Investigating the change in the photophysical properties of a trio of tetraphenylcyclopentadienone derivatives with varied groups on the aromatic rings in the 3- and 4-positions

Article abstract of DOI:10.1002/bio.3944

Organic compounds with electronic properties, such as a small band gap, are useful in areas ranging from organic field effect transistors to solar cells. Such organic compounds can possess conjugation and/or aromatic systems, with one example being tetrap

Full text of DOI:10.1002/bio.3944

Received: 11 May 2020
DOI: 10.1002/bio.3944
Revised: 14 August 2020
Accepted: 27 August 2020
R E S E A R C H A R T I C L E
Investigating the change in the photophysical properties of a
trio of tetraphenylcyclopentadienone derivatives with varied
groups on the aromatic rings in the 3- and 4-positions
Arnoldo J. Orozco1 | Kole Joachim1 | Henry D. Ruiz1 | Frank R. Fronczek2
Ralph Isovitsch1
|
1Department of Chemistry, Whittier College,
Abstract
Whittier, CA, USA
2Department of Chemistry, Louisiana State
University, Baton Rouge, LA, USA
Organic compounds with electronic properties, such as a small band gap, are useful in
areas ranging from organic field effect transistors to solar cells. Such organic
compounds can possess conjugation and/or aromatic systems, with one example
being tetraphenylcyclopentadienone and its derivatives. A trio of dramatically
coloured tetraphenylcyclopentadienone derivatives with varied substituents on the
aromatic rings in the 3- and 4-positions were prepared. Their identities were
confirmed using the usual methods, for example 1H nuclear magnetic resonance
(NMR) spectroscopy, and their purity quantified using elemental analysis. The X-ray
crystal structure of compound 2 was determined. Its notable structural features
involved the cyclopentadienone core with its distinct C−C and C=C bond lengths and
its overall nonplanarity, both of which served to mitigate its antiaromatic nature.
Chloroform solutions of compounds 2–4 exhibited absorption spectra with three
absorption bands at approximately 250, 350, and 500 nm that were assigned to
(π)!(π*) transitions. Computational chemistry methods assisted in assigning the
observed transitions to a specific molecular orbital combination in the structures of
2–4. Emission in the red end of the visible spectrum (550–625 nm) was observed
from chloroform solutions of all three of the prepared compounds.
Correspondence
Ralph Isovitsch, 13406 Philadelphia Street,
Department of Chemistry, Whittier College.
Whittier, CA 90601, USA.
Email: risovitsch@whittier.edu
K E Y W O R D S
absorption, crystal structure, emission, molecular orbitals, tetraphenylcyclopentadienone
1
|
INTRODUCTION
band gap energy of TACPDs and their easily executed synthesis
points to their being useful building blocks for molecular electron-
Tetraarylcyclopentadienones (TACPDs) have been synthesized and
their properties explored since the 1930s.[1,2] Their most striking
feature is their intense colour, which ranges from deep maroon to a
velvety dark purple, to almost black. This deep coloration was likely to
have been what spurred their first photophysical studies, whose goal
was to correlate changes in aryl ring substituents to changes in
absorption spectra. These first investigations discovered how various
groups, for example chloro- and methoxy-, could alter the colour, and
therefore the absorption spectra of TACPD derivatives.[3,4] The low
ics.[5,6] To facilitate the inclusion of TACPDs in small-scale devices, a
recent theoretical work has endeavoured to create a computationally
inexpensive routine that would permit facile prediction of the effect
of substituents on the band gap energy of TACPD derivatives with
varied aryl rings in the 2- and 5-positions.[7]
While some see TACPDs as potential building blocks for devices
such as solar cells and LEDs, others use them as a model to explore
the concept of antiaromaticity.[8] A notable example of work in this
area used X-ray crystallography to explore intermolecular interactions
Luminescence. 2020;1–9.
wileyonlinelibrary.com/journal/bio
© 2020 John Wiley & Sons, Ltd.
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OROZCO ET AL.
that stabilized the antiaromatic nature of TACPDs in the solid state
and computational methods to understand the maximum amount of
antiaromatic stabilization offered by various substituents.[9]
2.2 | Computational methods
Computational modelling was conducted in the Gaussian09
computational chemistry software package to rationalize the observed
photophysical properties of compounds 2–4.[13] Modelling of
compounds 2–4 was broken down into two steps: (i) ground state
While antiaromaticity is often viewed as a destabilizing force,
the antiaromatic nature of TACPDs can be a useful property. For
example, at the right temperature and with the correct dienophile,
TACPDs can act as dienes. Recent work has taken advantage of this
in the synthesis of blue-emitting fluorophores that are used to
label biomolecules and as the emissive layer in an organic light
emitting diode (OLED).[10,11] The antiaromatic resonance structure
of TACPDs contains a delocalized cyclopentadienyl ring that can act
as a versatile ligand for various metals. The most famous of these
are Shvo-type metal catalysts (hydroxycyclopentadienylruthenium
hydride dimers), which possess notable activity in hydrogen transfer
processes.[12]
geometry
and
vibrational
frequency
optimization;
and
(ii) quantification of excited state energies using TD-DFT methods.[14]
The Becke-3-LYP (B3LYP) hybrid functional was utilized in the
optimization of the molecular geometries. The optimization of com-
pounds 2–4 was attained with a 6-31G++ (d,p) level of theory. Excited
state information was calculated using the 6-311G+ (d,p) atomic basis
set with TD-DFT methods.[15] Optimization and absorption spectra
were properly modelled with corresponding solvation in chloroform
using the default Self-Consistent Reaction Field (SCRF) method of the
Polarizable Continuum Model (IEFPCM).[16] The steps used in the
computational studies of compounds 2–4 are common methods used
in the study of photophysics of organic molecules.[17,18]
TACPD derivatives are beautiful not only for their rich colour, but
also for their unique chemistry. Our study explored the synthesis and
photophysical characterization of interesting molecules towards the
goal of employing them in light harvesting devices. Here we present
the synthesis of a trio of TACPDs with differing substituents on the
aryl rings in the 3- and 4-positions: two that have methoxy groups are
constitutional isomers and one was substituted with fluorine atoms.
The crystal structure of the fluoro derivative is showcased. We also
discuss how the differing substituents of the trio affect their steady-
state absorption and emission spectra.
2.3 | Crystallographic data acquisition and
refinement
A dark red prism of 2 (0.44 × 0.38 × 0.36 mm) was used for data col-
lection at T = 90 K on a Bruker Kappa APEX-II DUO diffractometer
equipped with an Oxford Cryosystems Cryostream chiller and Mo
Kα radiation (λ = 0.71073 Å) from a TRIUMPH curved graphite
monochromator. The structure was solved using direct methods, and
structure refinement was carried out using SHELXL2014/7.[19] All
hydrogen atom locations were inferred from neighbouring sites.
2
| EXPERIMENTAL
2.1 | General methods, materials, and instruments
Compounds 2–4 were prepared using standard techniques. Solvents
and reagents were purchased from Sigma-Aldrich or Acros Organics,
and used as received. Melting points were obtained in open capillaries
and are uncorrected. NMR spectra (1H at 400 and 600 MHz, 19F at
600 MHz, and 13C at 100 MHz) were obtained from CDCl3 and
DMSO-d6 solutions of 2–4 using the residual solvent peak as the
internal standard for 1H and 13C spectra acquired on a Bruker
2.4 | General synthetic procedure and structure
characterization data for compounds 2–4
Equimolar amounts of diphenylacetone and the appropriate benzil
derivative were combined with 20 ml of absolute ethanol in a 50 ml
round-bottomed flask equipped with a magnetic stir bar. The reaction
mixture was heated and, upon reflux, 2.0 ml of 3 M ethanolic KOH
was added. After refluxing for 1.5 h, the reaction was cooled slowly,
and the crude product collected over vacuum filtration. Crystallization
was achieved via diffusion of cyclohexane in a chloroform solution of
the crude product.
AvanceNEO 400 NMR spectrometer or
a Bruker AVANCE III
600 MHz spectrometer. 19F NMR spectra were referenced to
trifluoroethanol. Infrared (IR) spectra were measured using an attenu-
ated total reflectance (ATR) attachment on a Perkin-Elmer Spectrum
Two FT-IR spectrometer. Elemental analyses were kindly performed
by M-H-W Laboratories (Tucson, Arizona, USA).
Absorption, emission, and excitation spectra were recorded at
room temperature from 5 to 6 × 10−5 M deoxygenated solutions
of 2–4 prepared with spectrophotometric grade chloroform using
2.4.1 | 3,4-Bis(4-fluorophenyl)-
2,5-diphenylcylcopenta-2,4-dienone (2)
a
Hewlett Packard 8453 diode array spectrometer, and a
HoribaJobinYvon FluoroMax-4 fluorometer. Emission and excitation
spectra were corrected for detector response using a correction curve
supplied by the fluorometer manufacturer. Scattered excitation light
was minimized from the excitation spectra using a Schott OG550 long
pass filter in front of the emission monochromator.
Yield: 42%. M.P. 213–214ꢀC; IR (ATR) 3030, 1715, 1601, 1504,
;
1490 cm−1 1H-NMR (600 MHz, DMSO-d6) ppm: 6.93 (d, 8H), 7.27
(m, 10H); 19F-NMR (600 MHz, DMSO-d6) ppm: −72.23. Anal. Calcd.
for C29H18OF2: C, 82.83; H, 4.32. Found: C, 82.68 H, 4.17; Rf = 0.93
(silica, THF).

OROZCO ET AL.
3
2.4.2 | 3,4-Bis(3-methoxyphenyl)-
2,5-diphenylcylcopenta-2,4-dienone (3)
Yield: 60%. M.P. 176–179ꢀC; IR 3045, 3006, 2937, 1706, 1601,
;
1571 cm−1 1H-NMR (400 MHz, CDCl3) ppm 7.28 (m, 10H), 7.13 (t,
2H), 6.83 (dd, 2H), 6.59 (d, 2H), 6.45 (s, 2H), 3.52 (s, 6H); 13C-NMR
(100 MHz, CDCl3) ppm 200.3, 159.0, 154.2, 134.4, 130.7, 130.1,
129.1, 128.0, 127.5, 125.3, 121.8, 115.1, 114.1, 55.1; Anal. Calcd. for
C
31H24O3: C, 83.75; H, 5.45. Found: C, 83.41; H, 5.17; Rf = 0.90
(silica, THF).
FIGURE 2 Left: The cyclopentadienone core common to
compounds 1–4. Right: An antiaromatic dipolar resonance structure
of the cyclopentadienone core
2.4.3 | 3,4-Bis(4-methoxyphenyl)-
2,5-diphenylcylcopenta-2,4-dienone (4)
Yield: 77%. M.P. 188–189ꢀC; IR 3048, 3006, 1900, 1698, 1601,
;
1490 cm−1 1H-NMR (400 MHz, CDCl3) ppm 7.26 (m, 10H), 6.88
(d, 4H), 6.73 (d, 4H), 3.81 (s, 6H); 13C-NMR (100 MHz, CDCl3) ppm
200.4, 159.8, 154.0, 131.2, 131.1, 130.2, 128.0, 127.2, 125.3, 124.7,
113.4, 55.2; Anal. Calcd. for C31H24O3: C, 83.75; H, 5.45. Found: C,
83.79; H, 5.28; Rf = 0.98 (silica, THF).
13C-NMR spectrum of 3 manifested one carbonyl resonance at
200.3 ppm and one methyl resonance at 55.1 ppm. Compound 4 had a
1H-NMR spectrum with an aromatic region that only had resonances
that were indicative of two types of aromatic protons: a multiplet that
integrated for 10 protons (characteristic of two monosubstituted ben-
zene rings) at 7.25 ppm and a doublet of doublets that integrated for
eight protons (characteristic of two para-disubstituted benzene rings)
at 6.72–6.89 ppm. All of the characterization data that were collected
for compounds 2 and 4 corresponded well with that reported in the
literature.[21,23,24]
3
| RESULTS AND DISCUSSION
3.1 | Synthesis, structural elucidation, and purity
assessment
Narrow melting ranges and well run thin-layer chromatography
(TLC) plates showing one product spot first substantiated the purity
of compounds 2–4. Their purity was further confirmed using elemen-
tal analyses that were within 0.34 of the theoretical values.
Compounds 2–4 had their structures unambiguously determined
using the usual spectroscopic techniques. The identity of 2 was
further substantiated using X-ray crystallography, which revealed
structural features that mitigate the antiaromatic character of the
dipolar resonance structure of its cyclopentadienone core, which has
been shown to be a major resonance contributor (Figure 2).[9]
Tetraarylcylopentadienones (Figure 1), are easily prepared via a base-
promoted condensation reaction between 1,3-diphenylpropanone,
and a bezil derivative.[20] We obtained compounds 2–4 in useful
yields (40–70%) as chunky crystals that varied in colour from a dark
red to purple-black.
Compound 1, studied for decades, is the unsubstituted parent of
compounds 2–4, which have substituents on their aromatic rings in
the 3- and 4-positions.[7,21] Compound 2 has inductively electron-
withdrawing fluoro groups. Compounds 3 and 4 are constitutional iso-
mers with methoxy groups. The position of the methoxy groups of
3 makes them inductively electron-withdrawing, while the positioning
of the methoxy group in 4 renders its electron-donating characteris-
tics via resonance stronger than its inductive electron-withdrawing
ability. The electron-donating or electron-withdrawing characteristics
of a substituent will be important in the consideration of the photo-
physical properties of the compounds in this study.[22]
3.2 | Molecular structure of compound 2
Diffusion of hexane vapours into a chloroform solution of compound
2 yielded large, dark red crystals that were eminently suitable for
X-ray analysis. The Oak Ridge Thermal Ellipsoid Plot (ORTEP) repre-
sentation of 2 is presented in Figure 3, while selected bond lengths
and angles are listed in Table 1. Crystallographic and refinement data
for 2 are summarized in Supporting InformationTable S1.
Spectroscopic data provided further evidence of the successful
syntheses of compounds 2–4. For example, the 19F-NMR spectra of
compound 2 manifested a single resonance at −72.23 ppm. The
The unit cell of 2 was found to contain two molecules of 2 that
lacked significant variations in their individual characteristics. Overall,
FIGURE 1 Tetraarylcyclopentadienones
investigated in this study. Compound 1: R = H,
compound 2: R = 4-fluoro, compound 3: R = 3-OCH3,
compound 4: R = 4-OCH3

4
OROZCO ET AL.
FIGURE 3 ORTEP plot of
compound 2 with thermal
ellipsoids drawn at the 50%
probability level
TABLE 1 Selected bond lengths [Å] and angles [ꢀ] for compound
2
minimization of the conjugation in the cyclopentadienone core of 2.
Another feature of 2 that helped to minimize its antiaromatic nature
was the slight nonplanarity of the central cyclopentadienone (C1–C5,
C30–C34) moieties, as indicated by dihedral angles that ranged from
−0.97(12) to 1.45(11)ꢀ.
O1–C1
1.2172 (12)
1.3565 (12)
1.3546 (13)
1.4941 (13)
1.5152 (12)
1.3598 (13)
1.5125 (12)
1.3542 (13)
1.2123 (13)
1.3626 (13)
1.3577 (13)
1.5016 (13)
1.5137 (14)
1.3579 (14)
1.5150 (13)
1.3530 (13)
C10–C11
1.4235 (13)
156.71 (11)
−51.45 (14)
156.29 (11)
129.13 (11)
132.55 (10)
135.59 (12)
132.14 (11)
138.38 (12)
−0.97 (12)
1.45 (11)
F1–C15
F2–C21
C1–C5
C3–C2–C6–C7
C4–C3–C12–C15
C32–C31–C35–C36
C33–C32–C41–C42
C3–C34–C18–C23
C4–C5–C24–C25
C32–C33–C47–C52
C33–C34–C53–C54
C2–C3–C4–C5
The X-ray crystal structure of 2, and related TACPD derivatives,
illustrated how structural features mitigate their inherent antiaromatic
instability.[8,9]
C1–C2
C2–C3
C3–C4
C4–C5
3.3 | Steady-state absorption and emission studies
O2–C30
F3–C44
F4–C50
C30–C34
C30–C31
C31–C32
C32–C33
C33–C34
Absorption spectra of TACPD derivatives have been studied since the
1950s.[1,3,4] At first, the question about TACPDs was how various
electron-donating and electron-withdrawing substituents affected
their colour. Recent studies have employed modern computational
methods to better understand the bonding of TACPDs so that they
can potentially be incorporated into microscale electronics.[7] Steady-
state photophysical data for compounds 1–4 are compared in Table 2,
with the literature data for compound 1 provided for comparison pur-
poses. The electronic absorption and emission spectra of 2–4 are
presented and compared in Figures 4 and 5.
C5–C1–C2–C3
C1–C2–C3–C4
−0.37 (11)
0.09 (11)
C34–C30–C31–C32
C30–C31–C32–C33
C31–C32–C33–C34
0.17 (11)
−0.40 (12)
the observed bond lengths and angles of 2 related nicely to similar
compounds reported in the literature.[9,25,26] The notable features of
the X-ray crystal structure of 2 are those that mitigate destabilization
from the antiaromatic nature of its cyclopentadienone core. For exam-
ple, the cyclopentadienone core of 2 was found to have C=C bonds
(C2–C3, C4–C5, C31–C32, C33–C34) that varied from 1.3530(13) to
1.3598(13) Å, and C–C bonds (C1–C5, C1–C2, C3–C4, C30–C34,
C30–C31, C32–C33,) that varied from 1.4941(13) to 1.5016(13) Å.
These quite different C=C and C–C bond lengths illustrated the
Absorption spectra of 1–4 measured from chloroform solution
have three prominent bands. The least intense of these three
(ε
= ) appeared in the visible region
900–1500 M−1 cm−1
(500–530 nm), and is understood to have arisen from a charge trans-
fer transition.[27] The assignment of this band as a charge transfer
transition (π!π*) was supported using literature absorption spectra of
1, in which slight solvatochromism was observed. In chloroform, this
peak was observed at 530 nm, in dioxane at 504 nm, and in benzene

OROZCO ET AL.
5
TABLE 2 Room temperature absorption and emission data for
compounds 1–4 in CHCl3 solution
Compound
λ
max,abs nm (ε)a
λmax,em nm (λexc)
1b
265 (33 500)
344 (9000)
530 (1500)
266 (31 600)
337 (6800)
502 (1300)
268 (33 500)
349 (6070)
505 (1300)
268 (30 800)
371 (10 400)
456 (1500)
507 (900)c
2
3
4
596 (520)
566 (508)
617 (507)
FIGURE 5 Room temperature emission spectra of compounds
2 (•-•), 3 (—), and 4 (---) in chloroform solution (λex ≈ 500 nm)
aMolar extinction coefficient units M−1 cm−1
bPlease see reference 21
cShoulder.
.
intensity of this band points to it arising from another π!π* transition.
A red-shifting of this band in compound 4 as compared with
compounds 1–3 implicated the methoxy group in 4 as having a
notable stabilizing effect on the energy of this transition, most likely
via resonance electron donation.[1,9]
The preceding data point to the involvement of the aryl rings in
the 3- and 4-positions of compounds 3 and 4 in these two π!π*
transition.
The third intense band at approximately 265 nm (ε ≈ 30 000–-
34 000 M−1 cm−1) was insensitive to the nature of the R-group in 1–
4, which would indicate it arose from a π!π* transition that involved
the aryl rings in the 2- and 5-positions, as their structure remained
constant in compounds 1–4.[9]
Compounds 2–4 manifested steady-state absorption spectra that
related well to the work done by others on related compounds.
To the best of our knowledge, no emission from TACPD deriva-
tives has been reported. Excitation of dilute chloroform solutions of
compounds 2–4 in their absorption band at approximately 500 nm
resulted in luminescence being observed in the red end of the visible
spectrum from 550 to 625 nm. This emission was observed to be
qualitatively weak, and was assigned to a π*!π transition, whose
orbitals are involved in the charge transfer transition that is the origin
of the intense colour of compounds.
FIGURE 4 Room temperature electronic absorption spectra of
compounds 2 (---), 3 (—), and 4 (•-•) in chloroform solution
at 512 nm.[1,7,21] The excited state of this charge transfer transition is
theorized to be the antiaromatic dipolar structure shown in
Figure 2.[1,8,9] The electron-withdrawing substituents in 2–4 caused
this band to blue shift when compared with 1, pointing to their
destabilizing effects on this transition. There was little variation in the
wavelength of this absorption in 2–4, pointing to a similarity in the
effect of their substituents on the energetics of this transition.
It is interesting to note that only in the spectrum of 4 was an
absorption band at 456 nm (ε = 1300 M−1 cm−1) resolved, which most
likely has arisen from a π!π* transition.
Excitation spectra (Figure S1) were measured at the emission
maxima to confirm the authenticity of the observed luminescence.
The obtained excitation spectra were comparable, with each having
two prominent bands. One band was at a shorter wavelength that
populated a higher energy excited state, which in turn populated the
lowest energy excited state through radiationless relaxation. The
other band was at a longer wavelength that populated the lowest
energy excited state. A notable difference was the spectrum of 4
having its longer wavelength band red shifted due to resonance inter-
actions with its methoxy group (see Section 3.4.1), which is indicative
A
second, more intense absorption band (ε
=
6000–
11 000 M−1 cm−1) was observed in the range 330–370 nm. The

6
OROZCO ET AL.
of the stabilization of its excited state. These spectra exhibited
emission at the excitation wavelengths used in this study, confirming
that the observed luminescence was from compounds 2–4.
3.4 | Computational studies of compounds 2–4
3.4.1 | Structure optimization of compounds 1–4
Attributing spectral features to molecular orbital transitions can be
done qualitatively using extinction coefficient magnitudes obtained
from absorption spectra. Computational methods allow these assign-
ments to be made in a more specific way by connecting the energies
of molecular orbital combinations in a compound to a specific absorp-
tion or emission band. Optimistically, this can lead to rationale tailor-
ing of a molecule for a desired photophysical property. The first step
in the computational study of 1–4 is the optimization of their ground
state geometries (Figure S2). These structures were validated through
a comparison of the theoretical bond lengths of their common struc-
tural feature, their cyclopentadienone cores, and the experimental
ones from X-ray crystal structures of 1, 2, and 4 (Table S2).[9] The cal-
culated bond lengths for 3 were compared with the experimental ones
from the X-ray structure of 4, one of its constitutional isomers. Differ-
ences between experimental and theoretical bond lengths varied from
0.0002–0.015 Å, with a mean absolute error of 0.0061 Å. Keeping in
mind that the experimental and theoretical values were obtained in
different environments, these differences were small, and validated
our choice of the B3LYP 6-31G++ (d,p) basis set to obtain the opti-
mized ground state structures of 1–4.[18]
The optimized geometries of compounds 1–4 provided support
for the dipolar resonance structure shown in Figure 2 via the slightly
longer than expected C=O bond. The average calculated length of the
C–O double bond in the cyclopentadienone cores of 1–4 was found
to be 1.2149 Å, which is slightly longer than the average bond length
of 1.208 Å for a C–O double bond, as reported by Allen and
colleagues.[28]
The optimized structures also provided support for the slight par-
ticipation of the methoxy oxygen atomic orbitals in the molecular
orbitals of compounds 3 and 4 through the shortening of their met-
hoxy Car–O bond lengths. The average calculated methoxy Car–O
bond length in 3 was 1.3593 Å, and in 4 was 1.3601 Å. Both of these
average values are slightly shorter than the 1.3714 Å length of a met-
hoxy Car–O in a similar compound, and the reported average Car–O
bond length of 1.370 Å for alkyl aromatic ethers.[28,29]
FIGURE 6 Molecular orbitals of compound 1 computed at the
6-311G + (d,p) level of theory in chloroform (IEFPCM). The figure
uses a 0.02 a.u. cutoff for the contour of the molecular orbitals
lowest unoccupied molecular orbital (LUMO)(π*) transition, with the
HOMO(π) constructed from orbitals based on the aryl rings in the 2-
and 5-positions, and the cyclopentadienone core with the LUMO(π*)
being constructed from orbitals located predominantly on the
cyclopentadienone core. The energy of this transition varied from
−2.74 eV (453 nm) for 1, −3.00 eV (413 nm) for 2, −2.84 eV (436 nm)
for 3, and −2.73 eV for 4 (454 nm). While these theoretical values are
less than the actual values by 77, 89, 66, and 42 nm, respectively, the
calculated shapes of molecular orbitals of the LUMO are still useful as
3.4.2 | Molecular orbital assignment of the
absorption spectral features of compounds 1–4
The optimized geometries of 1–4 allowed the construction of their
molecular orbitals (Figures 6–9), and a more detailed assignment of
their excited states. The absorption band observed at approximately
500 nm arose from a highest occupied molecular orbital (HOMO)(π)!

OROZCO ET AL.
7
FIGURE 7 Molecular orbitals of compound 2 computed at the
6-311G + (d,p) level of theory in chloroform (IEFPCM). The figure
uses a 0.02 a.u. cutoff for the contour of the molecular orbitals
FIGURE 8 Molecular orbitals of compound 3 computed at the
6-311G + (d,p) level of theory in chloroform (IEFPCM). The figure
uses a 0.02 a.u. cutoff for the contour of the molecular orbitals
they relate to the electron distribution in the dipolar antiaromatic
resonance form shown in Figure 2.
The molecular orbitals of compounds 1–4 have a HOMO(π) and
HOMO-1(π) energetically close together, separated by −0.84, −0.80,
−0.63, and −0.53 eV, respectively. This feature was useful in
assigning the next transition. The absorption band at approximately
330–370 nm was assigned to a HOMO-1(π)!LUMO(π*) transition,
with the HOMO-1(π) of 1, 2, and 4 being mostly delocalized, and that
of 3 located on the aryl rings in the 2- and 5-positions. The energy of
this transition ranged from −3.58 eV (346 nm) for 1, −3.57 eV
(347 nm) for 2, −3.44 eV (350 nm) for 3, and −3.26 eV for 4 (380 nm).
These theoretical values varied by at most 10 nm from the experimen-
4-methoxy group in 4, whose oxygen atom participated in the forma-
tion of the HOMO-1(π) and LUMO(π*). The effect of this was that the
energies of both were raised, with the energy of the HOMO-1 more
so (Figure 9). The overall effect of this was that HOMO-1 and LUMO
were brought closer together in energy, resulting in a red-shifted
absorption in 4 compared with 2 and 3.
The absorption band at approximately 265 nm was thought to
arise from a HOMO(π)!LUMO+1(π*) transition, with the LUMO+1
(π*) mainly residing on the aryl rings in the 2- and 5-positions. This
assignment was based on the energy of this band varying from −4.78
(259 nm) for 1, −4.80 eV (258 nm) for 2, −4.86 eV (255 nm) for 3, and
−4.79 eV for 4 (259 nm). A comparison of these theoretical values to
the experimental ones revealed that the largest difference was 13 nm.
This small variation supports the assignment of the transition.
tal ones, and provide
assignment.
a realistic foundation for the proposed
This energy of the HOMO-1(π)!LUMO(π*) transition in com-
pound 4 was notably different from that of compounds 2 and 3, both
theoretically and experimentally. This difference was caused by the

8
OROZCO ET AL.
ACKNOWLEDGEMENTS
Whittier College is acknowledged for support of this research
through the Chester and Olive McCloskey Endowed Chair in Chem-
istry. The upgrade of the X-ray diffractometer at the Louisiana State
University was made possible by Grant No. LEQSF (2011-2012)-
ENH-TR-01, which was administered by the Louisiana Board of
Regents.
ORCID
Ralph Isovitsch
https://orcid.org/0000-0002-2013-4386
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4
| CONCLUSION
A trio of dramatically coloured tetraarylcyclopentadienone derivatives
was prepared. Their identities were confirmed by the usual methods,
such as 13C NMR spectroscopy, and their purity quantified by elemen-
tal analysis. The X-ray crystal structure of compound 2 was deter-
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planarity, both of which served to mitigate its antiaromatic nature.
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SUPPORTING INFORMATION
Additional supporting information may be found online in the
Supporting Information section at the end of this article.
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