large
phenyl-nitrogen dihedral angle (θ4 ) 51°). For all four of the
amides, the calculated amide CdO bond length increases and
the Ph-CO bond length decreases for S1 vs S0. Changes in the
other bond lengths are dependent upon structure.
Figure 6. Absorption (solid line, λ ) 265 nm (sh), log ꢀ ) 3.11) and
phosphorescence (dash, 77 K) spectra of N-methyl-5,6,11,12-tetrahy-
drodibenz[b,f]azocine-6-one in MTHF (1 mM).
calculation are reported in Table 3 along with data for the crystal
structure of BA, which displays a planar amide group with
twisted phenyls. The barrier for 180° rotation about the amide
C-N bond with optimization at each dihedral angle was
calculated to be 6.5 kcal/mol with the Z isomer 1.3 kcal/mol
higher in energy than the E isomer (Figure 8).
The ground-state conformations of PI, MBA, and MDBA
calculated by AM1 are shown in Figure 7b-d, and their
structural parameters are reported in Table 3. The calculated
structure of PI, like that of BA, is fully planar (all dihedral angles
The singlet and triplet excited-state structures and energies
for BA have also been calculated for several restricted geom-
etries, including the calculated ground-state geometry (flat) and
several geometries in which the amide dihedral angle (θ1) is
fixed and all other structural parameters are minimized. The
results are shown in Figure 8. Starting with the planar ground-
state geometry (GS), the excited state geometry is allowed to
relax while retaining a totally planar structure (flat), and then
by fixing θ1 ) 180° but optimizing all other geometric
parameters (180). These small-amplitude geometric changes
decrease the S1 energy but increase the S0, S2, and T1 energies.
The minimized geometry for S1 with a fixed angle of θ1 ) 180°
is reported in Table 3 along with values for the other dihedral
angles and bond lengths, which are similar to those for the fully
optimized structure.
<2°). The calculated structure of MBA possesses a small amide
dihedral angle (θ1 ) 19° and θ2 ) 2°) but large phenyl-amide
angles (θ3 ) 44° and θ4 ) 66°). The structure of MDBA, like
that of MBA, is nonplanar, possessing moderate amide dihedral
angles, (θ1 ) 16° and θ2 ) 7°) and relatively large phenyl-
amide dihedral angles (θ3 ) 62° and θ4 ) 50°). Ground-state
geometries have also been optimized using Gaussian at the
3
-21G level. The results (not shown) are similar to those
obtained using AM1.
The optimized geometries of the lowest energy singlet states
of all four amides are reported in Table 3 and shown in Figure
With the optimized θ1 ) 180° S1 structure used as a starting
point, the value of θ1 is incrementally decreased to 10°, similar
to the ground-state value for MBA (Table 3). The results are
shown in Figure 8. The energy of the vibrationally excited
7a-d. The geometries were first optimized with the RCIS
/
0
method using a STO-3G basis set and then with a 3-21G basis
set (except in the case of MDBA). This two-step procedure was
necessary to obtain convergence for the 3-21G calculations. The
S1 geometries are notable in several respects. In the case of
BA and MBA, the amide group becomes nearly orthogonal (θ1
and θ2 ≈ 77°-100°) while the benzoyl and aniline groups
ground-state S is considerably higher than that of the opti-
mized ground-state S0 and changes only slightly with decreasing
θ1. The energy of S1 decreases as θ1 increases, reaching a
minimum value for θ1 ) 95°. The energy of T1 calculated using
the S1-minimized geometries is independent of the value of θ1.
The dependence of the T1 energy on θ1 has also been calculated

1980 J. Phys. Chem. A, Vol. 106, No. 10, 2002
Lewis and Liu
Figure 7. Optimized singlet ground-state and lowest excited-state geometry for BA (a), MBA (b), PI (c), and MDBA (d).
using the UHF method and displays a shallow minimum for a
twisted geometry (Figure 8).
Discussion
Benzanilide Fluorescence. We previously reported that the
fluorescence of BA at room temperature in MC solution consists
of a single broad band with a large Stokes shift (λmax ) 475
The calculated S0 f S1 transition energies of BA for the
ground-state, partially optimized (θ1 ) 180°), and fully opti-
mized S1 structures are reported in Table 4. The energies are
seen to decrease with increasing structural relaxation. The
transition energies of the fully optimized S1 structures for PI
and MBA are also reported in Table 4 along with the observed
values of the fluorescence maxima in the nonpolar solvent
methylcyclohexane (MC). The calculated values correspond to
7
nm). An even larger Stokes shift is observed for the fluores-
cence in MTHF solution (Figure 1a, λmax ) 520 nm). The
fluorescence decay time is shorter in MTHF (0.40 ns) than in
MC solution (0.86 ns). No fluorescence is observed at room
temperature in more polar nitrile or alcohol solvents. The
fluorescence decay of BA in both MC and MTHF solutions is
0
,0 transitions and thus are at higher energy than the values of
best fit by a single exponential (Table 2). Azumaya et al.4
λfl. Scaling the calculated energies by a factor of 1.5 provides
values that are in better agreement with the values of λfl.
ZINDO Calculations. We have previously employed semiem-
assigned the room-temperature fluorescence of BA to a single
excited state of TICT character on the basis of the absence of
Stokes-shifted fluorescence from analogues of BA that are
unable to undergo large-amplitude twisting about the amide
C-N bond. Our results are in agreement with this assignment.
BA displays strong fluorescence and phosphorescence at 77
K in MTHF with maxima at 430 and 416 nm, respectively.
Similar emission maxima were observed by O’Connell et al.1
in the more polar EPA glass. We have attributed the higher
energy of the fluorescence maximum in the glass vs solution
to the rigid nature of the glass, which prevents the large
amplitude geometric change necessary for formation of the
perical INDO/S SCF-CI calculations using the algorithm
11
developed by Zerner and co-workers to investigate the
electronic structure of the vertical (FC) excited singlet and triplet
states of BA. The results are summarized in Table 5 along with
the results of ZINDO calculations for the singlet and triplet states
of BA possessing the optimized S1 geometry. Twisting about
the amide bond is seen to result in a large decrease in S1, S2,
and T1 energies but only a small decrease in the S3 energy. All
of the twisted states have low oscillator strengths. Twisting is
seen to result in mixing of the character of the FC S1 and S3
states producing twisted singlet states with mixed n,B* and A,B*
character (see Table 5, footnote d). The character of the lowest
triplet changes from A,AB* to A,B* upon twisting. The ZINDO
calculated S1 energies for the optimized geometries of BA, PI,
and MBA are reported in Table 4 and are seen to be in
reasonable agreement with the scaled values obtained from RCIS
calculations.
8
relaxed TICT state. The fluorescence maximum in microcrys-
talline BA at both 77 and 298 K is similar to that in MTHF at
77 K (Table 1), in accord with the dependence of the Stokes
shift upon large amplitude geometry change.
To establish the connection between the room temperature
and 77 K luminescence of BA, a detailed investigation of the
temperature dependence was undertaken. MTHF was the solvent
of choice for these studies because it forms glasses with good

Luminescence of Benzanilides
J. Phys. Chem. A, Vol. 106, No. 10, 2002 1981
TABLE 4: Calculated and Observed S
1
State Transition
Energies for the Benzanilidesa
∆
E,
kcal
λ
calcd
nmb
,
λ
scaled
nmc
,
λ
ZINDO
nmd
,
λ,
nm
b
e
BA, FC
BA, relaxed
BA, opt
PI, opt
MBA, opt
132
99
90
106
89
217
290
320
269
322
326
434
479
403
483
430f
g
473
416
460
475
422
515
g
g
a
Structures shown in Scheme 1. b Singlet energy calculated using
c
d
Gaussian (RCIS method). Scaling factor of 1.5. Singlet energy
e
f
calculated using ZINDO. Experimental values. Data for MC at 77
K from ref 7. g Room-temperature data from ref 4.
TABLE 5: ZINDO-Calculated Energy, Wavelength,
Oscillator Strength, and Character of the Benzanilide
Excited States
BA statea
∆E, kcalb
λ
calcd, nmb
fc
characterd
S
S
S
S
S
S
1
1
2
2
3
3
, FC
, opt
, FC
, opt
, FC
, opt
79.9
60.4
102.1
86.0
103.6
101.4
68.1
358
473
280
332
276
282
420
634
0.0002
0.0002
0.08
0.0005
0.37
n,B*
e
n,B*; A,B*
A,A*
n,B*; A,B* f
A,B*
0.033
A,A*
A,AB*
A,B*
T
T
1
, FC
, opt
1
45.0
Figure 8. Calculated potential energy curves for benzanilide obtained
at various geometries. FC is the optimized planar ground-state geometry.
a
1
FC ) vertical (Franck-Condon), opt ) optimized S geometry.
The S
1
geometry is kept planar, but bond angles and bond lengths are
is fixed at 180°,
b
ZINDO-calculated energy and wavelength. c Oscillator strength. Char-
d
allowed to relax in flat. The amide dihedral angle θ
1
acter of the excited state, n ) nonbonding (CO), A ) aniline localized,
but all other bond lengths and bond angles are allowed to relax in
relaxed. With relaxed used as the starting point, the amide dihedral
e
B ) π benzoyl localized, AB ) π delocalized. 37% n,B*, 51% A,B*.
f
4
6% n,B*, 46% A,B*.
angle θ
1
is incrementally decreased from 180° to 20°. Solid lines are
optimized S
0
(9, AM1), S
1
(b, RCIS/3-21G), and T
1
(2, UHF/3-21G)
TABLE 6: Thermochromic Shiftsa
/
/
/
states. Dashed lines are the hot ground-state S (0), S (O), S (]),
0
2
3
low-temp shift,b
c
/
high-temp shift,
and T1 (4) states obtained from calculations using optimized S
1
-1
-1
cm /K
cm /K
geometries. Vertical arrows correspond to proposed geometries for
phosphorescence at 77 K and fluorescence at 77 and 298 K.
BA, S
BA, T
PI
1
-63
-12
-34
-29
13
1
optical properties and its physical properties are well-character-
ized over a broad temperature range.16,17 Moreover, all of the
1
11
MBA
a
Structures shown in Scheme 1. b Slope of Figure 3, T < 150 K.
amides remain soluble at low temperatures, thus avoiding
problems with amide aggregations, which have been reported
c
Slope of Figure 3, T > 150 K.
6
to occur in nonpolar glasses including MC. Warming the glass
from 77 to 160 K results in a continuous decrease in fluores-
cence intensity and a red shift in the emission maximum (Fig-
ures 2 and 3). On further warming a continuous blue shift
and increase in intensity are observed. The thermochromic
shifts obtained from the slopes of Figure 3 are summarized
in Table 6.
Two properties of the MTHF glass, its viscosity and dielectric
constant, are expected to affect the fluorescence maximum and
intensity. The viscosity of the glass decreases exponentially with
temperature. The increase in fluorescence intensity at higher
temperatures is consistent with the stronger fluorescence
observed in nonpolar vs polar solvents at room temperature. A
plot of νfl vs T at high temperatures (Figure 3) has a slope of
1
8
-1
13 cm /K. The thermochromic shift for BA in MTHF (Table
-
1
6) is smaller than the value of 21 cm /K reported for
1
8
4-dimethylaminobenzonitrile (DMABN). The excited-state
dipole moment for BA can be calculated from the thermochro-
mic shift, the ground-state dipole moment (µg ) 5.1 D), and
the estimated molecular radius (r ) 4.5 Å) using the method
2
0
3
increasing temperature from 3.7 × 10 P at 77 K to 2.1 × 10
1
8
P at 107.5 K, slightly below the glass-transition temperature
of Hagen et al. The resulting value of µe ) 15 D is somewhat
smaller than the value of µe ) 19.5 D obtained by this method
1
6
(
Tg ≈ 120 K). Nonexponential dependence of the viscosity
18
upon temperature is expected at higher temperatures. Below Tg,
the dielectric constant of MTHF has a constant value of ꢀ )
for the TICT state of DMABN. Thus, the relaxed singlet state
of BA can be characterized as possessing substantial ICT
character.
17
2
.6 , and thus, the shift in emission maximum should be
determined mainly by the solvent viscosity. A plot of νfl vs T
The phosphorescence behavior of BA in MTHF is also
temperature-dependent. The maximum displays a modest red
shift upon warming from 77 to 130 K, a plot of νph vs T having
a slope of 12 cm /K (Figure 3). This slope is much smaller
than that observed for the low-temperature fluorescence of BA,
suggesting that geometric relaxation is less extensive for the
triplet vs singlet state. No phosphorescence is observed at
temperatures greater than 130 K, plausibly because of diffusive
quenching of the long-lived triplet state by traces of oxygen or
other quenching impurities.
-
1
at low temperatures (Figure 3) has a slope of -63 cm /K.
Decreased viscosity should facilitate geometric relaxation of the
Franck-Condon excited state to the relaxed state, which emits
at longer wavelength.
Upon melting, the MTHF dielectric constant increases rapidly
to a maximum value of ꢀ ≈ 18 at 140 K and then continuously
decreases to a value of 7.0 at 298 K.17 The high solvent polarity
around 150 K should stabilize the geometrically relaxed TICT
state, thus accounting for the large red shift in λfl at this
-
1

1
982 J. Phys. Chem. A, Vol. 106, No. 10, 2002
Lewis and Liu
Benzanilide Potential Energy Surfaces. The low-tempera-
The calculated T1 potential energy surface differs from the
S1 surface in that the planar FC geometry is an energy minimum.
This is consistent with the relatively small thermochromic shift
observed for the phosphorescence maximum upon warming the
MTHF glass from 77 to 130 K (Figure 3).
ture viscosity dependence of BA luminescence suggests that
its fluorescence behavior is dependent upon the extent of
excited-state geometric relaxation. The calculated optimized
geometries of BA in the ground and lowest excited singlet states
are shown in Figure 7a, and the amide dihedral angles (Scheme
) and bond lengths are reported in Table 3. The calculated
gas phase) ground state is planar. In the crystal structure, BA
molecules are assembled into a one-dimensional hydrogen-
The effect of molecular geometry upon the luminescence
behavior of BA can be summarized with reference to Figure 8.
Electronic excitation populates the planar FC singlet state, which
can undergo intersystem crossing to yield the planar triplet state,
which emits with λph ) 416 nm. In rigid media (MTHF glass
or microcrystals), the FC singlet state undergoes small-amplitude
geometric relaxation effecting a decrease in the S1 energy and
an increase in the S0 energy. This results in emission with λfl
2
(
bonded tape in which both phenyl groups are twisted by ca.
9
3
0° from the amide plane. Crystal packing forces evidently
are sufficiently large to overcome the small barriers for twisting
about the phenyl-amide bonds. In solution, BA exists exclu-
sively (>99%) as the E isomer (Scheme 1). The calculated
)
430 nm, a slightly longer wavelength than λph (Figure 1). In
(
AM1) barrier for S0 (ground state) amide rotation is 6.5 kcal/
fluid solution, the lowest singlet state can undergo large-
amplitude geometric relaxation, resulting in a further red shift
in the fluorescence maximum to λfl ) 520 nm.
mol, and the E isomer is more stable than the Z isomer by 1.3
kcal/mol (Figure 8). This barrier is lower than that obtained
previously using MM2 calculations without optimization (16
Excited-State Electronic Configuration and Dynamics. We
previously reported the results of ZINDO calculations for the
planar Franck-Condon singlet and triplet states of BA (Table
7
kcal/mol) or the barrier estimated for MBA from NMR coales-
9
cence temperature for the E and Z isomers (13.3 kcal/mol).
The optimized S1 geometry has large amide dihedral angles
but small phenyl-amide dihedral angles (Table 3). Thus, the
planar benzoyl and anilide groups are approximately perpen-
dicular (Figure 7a). These changes are accompanied by a
decrease in the Ph-CO bond length and increases in the amide
CdO and C-N bond lengths. Twisting about the amide C-N
bond is the dominant large amplitude geometry change that
occurs upon relaxation of the planar Franck-Condon (FC)
singlet state to the optimized S1 geometry. The energy of the
FC excited-state S1 is found to decrease and the S0 energy to
increase as the planar geometry is allowed to relax in stages,
first by retaining a planar geometry but optimizing bond lengths
and angles (flat geometry) and then by retaining a dihedral angle
of θ1 ) 180° but removing all other geometric constraints (180°
relaxed geometry, Figure 8). Such small-amplitude geometry
changes plausibly can occur in the highly viscous MTHF glass
and the microcrystalline state and can account for the Stokes-
shifted fluorescence observed in these media (Figure 1).
5
). According to these calculations, the planar S1 state has
benzoyl-localized n,π* character (n,B*) similar to that of
acetophenone. The absence of fluorescence from the FC singlet
state is consistent with this assignment. The nearly-degenerate
S2 and S3 states have π,π* character, S2 being localized on
aniline (A,A*) and S3 being delocalized (A,B*). The character
of the π,π* T1 state is similar to that of S3. The relatively long
triplet lifetime (Table 2) and structured phosphorescence
observed in microcrystalline BA (Figure 1b) are consistent with
this assignment.
The two lowest energy excited singlet states with optimized
twisted geometry are predicted to have extensive configuration
interaction dominated by the n,B* and A,B* configurations. The
calculated singlet-state potential energy surfaces (Figure 8)
indicate the presence of an avoided crossing between the two
lowest singlet states that occurs upon low-amplitude geometric
relaxation. The relatively short singlet lifetime (τs ) 2.7 ns)
and occurrence of intersystem crossing at 77 K (Φisc g Φph )
Next, with the relaxed 180° geometry used as the starting
point, the effect of large-amplitude amide torsion was investi-
gated. The S1 energies decrease with increasing geometric
relaxation, reaching a minimum value for θ1 ) 95° (Figure 8,
bold line). The S0 energies calculated using the S1-minimized
geometries show little variation with θ1 (Figure 8, dashed line).
These S0 energies correspond to vibrationally excited ground-
state geometries and thus lie at higher energies than the AM1-
minimized ground state. The calculated S1-S0 energy gaps and
corresponding wavelengths for the FC, 180° relaxed, and fully
relaxed geometries of BA are reported in Table 3. Even for the
fully relaxed geometry, the calculated wavelength is consider-
ably shorter than the observed emission maxima in rigid media
0
.37) are consistent with the mixed n,π* and π,π* character of
the partially relaxed singlet state. On the assumption that
nonradiative decay at 77 K is dominated by intersystem crossing,
8
-1
a value of kisc ) 2.8 × 10 s can be estimated for BA (Table
1
). This value is distinctly smaller than that for the n,π* state
11 -1
of acetophenone (4 × 10 s ) but larger than that for the π,π*
7
-1 19,20
state of 1-phenylpropene (5 × 10 s ).
Solvation of the fully twisted singlet presumably enhances
the TICT character of the twisted singlet state in fluid solution.
This change is accompanied by a large decrease in the
fluorescence rate constant and increase in the nonradiative rate
constant (Table 1). These changes result in a much larger
decrease in Φfl than in τs at 298 vs 77 K.
(
Table 4). Application of a scaling factor of 1.5 to the calculated
singlet energies, provides scaled energies that correspond to
fluorescence maxima at 434 and 479 nm for the relaxed 180°
geometry and fully relaxed excited states, respectively. These
values are in good agreement with the emission maxima in rigid
media and in fluid solution. The singlet energy calculated for
the optimized singlet-state geometry using ZINDO (447 nm) is
similar to the scaled value. The appearance of the S1 potential
energy surface in Figure 8 indicates that there is no barrier for
large-amplitude twisting about the amide bond. The temperature
dependence of the fluorescence intensity and maxima below
Fluorescence of PI, a Planar E Analogue. According to
the AM1 calculations, PI is planar in the ground state with the
two phenyl groups fixed in the E configuration (Figure 7b).
Twisting about the amide bond is restricted by the lactam ring.
4
Azumaya et al. observed that the fluorescence maximum for
PI in MC solution occurs at 422 nm, considerably blue-shifted
with respect to the emission of BA (477 nm in MC). In addition,
unlike BA, which is nonfluorescent in ethanol solution, more
intense emission with a maximum of 442 nm is observed for
4
PI in ethanol vs MC solution. The behavior of PI was attributed
1
50 K are consistent with the presence of a small, viscosity-
to its rigid structure, which precludes extensive twisting about
imposed barrier for large-amplitude amide twisting.
the amide C-N bond. We observe a fluorescence maximum at

Luminescence of Benzanilides
25 nm for PI in MTHF at 298 K (Figure 4), intermediate
J. Phys. Chem. A, Vol. 106, No. 10, 2002 1983
4
the relaxed and FC S1 geometries. The high-temperature shift
is similar to that for BA, indicative of TICT states with similar
dipole moments for MBA and BA. No fluorescence is observed
for MDBA in MTHF at 77 K or in the microcrystalline solid.
between the values for MC and for ethanol solution.
1
O’Connell et al. observed fluorescence from PI in the polar
mixed solvent EPA both at room temperature and in a rigid
glass at 77 K. Upon warming from 77 K to room temperature,
the emission maximum shifts from 410 to 450 nm and the
intensity decreases ca. 50-fold. We observe emission maxima
at 400 nm for PI in a MTHF glass and at 405 nm in
microcrystalline PI both at 77 and 298 K (Figure 4, Table 1).
Upon warming the glass from 77 to 100 K, there is a decrease
in the fluorescence intensity and a red shift in the maximum
from 400 to 430 nm (Figure 3) attributed to small-amplitude
geometric relaxation. The thermochromic shift for PI (Table 6)
is approximately half that of BA. Above Tg, the emission
intensity continues to decrease with increasing temperature, but
unlike BA, there is little further red shift. The absence of
thermochromism at higher temperature indicates that the singlet
state of PI is relatively nonpolar, in accord with its inability to
form a TICT state.
According to the RCIS-minimized calculations, the PI singlet
state is less planar than the ground state (Figure 7b). Deforma-
tion of the lactam ring is pronounced, with changes in both
amide dihedral angles, θ1 and θ2, and in the phenyl-CO dihedral
angle, θ3 (Table 2). However, the phenyl-amide dihedral angle,
θ4, remains small. Thus, we assign the small red shift observed
upon warming of the MTHF glass to geometric relaxation of
the lactam ring. The scaled value of the PI S0-S1 energy gap
calculated using the minimized S1 geometry corresponds to 403
nm, and the value obtained from a ZINDO calculation is 416
nm, both slightly shorter wavelengths than the 422 nm emission
maximum in MC solution.
The minimized geometry of the lowest singlet state of MBA
is shown in Figure 7c. As is the case for BA, the amide group
is twisted in the singlet state (θ ) 101° and θ ) 79°) and the
1
2
benzoyl and aniline groups are more nearly planar than in the
ground state (θ ) 2° and θ ) 6°). Thus, large amplitude
3
4
motions are necessary to convert the FC excited state to the
relaxed singlet. The smaller Stokes shift observed at 77 K in
crystalline MBA than in MTHF glass may reflect the greater
rigidity of the former environment. Assuming that the S1
potential energy surface for MBA resembles that for BA (Figure
8), there should be no barrier other than that imposed by the
medium for amide rotation from the FC state (θ ) 19°) to the
1
fully optimized S geometry (θ ) 101°).
1
1
The minimized geometry of the singlet state of MDBA is
shown in Figure 7d. The amide dihedral angles are smaller than
those for MBA (θ ) 62° and θ ) 43°), presumably due to
1
2
restricted torsion in the dibenzazocineone ring system.15 The
benzoyl group dihedral angle is small (θ ) 3.0°), as is the
3
case for the other benzanilides; however, the phenyl-nitrogen
dihedral angle is much larger (θ ) 51°) than for the other
4
benzanilides.
The singlet decay time for MBA at 77 K is similar to that of
BA in both MTHF and the crystalline state. However, both the
fluorescence quantum yield and rate constant are considerably
smaller for MBA. This may reflect the fact that the less-planar
MBA has smaller Franck-Condon factors for radiative decay.
Nonradiative decay rates are similar for BA and MBA at both
77 and 298 K. Our failure to detect fluorescence from the
nonplanar MDBA could reflect a low value of kfl.
The singlet lifetime of PI at 77 K is somewhat longer than
that of BA, as a consequence of its slower nonradiative rate
constant (Table 1). The fluorescence rate constant for PI is
similar to that for BA, resulting in a larger value of Φfl for PI.
At 298 K, the singlet lifetime of PI is too short to measure with
All of the benzamides display phosphorescence at 77 K both
in MTHF glasses and in microcrystalline solids. Phosphores-
cence in MTHF is structureless, whereas the phosphorescence
of crystalline BA and PI is structured. The E amides, BA and
PI, which are expected to have planar triplet states, have similar
4
our lifetime apparatus (<0.1 ns). Azumaya et al. report a
lifetime of less than 30 ps in MC solution.
Fluorescence of MBA and MDBA, Nonplanar Z Ana-
logues. The AM1-calculated ground-state structure of MBA is
highly nonplanar (Figure 7c) and is similar to its crystal structure
values of λph and τ in MTHF glasses (Table 2). The longer
wavelength emission of MBA may reflect either differences in
T
geometry or the effect of N-methylation on the frontier orbital
energies. The higher energy of the MDBA phosphorescence is
somewhat surprising in view of the similarity of its ground-
state structure to that of MBA (Table 3). The more rigid structure
of MDBA may break the conjugation between the benzoyl and
aniline groups. The value of λph for MDBA is, in fact, similar
to that for benzamide at 77 K in a MTHF glass. Like MDBA,
benzamide is nonfluorescent at both 77 and 298 K in MTHF.
9
a
(Table 3). Both the calculated and observed amide dihedral
angles are small (θ1 ) 15° and θ2 ) 2°), whereas both phenyl-
amide dihedral angles are large (θ3 ) 45° and θ4 ) 62°). This
is consistent with larger amide vs benzoyl or aniline resonance
energies. The calculated ground-state structure of MDBA
(Figure 7d) is similar to that of MBA, with small amide dihedral
angles (θ1 ) 16° and θ2 ) 7°) and large phenyl-amide dihedral
angles (θ3 ) 62° and θ4 ) 60°).
Concluding Remarks. The combination of experimental
investigation of the temperature-dependent luminescence of BA
and several analogues with computational studies of the ground-
and excited-state potential energy surfaces and electronic
configuration has permitted further elucidation of the complex
excited-state behavior of these molecules. Studies of the
temperature dependence of BA luminescence over the entire
77-298 K temperature range serve to connect the results of
previous studies obtained at a single temperature. The thermo-
chromic behavior of BA displays two distinct temperature
regimes: a low-temperature regime in which decreasing viscos-
ity of the MTHF glass permits increased geometric relaxation,
resulting in a red shift and decreased fluorescence intensity and
a high-temperature regime in which decreasing solvent polarity
results in a blue shift and increased fluorescence intensity. An
excited-state dipole moment of 15 D is estimated from the high-
Very weak Stokes-shifted fluorescence is observed for MBA
in fluid solution with fluorescence maxima of 518 nm in MC
and 550 nm in MTHF. This emission was attributed by
Azumaya et al. to a TICT state. No fluorescence is observed
for MBA at 298 K in polar solvents. MDBA is nonfluorescent
in both nonpolar and polar solvents.
4
The fluorescence maximum of MBA in MTHF at 77 K is at
6
5
00 nm, similar to the value observed by Heldt et al. for MC
solution. The fluorescence maxima in microcrystalline MBA
at both 77 and 298 K are at shorter wavelengths, 460 and 480
nm, respectively (Table 1). Upon warming the MTHF glass from
7
7 to 343 K, changes in fluorescence maxima and intensities
similar to those for BA are observed (Figure 3). The low-
temperature thermochromic shift is considerably smaller than
that for BA (Table 6), indicative of a smaller difference between

1984 J. Phys. Chem. A, Vol. 106, No. 10, 2002
Lewis and Liu
temperature thermochromism data. In contrast, BA phospho-
rescence shows only modest thermochromic shifts at low
temperatures, and no phosphorescence is observed in fluid
solution.
References and Notes
(
1) O’Connell, E. J., Jr.; Delmauro, M.; Irwin, J. J. Photochem.
Photobiol. 1971, 14, 189.
2) Tang, G.-Q.; MacInnis, J.; Kasha, M. J. Am. Chem. Soc. 1987,
(
These results can be understood using the calculated ground-
and excited-state potential energy surfaces and excited-state
configurations. The geometry of the π,π* triplet state is similar
to that of the ground state, and thus, its emission maximum
changes only slightly as the MTHF glass is warmed from 77
K. In contrast, the Franck-Condon n,π* singlet state undergoes
small-amplitude geometric changes even in the 77 K glass or
crystalline solid. The resulting decrease in S1-S0 energy gap
shifts the fluorescence maximum to longer wavelength than the
phosphorescence maximum. Above the solvent glass transition
temperature, the S1 state can undergo large-amplitude twisting
about the amide C-N bond resulting in the formation of a TICT
state that is responsible for the weak Stokes-shifted fluorescence
observed in fluid solution. The luminescence behavior of PI,
MBA, and MDBA can also be correlated with their ground-
and excited-state geometries. Small-amplitude singlet-state
geometric relaxation occurs even for the cyclic analogue PI.
The potential energy surfaces for BA bear some similarity
109, 2531-2533.
(3) Heldt, J.; Gormin, D.; Kasha, M. J. Am. Chem. Soc. 1988, 110,
8
255-8256.
4) Azumaya, I.; Kagechika, H.; Fujiwara, Y.; Itoh, A.; Yamaguchi,
K.; Shudo, K. J. Am. Chem. Soc. 1991, 113, 2833-2838.
(
(
(
5) Lucht, S.; Stumpe, J.; Rutloh, M. J. Fluoresc. 1998, 8, 153-166.
6) Heldt, J.; Heldt, J. R.; Szatan, E. J. Photochem. Photobiol., A 1999,
121, 91-97.
(7) Lewis, F. D.; Long, T. M. J. Phys. Chem. A 1999, 102, 5327.
(8) Lewis, F. D.; Liu, W.-Z. J. Phys. Chem. A 1999, 103, 9678.
(9) (a) Itai, A.; Toriumi, Y.; Tomioka, N.; Kagechika, H.; Azumaya,
I.; Shudo, K. Tetrahedron Lett. 1989, 30, 6177. (b) Azumaya, I.; Yamaguchi,
K.; Kagechika, H.; Saito, S.; Itai, A.; Shudo, K. Yakugaku Zasshi 1994,
114, 414. (c) Azumaya, I.; Kagechika, H.; Yamaguchi, K.; Shudo, K.
Tetrahedron 1995, 51, 5277.
(10) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi,
I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M.
W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon,
M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.7; Gaussian,
Inc.: Pittsburgh, PA, 1998.
2
1
to the well-characterized surfaces for the stilbenes in that
twisting about the central C-N or CdC bond results in a
maximum on the ground state and shallow minima in the excited
singlet and triplet surfaces. There are, however, several signifi-
cant differences in the behavior of the benzamides and stil-
2
1
benes. First, the relaxed BA singlet undergoes intersystem
crossing at 77 K, whereas the stilbene singlet does not. This
difference plausibly reflects the n,π* character of the relaxed
BA singlet. Second, there is a small barrier for twisting of singlet
stilbene but not for twisting of BA, resulting in the observation
of fluorescence from the FC state of stilbene but not from BA.
Third, the twisted singlet of stilbene is nonfluorescent as a
consequence of its very short singlet lifetime, whereas twisted
BA is weakly fluorescent and has a moderately long singlet
lifetime. This difference may reflect the higher S0 energy for
twisted stilbene, which results in a smaller S0-S1 energy gap.
Presumably, BA, like stilbene, undergoes efficient E,Z photo-
isomerization. However, the low barrier for BA thermal isomer-
ization (Figure 8) would require the use of low-temperature
methods to detect the Z isomer of BA or the E isomer of MBA.
(11) Zerner, M. C.; Loew, G. H.; Kirchner, R. F.; Mueller-Westerhoff,
U. T. J. Am. Chem. Soc. 1980, 102, 589.
(12) Li, R.; Lim, E. C. J. Chem. Phys. 1972, 57, 605.
(
13) James, D. R.; Siemiarczuk, A.; Ware, W. R. ReV. Sci. Instrum.
1
992, 63, 1710.
(14) Grigg, R.; Gunaratne, H. Q. N.; Sridharan, V. J. Chem. Soc., Chem.
Commun. 1985, 17, 1183.
(15) Chimirri, A.; Grasso, S.; Romeo, G.; Zappala, M.; Fenech, G.
Heterocycles 1985, 23, 895.
(16) Furutsuka, T.; Imura, T.; Kojima, T.; Kawabe, K. Technol. Rep.
Osaka UniV. 1974, 367.
(17) Ling, A. C.; Willard, J. E. J. Phys. Chem. 1968, 72, 1018.
(18) Hagan, T.; Pilloud, D.; Suppan, P. Chem. Phys. Lett. 1987, 139,
499.
(19) McGarry, P. F.; Doubleday, C. E., Jr.; Wu, C.-H.; Staab, H. Z.;
Turro, N. J. J. Photochem. Photobiol. A 1994, 77, 109.
20) Lewis, F. D.; Bassani, D. M.; Caldwell, R. A.; Unett, D. J. J. Am.
Chem. Soc. 1994, 116, 10477.
(
(
21) (a) Waldeck, D. H. Chem. ReV. 1991, 91, 415. (b) Photochromism,
Acknowledgment. Funding for this project was provided
Molecules and Systems; Saltiel, J., Sun, Y.-P., Eds.; Elsevier: Amsterdam,
by NSF Grants CHE-9734941 and CHE-0100596.
1990.