Predicting the UV-vis spectra of tetraarylcyclopentadienones: Using DFT molecular orbital energies to model electronic transitions of organic materials

Article abstract of DOI:10.1021/jo701676x

(Chemical Equation Presented) Tetraphenylcyclopentadienone, due to its intrinsically low HOMO-LUMO gap, has been suggested as a valuable repeat unit in conducting polymers for nanoscale electronics. The HOMO and LUMO of tetraphenylcyclopentadienone appear

Full text of DOI:10.1021/jo701676x

Predicting the UV-Vis Spectra of Tetraarylcyclopentadienones:
Using DFT Molecular Orbital Energies to Model Electronic
Transitions of Organic Materials
Robert G. Potter and Thomas S. Hughes*
Department of Chemistry, Cook Physical Sciences Building, 82 UniVersity Place, UniVersity of Vermont,
Burlington, Vermont 05405
thomas.s.hughes@uVm.edu
ReceiVed August 15, 2007
Tetraphenylcyclopentadienone, due to its intrinsically low HOMO-LUMO gap, has been suggested as
a valuable repeat unit in conducting polymers for nanoscale electronics. The HOMO and LUMO of
tetraphenylcyclopentadienone appear to be associated with the relevant π orbitals of unsubstituted
cyclopentadienone. Using previously developed carbonylative coupling reactions, a series of tetraaryl-
cyclopentadienones was synthesized, accessing a range of substituents not previously available. The UV-
vis spectra of these molecules were compared to their calculated wave functions and predicted transitions.
A quantitative structure-activity relationship was discovered that may greatly simplify prediction of
band gaps for oligomers and polymers built from these tetraarylcyclopentadienones.
Introduction
materials for LEDs4 or photovoltaics.5 In particular, the fulvalene
and cyclopentadienone scaffolds have been suggested as useful
repeat units6 due to the low HOMO-LUMO gap of these
monomers and subsequent reduction of the band gap upon the
increase of conjugation upon oligomerization.7,8 Cyclopentadi-
enones in particular are also synthetic precursors of phenylenes,9
Molecules that contain strongly absorbing chromophores not
only have found use as pigments and dyes, but are also of current
interest as building blocks for low band gap conjugated
oligomers and polymers.1 Such materials are of particular
interest for their use as “organic metals”,2 as semiconductors
for use in field effect transistors,3 and as emissive or absorbing
(4) Harrison, M. G.; Friend, R. H. In Electronic Materials: The Oligomer
Approach; Mu¨llen, K., Wegner, G., Eds.; Wiley-VCH: Weinheim, Ger-
many, 1998. Robertson, N. Angew. Chem. 2006, 45, 2338. Shih-Chun Lo,
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Neugebauer, H.; Sariciftci, N. S. Chem. ReV. 2007, 107, 1324.
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Bre´das, J. L. In Electronic Materials: The Oligomer Approach; Mu¨llen,
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10.1021/jo701676x CCC: $40.75 © 2008 American Chemical Society
Published on Web 03/26/2008
J. Org. Chem. 2008, 73, 2995-3004
2995

Potter and Hughes
SCHEME 1. Tetraarylcyclopentadienones Synthesized and
Parent Cyclopentadienone
which are target materials for the same applications. It is
therefore highly desirable to obtain a detailed understanding of
the electronic structure of the tetraarylcyclopentadienone moiety,
in particular the HOMO-LUMO gap, and how that gap can be
tuned by substitution of the aryl groups. Herein is presented a
spectroscopic and molecular orbital study of a series of
substituted tetraarylcyclopentadienones (TACPDs).
Recent developments in time-dependent density functional
theory10 have allowed predictions of electronic transitions of
conjugated oligomers,11 although systematic errors arising from
long-range electron correlation are problematic for longer
oligomers.7 However, if CI calculations indicate that certain
transitions are primarily single-electron excitations, simple DFT
can in principle give reasonable predictions of the transition
energies. This has been demonstrated by several groups for a
diverse set of molecules.12 Ab initio MO and DFT computations
were therefore performed to model these transitions, and to
determine the level of theory required to predict the magnitude
of the HOMO-LUMO gap. A clear relationship between the
calculated electronic structure and the spectra was identified.
These results not only add to the wealth of experimental data
reported by other groups,13 but also provide a more quanti-
tative explanation of the substituent effects on the electronic
transitions of TACPDs that may guide the design of TACPD
oligomers.
There are few reported examples of TACPDs bearing
electron-withdrawing groups, and any investigation of the effect
of substituents on the electronic spectra of TACPDs should
include members of this class. The use of appropriate carbo-
nylative coupling reactions has allowed access to previously
unreported TACPDs with a range of electron-donating or
electron-withdrawing groups. This extended library of TACPDs
has allowed investigation of the electronic properties, and in
particular the electronic transitions.
of 2 in the absence of a solid matrix.17 Substitution of the
cyclopentadienone ring with phenyl rings provides enough steric
bulk to prevent intermolecular interaction to the extent that 1a
is stable at modestly high temperatures, but will react at high
temperature when an appropriate dienophile is present.18
The UV-vis spectra of substituted TACPDs have been
recorded earlier, and the electronic transitions have been
assigned to different valence-bond structures associated with
the exited states.13 While VB theory is well-known for its utility
for describing reaction mechanisms and other circumstances
where strict knowledge of electronic structure is not necessary,
the quantitative nature of molecular orbital (MO) theory and
its derivation from first principles make it a better suited model
for the analysis and prediction of electronic structure and UV-
vis transitions. A marriage of VB and MO theories has long
been pursued due to the valuable and quantifiable predictions
of the two models.19
The purpose of this study is to readdress the electronic spectra
of TACPDs by using an MO description of the ground and
excited states which may quantitatively predict the observed
phenomena. The relationship between the electronic properties
of the substituent, which could in principle be modeled as a
function of group electronegativity20 or a Hammett linear free
energy parameter,21 and the corresponding absorption maxima
were analyzed to find a quantitative relationship. To describe
the effect of various substituents on the π MOs of the TACPDs,
the correlations between the molecular orbitals of 1a and the
well-known parent compound 2 were established.
Tetraphenylcyclopentadienone (1a), first synthesized by
Dilthey and co-workers,14 is a stable analogue of the very
unstable cyclopentadienone 2 that was not isolated until years
later by matrix isolation in argon.15 The reactivity of the parent
compound can be explained by the presence of an antiaromatic
valence bond (VB) structure (Scheme 1) as one of the primary
resonance forms, a model that has been supported by DFT
calculations.16 The compound spontaneously undergoes an
intermolecular Diels-Alder cycloaddition with another molecule
Results and Discussion
Synthesis of Substituted Tetraarylcyclopentadienones. By
using carbonylative coupling reactions developed by Collman,22
des Abbes,23 and van Leusen24 followed by Knovenagel
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F. C.; Koizumi, Y.; Seki, S.; Saeki, A.; Tagawa, S.; Dudek, S. P.; Siebbeles,
L. D. A. J. Phys. Chem. B 2006, 110, 5984. Toivonen, T. L. J.; Hukka, T.
I. J. Phys. Chem. A 2007, 111, 4821.
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2996 J. Org. Chem., Vol. 73, No. 8, 2008

UV-Vis Spectra of Tetraarylcyclopentadienones
SCHEME 2. Synthesis of Series 1b-h
condensations,25 a variety of different TACPDs were synthesized
to expand the library of UV-vis data previously reported for
these compounds (Scheme 2). A convenient method of synthe-
sizing TACPDs involves the ethanolic potassium hydroxide-
mediated Knovenagel condensation between a benzil and a 1,3-
diaryl-2-propanone.13,14 The placement of substituents on the
aryl rings in 1 simply requires the use of appropriately
substituted benzils or 1,3-diaryl-2-propanones. Since Becker
observed a greater shifting of the longest wavelength transition
upon substitution of the 2- and 5-phenyl rings,13c substitution
of the 1,3-diaryl-2-propanone synthons was undertaken.
Preparation and use of the Collman reagent toward the
preparation of 1,3-diaryl-2-propanones 3 from the corresponding
benzyl bromides has been reported earlier.26 The yields of 3
were moderate to high when neutral or electron-donating sub-
stituents were present, but low to zero when electron-withdraw-
ing substituents were present. Strict purification of all starting
materials and solvents was necessary for the synthesis and
reactions of Na2Fe(CO)4 toward the synthesis of 3e, 3f, and
3g. To avoid the poor yields of the Collman couplings that give
3b and 3c, which bear nitro and methyl ester groups, respec-
tively, the tosylmethyl isocyanide-mediated carbonylative cou-
pling methodology of van Leusen was used with benzyl
bromides bearing these strongly electron-withdrawing groups.
Ketones 3b, 3c, and 3h were synthesized by treating 2 equiv-
alents of 4-nitrobenzyl bromide, 4-methoxycarbonylbenzyl bro-
mide, and 4-chlorobenzyl bromide, respectively, with base and
tosylmethylisocyanide under phase-transfer conditions, followed
by acid hydrolysis of the resulting dibenzyltosylisocyanides. To
probe the effect of strongly electron-donating substituents on
the optical spectra of TACPDs, an amino-substituted 1,3-diaryl-
2-propanone was desired. However, the difficulty encountered
in preparing 4-aminobenzyl bromide necessitated an alternate
route involving the reductive hydrogenation of 3b, which
unfortunately gave a complex mixture of oligomeric products
that appeared to involve the condensation of the reduced amino
groups with the carbonyl groups. Addition of di-tert-butyl
dicarbonate to the hydrogenation mixture did afford the Boc-
protected diamine 3d in quantitative yield from 3b.
raphy afforded TACPDs 1b-h. It should be noted that THF
was substituted as solvent in the synthesis of 1c to avoid
transesterification of the ester moiety with the solvent. Yields
of the Knovenagel condensations were moderate to high in most
cases, and the products are stable for over a year if stored in
the solid phase. However, solutions of many of these compounds
decompose within weeks.
UV-Vis Spectroscopy of Tetraarylcyclopentadienones.
UV-vis spectra for the substituted TACPDs are shown in Figure
1. All of the spectra were recorded in dioxane, at concentrations
of 0.02-0.04 mmol/L. Each spectrum was fitted by using a
sum of Gaussian functions;27 the root-mean-square error of the
sum of Gaussians compared to the raw spectrum was minimized
as the height, width, and λmax of each Gaussian was varied. The
number of Gaussians used to fit each spectrum was determined
by evaluating the root mean square; if no appreciable improve-
ment in the fit was observed upon adding an additional Gaussian
function, the next lowest number of functions was used.
The resulting absorption maxima are given in Table 1, along
with the fitted extinction coefficients. All of the TACPDs
exhibited three electronic transitions, which we have designated
as λ1 through λ3. A fourth absorption maximum, λ4, was
observed for all of the TACPDs except for 1a and 1f. Each of
these absorption maxima changed upon substitution of the para
positions of the 2- and 5-phenyl rings. The features at 210 nm
in all of the spectra recorded were fitted in each spectrum by a
Gaussian function with a λmax of 182 nm and a very large
extinction coefficient, which suggests they correspond to the
dioxane solvent.
The lowest energy transitions, λ1, exhibited a bathochromic
shift of 52 nm upon substitution of the electron-deficient
TACPD, 1b, with a λ1max of 492.5 nm, to the electron-rich 1d,
with a λ1
of 544.7 nm. The extinction coefficients varied
max
from 1300 to 2000 L mol-1 cm-1, and showed no correlation
to the electron demand of the substituents. The second-lowest
energy transitions, λ2, exhibited a 53 nm bathochromic shift,
of larger energy, with increasing electron-donating capacity of
the substituent, from 323.5 nm for electron-poor 1b to 376.1
nm for electron-rich 1d. The spectrum of 1e had previously
been measured separately in isooctane and benzene,13d which
allowed the observation of absorptions at 370 and 340 nm,
respectively. In dioxane, two closely overlapping absorptions
at 375 and 329 nm were fit with the Gaussian functions. A
similar pair of absorptions was observed in dioxane for 1d,
The syntheses of 1b-h were conducted with KOH in
refluxing ethanol. Quenching the reaction with a mild acid
followed by simple aqueous extraction and column chromatog-
(25) Johnson, J. R.; Grummitt, O. Organic Syntheses; Wiley: New York,
1955; Collect. Vol. III, p 806.
(26) Potter, R. G.; Hughes, T. S. Org. Lett. 2007, 9, 1187.
(27) Hu, Y.; Liu, J.; Li, W. Anal. Chim. Acta 2005, 538, 383.
J. Org. Chem, Vol. 73, No. 8, 2008 2997

Potter and Hughes
FIGURE 1. UV-vis spectra of 1a through 1h in dioxane. The inset shows the visible region of the spectra, and λ1 for each compound.
max
TABLE 1. Absorption Maxima and Extinction Coefficients for UV/VIS Spectra of 1a-h
a
compd
λ1
ꢀ1
λ2
ꢀ2
λ3
ꢀ3
λ4
max
ꢀ4
max
max
max
1a (R ) H)
503.8
492.5
496.0
544.7
539.7
518.1
513.0
510.0
1410
1960
1950
1960
1650
1340
1330
1910
331.6
323.5
341.0
376.1
375.3
338.6
345.2
330.7
7080
7260
6720
3060
3160
5330
4620
7700
259.4
321.1
284.3
277.9
276.5
262.2
271.1
264.5
26000
14700
35000
25300
7390
23300
10000
33800
1b (R ) NO2)
1c (R ) CO2CH3)
1db (R ) NHBoc)
1eb (R ) OCH3)
1f (R ) CH3)
1g (R ) Br)
248.9
233.5
242.9
255.9
9530
14900
12100
16800
253.7
231.7
17100
3560
1h (R ) Cl)
a λmax reported in nm, and ꢀ in L mol-1 cm-1 b 1d and 1e also exhibited absorptions at 328.5 and 319.7 nm with extinction coefficients of 3950 and 4250
.
L mol-1 cm-1, respectively.
which also bears electron-donating substituents. The large range
of λ2
observed in this set of TACPDs makes it clear that
isolobal28 to the unsubstituted 2, and one that can be treated as
the “ligand group” surrounding 2. Both 2 and this structure with
orthogonal π systems (1a(C2W)) are defined by the C2V point
group. The four frontier π MOs of cyclopentadienone are
depicted in Figure 2, as are the four frontier π MOs of 1a(C2W),
which contain components of these MOs from the cyclopenta-
dienone core mixed with very small contributions of the
orthogonal π MOs from the pendent phenyl groups. A signifi-
cant gap exists between the highest occupied and the lowest
unoccupied π MOs of 2 and 1a(C2W) that places the electronic
transition in the UV manifold. The HF/6-31G(d)//B3LYP/6-
31G(d) energies and geometries are shown for each of the MOs
involved, and agree with previous experimental data for 2.29
As the phenyl rings are rotated with respect to the plane of
the cyclopentadienone core, the π orbitals of the two types of
rings begin to mix. The orbitals and energies of 1a shown to
the right of Figure 2 are for the minimum energy structure, in
which the less sterically crowded 2- and 5-phenyl rings have a
37° dihedral angle with respect to the cyclopentadienone core,
and the more sterically crowded 3- and 4-phenyl rings have a
dihedral angle of 52°. The gray lines indicate MOs of 1a(C2)
that arise from small contributions of the cyclopentadienone π
orbitals mixing with the pendent phenyl π orbitals, while the
black lines indicate the frontier orbitals of 1a(C2) that are
max
this transition is affected by substituents on the 2- and 5-phenyl
rings, contrary to what was claimed earlier13b,d on the basis of
a more limited set of spectra. Although all substituents shift
the λ3
257 nm absorption of the unsubstituted TACPD 1a
max
bathochromically, again contrary to previous claims,13d a clear
relationship between λ3 and both the position and electron-
max
donating capacity of the substituent was not evident in this larger
set of substituted TACPDs. It should be noted that λ3 for 1b
initially appeared to be absent, but the broad absorption
maximum at ∼323 nm for 1b was fitted well by the sum of
two gaussians, one centered at 321 nm and the other at 323
nm. To develop a tool for quantitatively predicting the spectra
of TACPDs, in particular the long wavelength transition λ1, the
molecular orbitals and electronic transitions of 1a and its parent
compound 2 were calculated.
Electronic Structure and Electronic Transitions of Tet-
raarylcyclopentadienones. TACPDs can be considered as
perturbations of the cyclopentadienone core 2 by substitution
of each hydrogen with an aryl ring, which can potentially lower
the C2V symmetry of 2 depending on their orientation. To
demonstrate this descent in symmetry, consider 2 substituted
with four phenyl rings that are each perpendicular to the inner
cyclopentadienone ring, such that the dihedral angle θ ) 90°.
This molecule consists of two orthogonal π frameworks, one
(28) Hoffmann, R. Nobel Lecture, 1981.
(29) Koenig, T.; Smith, M.; Snell, W. J. Am. Chem. Soc. 1977, 99, 6663.
2998 J. Org. Chem., Vol. 73, No. 8, 2008

UV-Vis Spectra of Tetraarylcyclopentadienones
interaction between the rings has a different effect on the
energies of what are the HOMO and LUMO for 1a(C2). Figure
4 shows that the HOMO is greatly destabilized as the π systems
begin to interact, while the energy of the LUMO is essentially
unchanged. This is presumably the result of the relatively small
contribution of the phenyl π orbitals in the LUMO, as can be
seen in Figure 2.
A thorough review of the UV-vis spectra of 2 and its
theoretical description has been reported earlier along with other
fulvene-like molecules.30 Two major absorption bands were
calculated at the INDO/S-CI level that matched observed
absorptions,15 a relatively weak absorption from the HOMO f
LUMO (A2π f B1π*) transition of B2 symmetry, as well as
a relatively strong HOMO f LUMO+2 (A2π f B1π*)
transition also of B2 symmetry. These results were reproduced
at the CIS/6-31G(d)//B3LYP/6-31G(d) level of theory, as shown
in Table 2. The lower energy transition of B2 symmetry
consisting of a HOMO f LUMO (A2π f B1π*) excitation is
the same as that found in the INDO/S model. However, the
stronger band was found at the higher level of theory to be a
mixed transition of A1 symmetry consisting primarily of the
HOMO-2 f LUMO (B1π f B1π*) excitation mixed with the
HOMO f LUMO+1 (A2π f A2π*) excitation. Only seven
transitions were reported in the INDO study, so up to 30
transitions were calculated in the present study. In this larger
set of transitions, another was found consisting of a HOMO f
LUMO+1 (A2π f A2π*) excitation, with a similar transition
dipole moment, and at a slightly higher energy. Since the
spectrum of 1a has three absorptions, the identification of this
third transition in the parent compound served as a reference
point for the third transition present in 1a.
1a(C2W) is predicted to have the same transitions arising from
the π MOs of the cyclopentadienone core as 2 (Figure 2),
namely those consisting of the A2π f B1π*, B1π f B1π*,
and A2π f A2π* excitations, along with many higher energy
transitions associated with the phenyl rings. Again, the orthogo-
nality of the π systems in 1a(C2W) prevents significant mixing
of the central cyclopentadienone ring π orbitals with those on
the pendant phenyl rings, which results in similar transitions to
those found for 2; the slight destabilization of occupied π MOs
and the slight stabilization of the unoccupied π MOs lower the
energies of the three transitions. The transition arising from the
HOMO-1 f LUMO (B2σ f B1π*) excitation is A2 and is
formally disallowed. The transition arising from the HOMO f
LUMO+1 (A2π f B2σ*) excitation of B1 symmetry is
allowed, but is predicted to have a near zero probability in the
CIS/6-31G(d)//B3LYP/6-31G(d) model.
FIGURE 2. Correlation diagram of HF/6-31G(d) molecular orbitals
of 2, 1a(C2W) and 1a(C2). Left: Upon substitution of the cyclopenta-
dienone with orthogonal phenyl rings, the core π MOs remain nearly
isoenergetic. Rotation of the phenyl rings to allow interaction of the π
systems destabilizes the Aπ and Bπ while stabilizing the Aπ*. Note
that the energy of the Bπ* orbital does not change significantly upon
interaction with the pendent phenyl groups.
predominantly derived from the cyclopentadienone π MOs. As
with many biaryl systems, the overlap between the adjacent π
systems would reach its maximum when the two rings are
coplanar, but this places the neighboring phenyl rings into van
der Waals contact. The mixing of the π orbitals of the pendent
phenyl rings destabilizes the A2π HOMO (which is now Aπ
in the C2 point group), and at the same time it stabilizes two
other lower lying Aπ MOs (indicated by the gray lines in Figure
2). This mixing also stabilizes, albeit to a much lesser extent,
the B1π* LUMO (now Bπ in the C2 point group) while
destabilizing two other higher lying Bπ MOs. Two other
relevant cyclopentadienone π MOs are also affected by the
interaction with the phenyl π MOs; the occupied B1π is greatly
destabilized and the unoccupied A2π* orbital is greatly stabi-
lized. The result is that these orbitals which were involved in
the high intensity but short wavelength transitions in 2 become
close enough in energy that at least three transitions involving
these MOs move into the UV-vis region.
Plots of the total energy of 1a as a function of θ as well as
the energies of the highest occupied Aπ and lowest unoccupied
Bπ* MOs are shown in Figure 3. To scan the relative total
energy as a function of θ, all four phenyl ring dihedrals were
constrained to be identical by the symmetry of the Z-matrix.
Despite this artificial constraint on the geometry of 1a, the
minimum energy agrees well with the fully optimized B3LYP/
6-31G(d) geometry and other maxima correspond to expected
boundary conditions; 1a(C2W) exists as a local energy maximum,
and the total energy of 1a rises rapidly as θ approaches zero,
which places neighboring phenyl rings in van der Waals contact.
Changing the dihedral θ from orthogonality and increasing π
When 1a is relaxed to C2 symmetry, the preferred geometry
at every level of theory examined, the same three major
transitions are found at the CIS/6-31G(d)//B3LYP/6-31G(d)
level of theory. Since the mixing of the phenyl π orbitals
destabilizes the highest occupied B1π and A2π orbitals and
stabilizes the lowest unoccupied A2π* orbital, the transition
energies are all calculated to be smaller than those of 1a(C2W).
The transition arising from the HOMO f LUMO (Aπ f Bπ*)
excitation remains present throughout the descent in symmetry
from 2, but becomes mixed with a transition from a lower lying
orbital of like symmetry (see the Supporting Information for
the full listing of the excitations involved) that also contains
significant phenyl and cyclopentadienone π overlap. The HOMO
f LUMO+1 (Aπ f Aπ*) and HOMO-1 f LUMO (Bπ f
(30) Buemi, G.; Zuccarello, F.; Raudino, A. THEOCHEM 1981, 76, 137.
J. Org. Chem, Vol. 73, No. 8, 2008 2999

Potter and Hughes
FIGURE 3. Phenylcyclopentadienone dihedral angular dependence of relative total energy and MOs of 1a. Left: HF/63-1G(d) total energy of 1a
as a function of the four dihedral angles θ. The maximum at θ ) 0° represents the lack of π overlap between the core and phenyl rings. Right:
Highest occupied Aπ and lowest unoccupied Bπ* energies as a function of θ. As the dihedral decreases from orthogonality (θ ) 90°), the Aπ is
destabilized while the Bπ* is nearly isoenergetic.
FIGURE 4. Assignment of λ1, λ2, and λ3 to single electron excitations. Left: Effect of the phenyl substitution on the energies of the relevant
orbitals and thus transitions: energies of all transitions are lowered. Right: Effect on the orbital energies of electron-donating substituents on the
phenyl rings: energies of λ1 and λ2 are lowered, but λ3 is not necessarily lowered.
TABLE 2. Electronic Transitions Calculated with CIS/6-31G(d) Corresponding to Absorption Maxima
λ1: A2π f B1π* a
λ2: B1π f B1π*
λ3: A2π f A2π*
E (kcal mol-1
)
µb (D)
cc
E (kcal mol-1
)
µ (D)
c
E (kcal mol-1
)
µ (D)
c
2
92.8
74.9
73.9
0.029
0.057
0.069
0.70
0.69
0.65
177.7
162.9
116.9
0.573
1.099
0.697
0.68
0.60
0.62
220.5
182.4
161.3
0.594
1.372
0.888
0.67
0.28
0.35
1a(C2W)
1a(C2)
a Three major absorption maxima and their corresponding primary single electron excitation. b Electric transition dipole moment in Debyes. c Coefficient
of indicated single electron excitation in overall wave function.
Bπ*) excitations mix, and give rise to a lower energy transition
that is primarily the lower energy Aπ f Aπ* excitation and a
higher energy transition that is primarily the Bπ f Bπ*
excitation, although the latter has significant mixing of several
other MO excitations.
TACPDs studied. However, the level of theory required to make
a quantitative prediction of the electronic spectrum had to be
determined. The transitions were identified at the CIS/6-31G-
(d)//B3LYP/6-31G(d) level of theory, and in principle more
accurate energies could be obtained from a configuration
interaction computation that includes doubles, triples, and
quadruples or a multiconfigurational self-consistent field cal-
culation such as a CASPT231 in which the active space includes
Correlation of MO Energies and UV/Vis Spectra. To
correlate the computed transitions with those observed in the
spectra, computations were required on each of the eight
3000 J. Org. Chem., Vol. 73, No. 8, 2008

UV-Vis Spectra of Tetraarylcyclopentadienones
FIGURE 5. Correlation of the observed and calculated electronic
transitions of 1a-h, using B3LYP/6-31G(d) calculated MO energies.
FIGURE 6. Correlation of the observed and calculated electronic
transitions of 1a-h, using TD-B3LYP/6-31G(d) calculated excitation
energies.
the relevant π MOs. TDDFT could also be employed to calculate
the lowest relevant electronic transitions. However, since each
of the three transitions identified involves one predominant
single electron excitation, it is reasonable to expect that there
would be a correlation between the difference of the relevant
orbital energies and the observed transitions.
The observed λ1max values show a linear correlation with the
HOMO-LUMO gaps calculated for 1a-h, as can be seen in
Figure 5. The correlation was observed for the two levels of
theory performed, the HF/6-31G(d) and the B3LYP/6-31G(d)
methods. The correlation is best for the DFT method, with a
linear least-squares fit having an R2 value of 0.95, while for
the HF methods, the R2 value was 0.89. Higher levels of theory
could be expected to give more accurate orbital energies or more
detailed descriptions of the allowed optical transitions, but would
not likely improve the fit of the experimental data to an extent
that would justify the computational expense.
to within 5 nm.32 The MOs of 1a were calculated at the B3LYP/
6-311+G(2d,p) level, and the HOMO-LUMO gap was found
to be 1 kcal/mol more accurate for λ1, 0.5 kcal/mol less accurate
for λ2, and 3.5 kcal/mol less accurate for λ3. While it is
reasonable to expect that the more complete basis set may give
more accurate MO energies for some if not all of the TACPDs
discussed herein, the error inherent in approximating electronic
transitions as single electron excitations may overwhelm this
benefit. Similarly, alternative functionals may give more accu-
rate energies, but this may not be reflected after the MO energy
gap approximation is made.
At the B3LYP/6-31G* level, the two higher energy transi-
tions, λ2 and λ3, also show a straight-line dependence on the
calculated HOMO-1-LUMO and HOMO-LUMO+1 gaps,
respectively. Not only do the values for each individual transition
follow a well-behaved linear correlation with the computed MO
gaps, but also all fall on the same line with a slope very close
to one when plotted together, indicating that the DFT MO gaps
are very good predictors of the actual UV/vis transitions across
a large range of energies. A similar straight-line plot is obtained
with the HF computed MO gaps, but with larger systematic
errors. The straight-line correlations observed suggest that the
three major electronic transitions for 1a-h, and by extension
1a(C2W) and 2, are best described by the single electron
excitations shown in Figure 4.
A major goal of this study was to identify a computationally
inexpensive method to predict the electronic spectra of TACPDs.
The correlations described above can now be compared to the
VB models previously described by Becker and co-workers
which proposed that the two longest wavelength transitions of
1a correspond to the two charge transfer states depicted in
Scheme 3, both of which eliminate the antiaromatic VB
depiction of the ground state. The polarization of the CIS/6-
31G(d) HOMO f LUMO excitation-derived transition is
consistent with the polarization required for λ1 in Scheme 3,
and the same is true for the CIS/6-31G(d) HOMO-1 f LUMO
To demonstrate this, TDDFT (B3LYP/6-31G(d)) computa-
tions were carried out on 1a-1h to determine the three
electronic transition energies, and the linear least-squares
correlation is 0.98, as is shown in Figure 6. While this is clearly
a better correlation than that obtained for the simple DFT
method, the TDDFT methods required 6-10 times more
computational time, despite the calculation of only the three
lowest energy transitions, which included only λ1 and λ2. A
further comparison of the DFT MO method to the TDDFT
shows that there is a larger systematic error in the DFT MO
correlations; the largest computed error and average error for
λ1 for the DFT MO method are 5.9 and 4.9 kcal/mol,
respectively, while they are 2.9 and 2.0 kcal/mol for the TDDFT
method. However, if the systematic errors in both methods are
compensated for by performing linear correlations of both data
sets for λ1, these errors are reduced to 1.2 and 0.5 kcal/mol for
the DFT MO method and 1.1 and 0.4 kcal/mol for the TDDFT
method. Recent TDDFT methodological studies have shown that
the choice of functional and basis setsin particular 6-311+G-
(2d,p)scan play a significant role in achieving high accuracy,
(32) Jacquemin, D.; Preat, J.; Wathelet, V.; Perpe`te, E. A. THEOCHEM
2005, 731, 67. Jacquemin, D.; Preat, J.; Wathelet, V.; Fontaine, M.; Perpe`te,
E. A. J. Am. Chem. Soc. 2006, 128, 2072. Jacquemin, D.; Preat, J.; Wathelet,
V.; Perpe`te, E. A. J. Chem. Phys. 2006, 124, 074104.
(31) Andersson, K.; Malmqvist, P.-Å.; Roos, B. O. J. Chem. Phys. 1992,
96, 1218. Serrano-Andre`s, L.; Roos, B. O. Chem.-Eur. J. 1997, 3, 717.
J. Org. Chem, Vol. 73, No. 8, 2008 3001

Potter and Hughes
SCHEME 3. Valence Bond Descriptions of Transitions λ1 and λ2
TABLE 3. Electronic Transitions Calculated with Modeling of
Solvation CIS/6-31G(d) Corresponding to Absorption Maxima with
Average (unsigned) Errors in kcal mol-1 and Largest Unsigned
Error in Parentheses
Although no 3- or 4-phenyl-substituted analogues were syn-
thesized in the current study, computations of such analogues
for which spectra had previously been recorded gave similar
results to the 1a-h series (see the Supporting Information).
While the DFT MO correlation to observed spectral absorp-
tions is quantitative in nature, the simple and very elegant VB
models described earlier are useful in making qualitative
predictions, and the buildup of positive charge on the pendent
phenyl rings in each VB description of the electronic transitions
suggests that, as observed, electron-donating groups will stabilize
the excited state while electron-withdrawing groups will desta-
bilize the excited state, thereby blue-shifting the transition. This
is exactly what is observed in the 1a-h series, and it is
reasonable to search for a simple parameter that captures the
electron demand of the substituents as a model for the transition
energies, and by extension, the HOMO-LUMO gap in these
materials.
λ1: A2π f
B1π* a
λ2: B1π f
B1π*
λ3: A2π f
A2π*
B3LYP/6-31G*
SCRF B3LYP/6-31G*
4.8 (5.9)
4.9 (6.0)
4.7 (9.2)
4.3 (9.0)
1.0 (2.1)
1.0 (2.7)
a Three major absorption maxima and their corresponding primary single
electron excitation.
The plots of λ1max versus either Hammett σ values21 or group
electronegativities20 gave random scatter, which suggested that
the substituents were playing a greater role than just an inductive
perturbation of the MOs involved in the transition. The σ+
parameter33 has been determined for different substituents in
the ionization of phenyldimethylchloromethane where a formal
positive charge is present at the ipso carbon. Such a positive
charge resembles those in the charge transfer states shown in
Scheme 3. A plot of σ+ versus the λ1
values is shown in
max
Figure 7, and although the correlation is not as precise as for
the DFT MO method, it is still accurate enough to provide a
rough prediction of the HOMO-LUMO gap for a TACPD
substituted at the para positions of the 2- and 5-phenyl rings.
The VB model also suggested that a charge transfer was
involved in λ2, and Figure 7 also shows a reasonable correlation
between σ+ values and the observed λ2max for 1a-h. A similar
FIGURE 7. Correlation of observed electronic transitions with σ+.
Correlations are found for λ1 and λ2, but not for λ3.
excitation-derived transition polarization and that required for
λ2. Thus, as has been stated earlier the valence bond versus
molecular orbital theory dilemma is in fact an ill-posed one,
and should be replaced by the notion that they are complimen-
tary.19
Given the polarization suggested by the VB depictions and
calculated at the CI and TDDFT levels, it is reasonable to
consider the effect of solvation on the energies of the electronic
transitions. SCRF B3LYP/6-31G* calculations were carried out
on 1a-h, and the results are shown in Table 3. Modeling of
the solvation with the SCRF method did not improve the fit of
the MO energy differences to the electronic transition energies
appreciably. The straight-line correlation did improve slightly
upon inclusion of the solvation effects, increasing R2 from 0.978
to 0.984. Overall, this does not appear to be a significant enough
improvement in the predictive power of the MO energy
differences to justify the computational cost.
In the smaller set of TACPDs Becker and co-workers
examined, the VB model was proposed because substitution of
the 2- and 5-phenyl rings predominantly shifted the high
wavelength absorption λ1 and substitution of the 3- and 4-phenyl
rings predominantly shifted λ2. By examining a larger set of
substituents, including some electron-withdrawing groups,
perturbations of all three major transitions were observed.
plot for the λ3
values shows no such correlation, however.
max
Examination of the MOs in Figure 2 shows that the Aπ LUMO
has relatively little contribution from the pendent phenyl ring
π orbitals, and as a result, the energy of this orbital changes to
a lesser extent upon substitution of those phenyl rings. The other
three relevant orbitals, the Bπ HOMO-1, the Aπ HOMO, and
the Aπ* LUMO+1, have energies that change more upon
substitution of the pendent phenyl rings. The result is that λ1
and λ2, which both involve excitation from a variable-energy
MO to the relatively constant LUMO, show a stonger depen-
dence on the phenyl substituents than does λ3, which involves
an excitation from and to MOs whose energies vary with
substitution. This is shown in Figure 4, in which 1a is substituted
with electron-donating substituents to give a generic 1x; λ1 and
λ2 both decrease but λ3 remains constant.
Conclusions
The dark purple color of the TACPDs arises from the very
broad visible absorption band around 510 nm that has been
(33) McDaniel, D. H.; Brown, H. C. J. Org. Chem. 1958, 23, 42.
3002 J. Org. Chem., Vol. 73, No. 8, 2008

UV-Vis Spectra of Tetraarylcyclopentadienones
acetate in hexanes on silica) and subsequent recrystallization in
ethanol and ethyl acetate provided the desired ketone (0.518 g, 62%,
mp 139 °C). 1H NMR δ 7.99 (d, J ) 8.6 Hz, 4H), 7.21 (d, J ) 8.6
Hz, 4H), 3.91 (s, 6H), 3.80 (s, 4H); 13C NMR δ 203.7, 166.8, 138.7,
130.0, 129.6, 52.1, 49.2; FTIR (neat, cm-1) 2954.86, 1707.01,
1629.29, 1575.20, 1510.58, 1434.35, 1416.32, 1275.18, 1177.10,
1100.14, 1052.23, 1017.73, 964.64; PCIMS (m + 1)/z calcd
327.1154, found 327.1.
represented by a formal charge transfer absorption. CIS
computations determined that this transition consists primarily
of the HOMO to LUMO single electron excitation, and the good
correlation of the DFT computed HOMO-LUMO gap with the
observed absorptions of 1a-h corroborated this assignment.
Similar correlations of computed MO energy gaps to observed
absorptions allowed assignment of two other electronic transi-
tions to single-electron excitations. The use of this model is
computationally less demanding than CI or CAS computations
of the electronic transitions, and can serve as a relatively rapid
screen for determining the shifts of TACPD HOMO-LUMO
gaps arising from substitutions on the aryl rings, or in principle
band gaps of larger molecules for which more detailed
computations would be prohibitively costly. Perturbation and
prediction of their HOMO-LUMO gaps are advantageous if
these TACPDs are to be used as electronic materials. Though
the production of high molecular weight oligomers is still under
study, they may have utility as electronic materials due pri-
marily to the tunable nature of the HOMO-LUMO gap of the
monomer.
1,3-Bis(4-(tert-butoxycarbonylamino)phenyl)propan-2-one (3d).
1,3-Bis(4-nitrophenyl)-propan-2-one (0.020 g, 0.067 mmol), Pd/C
(0.014 g, 10 wt %), and di-tert-butyl dicarbonate (0.100 g, 0.458
mmol) were dissolved in 15 mL of ethyl acetate, to which 0.243 g
(3.28 mmol) of Ca(OH)2 was added. The reaction was agitated in
a Parr shaker for 18 h under 58 psi of H2. Column chromatography
(hexanes on silica gel) gave the desired product (0.027 g, 99.9%,
mp 127 °C). 1H NMR δ 7.30 (d, J ) 8.4 Hz, 4H), 7.05 (d, J ) 8.4
Hz, 4H), 3.48 (br s, 2H), 3.63 (s, 4H), 1.51 (s, 18H); 13C NMR δ
206.0, 152.7, 137.3, 130.0, 128.5, 118.8, 48.3, 28.3; FTIR (cm-1
,
neat) 3329.69, 2926.12, 1717.64, 1699.84, 1593.93, 1520.14,
1456.90, 1413.86, 1391.91, 1366.25, 1317.50, 1231.95, 1152.17,
1051.44, 1051.44, 1018.35, 830.09, 771.11, 659.79, 503.45, 403.46;
HRMS (m + Li)/z calcd 447.2489, found 447.2466.
2,5-Bis(4-nitrophenyl)-3,4-diphenylcyclopenta-2,4-dienone (1b).
3b (0.156 g, 0.520 mmol), benzil (0.471 g, 2.240 mmol), and
potassium hydroxide (0.039 g) were dissolved in 10 mL of ethanol
and the solution was brought to reflux for 2 h prior to cooling to
room temperature and quenching with saturated ammonium chloride
in 0.5% HCl. The aqueous layer was extracted with three portions
of hexanes. Flash chromatography (5% ethyl acetate in hexanes)
yielded 1b as a bright purple crystalline solid (0.199 g, 81%, mp
Experimental Section
1,3-Bis(4-nitrophenyl)-propan-2-one34 (3b). 4-Nitrobenzyl bro-
mide (0.956 g, 4.425 mmol), tetrabutylammonium iodide (0.186
g, 0.504 mmol), and tosylmethylisocyanide (0.432 g, 2.213 mmol)
were dissolved in 50 mL of methylene chloride and the solution
was stirred vigorously until the additives were completely dissolved.
After 5 min, 20 mL of 40% aqueous NaOH was added and the
reaction mixture was stirred as rapidly as possible for 4 h prior to
neutralization with HCl. The resulting mixture was transferred to
a separatory funnel and extracted with three portions of methylene
chloride. Solvent was removed from the combined organic layers
and dissolved in 8 mL of methylene chloride. HCl (1.0 mL, 50%)
was added and the resulting red solution was stirred for 30 min.
The hydrolysis was quenched with aqueous sodium bicarbonate
and partitioned between water and methylene chloride. The organic
layer was removed and the aqueous layer was extracted with three
portions of methylene chloride. Chromatography (methylene chlo-
ride on silica gel) followed by recrystallization from ethanol/ethyl
acetate gave the desired product (0.379 g, 57%, mp 178-179 °C).
1H NMR δ 8.21 (d, J ) 8.8 Hz, 4H), 7.34 (d, J ) 8.8 Hz, 4H),
3.93 (s, 4H); 13C NMR δ 201.9, 147.4, 140.4, 130.5, 123.9, 49.0;
FTIR (neat, cm-1) 1719.94, 1598.68, 1507.55, 1407.68, 1342.06,
1214.37, 1177.94, 1108.83, 1051.83, 1017.22, 855.29, 826.77,
790.06, 736.43, 707.67, 656.61, 631.25; PCIMS (m + 1)/z calcd
300.0746, found 300.2.
1,3-Bis(4-methoxycarbonylphenyl)propan-2-one35 (3c). Meth-
yl-4-(bromomethyl)benzoate (1.169 g, 5.103 mmol), tosylmethyl-
isocyanide (0.483 g, 2.47 mmol), and tetrabutylammonium iodide
(0.231 g, 0.625 mmol) were dissolved in 35 mL of methylene
chloride and the solution was stirred as rapidly as possible until it
was completely homogeneous. After 5 min, 20 mL of 40% aqueous
NaOH was added slowly to maintain the high stirring rate. The
reaction was stirred vigorously for 4 h prior to neutralization with
HCl. The resulting mixture was transferred to a separatory funnel
and extracted with three portions of methylene chloride. Solvent
was removed from the combined organic layers and dissolved in 8
mL of methylene chloride. HCl (1.0 mL, 50%) was added and the
resulting red solution was stirred for 30 min. The hydrolysis was
quenched with aqueous sodium bicarbonate and partitioned between
water and methylene chloride. The organic layer was removed and
the aqueous layer was extracted with three portions of methylene
chloride. Two column chromatographic purifications (10% ethyl
1
245 °C). H NMR δ 8.10 (d, J ) 9 Hz, 4H), 7.41 (d, J ) 9 Hz,
4H), 7.34 (t, J ) 7.5 Hz, 2H), 7.24 (t, J ) 7.5 Hz, 4H), 6.91 (dd,
J ) 8 Hz, 1 Hz, 4H); 13C NMR δ 197.9, 157.5, 146.9, 137.0, 131.7,
130.8, 129.7, 129.0, 128.6, 124.2, 123.3; FTIR (neat, cm-1) 3065.05,
2924.86, 2849.55, 1708.99, 1591.87, 1511.00, 1441.65, 1338.56,
1307.07, 1114.41, 862.99, 745.92, 718.01, 697.11; HRMS (m +
Li)/z calcd: 481.1376, found 481.1357.
2,5-Bis(4-methoxycarbonylphenyl)-3,4-diphenylcyclopenta-
2,4-dienone (1c). 3c (0.119 g, 0.365 mmol), benzil (0.102 g, 0.485
mmol), and potassium hydroxide (0.025 g) were dissolved in
10 mL of THF and the solution was brought to reflux for 2 h prior
to cooling to room temperature and quenching with saturated
ammonium chloride in 0.5% HCl. The aqueous layer was extracted
with three portions of hexanes. Flash chromatography (5% ethyl
acetate in hexanes on silica) yielded 1c as a bright purple crystalline
1
solid (0.084 g, 71%, mp 203-205 °C). H NMR δ 7.91 (d, J )
8.2 Hz, 4H), 7.31 (d, J ) 8.3 Hz 4H), 7.26-7.30 (m, 2H), 7.19 (t,
J ) 7.5 Hz 4H), 6.91 (dd, J ) 8.5 Hz, 1 Hz, 4H), 3.89 (s, 6H); 13
C
NMR δ 198.9, 166.8, 156.1, 135.2, 132.4, 130.0, 129.2, 129.1,
129.0, 128.9, 128.2, 124.9, 52.0; FTIR (neat, cm-1) 2949.94,
1730.17, 1706.79, 1604.17, 1435.12, 1406.20, 1274.84, 1186.27,
1101.38, 1074.02, 1018.52, 959.01, 863.85; HRMS (m + Li)/z calcd
507.1784, found 507.1777.
2,5-Bis(4-(tert-butoxycarbonylamino)phenyl)-3,4-diphenylcy-
clopenta-2,4-dienone (1d). 3d (0.104 g, 0.264 mmol), benzil (0.115
g, 0.547 mmol), and potassium hydroxide (0.011 g) were dissolved
in 10 mL of ethanol and the solution was brought to reflux for 2 h
prior to cooling to room temperature and quenching with saturated
ammonium chloride in 0.5% HCl. The aqueous layer was extracted
with three portions of hexanes. Flash chromatography (5% ethyl
acetate in hexanes) yielded 1d as a bright purple crystalline solid
(0.049 g, 30%, mp 137 °C). 1H NMR δ 7.20-7.25 (m, 6H), 7.14-
7.19 (m, 8H), 6.91 (dd, J ) 8.3 Hz, 1 Hz, 4H), 6.44 (br s, 2H),
1.50 (s, 18H); 13C NMR δ 200.9, 153.7, 152.5, 137.6, 133.3, 130.8,
129.2, 128.3, 128.0, 125.5, 124.4, 117.9, 74.7, 28.3; FTIR (neat,
cm-1) 3362.22, 2975.29, 2930.17, 1706.62, 1585.65, 1518.83,
1455.04, 1408.19, 1392.32, 1366.39, 1316.44, 1230.19, 1152.31,
1084.12, 1051.61, 1027.15, 846.49, 769.07, 730.46, 702.37; HRMS
(m + Li)/z calcd 621.2941, found 621.2913.
(34) Campaigne, E.; Edwards, B. E. J. Org. Chem. 1962, 27, 3760.
(35) Inaba, S.; Rieke, R. D. J. Org. Chem. 1985, 50, 1373.
J. Org. Chem, Vol. 73, No. 8, 2008 3003

Potter and Hughes
Computational Details. Geometry optimization of the cyclo-
pentadienones was carried out with the Gaussian 03 software
package36 at the B3LYP/6-31G(d) level of theory,37 and the minima
were confirmed by the presence of no imaginary frequencies in
frequency calculations. Single point energies were then calculated
at the HF/6-31G(d)38 level, using the B3LYP/6-31G(d) optimized
geometry. Despite the failure of DFT methods for calculating
eigenvalues and enthalpies of formation reported earlier for other
large organic molecules39 the B3LYP hybrid functional worked well
with the rather simple geometries of the TACPDs. Prediction of
the first 30 electronic states was performed by using a time-
independent CIS40/6-31G(d) model, using a frozen core and direct
calculation of integrals. The symmetry enforced structure of 1a-
(C2W) was optimized by a symmetry-constrained Z-matrix. The
optimized parameters from this structure were then held fixed while
all four of the pendent phenyl rings were rotated from θ ) 90° to
0° to generate the energy plot in Figure 3.
Acknowledgment. We thank the University of Vermont for
financial support of this work and Dr. Kelvin Chu in the Physics
Department at the University of Vermont for computational
cycles. Mass spectrometry was provided by the Washington
University Mass Spectrometry Resource with support from the
NIH National Center for Research Resources (Grant No.
P41RR0954).
Supporting Information Available: Complete ref 25; NMR
spectra; UV-vis spectra of novel compounds; detailed description
of computational details including optimized Cartesian coordinates
and/or Z-matrices for all TACPDs and CIS output for 2, 1a(C2W),
and 1a(C2); plots of computed MO energy differences versus
observed absorption spectra for the HF method and 1a-h, and for
the HF method and tetraphenylcyclopentadienones substituted at
p-3-phenyl and p-4-phenyl from the literature. This material is
available free of charge via the Internet at http://pubs.acs.org.
(36) Frisch, M. J.; et. al. Gaussian 03; Gaussian Inc.: Wallingford, CT,
2004.
(37) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Becke, A. D.
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