Structures of (2E,5E)-2-(4-cyanobenzylidene)-5-(4-dimethylaminobenzylidene)cyclopentanone and (2E,5E)-2-benzylidene-5-cinnamylidenecyclopentanone

Article abstract of DOI:10.1016/j.molstruc.2017.06.032

The X-ray crystal structures of (2E,5E)-2-(4-cyanobenzylidene)-5-(4-dimethylaminobenzylidene)cyclopentanone (I) and (2E,5E)-2-benzylidene-5-cinnamylidenecyclopentanone (II) are presented, compared to the gas phase structures calculated using density funct

Full text of DOI:10.1016/j.molstruc.2017.06.032

Accepted Manuscript
Structures of (2E,5E)-2-(4-cyanobenzylidene)-5-(4-
dimethylaminobenzylidene)cyclopentanone and (2E,5E)-2-benzylidene-5-
cinnamylidenecyclopentanone
Christopher A. Zoto, John C. MacDonald
PII:
S0022-2860(17)30811-6
10.1016/j.molstruc.2017.06.032
MOLSTR 23918
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 20 March 2017
Revised Date: 7 May 2017
Accepted Date: 7 June 2017
Please cite this article as: C.A. Zoto, J.C. MacDonald, Structures of (2E,5E)-2-(4-
cyanobenzylidene)-5-(4-dimethylaminobenzylidene)cyclopentanone and (2E,5E)-2-benzylidene-5-
cinnamylidenecyclopentanone, Journal of Molecular Structure (2017), doi: 10.1016/
j.molstruc.2017.06.032.
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ACCEPTED MANUSCRIPT
The X-ray crystal structures of (2E,5E)-2-(4-cyanobenzylidene)-5-(4-
dimethylaminobenzylidene)cyclopentanone (I) and (2E,5E)-2-benzylidene-5-
cinnamylidenecyclopentanone (II) are presented, compared to the gas phase structures calculated
using density functional theory, and discussed in the context of the photophysical behavior
ꢀ
exhibited by I and II. Compound I crystallizes in the triclinic space group P1 with a = 6.8743(2)
Å, b = 8.8115(2) Å, c = 14.9664(4) Å, = 77.135(2)°, = 81.351(2)°, = 80.975(2)°, and Z = 2,
and exhibits a planar structure. Compound II crystallizes in the monoclinic space group C2/c
with a = 33.4281(10) Å, b = 11.9668(4) Å, c = 7.8031(2) Å, = 92.785(2)°, and Z = 8, and
adopts a nonplanar structure in the solid state and calculated structure.
α
β
γ
β

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Structures of (2E,5E)-2-(4-cyanobenzylidene)-5-(4-
dimethylaminobenzylidene)cyclopentanone and (2E,5E)-2-benzylidene-5-
cinnamylidenecyclopentanone
Christopher A. Zoto*, John C. MacDonald
Department of Chemistry and Biochemistry, Worcester Polytechnic Institute, 100 Institute Road,
Worcester, MA 01609, USA
*Email address: czoto@hotmail.com; Telephone: (508)-233-5084
Keywords: 2,5-diarylidenecyclopentanone, X-ray crystal structure, DFT, crystal packing,
spectroscopic, photophysical
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Abstract
The X-ray crystal structures of (2E,5E)-2-(4-cyanobenzylidene)-5-(4-
dimethylaminobenzylidene)cyclopentanone (I) and (2E,5E)-2-benzylidene-5-
cinnamylidenecyclopentanone (II) are presented, compared to the gas phase structures calculated
using density functional theory, and discussed in the context of the photophysical behavior
ꢀ
exhibited by I and II. Compound I crystallizes in the triclinic space group P1 with a = 6.8743(2)
Å, b = 8.8115(2) Å, c = 14.9664(4) Å, = 77.135(2)°, = 81.351(2)°, = 80.975(2)°, and Z = 2,
and exhibits a planar structure. Compound II crystallizes in the monoclinic space group C2/c
with a = 33.4281(10) Å, b = 11.9668(4) Å, c = 7.8031(2) Å, = 92.785(2)°, and Z = 8, and
α
β
γ
β
adopts a nonplanar structure in the solid state and calculated structure.
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1. Introduction
The class of organic compounds called 2,5-diarylidenecyclopentanones have received
attention in various types of applications, including their use as fluorescent probes for solvent
polarity [1], fluoroionophores [2], nonlinear optical materials [3], and photosensitizers [4]. Our
previous investigations of (2E,5E)-2-(4-cyanobenzylidene)-5-(4-
dimethylaminobenzylidene)cyclopentanone (I) [5], symmetrically-unsubstituted 2,5-
diarylidenecyclopentanones [6, 7], and alkylamino-substituted 2-arylidene- and 2,5-
diarylidenecyclopentanones [8] in solution have shown that the spectroscopic and photophysical
properties of these compounds change to varying degrees with respect to solvent polarity. In
particular, nonsymmetrically-substituted cyclopentanones such as compound I and alkylamino-
substituted 2-arylidene- and 2,5-diarylidenecyclopentanones [8] exhibit solvatochromism and
photoinduced internal charge transfer (ICT) due to the push-pull effect of the electron-donating
and electron-withdrawing groups through the conjugated backbone. It was demonstrated through
experimental and computational work that both I [5] and the alkylamino-substituted 2,5-
diarylidenecyclopentanones [8] undergo photoinduced ICT. The ICT state is an excited state
characterized by the transfer of electron density from the electron donor end of the molecule to
the electron acceptor end of the molecule upon photoexcitation. The nature of ICT excited states
gives rise to lower energies of fluorescence than the locally excited (LE) states, such that the
fluorescence of an ICT state is red shifted relative to the LE state. Furthermore, we have shown
that (2E,5E)-2,5-bis(4-dimethylaminobenzylidene)cyclopentanone acts as a direct
photosensitizer of singlet state oxygen (1∆g), a highly electrophilic oxygen species, and reacts
with 1∆g in oxygenated solutions, in addition to undergoing (E,E) ➔ (E,Z) photoisomerization in
deoxygenated solutions [9].
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Here, we present the crystal structures of two nonsymmetrically-substituted
cyclopentanones, (2E,5E)-2-(4-cyanobenzylidene)-5-(4-
dimethylaminobenzylidene)cyclopentanone (I) and (2E,5E)-2-benzylidene-5-
cinnamylidenecyclopentanone (II), shown in Figure 1, and the corresponding structures
predicted by DFT. Our previous work comparing calculated DFT structures of structurally-
related substituted cyclopentanones to the corresponding experimental crystal structures has
revealed that DFT generally is able to predict the LE structures of nonsymmetrically-substituted
cyclopentanones quite accurately [8]. Establishing a computational method such as DFT that can
predict the structures of substituted cyclopentanones reliably is important in the context of our
work not only for cases where the LE structures of substituted cyclopentanones cannot be
determined experimentally using X-ray diffraction, but also because knowing the structures,
relative energies, and changes in the electronic distributions of ꢁ electrons in the molecular
orbitals of ground state HOMOs and excited state LUMOs is essential for understanding both the
photophysical and solvatochromic properties of substituted cyclopentanones, like that reported in
[8]. This work was carried out to determine whether knowledge of the molecular structures of I
and II would provide further insight toward understanding the spectroscopic and photophysical
properties of these class of compounds as well as their chemical reactivity and mechanistic
behavior in single- and multi-component reactions.
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2. Experimental
All reagents and solvents were purchased from Sigma-Aldrich, Alfa Aesar, and Pharmco
and were used without further purification. Purity of all intermediate and target compounds was
confirmed by thin layer chromatography. The percent yields of the intermediate and target
compounds were not reported at the time the synthetic workups were carried out but were found
to range between moderate to high (50% - 100%). NMR spectra were obtained with a Bruker®
AVANCE III 500 (500 MHz) NMR spectrometer. ATR-IR spectra were measured using a
Perkin-Elmer® Spectrum One IR spectrometer.
Synthesis and structural characterization of compounds I and II
Compound I was synthesized in two steps (Figure 2(a)). The first step involved
synthesizing the precursor intermediate (E)-2-(4-dimethylaminobenzylidene)cyclopentanone
(IA). Cyclopentanone (0.42 g, 5.0 mmol), 4-dimethylaminobenzaldehyde (0.75 g, 5.0 mmol),
and N,N-dimethylammonium-N’,N’-dimethylcarbamate (DIMCARB) (27.5 mmol, 3.7 g) in
CH2Cl2 (5.5 mL) were stirred for 12 hours at RT. The solvent was then removed under reduced
pressure, the organic phase was extracted with CH2Cl2, and the combined organic layer extracts
were dried over anhydrous sodium sulfate, filtered, and evaporated under reduced pressure. The
crude product residue was purified by silica gel column chromatography using a gradient
approach of hexanes/ethyl acetate, beginning with 100% hexanes, gradually increasing the
eluting solvent polarity with ethyl acetate, to afford pure IA as a yellow solid. 1H NMR (CDCl3):
δ (ppm) 1.94 (p, 2H), 2.30 (t, 2H), 2.87 (td, 2H), 2.95 (s, 6H), 6.62 (d, 2H), 7.28 (s, 1H), 7.38 (d,
2H); 13C NMR (CDCl3): δ (ppm) 19.1, 28.4, 28.7, 36.8, 39.1, 110.8, 122.3, 130.0, 131.5, 132.3,
149.9, 207.1.
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The second step involved a base-catalyzed crossed-aldol condensation of an equimolar
amount of IA (0.45 g, 2.1 mmol) and 4-cyanobenzaldehyde (0.27 g, 2.1 mmol) in MeOH
containing 2.5 % (w/v) NaOH (0.6 mL). A red precipitate emerged immediately from the
reaction mixture, which was then stirred for 12 hours at RT. The precipitate was collected by
vacuum filtration, washed with cold MeOH, and then purified by silica gel column
chromatography using a gradient approach of hexanes/ethyl acetate, beginning with 100%
hexanes, gradually increasing the eluting solvent polarity with ethyl acetate, to afford pure I as a
red solid. Crystals of I decomposed at ~260°C prior to melting. 1H NMR (CDCl3): δ (ppm) 2.99
(s, 6H), 3.03 (s, 4H), 6.66 (d, 2H), 7.42 (s, 1H), 7.47 (d, 2H), 7.55 (s, 1H), 7.60 (d, 2H), 7.62 (d,
2H); 13C NMR (CDCl3): δ (ppm) 25.56, 25.61, 39.1, 110.7, 110.9, 117.7, 122.4, 128.6, 129.6,
130.5, 131.3, 132.1, 135.4, 139.7, 140.9, 150.3, 194.3. IR: ν (cm-1) 2907, 2224, 1682, 1612,
1574, 1528, 1444, 1414, 1372, 1321, 1285, 1254, 1231, 1180, 1165, 1119, 1059, 988, 944, 828,
812.
Compound II was synthesized in two steps (Figure 2(b)). The first step involved
synthesizing the precursor intermediate (E)-2-benzylidenecyclopentanone (IIA). Cyclopentanone
(0.42 g, 5.0 mmol), benzaldehyde (0.53 g, 5.0 mmol), and DIMCARB (27.5 mmol, 3.7 g) in
CH2Cl2 (5.5 mL) were stirred for 12 hours at RT. The solvent was then removed under reduced
pressure. To the residue was added 0.5 M H2SO4 (10 mL) and extracted with ethyl acetate. The
combined organic layer extracts were dried over anhydrous sodium sulfate, filtered, and
evaporated under reduced pressure. The crude product residue was purified by silica gel column
chromatography using a gradient approach of hexanes/ethyl acetate, beginning with 100%
hexanes, gradually increasing the eluting solvent polarity with ethyl acetate, to afford pure IIA as
a yellow solid. 1H NMR (CDCl3): δ (ppm) 1.96 (p, 2H), 2.34 (t, 2H), 2.91 (td, 2H), 7.29-7.36 (m,
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4H), 7.46 (d, 2H); 13C NMR (CDCl3): δ (ppm) 20.2, 29.4, 37.8, 128.7, 129.3, 130.5, 132.3,
135.5, 136.1, 208.1.
The second step involved a base-catalyzed crossed-aldol condensation of an equimolar
amount of IIA (0.75 g, 4.3 mmol) and (E)-cinnamaldehyde (0.57 g, 4.3 mmol) in EtOH
containing 2.5 % (w/v) NaOH (2.0 mL). A yellow precipitate emerged immediately from the
reaction mixture, which was then stirred for 14 hours at RT. The precipitate was collected by
vacuum filtration, washed with cold EtOH, and dried. Recrystallization from EtOH (2x) afforded
pure II as yellow flakes. Crystals of II melted at 192°C. 1H NMR (CDCl3): δ (ppm) 2.85-2.92
(m, 2H), 3.00-3.05 (m, 2H), 6.92-6.94 (m, 2H), 7.24 (d, 2H), 7.30 (t, 3H), 7.37 (t, 3H), 7.44-7.46
(m, 3H), 7.52 (d, 2H). 13C NMR (CDCl3): δ (ppm) 24.4, 26.2, 124.7, 127.3, 128.8, 129.1, 129.3,
130.7, 133.3, 135.9, 136.5, 138.5, 138.6, 141.6, 195.7. IR: ν (cm-1) 3059, 1682, 1624, 1590,
1514, 1491, 1465, 1447, 1338, 1281, 1240, 1181, 1154, 1104, 969, 919, 894, 837.
Determination of the crystal structures of I and II
Suitable single crystals of I and II were mounted on a Bruker-AXS Kappa APEX CCD
diffractometer. Diffraction data were collected at RT using graphite monochromated Mo-Kα
radiation (λ = 0.71073 Å) by the omega-scan technique. Empirical absorption corrections were
applied using the SADABS program [10]. The unit cells and space groups were determined
using the SAINT+ program [11]. Structures were solved by direct methods and refined by full
matrix least-squares using the SHELXTL program [12]. Refinement was based on F2 using all
reflections. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms on carbon
atoms were all located in the difference maps and subsequently placed at idealized positions and
given isotropic U values 1.2 times that of the carbon atom which they were bonded. Both
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structures were refined to R1 values of < 5%. Mercury 2.4 software was used to examine the
molecular structures and crystal packing [13].
Quantum chemical calculations using density functional theory
Gaussian 09® [14] was used to perform DFT and TD-DFT calculations, carried out at the
B3LYP/6-311+G(d,p) (I) and B3LYP/6-31G(d) (II) levels of theory. Geometry optimizations
were carried out in the gas phase and minimum energy structures were confirmed by obtaining
all positive frequencies for the calculated modes of vibration. The DFT gas phase geometry was
used in all solvent calculations without further optimization. Solvent effects were computed
using the Self-Consistent Reaction Field Polarizable Continuum Model (SCRF PCM) options.
For TD-DFT, the first five lowest excited singlet and triplet states were calculated and the
excited state orbital configurations and excited state energies were determined. Only the first
three lowest excited singlet states (S1, S2, and S3, respectively) are reported in this manuscript.
3. Results and Discussion
Crystal structures and DFT structures of I and II
The crystal structures and crystal packing of I and II with anisotropic displacement
parameters and atomic numbering are shown in Figure 3. A summary of the crystallographic data
and refinement parameters of I and II is given in Table 1. Compound I crystallizes in the triclinic
ꢀ
space group P1 with a = 6.8743(2) Å, b = 8.8115(2) Å, c = 14.9664(4) Å,
81.351(2)°, = 80.975(2)°, and Z = 2. The molecular structure of I is fully extended in the (E,E)
conformation and is essentially flat with the central cyclopentanone ring and phenyl rings on
α = 77.135(2)°, β =
γ
both ends of the molecule nearly coplanar. The torsion angles within the cyclopentanone ring are
3.0° (C9-C10-C11-C12), -2.7° (C13-C9-C10-C11), and -2.4° (C13-C12-C11-C10) such that the ring is
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essentially flat with the saturated carbons exhibiting no significant torsional buckling. The angles
between the mean plane defined by the central cyclopentanone ring and methyl groups (C8-C14)
and the mean planes of the amino-substituted and nitrile-substituted phenyl rings are 4.3° and
2.1°, respectively. Phenyl rings in dibenzylidene compounds such as I generally are expected to
be coplanar or close to coplanar with conjugated backbone in order to promote orbital overlap
and optimize conjugation throughout the ꢁ system. For example, the cyclopentanone and phenyl
rings in the crystal structure of the unsubstituted analogue of I, (2E,5E)-2,5-
dibenzylidenecyclopentanone, are coplanar with no torsional distortion along the conjugated
backbone [15]. Similar to that structure, eclipsing of the methylene groups on the
cyclopentanone ring in I allows the phenyl rings to remain coplanar by minimizing repulsive
interactions between the methylene hydrogen atoms and ortho hydrogen atoms on the phenyl
rings, as shown in the space-filling model on the left in Figure 3c.
Molecules of I pack at van der Waals separation in the unit cell with no short contacts of
consequence. Molecules stack on top of one another along the b axis, [010], where alternating
molecules within stacks are related by inversion (Figure 3d). That packing arrangement positions
the dimethylamino-substituted phenyl ring over the cyano-substituted phenyl ring of adjacent
neighbors within a stack, as shown in Figure 3e. Alternate stacking of electron-rich and electron-
deficient phenyl rings overlapping at a separation of 3.6 Å suggests that ꢁ donor-acceptor
interactions between the lowest-energy empty molecular orbital (LUMO) and highest-energy
filled molecular orbital (HOMO) of I should play a role influencing molecular aggregation
during nucleation of crystals to favor centrosymmetric packing. Shown in Figure 4 are the
molecular orbitals for the LUMO and HOMOs of compound I calculated using TD-DFT. Taking
into consideration that the LUMO (ꢁ*) resides predominantly on the cyclopentanone and cyano-
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substituted phenyl rings, and the HOMO (ꢁ) resides predominantly on the cyclopentanone and
dimethylamino-substituted phenyl ring, it is not surprising to observe that adjacent molecules of
I adopt a centrosymmetric packing arrangement at least in part to maximize favorable overlap
between the LUMO and HOMO. Kitaigorodski also has shown that molecules related via a
center of symmetry usually pack more efficiently compared to molecules related by other
symmetry operators [16].
As shown in Table 2, comparison of the molecular geometries obtained from the
experimental crystal structure and the calculated DFT ground state structure shows good
agreement between bond lengths and bond angles as a whole. Differences between experimental
and calculated bond lengths varied between 0.001 Å – 0.023 Å and bond angles between 0.01° –
3.5°. The calculated DFT geometry for I predicts a planar conjugated backbone and both phenyl
rings rotated out of the plane of the central cyclopentanone ring by just 6° such that the
conformation of DFT structure is consistent with that of the crystal structure. That geometry also
is consistent with the structures of symmetrically-disubstituted cyclopentanones determined
previously using DFT calculations [8].
Compound II crystallizes in the monoclinic space group C2/c with a = 33.4281(10) Å, b
= 11.9668(4) Å, c = 7.8031(2) Å,
β = 92.785(2)°, and Z = 8. As shown on the right in Figure 3,
the molecular structure of II has a twisted rather than planar conformation with both phenyl rings
rotating out of the plane of the conjugated backbone. The mean planes of the phenyl rings in the
benzylidene and cinnamylidene substituents rotate significantly out of the mean plane defined by
the central cyclopentanone and acyclic carbon atoms (C1-C8 and C15) by 43.6 ° and 20.6°,
respectively. The torsion angles within the cyclopentanone ring are -20.1° (C1-C2-C3-C4), -17.5°
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(C1-C5-C4-C3), and 22.8° (C2-C3-C4-C5), reflecting significant buckling of the central ring. The
mean planes of the phenyl groups on either end are twisted with respect to one another by 59.3°.
Considering that compound II contains no obvious structural features that might cause
repulsive steric interactions, major twisting of the phenyl ring on the benzylidene substituent
likely is due to intermolecular interactions caused by crystal packing. Molecules pack at van der
Waals separation in the unit cell with no intermolecular contacts less than the VDW contact
distances. The molecules stack with alternating molecules related by glide symmetry along the c
axis, [001], as shown in Figure 3d. Within a stack, molecules align in parallel positioned with the
benzylidene groups overlapping and the cinnamylidene groups overlapping. Phenyl rings on the
benzylidene substituents align edge-to-face forming a C-H...ꢁ interaction involving C17-H17 on
one phenyl group pointing toward the ꢁ cloud on an adjacent ring, C16-C21, as shown in Figure
3e. The contact distances of C17…centroid of ring = 3.9 Å and H17…centroid of ring = 3.1 Å are
consistent with C-H...ꢁ edge-to-face interactions observed in the crystal structure of benzene
[17]. That type of intermolecular interaction is energetically favorable and is commonly
observed when aromatic ring systems pack in crystalline solids [18]. For example, Marjani has
reported that the 4-aminophenyl group in the crystal structure of N-(2-
pyridylmethylene)benzene-1,4-diamine twists 25° out of the molecular plane to form C-H...ꢁ
interactions with adjacent molecules [19].
Comparison between the geometry of II in the crystalline state and its calculated DFT
geometry shows reasonable agreement in the bond lengths and bond angles, as shown in Table 3.
Similar to the structure of I, differences between experimental and calculated bond lengths
varied between 0.001 Å – 0.023 Å with bond angles varying between 0.01° – 3.5°. Notably, the
phenyl substitutents in the DFT structure exhibit far less twisting, rotating just ~10° out of the
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mean plane of the central cyclopentanone ring, which supports our hypothesis that greater
twisting of the phenyl groups in the solid state is due largely to effects of crystal packing and, in
the case of the benzylidene group, formation of a C-H...ꢁ interaction. Close examination of the
X-ray and DFT bond lengths in the carbon backbone reveals an interesting trend. The DFT
values for double bonds (C5-C6, C7-C8, and C2-C15) in the valence bond structure of II (Figure 1)
are longer by 0.004-0.023 Å than the corresponding bonds in the crystal structure, while the DFT
values for single bonds (C6-C7 and C15-C16) in the valence bond structure are shorter by 0.001-
0.004 than the corresponding bonds in the crystal structure. Although the magnitude of the
differences in those bond lengths is small, the trend of alternating longer C=C and shorter C-C
bonds suggests that the DFT structure exhibits bond orders with a higher degree of conjugation
in the carbon backbone compared to the crystal structure. That finding is consistent with the
conformation of II exhibiting greater deviation from planarity in the crystalline solid, where
rotation of the phenyl rings and buckling of the central cyclopentanone ring should inhibit
optimal overlap of p orbitals along the central ꢁ system. It is interesting to note that the bond
lengths for compound I (Table 2) do not show the same trend of alternating shorter C=C and
longer C-C bond lengths for the DFT values, which makes sense given the close structural
similarity and nearly planar conformations of the DFT and crystal structures of I.
Photophysical properties of compounds I and II
Knowledge of the structures of these compounds provides some degree of insight to
understanding the spectroscopic and photophysical properties exhibited by I and II. Structural
features such as polyene chain length and the presence of electron-donating and electron-
withdrawing groups have been shown to effect the spectroscopic and photophysical properties
for this class of compounds. Compound I is an example of a “donor-acceptor-acceptor” molecule
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that we previously have demonstrated exhibits solvatochromism and photoinduced internal
charge transfer (ICT) [5] resulting from the presence of the strongly electron-donating
dimethylamino group and the strongly electron-withdrawing cyano group on opposite ends of the
conjugated carbon backbone. Figure 5 shows absorption and fluorescence spectra of I at room
temperature in methanol, chloroform, and carbon tetrachloride. We found that I undergoes
bathochromic (red) shifts as solvent polarity increases as indicated by the shifts in absorption
maxima from 446 nm (carbon tetrachloride) to 473 nm (methanol) and fluorescence maxima
from 513 nm (carbon tetrachloride) to 693 nm (methanol). Shown in Figure 4, the computed
molecular orbitals of I reveal the ICT (ꢁ, ꢁ*) nature of the S1 excited state, where ꢁ electron
density is transferred from the HOMO (ꢁ) on the side bearing the dimethylamino group to the
LUMO (ꢁ*) on the side bearing the cyano group upon photoexcitation. The HOMO-2 (n) orbital
was determined to be a nonbonding orbital centered on the carbonyl group. Although we have
studied the photophysical behavior of I only in solution and not in the solid state, the similarity
in the molecular structure of I both in the solid state (crystal structure) and in the gas phase
(calculated DFT structure) suggest that the observed planar structure of compound I is the lowest
energy conformation. Therefore, it is reasonable to expect that molecular structure predominates
in solution and represents the most stable ground state conformation giving rise to the
photophysical behavior we have observed.
In contrast to I, compound II contains unsubstituted phenyl moieties and exhibits a
smaller degree of ICT due to the absence of strongly electron-donating and electron-withdrawing
groups. As depicted in Figure 4, the computed molecular orbitals of II show that in the HOMO
(ꢁ), ꢁ electron density resides largely on the cinnamylidene group. Upon photoexcitation to the
S1 state, ꢁ electron density in the LUMO (ꢁ*) shifts slightly onto the conjugated backbone and
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carbonyl group in the center of the molecule rather than transferring completely to the opposite
side as in compound I. Our previous studies of the spectroscopic properties of II in various
solvents (Figure 6) showed that II fluoresces only in protic solvents such as alcohols and acetic
acid in which photoexcitation from S0 to S1 involve ꢁ to ꢁ* transitions rather than n to ꢁ*
transitions [20]. Nonpolar solvents that were unable to induce fluorescence upon photoexcitation
necessarily involved n to ꢁ* transitions where the absence of fluorescence from S1 is attributed to
efficient intersystem crossing of the singlet ꢁ* states to triplet ꢁ* states in accordance to El-
Sayed’s rule [21]. We have demonstrated similar photophysical behavior previously for a related
family of three symmetrically unsubstituted 2,5-diarylidenecyclopentanones [22]. As with I, we
have studied the photophysical properties of compound II only in solution and not in the solid
state. Our finding that the structure is nonplanar both in the solid state (crystal structure) and the
gas phase (calculated DFT structure) indicate that molecules of II will adopt a twisted structure
in solution. Given the discrepancies between the crystal and calculated structures with regard to
the degree of rotation of the phenyl substituents and buckling present in the cyclopentanone ring,
we can conclude only that the lowest energy conformation likely also is nonplanar in solution in
the ground state.
TD-DFT spectral calculations were carried out both in the gas phase and in solvent,
employing the SCRF PCM method. Table 4 shows the TD-DFT calculations for the first three
lowest lying excited singlet states (S1, S2, and S3, respectively) in the gas phase and in solvent
(chloroform for I and ethanol for II). Direct comparison between the experimental UV-Visible
absorption spectra, plotted together with the TD-DFT calculations are shown in Figure 7. For I,
the S0 ꢀ S1 transition was observed at 468 nm in chloroform and is predicted by TD-DFT to be
a strong transition appearing at 473 nm in the gas phase and 515 nm in chloroform, arising from
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the HOMO (π) ꢀ LUMO (π*) orbital configuration. The S0 ꢀ S2 transition is computed to be a
forbidden n ꢀ π* excitation arising from the HOMO-2 (n) ꢀ LUMO (π*), where HOMO-2 is
nonbonding orbital localized on the carbonyl oxygen atom. The calculated wavelengths are 426
nm in the gas phase and 403 nm in chloroform. Absorption to the S2 state has not been observed
owing to its forbidden nature. Excitation to S3 is predicted to occur at 367 nm in the gas phase
and 376 nm in chloroform with major Configuration Interaction (CI) configurations HOMO-1 ꢀ
LUMO and HOMO ꢀ LUMO + 1, corresponding to a π ꢀ π* transition delocalized over the
entire molecule. The band observed at λmax = 322 nm is assigned to this computed excitation.
For II, in the gas phase, the S0 ꢀ S1 excitation was predicted by TD-DFT to be a
forbidden (n, π*) state in the gas phase with a calculated transition energy of 23,041 cm-1 (λ 434
nm) and HOMO-2 ꢀ LUMO as the major CI configuration. The S0 ꢀ S2 excitation was
predicted to be a spin allowed (π, π*) state with a calculated transition energy of 25,316 cm-1 (λ
395 nm), and with HOMO ꢀ LUMO as the major CI configuration. Lastly, the S0 ꢀ S3
excitation was also predicted to be (π, π*), with a calculated transition energy of 28,736 cm-1 (λ
348 nm), with HOMO-1 ꢀ LUMO as the major CI configuration. In ethanol, the lowest lying
1(π, π*) state shifts below that of 1(n, π*), making it the S1 state, thereby inducing fluorescence,
as was experimentally observed (Figure 6). From TD-DFT, calculated transition energies for S1,
S2, and S3 were 23,810 cm-1 (λ 420 nm), 24,390 cm-1 (λ 410 nm), and 28,329 cm-1 (λ 353 nm).
Conclusions
The single crystal X-ray structures of two 2,5-diarylidene cyclopentanone compounds
have been measured and solved by X-Ray Diffractometry. Excellent agreement was established
between the experimental single crystal X-ray structure and the predicted DFT structure of I and
15

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II in both bond lengths and bond angles. The single crystal X-ray structure of Ι was found to be
mainly planar, with angles between the mean plane defined by the central cyclopentanone ring
and methyl groups (C8-C14) and the mean planes of the amino-substituted and nitrile-substituted
phenyl rings of 4.3° and 2.1°, respectively. Alternate stacking of electron-rich and electron-
deficient phenyl rings overlapping at a separation of 3.6 Å suggests that π donor-acceptor
interactions between the LUMO and HOMO should play a role of influencing molecular
aggregation during nucleation of crystals to favor centrosymmetric packing. The single crystal
X-ray structure of II was found to be nonplanar, with mean planes of the phenyl rings in the
benzylidene and cinnamylidene ends of the molecule rotating out of the mean plane defined by
the central cyclopentanone ring and acyclic carbon atoms (C1-C8 and C15) by 43.6° and 20.6°,
respectively. The mean planes of the phenyl groups on either end are twisted with respect to one
another by 59.3°. Furthermore, surveys of the spectroscopic, photophysical, and reactivity
properties of these compounds show and explain how molecular structure plays an important role
in determining the outcomes of these properties.
Supporting Information Available
The Crystallographic Information Files (CIFs) for both compounds I and II are provided
in the Supplementary Information. The crystallographic data is from The Cambridge
Crystallographic Data Centre (CCDC), 12 Union Road, Cambridge CB2 1EZ, UK, Fax:
+44(0)1223−336033.
16

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18

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Figure Captions
Fig. 1. Chemical structures of compounds I and II.
Fig. 2. Reaction schemes for the syntheses of (a) I and (b) II.
Fig. 3. Views of the crystal structures of compounds I (left) and II (right) showing the molecular
structures viewed (a) from above and (b) edge-on with anisotropic displacement parameters at
the 50% probability level, (c) space-filling models of the molecular structures, (d and e) crystal
packing. The green sphere indicates the centroid of the C16-C21 phenyl ring in II.
Fig. 4. Molecular orbitals of I and II computed by TD-DFT.
Fig. 5. Absorption spectra (left) and fluorescence spectra (right) of compound I in (a) methanol,
(b) chloroform, and (c) carbon tetrachloride.
Fig. 6. Absorption spectra (left) and fluorescence spectra (right) of II in (a) glacial acetic acid,
(b) methanol, (c) ethanol, (d) 1-propanol, (e) 2-propanol, and (f) 1-butanol.
Fig. 7. Experimental absorption spectra of (a) I (in chloroform) and (b) II (in ethanol) plotted
together with the TD-DFT calculated results. The forbidden 1(n, π*) state is represented by the
filled diamond.
19

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Table Captions
Table 1. Crystallographic data and refinement parameters for compounds I and II.
Table 2. Comparison of geometries for compound I determined by X-ray diffraction in the solid
state and by DFT calculations (B3LYP/6-31G(d) level of theory) in the gas phase. ∆ indicates the
difference between corresponding geometric values determined by X-ray and DFT (X-ray–DFT).
Table 3. Comparison of geometries for compound II determined by X-ray diffraction in the solid
state and by DFT calculations (B3LYP/6-31G(d) level of theory) in the gas phase. ∆ indicates the
difference between corresponding geometric values determined by X-ray and DFT (X-ray–DFT).
Table 4. TD-DFT spectral calculations of S1, S2, and S3 in gas and solvent (chloroform for I and
ethanol for II).*
20

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Crystal Form
Compound
I
II
C21H18O
286.35
Formula
Formula weight (g mol-1)
Crystal system
Space group
Color and habit
Crystal size
a (Å)
C22H20N2O
328.40
Triclinic
P-1
Red, needles
Monoclinic
C2/c
Yellow, needles
0.05 mm × 0.20 mm × 0.50 mm 0.15 mm × 0.20 mm × 1.00 mm
6.8743(2)
8.8115(2)
14.9664(4)
77.135(2)
81.351(2)
80.975(2)
866.56(4)
2
33.4281(10)
11.9668(4)
7.8031(2)
90.00
92.785(2)
90.00
b (Å)
c (Å)
α (°)
β (°)
γ (°)
Volume (Å3)
Z
3117.77(16)
8
0.71073
1.259
296(2)
0.71073
1.220
296(2)
λ (Å)
ρcalc (g cm-3)
Temperature (K)
F (000)
348
1216
2.39 – 28.47
1.22 – 28.30
θ range for data
collection (°)
Ranges of Miller indices
-9 ≤ h ≤ 9
-11 ≤ k ≤ 11
-20 ≤ l ≤ 19
0.078
-44 ≤ h ≤ 44
-15 ≤ k ≤ 15
-10 ≤ l ≤ 9
0.073
Absorption Coefficient
(mm-1)
Reflections collected
Independent reflections
Reflections [I > 2σ(I)]
Data/restraints/parameters
Goodness of fit on F2
R (all data)
16663
4370 [Rint = 0.0279]
2395
23582
3876 [Rint = 0.0275]
2519
4370/0/228
1.004
R1 = 0.0496
wR2 = 0.1460
3876/0/200
1.023
R1 = 0.0414
wR2 = 0.1112

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Bond Angles (°)
X-ray
Bond Lengths (Å)
X-ray DFT
∆
DFT
120.4
120.2
179.9
119.8
120.3
119.4
121.7
117.6
117.8
121.2
130.9
124.6
131.6
119.3
106.3
107.9
125.2
109.1
106.1
109.7
130.7
126.9
131.6
119.5
118.3
125.4
122.6
122.0
116.2
120.8
117.0
121.5
121.4
120.0
120.2
121.5
118.8
∆
0.6
-0.4
-1.9
-0.01
0.5
-0.2
0.4
-0.2
0.7
-0.6
1.2
-0.4
0.8
-0.5
0.2
1.0
-0.5
-0.3
0.5
-0.6
1.1
-0.4
1.1
-0.4
0.1
C1-C2
C1-N1
C2-C3
C2-C7
C3-C4
C4-C5
C5-C6
C5-C8
C6-C7
C8-C9
C9-C10
C9-C13
1.440(3) 1.429
1.131(3) 1.156
1.383(3) 1.403
1.383(3) 1.403
1.363(3) 1.385
1.396(3) 1.410
1.394(3) 1.410
1.453(2) 1.459
1.374(3) 1.386
1.336(2) 1.347
1.498(2) 1.507
1.485(2) 1.500
0.011
C1-C2-C3
C1-C2-C7
C2-C1-N1
C2-C3-C4
C2-C7-C6
C3-C2-C7
C3-C4-C5
C4-C5-C6
C4-C5-C8
C5-C6-C7
C5-C8-C9
C6-C5-C8
C8-C9-C10
C8-C9-C13
C9-C10-C11
C9-C13-C12
C9-C13-O1
C10-C9-C13
C10-C11-C12
C11-C12-C13
C11-C12-C14
C12-C13-O1
C12-C14-C15
C13-C12-C14
C14-C15-C16
C14-C15-C20
C15-C16-C17
C15-C20-C19
C16-C15-C20
C16-C17-C18
C17-C18-C19
C17-C18-N2
C18-C19-C20
C18-N2-C21
C18-N2-C22
C19-C18-N2
C21-N2-C22
121.0(2)
119.8(2)
178.0(3)
119.8(2)
120.8(2)
119.2(2)
122.1(2)
117.4(2)
118.5(1)
120.6(2)
132.1(1)
124.2(1)
132.4(1)
118.8(1)
106.5(1)
108.9(1)
124.7(1)
108.8(1)
106.6(1)
109.1(1)
131.8(1)
126.5(1)
132.7(1)
119.1(1)
118.4(1)
125.7(1)
122.7(2)
122.0(2)
115.9(2)
121.0(2)
116.8(2)
121.0(2)
121.7(2)
120.6(2)
120.9(2)
122.2(2)
118.2(2)
-0.025
-0.020
-0.020
-0.022
-0.014
-0.016
-0.006
-0.012
-0.011
-0.009
-0.015
-0.012
-0.008
-0.013
-0.012
0.005
-0.006
-0.013
-0.012
-0.011
-0.015
-0.019
-0.005
-0.009
-0.002
-0.019
C10-C11 1.545(2) 1.557
C11-C12 1.500(2) 1.508
C12-C13 1.467(3) 1.480
C12-C14 1.343(2) 1.355
C13-O1 1.227(2) 1.222
C14-C15 1.441(2) 1.447
C15-C16 1.398(2) 1.411
C15-C20 1.399(3) 1.411
C16-C17 1.371(3) 1.382
C17-C18 1.401(3) 1.416
C18-C19 1.397(3) 1.416
C18-N2 1.372(3) 1.377
C19-C20 1.374(3) 1.383
C21-N2 1.453(3) 1.455
C22-N2 1.436(4) 1.455
0.3
0.1
0.01
-0.3
0.2
-0.2
-0.5
0.3
0.6
0.7
0.7
-0.6
Dihedral Angles (°)
X-ray
-3.8
1.2
DFT
7.1
4.6
C10-C9-C5-C6
C11-C12-C15-C20

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Bond Lengths (Å)
X-ray DFT
Bond Angles (°)
∆
X-ray
DFT
∆
-1.0
2.3
-1.1
2.4
-1.2
-0.3
-1.1
-3.5
-1.5
-0.4
-1.3
0.5
-0.7
0.6
-0.1
0.6
-0.6
-0.3
0.05
-0.02
0.2
0.3
-0.1
1.5
-1.7
-0.7
0.1
0.3
0.3
0.2
-0.2
C1-C2
C1-C5
C1-O1
C2-C3
C2-C15
C3-C4
C4-C5
C5-C6
C6-C7
C7-C8
C8-C9
C9-C10
C9-C14
1.490(2) 1.493
1.478(2) 1.484
1.225(2) 1.227
1.504(2) 1.512
1.336(2) 1.351
1.541(2) 1.560
1.503(2) 1.510
1.336(2) 1.354
1.440(2) 1.439
1.333(2) 1.356
1.463(2) 1.460
1.388(2) 1.409
1.395(2) 1.410
-0.003
-0.006
-0.002
-0.008
-0.015
-0.019
-0.007
-0.018
0.001
C1-C2-C3
C1-C2-C15
C1-C5-C4
C1-C5-C6
C2-C1-C5
C2-C1-O1
C2-C3-C4
C2-C15-C16
C3-C2-C15
C3-C4-C5
108.3(1)
121.5(1)
108.7(1)
123.5(1)
107.6(1)
125.8(1)
104.9(1)
127.7(1)
130.0(1)
105.3(1)
127.8(1)
126.6(1)
125.6(1)
123.2(1)
127.8(1)
119.3(1)
122.8(1)
121.0(1)
120.9(1)
117.9(1)
120.2(2)
119.8(2)
120.3(2)
119.3(1)
122.8(1)
120.7(1)
120.9(1)
118.0(1)
120.3(2)
119.7(2)
120.3(1)
109.3
119.2
109.8
121.1
108.8
126.1
106.0
131.2
131.5
105.7
129.1
126.1
126.3
122.6
127.9
118.7
123.4
121.3
120.9
117.9
120.0
119.5
120.4
117.8
124.5
121.4
120.8
117.7
120.0
119.5
120.5
-0.023
0.003
C4-C5-C6
C5-C1-O1
C5-C6-C7
C6-C7-C8
-0.021
-0.015
-0.010
-0.022
-0.023
-0.012
0.004
-0.014
-0.016
-0.009
-0.018
-0.020
-0.012
C10-C11 1.383(2) 1.393
C11-C12 1.374(3) 1.396
C12-C13 1.376(2) 1.399
C13-C14 1.378(2) 1.390
C15-C16 1.464(2) 1.460
C16-C17 1.397(2) 1.411
C16-C21 1.395(2) 1.411
C17-C18 1.382(2) 1.391
C18-C19 1.379(2) 1.397
C19-C20 1.377(2) 1.397
C20-C21 1.381(2) 1.393
C7-C8-C9
C8-C9-C10
C8-C9-C14
C9-C10-C11
C9-C14-C13
C10-C9-C14
C10-C11-C12
C11-C12-C13
C12-C13-C14
C15-C16-C17
C15-C16-C21
C16-C17-C18
C16-C21-C20
C17-C16-C21
C17-C18-C19
C18-C19-C20
C19-C20-C21
Dihedral Angles (°)
X-ray
DFT
C1-C2-C3-C4
C1-C5-C4-C3
C21-C16-C9-C14
C4-C6-C9-C14
C3-C15-C16-C21
-20.1
-17.5
52.8
12.9
35.0
9.9
10.3
-15.4
-1.1
-9.7

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I
Gas
State
Energy (cm-1)
f
MO Configuration Interation (CI)
0.73
0.00
0.84
S1 (π, π*) 21,142 (λ 473 nm)
S2 (n, π*) 23,474 (λ 426 nm)
S3 (π, π*) 27,248 (λ 367 nm)
HOMO (π) ꢀ LUMO (π*)
HOMO – 2 (n) ꢀ LUMO (π*)
HOMO – 1 (π) ꢀ LUMO (π*)
HOMO (π) ꢀ LUMO + 1 (π*)
Chloroform
0.94
0.00
S1 (π, π*) 19,417 (λ 515 nm)
S2 (n, π*) 24,814 (λ 403 nm)
S3 (π, π*) 26,596 (λ 376 nm)
HOMO (π) ꢀ LUMO (π*)
HOMO – 2 (n) ꢀ LUMO (π*)
HOMO – 1 (π) ꢀ LUMO (π*)
HOMO (π) ꢀ LUMO + 1 (π*)
0.86
II
Gas
State
Energy (cm-1)
f
MO Configuration Interation (CI)
HOMO - 2 (n) ꢀ LUMO (π*)
HOMO (π) ꢀ LUMO (π*)
0.00
1.41
S1 (n, π*) 23,041 (λ 434 nm)
S2 (π, π*) 25,316 (λ 395 nm)
S3 (π, π*) 28,736 (λ 348 nm)
0.018
Ethanol
1.58
0.0035
0.044
HOMO – 1 (π) ꢀ LUMO (π*)
S1 (π, π*) 23,810 (λ 420 nm)
S2 (n, π*) 24,390 (λ 410 nm)
S3 (π, π*) 28,329 (λ 353 nm)
HOMO (π) ꢀ LUMO (π*)
HOMO – 2 (n) ꢀ LUMO (π*)
HOMO – 1 (π) ꢀ LUMO (π*)
*The corresponding energies in units of wavelength (λ, nm) are given in parentheses.

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O
O
O
DIMCARB
H
(i)
+
+
CH2Cl2, RT
(H3C)2N
(H3C)2N
IA
O
NaOH
H
(ii)
IA
I
MeOH, RT
NC
(a)
O
O
O
DIMCARB
(i)
H
+
CH2Cl2, RT
IIA
O
NaOH
(ii)
IIA
+
H
II
MeOH, RT
(b)

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