Ortho-branched ladder-type oligophenylenes with two-dimensionally π-conjugated electronic properties

Article abstract of DOI:10.1021/ja202144v

The synthesis, photochemical and electrochemical properties, and electronic structures of a series of star-shaped ladder-type oligophenylenes Sn (n = 7, 10, 13, 16, 19, and 22), including one multibranched case S19mb, are reported and compared with the linear para-phenylene ladders Rn (n = 2-5 and 8) and the stepladder analogues SFn (n = 10, 16, and 22). The n value refers to the number of π-conjugated phenylene rings. Functionalized isotruxenes are the key synthetic building blocks, and S22 is the largest monodispersed ladder-type oligophenylene known to date. The Sn systems possess the structural rigidity of Rn and the ortho-para phenylene connectivity of SFn. Consequently, Sn represents the first class of branched chromophores with fully two-dimensional conjugation in both ground- and excited-state configurations. Evidences include the excellent linear correlations for the optical 0-0 energies or the first oxidation potentials of Sn and Rn against the reciprocal of their n values, delocalized HOMO and LUMO based on density functional theory calculations, and molecule-like fluorescence anisotropy. The resulting model of effective conjugation plane (ECP) for the two-dimensional π-conjugated systems compliments the concept of effective conjugation length (ECL) for one-dimensional oligomeric systems. Other implications of the observed structure-property relationships are also included.

Full text of DOI:10.1021/ja202144v

ARTICLE
pubs.acs.org/JACS
Ortho-Branched Ladder-Type Oligophenylenes with
Two-Dimensionally π-Conjugated Electronic Properties
Hsin-Hau Huang, Ch. Prabhakar, Kuo-Chun Tang, Pi-Tai Chou,* Guan-Jhih Huang, and Jye-Shane Yang*
Department of Chemistry, National Taiwan University, Taipei, 10617 Taiwan
S Supporting Information
b
ABSTRACT: The synthesis, photochemical and electrochemical properties,
and electronic structures of a series of star-shaped ladder-type oligophenylenes
Sn (n = 7, 10, 13, 16, 19, and 22), including one multibranched case S19mb, are
reported and compared with the linear para-phenylene ladders Rn (n = 2ꢀ5
and 8) and the stepladder analogues SFn (n = 10, 16, and 22). The n value refers
to the number of π-conjugated phenylene rings. Functionalized isotruxenes are
the key synthetic building blocks, and S22 is the largest monodispersed ladder-
type oligophenylene known to date. The Sn systems possess the structural
rigidity of Rn and the orthoꢀpara phenylene connectivity of SFn. Conse-
quently, Sn represents the first class of branched chromophores with fully two-
dimensional conjugation in both ground- and excited-state configurations. Evidences include the excellent linear correlations for the
optical 0ꢀ0 energies or the first oxidation potentials of Sn and Rn against the reciprocal of their n values, delocalized HOMO and
LUMO based on density functional theory calculations, and molecule-like fluorescence anisotropy. The resulting model of effective
conjugation plane (ECP) for the two-dimensional π-conjugated systems compliments the concept of effective conjugation length
(ECL) for one-dimensional oligomeric systems. Other implications of the observed structureꢀproperty relationships are also
included.
’ INTRODUCTION
their solubility and conformational planarity and rigidity. The so-
called stepladder and ladder-type polyphenylenes correspond to
the partly and fully bridged systems, respectively. Recently, an
intriguing position effect of ring-bridging was observed for in-
denofluorenes (1, R = H), where the anti-isomer 1a displays red-
shifted absorption and fluorescence maxima (12ꢀ13 nm) with
respect to the syn-isomer 1s.24 While thispositioneffect reflects the
differences in linearity of the phenylene backbone (1a > 1s) or the
pattern of substitutions (para in 1a vs ortho in 1s), the role of this
position effect in a larger ladder system remains to be evaluated.
Monodispersed conjugated oligomers (MCO) constitute an
important class of organic electronic materials.1ꢀ15 They possess
the merits of molecule-like structural purity and polymer-like
flexibility, thermal stability, and film-forming properties. Many
recent efforts have been devoted to synthesis of multidimen-
sional MCO such as two-dimensional stars and discs as well as
three-dimensional cruciforms and dendrimers.8ꢀ16 The extra
dimensions in MCO hold great promise for improved solubility,
new film morphology, and/or decreased electronic anisotropy.
However, their effective conjugation length (ECL)17,18 is often
poorly defined or confined in a short one-dimensional segment.
This stems from the fact that electronic coupling among the
branched segments strongly depends on the degree of steric
congestion and electronic communications through the branch-
ing units. Large torsion angles3,19 and cross-conjugated linkers
(e.g., meta-phenylene)2,13,14 in the conjugated backbone could
severely hamper or even truncate the exciton delocalization.
Accordingly, the steric and electronic characters of the branching
units play a pivotal role in developing MCO of truly multi-
dimensional electronic conjugation.
An extension of the one-dimensional 1s by adding another
indeno group to the central ring leads to the two-dimensional
isotruxene 2 with an orthoꢀpara phenylene connectivity. We
have shown that 2 is a branching unit that allows strong
electronic coupling between the ortho and the para branches.15
A combination of ladder-type para-phenylene segments and
Phenylene-based conjugated oligomers4ꢀ10,14ꢀ16 and poly-
mers9,20ꢀ23 are promising candidates for organic electronics
due to their high photoluminescence quantum efficiency, pro-
minent charge-carrier mobility, and great electrochemical and
thermal stability. The ring-bridging approach that connects
adjacent phenylene rings at the ortho positions with solubili-
zer-substituted saturated carbons can simultaneously enhance
Received: March 9, 2011
Published: April 28, 2011
r
2011 American Chemical Society
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Figure 1. Schematic representation of the structures of oligophenylenes Sn, Rn, and SFn, where the black dots represent the backbone phenylene rings
with a number of n. The difference in colors between the black dots represents the difference of solubilizing substituents on the saturated carbons. See
Supporting Information Charts S1ꢀS4 for chemical structures of Sn.
2 should lead to systems fulfilling the needs for two-dimensional
conjugation. Indeed, we recently communicated16 the first
example of fully conjugated stars with the isotruxene-derived
ladder-type oligophenylenes Sn, where S and n denote the shape
(i.e., star) of the structure and the total number (i.e., 4, 7, 10, 13,
and 16) of phenylene rings in the π-conjugated backbone,
respectively (Figure 1). The extra dimension added by the ortho
branch in Sn elongates the ECL and lowers the energy gap,
leading to red-shifted fluorescence for S16 vs a linear long-chain
ladder polymer21 (470 nm vs 464 nm). Unlike the ECL of ∼12
phenylene rings for one-dimensional ladders (Rn),22 the absence
of saturation in the emission wavelength of S16 led us to extend
the concept of ECL to a two-dimensional cyclic region that
defines the boundary of exciton delocalization in Sn. We will call
it effective conjugation plane (ECP) hereafter.
Several important questions remained open in the study of Sn.
First, we have proposed16 that ECP is a cyclic extension of ECL:
namely, the diameter of ECP for Sn equals the ECL for the linear
ladder Rn. This in turn suggests that saturation in electronic
transition energy for the Sn series will occur at the size of S19 or
S22. However, this prediction is not yet verified, because
previous attempts to prepare Sn larger than S16 with the same
solubilizers were unsuccessful. Second, despite the presence of
effective two-dimensional conjugation in Sn containing one
ortho branch, it is unknown if the same phenomenon is also
present in systems of multiple ortho branches. This is another
crucial point for establishing the ECP model. Third, neither the
general picture of electronic structure nor the origins of allowed
transitions in the absorption spectra of Sn are fully characterized.
In the present work, we address all these questions. Two larger
systems S19 and S22 and one multi-branched system S19mb
containing four ortho branching cores have been successfully
prepared by changing the solubilizers and/or the synthetic
strategy. In addition, DFT and TD-DFT calculations and fluor-
escence anisotropy measurements on Sn have been conducted to
characterize their electronic structures. Our results confirm the
proposed ECP model and show that ortho-conjugation is as
effective as para-conjugation provided that the planarity and
rigidity of the π-conjugated backbone is sufficiently high. The
backbone planarity is more important than the position of
bridging atoms in determining the optical properties of these
ladder-type oligomers. The electronic character of these
branched oligophenylenes is molecule-like rather than poly-
mer-like in terms of the single-chromophore and two-
dimensional optical transition behavior.
’ RESULTS AND DISCUSSION
Synthesis. The synthesis of S7, S10, S13, S16, and S19mb
adopted a convergent synthetic strategy by linking an isotruxene
core and three corresponding arms. The synthetic details for the
former four compounds have been communicated,16 and the
strategy for S19mb is illustrated in Scheme 1. The Suzuki-
Miyaura cross coupling of the boronic acid functionalized
isotruxene core S40BA and the bromo and ester functionalized
5-phenylene branched arm A5bBr formed the ester-containing
19-phenylene precursor pre-S19mb. The subsequent nucleophi-
lic carbonyl addition and intramolecular FriedelꢀCrafts alkyla-
tion reactions afforded the fully ring-bridged ladder scaffold of
S19mb. Both the reactants S40BA and A5bBr contain the
isotruxene moieties and can be prepared from the parent
isotruxene and isotruxene monobromide ITBr, respectively.
We have developed facile synthetic methods for the parent
isotruxene (i.e., 2 with R = H).25 We also showed that they can
be modified to form prefunctionalized isotruxenes with func-
tional groups at selected phenylene ring(s).26 The synthesis of
ITBr provided a new example (Supporting Information [SI]).
Scheme 2 shows the conversion of ITBr to A5bBr. Base-
promoted alkylations of ITBr were conducted with t-BuOK in
the presence of catalytic amounts of 18-crown-6. The resulting
S4Br was converted to boronic ester S4BE by the Miyaura
boration reaction and then to A5bBr by the Suzuki coupling with
the ring-bridging precursor diethyl 2,5-dibromobenzene-1,
4-dioate (pre-RB). The reactant S40BA was prepared by follow-
ing the previously reported method16 for its analogue S4BA. In
brief, the synthesis was accomplished in three steps starting with
the parent isotruxene: namely, alkylations on the saturated
carbons, brominations on the terminal phenylenes, and bromo-
to-boronic acid transformation. The difference between S40BA
and S4BA is the length of the alkyl substituents (hexyl vs ethyl)
on the saturated carbons. Hexyl chains are no doubt better
solubilizers than ethyl chains for a polymer system. However, we
have adopted the short-chain isotruxene building blocks S4Br
and S4BA in our previous synthesis of star-shaped isotruxene
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Scheme 1
Scheme 2
derivatives, including S7, S10, S13, and S16. These isotruxene
derivatives possess sufficiently good solubility in organic sol-
vents, presumably due to their unsymmetrical structures. A
particular reason for using ethyl instead of hexyl groups at the
first onset is that the hexyl substituted S40 (i.e., 2 with R = C6H13)
is an oily liquid, but the ethyl substituted S4 (i.e., 2 with R =
C2H5) is a solid at room temperature. The latter has the
advantages of facile compound purification, storage, and transfer.
However, the use of ethyl solubilizers has met its limitation in
preparing the larger Sn systems S19 and S22. We thus adopted
S40BA for the synthesis of S19 and S22 as well as S19mb in
this work.
The synthesis of S19 and S22 was accomplished by a mixed
convergent and divergent method, which is represented by
the case of S22 (Scheme 3). It is divergent because the entire
phenylene backbone was constructed through two stages. The
first stage is the Suzuki-Miyaura cross coupling of S40BA and the
TMS group-protected 3-phenylene arm A3TMSBr that forms
the 13-phenylene intermediate pre-S13TMS bearing a TMS
group at each terminal phenylene. The TMS groups were
then replaced by iodine atoms with iodine monochloride. The
resulting pre-S13I was then subjected to the second-staged
cross coupling reactions with the boronic ester-incorporated
3-phenylene arm A3BE to afford the 22-phenylene precursor
pre-S22. The final step of ring-bridging reaction to form S22 is
the same as that for S19mb. The overall synthesis is also
convergent because not only the above-mentioned reactant
S40BA but alsoA3TMSBr and A3BE requires multistep syntheses.
The synthesis of A3TMSBr is shown in Scheme 4. Treatment of
2,7-dibromo-9,9-dihexylfluorene (FBr2) with 1 equiv of n-BuLi
followed by reaction with chlorotrimethylsilane gave compound
FTMSBr. After the bromo-to-boronic acid transformation, the
resulting FTMSBA was subjected to the Suzuki-Miyaura coupling
reaction with pre-RB in the final step of Scheme 2 to form
A3TMSBr. The synthesis of A3BE also involved the Suzuki-
Miyaura coupling reaction and the Miyaura boration reaction with
the reaction conditions resembling those in Scheme 2 (see
Supporting Information). To the best of our knowledge, S22 is
the largest monodispersed oligophenylene reported to date.
It should be noted that the solubilizers on the ring-bridging
saturated carbons in S19, S22, and S19mb are different from
those in S4, S7, S10, S13, and S16 not only in the central
isotruxene core but also in the arms. Such differences are
represented with different colors in the represented structures
schematically depicted in Figure 1. Unlike the ethyl and p-tolyl
substituents in the smaller Sn systems, the mixed substituents of
hexyl and 4-butylphenyl groups are believed to be crucial in
promoting the solubility of S19, S22, and S19mb.
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Scheme 3
Scheme 4
Some of the above reactions might deserve comments,
although the reaction conditions are not completely optimized.
For the Suzuki coupling reactions, it generally requires higher
loading of the Pd catalyst (5ꢀ7 mol % per CꢀBr bond) for
substrates containing the isotruxene core to obtain an optimal
yield. This situation was also reported for truxene derivatives.13b
For the two-step ring-bridging reactions for pre-Sn f Sn
conversion, the yield decreases as the n value increases and in
the order S7 (72%) > S10 (66%) > S13 (50%) > S16 (46%)
∼ S19mb (43%) > S22 (38%) ∼ S19 (35%). The lower yield for
larger Sn could be attributed to increased number of reaction
sites (i.e., 3, 6, and 12 for S7, S10ꢀS16 and S19mb, and S19ꢀ22,
respectively), decreased solubility of the precursor pre-Sn or the
alcohol intermediate, and/or increased skeletal branching and
solubilizer size (e.g., S19mb vs S16).
Electronic Properties. The difference in solubilizers among
the Sn series should have little or no effects on their electronic
properties, because the alkyl and aryl substituents are nearly
perpendicular to the phenylene backbone based on the previously
communicated X-ray crystal structures of S4 and S7.16 A support
of this argument is from the same electronic spectra recorded for
S4 and S40 (not shown). The phenomenon of negligible
substituent effects on electronic spectra is also present for the
linear ladder Rn.6,7 Therefore, the following discussion on the
photochemical and electrochemical properties of Sn will focus on
the effect of the phenylene number n. In this case, S19mb can be
considered as a skeletal isomer of S19, and the difference between
them would reflect the effect of terminal branching. The impact of
molecular dimensions will be elucidated by comparing Sn with
the linear ladder Rn (n = 2ꢀ5 and 8).6,7,16 The effect of molecular
planarity and rigidity will be addressed by comparison with our
previously reported15 stepladder systems SFn (n = 10, 16, and 22,
Figure 1).
The normalized absorption and fluorescence spectra for Sn in
dichloromethane are shown in Figure 2 and the corresponding
spectral data are summarized in Table 1. All Sn, except for S4,
display well-defined 0ꢀ0 vibronic bands in the absorption
spectra and 0ꢀ0 and 0ꢀ1 bands in the fluorescence spectra.
The small Stokes shifts (Δvst) between the absorption and
fluorescence 0ꢀ0 bands is consistent with the rigid ladder-type
structures. Both the absorption and fluorescence spectra undergo
bathochromic shift with increasing the number of phenylene
rings up to n = 19. The small or no difference in the 0ꢀ0
wavelengths (Δλ e 1 nm) between S19 and S22 reveals that the
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effective conjugation size of Sn is S19. More information on this
subject will be addressed in the section of ECP model. Both the
values of Δvst and the intensity ratio of 0ꢀ1 versus 0ꢀ0 band
(I01/I00) decrease in the order S7 > S10 > S13 > S16 > S19mb >
S19 > S22. This is consistent with the expected scenario of a
smaller extent of structural relaxation and vibrational coupling in
the fluorescing state for larger π systems.15,27 However, unlike
most one-dimensional rigid π systems such as Rn, the absorption
and fluorescence spectra have poor mirror images due to the
presence of more absorption bands. For example, the absorption
spectrum of S7 displays an intense band at 356 nm in addition to
the 0ꢀ0 (412 nm) and 0ꢀ1 (390 nm) bands. Evidently, the
presence of an ortho branch is essential for observing these
additional optically allowed short-wavelength bands. The ortho
segment in S7, S10, S13, and S22 contains 3, 4, 5, and 8 para-
linked phenylene rings, corresponding to R3, R4, R5, and R8,
respectively. The observed short-wavelength bands of the former
group are at longer wavelengths than the 0ꢀ0 absorption bands
of latter group (i.e., 356, 386, 402, and 442 vs 347, 375, 396, and
433 nm, Table 1). Thus, the short-wavelength bands for Sn
should not be due to a localized transition of the ortho-branched
arm. On the basis of the TD-DFT calculations (vide infra), these
bands involve molecular orbitals localized in the two meta-
related arms.
The fluorescence quantum efficiencies (Φfl)28 and fluores-
cence lifetime (τfl) for Sn in CH2Cl2 are also shown in Table 1.
Except for S4 that displays a lower value of Φfl (0.63), the other
Sn have similar fluorescence quantum yields (Φfl = 0.80ꢀ0.84).
In contrast, the values of τfl show a progressive decrease from
S4 to S22, although S19mb has the same lifetime as S16. Thus,
the radiative decay rate constant kfl (kfl = Φfl/τfl) increases as the
n increases. The smaller kfl for S19mb vs S19 highlights the
negative effect of skeletal branching on kfl. Skeletal branching
might also account for the smaller kfl for Sn vs Rn of the same
number of phenylene rings (e.g., S4 vs R4).
The electrochemical behavior of Sn has been investigated by
cyclic voltammetry (CV) and differential pulse voltammetry
(DPV). As shown in Figure 3, the number of reversible anodic
waves increases from one for S4 to four for S13. Several
reversible anodic waves are also present for the larger Sn, but
the peaks become less resolved. Nevertheless, it can be con-
cluded that the larger is the Sn size, the more charges can be
accommodated. The oxidation potentials (Eox) relative to the
ferrocene redox couples based on the DPVs are shown in Table 1.
As the n value increases, both the first and the second oxidation
potentials (Eox1 and Eox2) of Sn undergo negative shifts (i.e., S4 >
S7 > S10 > S13 > S16 > S19 = S19mb > S22). The same trend
also hold for the size of peak splitting between Eox1 and Eox2
Figure 2. Normalized absorption (black) and fluorescence (red) spec-
tra for Sn in CH2Cl2 at room temperature. The two vertical blue lines
denote the positions of 400 and 440 nm for the discussion of the results
of time-resolved fluorescence anisotropy.
Table 1. Photophysical and Electrochemical Data for Sn and Rn in CH2Cl2 at Room Temperature
compd
λabsa (nm)
log εb
λflc (nm)
Δνstd (cmꢀ1
)
Φfl
τfl (ns)
kfle (108 sꢀ1
)
Eoxf (V)
S4
306 336 (352)
356 390 (412)
386 415 (439)
402 426 (452)
411 430 (458)
411 425 (459)
414 433 (461)
418 442 (462)
(305)
4.46 (4.19)
5.03 (4.81)
5.16 (5.03)
5.27 (5.21)
5.31 (5.27)
5.31 (5.26)
5.41 (5.33)
5.52 (5.41)
(3.85)
(382)g
2231
907
704
572
557
555
506
458
736
490
489
439
0.63
0.81
0.84
0.83
0.80
0.81
0.81
0.82
0.28
0.38
0.78
0.81
3.22
1.97
1.38
1.16
1.03
1.03
0.91
0.83
4.04
2.01
1.36
1.12
1.96
4.11
6.09
7.16
7.77
7.86
8.90
9.88
0.69
1.89
5.74
7.23
(0.79)
S7
(428) 449
(453) 481
(464) 495
(470) 500
(471) 503
(472) 506
(472) 507
(312)
(0.57) 1.05
S10
S13
S16
S19mb
S19
S22
R2
(0.48) 0.85 1.14
(0.44) 0.75 0.92 1.15
(0.43) 0.70 0.80 0.96
(0.41) 0.66 0.84
(0.41) 0.66 0.80
(0.38) 0.59 0.89
R3
314 (347)
(4.63)
(353) 366
(382) 401
(403) 425
(441) 469
1.02
0.83
0.75
R4
357 (375)
(4.88)
R5
R8h
375 (396)
(5.04)
410 (433)
a Peak maxima of the absorption bands with the 0ꢀ0 band in the parentheses. b Extinction coefficients of the highest peak and the 0ꢀ0 absorption band
with the latter in parentheses. c Maxima of the 0ꢀ0 and 0ꢀ1 fluorescence bands with the former in parentheses. d Stokes shift defined by the difference of
the 0ꢀ0 absorption and fluorescence peak maxima. e kfl = Φfl/τfl. f Oxidation potentials vs Fc/Fcþ with the first oxidation potential (Eox1) in parentheses.
g Estimated from the blue edge of the fluorescence plateau. h Data from ref 7.
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Figure 3. Cyclic voltammogram (black) and differential pulse voltam-
mogram (red) for oxidiation of Sn in CH2Cl2 with electrolyte 0.1 M
Bu4NPF6 at a scan rate of 100 mV sꢀ1
.
(ΔEox = Eox2 ꢀ Eox1). The smaller ΔEox for larger Sn can be
attributed to a decreasing Coulombic repulsion between the two
charges. The behavior of n-dependent charge accommodation
and peak splitting has also been observed for the stepladder
analogues of Sn (i.e., SFn).15 However, the Eox1 of SFn (0.60 V vs
Fc/Fcþ) is independent of the n values (10, 16, and 22) and lies
in between that of S4 (0.79 V) and S7 (0.57 V), the result of
which reflects the important effect of structural planarity (or
conformational freedom in torsions) on electronic properties.
ECP Model. MCO are ideal models for elucidating funda-
mental electronic properties of π-conjugated polymers. Correla-
tion plots of photophysical or electrochemical data against the
number of repeat units (n) or its reciprocal (1/n) are particularly
informative.17,18,29 For example, the ECL and the limiting band-
gap of many one-dimensional systems have been determined on
the basis of the optical transition energy vs 1/n plots.17 In
addition, the interplay of charge transfer and conjugation
(particle in a box) interactions in donorꢀacceptor substituted
MCO can be probed with the spectral peak maxima vs n plots.29
With the correlations known for the parent systems, any deviation
of related compounds might reveal the conformational or sub-
stituent effects. As such, data of the electronic 0ꢀ0 energy and the
first oxidation potential of Rn and Sn in CH2Cl2 are plotted
against 1/n, where n is the number of phenylene rings for
consideration. To study the temperature and solvent effects on
the 0ꢀ0 energies and thus the correlation plots, we have also
determined the fluorescence and excitation spectra of Rn and Sn
in methylcyclohexane (MCH) at 77 and 300 K (Figure S1, SI).
The corresponding 0ꢀ0 band energies are shown in Table S1,SI.
Plots aꢀf in Figure 4 show the excellent linear correlations
(r2 > 0.997) for the 0ꢀ0 band energies of absorption,
Figure 4. Linear plots of the 0ꢀ0 band energy (eV) ofthe (a) absorption
and (b) fluorescence in CH2Cl2 at room temperature, the (c) excitation
and (d) fluorescence in MCH at 300 K, the (e) excitation and
(f) fluorescence in MCH at 77 K, and the plots of (g) the first oxidation
potential (Eox1) and (h) HOMO and LUMO energy levels (eV) versus
the reciprocal of number of the backbone phenylene rings (n = n) in
Sn (red squares) and Rn (black squares). The red open circle specifically
denotes S4 for the purpose of discussion.
fluorescence, or excitation spectra against 1/n with n = n for both
Sn (n = 7, 10, 13, 16, 19, 22) and Rn (n = 2ꢀ5 and/or 8). In
contrast, poorer correlations (r2 < 0.990, Figure S2 in SI) were
found with considering only the para-conjugated oligophenylene
chain of Sn (i.e., n = (2n þ 1)/3 = 5, 7, 9, 11, 13, 15 for S7, S10,
S13, S16, S19, and S22, respectively, and n = 11 for S19mb). This
explicitly shows that electronic excitation is effectively delocalized
over the whole two-dimensional π-backbone of Sn in both the
ground and the fluorescing state configurations. In other words,
the ortho conjugation is as effective as the para conjugation in Sn
in lowering the energy gap, and each Sn molecule behaves as a
single chromophore. Fully two-dimensional exciton delocaliza-
tion has not been observed for other branched MCO,30,31
including the stepladder systems SFn.15 Oligophenylenes SF16
and SF22 display the same fluorescence 0ꢀ0 energy at 438 nm
(2.83 eV), which is red-shifted by only 8 nm from the 0ꢀ0 band of
SF10 (430 nm, 2.88 eV).15 We might conclude that it requires
orthoꢀpara connectivity and torsion-constrained planar π-scaf-
folds to achieve two-dimensional exciton delocalization.32 In fact,
only few known two-dimensional systems can fulfill both criteria.
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reduced in the excited state due to structural relaxation
(planarization), which accounts for the decreased deviation in
the fluorescence plots. The occurrence of structural relaxation is
also consistent with the structureless fluorescence spectra and
the large Stokes shift for S4 at room temperature. When the
compounds are in the MCH glass at 77 K, structural relaxation
toward the planar geometry in S1 is somewhat hampered. This
accounts for the increased deviation in the fluorescence energy
plot. Since structural relaxation is reduced at 77 K, the more
planar conformers in the ground state would contribute more to
the excitation spectra. This would lead to a red-shifted excitation
spectrum as compared to that recorded at 300 K and thus a
smaller deviation for S4 in Figure 4e. Although the isotruxene
cores in S7 and the larger Sn systems are also nonplanar, the
steric effect appears to be negligible. This can be attributed to a
dilution of wave functions in the isotruxene core in Sn larger than
S4. Note that the pattern of ring-bridging substituents in S4 vs
R4 is more different than that in 1s vs 1a. The phenomenon of
same fluorescence 0ꢀ0 energy but different absorption 0ꢀ0
energy for S4 and R4 (Table 1) suggests that backbone planarity
is more important than substituent pattern in determining the
electronic properties.
Figure 5. Model proposed for the effective conjugation plane (ECP, a
two-dimensional region covered by the yellow circle) of conjugated
systems illustrated with (a) R12, (b) S19, and (c) S19mb.
Disc-shaped polycyclic aromatic hydrocarbons (PAHs)33ꢀ35
such as coronene and hexa-peri-hexabenzocoronene (HBC) are
also qualified for two-dimensional exciton delocalization. Two-
dimensional PAHs with a continued small curvature would lead
to three-dimensional bowl- (e.g., corannulene)36 or ball-shaped
(e.g., C60)37 structures. Their low HOMOꢀLUMO gaps appear
to demonstrate an effective two- or three-dimensional exciton
delocalization. Nevertheless, these PAHs are not oligomers of
well-defined repeat unit and thus cannot be analyzed like Sn with
the correlation plots.
The observation of two-dimensional conjugation in the
excited state of Sn has led us to propose the ECP model for
the description of exciton coherence size (Figure 5). By an
analogy to ECL that defines the maximum length of exciton
delocalization on one-dimensional systems, the ECP is the
maximum region of exciton delocalization in a two-dimensional
system. The ECL for Rn has been shown to be approximately
the size of R12 on the basis of single-molecule spectroscopic
analysis of R11 and long-chain Rn (n ≈ 62 and 165).22
According to Figure 4 and Table 1, S19 is approaching the
ECP for Sn, as both S19 and S22 have the same λfl of 472 nm.
The slightly bended para chain in S19 contains 13 phenylene
rings, which has a similar end-to-end distance to the ECL for Rn
(i.e., the diameter of the yellow circle in Figure 5). This appears
to suggest that ECP and ECL have a common origin: namely,
exciton delocalization is multidimensional in nature with a
specific size (e.g., a 3D sphere or 2D circle), but only suitable
multidimensional systems can manifest this nature. To the best
of our knowledge, Sn is the first system that uncovers the two-
dimensional boundary of exciton delocalization. Nevertheless,
our results indicate that the ECL of one-dimensional systems
allows one to define the diameter.
In the context of the ECP model, we can address the impli-
cations of deviation of S4 from the linear plots in Figure 4aꢀf.
Regarding the structureless fluorescence for S4 at room tem-
perature, the fluorescence 0ꢀ0 energy was estimated by the blue
edge of the fluorescence plateau. The same degree of deviation
for S4 in CH2Cl2 at room temperature vs MCH at 300 K (plots a
and b vs c and d in Figures 4, respectively) indicates a negligible
solvent polarity effect. However, at 77 K the deviation becomes
smaller and larger in the excitation (Figure 4e) and the fluores-
cence energy plot (Figure 4f), respectively. All these observations
can be attributed to a conformation effect. According to the X-ray
crystal structures of R5,6 S4,16 and S7,16 the arms of Sn are planar
but the isotruxene core has a small twist (9.8ꢀ for S4 and 18.6ꢀ for
S7) between the two ortho-branched phenylene rings due to
steric hindrance. This backbone twisting in the ground state can
account for the blue shift of absorption and excitation spectra for
S4 vs R4, although it is not large enough to inhibit exciton
delocalization. However, the backbone twisting is expected to be
The above discussion also leads to the conclusion that the small
blue shifts of the 0ꢀ0 absorption and fluorescence band
(3ꢀ4 nm) for S19mb vs S19 mainly result from a less planar
backbone. The participation of the three terminal ortho-branched
phenylene rings in S19mb in the conjugation interactions is
evidenced by the red-shifted 0ꢀ0 absorption and fluorescence
bands and lower Eox1 value as compared to S16 (Table 1).
However, the presence of four isotruxene groups in S19mb
makes the density of ortho branching (number of ortho
branches/number of phenylene rings = 0.21) to an extent similar
to that in S4 (0.25). Accordingly, the steric effect cannot be
neglected in S19mb.
A linear correlation between Eox1 and 1/n is also observed for
Sn and Rn (Figure 4g). Figure 4h shows the corresponding plots
with the HOMO and LUMO energy levels that are derived
according to eqs 1 and 2:38
EHOMO ¼ ꢀ ð4:8 þ Eox1
Þ
ð1Þ
ELUMO ¼ EHOMO þ E0,0
ð2Þ
where E0,0 is the 0ꢀ0 transition energy obtained from the
intersection of normalized absorption and fluorescence 0ꢀ0
bands. The results show that increasing the chain length desta-
bilizes the HOMO but stabilizes the LUMO and thus reduces the
optical bandgap. A larger slope for the HOMO (ꢀ2.23) vs the
LUMO (0.99) plot also reveals that the HOMO energy is more
sensitive than that of the LUMO to the chain length, as is the case
in linear oligophenylenes.4
Electronic Structure. We have shown that, unlike the multi-
chromophoric nature of most multidimensional MCO,31 the Sn
represent the first example of single chromophores with
branched arms. To gain insight into the electronic structure
and spectra of Sn, we have carried out DFT calculations for the
ground-state geometry optimization and TD-DFT calculations
for the absorptions in the B3LYP/6-31G** level.39,40 To expedite
the calculations, all the solubilizers on the saturated carbons are
replaced with methyl groups. The energy diagram of the highest
occupied molecular orbital HOMO (H) and lowest unoccupied
molecular orbital LUMO (L) and the nearby orbitals Hꢀ2,
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Hꢀ1, Lþ1, and Lþ2 for Sn are shown in Figure 6. These frontier
molecular orbitals (FMO) for S19 are shown in Figure 7, and the
corresponding FMO for the other Sn are shown in SI: Figure S3.
The calculated absorption wavelength (λabs,cal), oscillator
strength (f), and the configuration description of the lowest
three singlet excited states (S1ꢀS3) are shown in Table 2. For
comparison, the corresponding calculations for R2ꢀR5 were
also carried out, and the results are shown in SI: Table S2 and
Figure S3.
Despite a large difference in the number of phenylene rings (n =
4ꢀ22) and ortho-branching (4 in S19mb but 1 in the others) in
Sn, they have a great similarity in the calculated electronic
structure. There are two allowed transitions for each Sn, which
are S0 f S1 and S0 f S3 for S4, S7, and S10 but the latter
transition becomes S0 f S2 for the larger Sn. Disregarding the
second allowed transition being S0 f S2 or S0 f S3, it possesses a
larger f value than the S0 f S1 transition for all cases. This is
consistent with the absorption spectra, where the maximum is
always located at shorter wavelengths than the 0ꢀ0 absorption
bands for Sn (Figure 2). For all Sn, the S1 state is mainly
contributed by the H f L configuration and the allowed S2 or
S3 excitation is dominated by two configurations, Hꢀ1 f L and
H f Lþ1, although there are more configuration interactions as
the size of Sn increases. The increased participation of the
configurations associated with the Hꢀ2 and Lþ2 orbitals in
the allowed transitions for larger Sn could be attributed to the
significantly decreased energy difference between orbitals Hꢀ2
and Hꢀ1 and orbitals Lþ1 and Lþ2 (Figure 6). As represented
by the case of S19 (Figure 7), the H, L, Hꢀ2 and Lþ2 cover all
the phenylene backbone with larger electron density in the
central vs terminal phenylene rings for the former two orbitals
but an opposite electron distribution is observed for the latter
two orbitals. In contrast, the molecular orbitals Hꢀ1 and Lþ1
have electron density localized on the two arms that are meta-
related with respect to the central phenylene ring. Evidently, the
S0 f S1 is a completely delocalized transition, in agreement with
the conclusion based on the 1/n-correlated 0ꢀ0 energy plots
(Figure 4). Although the second allowed S0 f S2 transition is
somewhat localized in the two meta-related arms, the two major
configurations involve both the delocalized molecular orbitals
H and L and have opposite direction of electron motions: from
the localized Hꢀ1 to the delocalized L and from the delocalized
H to the localized Lþ1.
Another interesting feature for the two allowed transitions for
Sn is their distinct orientation of the transition dipole moments,
which is different from the linear analogues Rn. As represented
by S19 again, the orientations of the two allowed transition
moments are depicted in Figure 8. The other Sn all display
similar orientations for the two allowed transitions (Figure S4 in
SI). These two allowed transition dipoles defines an angle (θ) in
the range 65ꢀ73ꢀ for Sn. In contrast, there is only one allowed
transition S0 f S1 that is oriented along the long molecular axis
for R5, and the calculated oscillator strengths for the transitions
to S2 and higher singlet states are essentially zero (Table S2).
This character is retained for larger Rn but not for the smaller
systems. For example, R2 (9,9-dialkylfluorene) has transitions
moments oriented along the short (S3 and S5) molecular axis in a
comparable size as those along the long (e.g., S1, S2, and S4)
molecular axis (Figure S4). Evidently, the electronic structure
of R2 resembles most small molecule chromophores with
Figure 6. B3LYP/6-31G** orbital energy level diagram for Sn and R5.
Figure 7. B3LYP/6-31G** level calculated structure and the frontier molecular orbitals of S19. Only atomic charge densities with 1% or higher
contribution are included.
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Table 2. B3LYP/6-31G** Level Calculated Absorption
Energy, Oscillator Strength, and Description for the Lowest
Three Singlet Excited States of Sn
weightd
(%)
a
excited
state
λabs,cal
compd
(nm)
fb
configurationc
S4
S1
S2
343
328
0.404 H f L
0.004 Hꢀ1 f L
H f Lþ1
98
49
48
46
48
99
50
45
46
52
98
68
26
27
69
96
59
35
35
58
94
52
37
40
52
92
41
5
S3
295
0.522 Hꢀ1 f L
H f Lþ1
S7
S1
S2
410
371
0.888 H f L
0.006 Hꢀ1 f L
H f Lþ1
1.170 Hꢀ1 f L
H f Lþ1
1.425 H f L
0.114 Hꢀ1 f L
H f Lþ1
1.710 Hꢀ1 f L
H f Lþ1
1.949 H f L
2.420 Hꢀ1 f L
H f Lþ1
0.030 Hꢀ1 f L
H f Lþ1
2.497 H f L
3.059 Hꢀ1 f L
H f Lþ1
0.013 Hꢀ1 f L
H f Lþ1
2.522 H f L
2.873 Hꢀ1 f L
Hꢀ1 f Lþ2
H f Lþ1
0.007 Hꢀ1 f L
H f Lþ1
3.041 H f L
3.691 Hꢀ2f Lþ1
Hꢀ1 f L
Hꢀ1 f Lþ2
H f Lþ1
0.007 Hꢀ1 f L
H f Lþ1
Figure 8. Orientation of the transition dipoles for the B3LYP/6-31G**
level calculated S0 f S1 and S0 f S2 absorptions of S19.
S3
355
two-dimensionally spread transition moments, but R5 behaves
as linear π-conjugated polymers, which are characterized by
one-dimensional polarization that leads to a single intense
structured absorption band. In this context, the behavior of Sn
is more molecule-like than polymer-like. The two-dimensional
optical transition character for Sn is indeed borne out by their
wavelength-dependent fluorescence anisotropy (vide infra).
The calculated absorption wavelength for the S1 state is in the
order S22 > S19mb > S19 > S16 > S13 > S10 > S7 > S4, which is
consistent with the experimental observation except for the
position of S19mb. The observed 0ꢀ0 absorption energy for
S19mb is 2 nm lower than that of S19, but the opposite (2 nm
higher) was predicted by the TD-DFT calculations. This
discrepancy could be attributed to the uncertainty raised by
the level of theoretical approach. If this factor can be ruled out,
other possible differences may lie in the backbone planarity. The
calculated structures are expected to have more planar pheny-
lene backbone in the isotruxene moieties because of the use of
smaller methyl substituents. This in turn suggests that with a
more planar phenylene backbone the phenylene connection in
S19mb would lower the energy gap more than that in S19. This
might be attributed to the smaller diameter of S19mb vs S19
(Figure 5) in view of the fact that the HOMO and LUMO have
more electron density in the center than the terminals
(Figure 7). Recall that the S0 f S1 transition is mainly from
the HOMO f LUMO configuration. The calculated results also
support the conclusion from the energy plots (Figure 4) that
ortho conjugation is inherently as effective as para conjugation.
The commonly observed weaker electronic coupling through an
ortho-phenylene bridge results from a steric effect that decreases
the backbone planarity.
Fluorescence Anisotropy. To verify the TD-DFT predicted
two-dimensional optical transition character of Sn, we have
measured the time-resolved fluorescence anisotropy for Sn
(n g 7) in CH2Cl2 with two different excitation wavelengths
(400 and 440 nm, the blue lines in Figure 2). For comparison, the
corresponding measurement was carried out for R5. Figure 9
shows the polarized fluorescence decay curves for S19 and R5
recorded at angles of 0ꢀ (parallel, I ), 54.7ꢀ (magic angle, IMA),
and 90ꢀ (perpendicular, I^) with respect to the polarization
direction of the excitation laser. The corresponding decay curves
for the other compounds are provided in the Supporting
Information (Figure S5). Measured polarized fluorescence decay
curves are carefully scaled by the tail-matching method to correct
different grafting efficiencies for I and I^, in order to calculate
S10
S1
S2
444
396
S3
393
S13
S1
S2
462
418
S3
410
S16
S1
S2
473
434
S3
420
S19mb
S1
S2
481
446
44
49
43
91
5
S3
427
S19
S1
S2
479
445
47
5
36
42
47
88
6
S3
426
S22
S1
S2
484
454
3.605 H f L
4.317 Hꢀ2 f Lþ1
Hꢀ1 f L
43
7
Hꢀ1 f Lþ2
H f Lþ1
35
15
40
35
S3
431
0.019 Hꢀ2 f L
Hꢀ1 f L
H f Lþ1
a Calculated absorption energy. b Oscillator strength. c H and L stand for
HOMO and LUMO, respectively. The second and third highest
occupied molecular orbitals are denoted as Hꢀ1 and Hꢀ2 and the
second and third lowest unoccupied molecular orbitals as Lþ1 and Lþ2,
respectively. d Only configurations with 5% or greater contribution are
included.
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Figure 10. Fluorescence anisotropy for Sn and R5 in CH2Cl2 with the
excitation wavelengths at 400 (solid circles and lines) and 440 nm (open
circles and dashed lines). The number for each data point denotes
the fraction of contribution from the S0 f S1 absorption (f1(400 nm)
for the solid circles and f1(440 nm) for the open circles) calculated
based on eq 5.
S13 and S16. At 400 nm, R5 and S7 can also be determined, and
they display higher anisotropy than the other Sn.
The results shown in Figure 10 can be rationalized by the
changing contributions of the two allowed transitions (S0 f S1
and S0 f S2 or S3), which have transition moments polarized at
an angle (θ) of 65ꢀ73ꢀ to each other based on TD-DFT
calculations (vide supra). The validity of a large calculated θ
angle between the two allowed transitions is promptly supported
by the negative r values observed for S13 and S16, since θ values
smaller than the magic angle 54.7ꢀ will lead to only positive r
values. Assuming that the emission dipole moment is collinear to
the S0 f S1 transition dipole (i.e., θ = 0), the S0 f S1 and S0 f
S2 transitions would lead to theoretical r values of 0.4 and ꢀ0.11
to ꢀ0.15, respectively, according to eq 3. In general, the observed
limiting anisotropy is less than the theoretical value 0.4. The high
r value of 0.35 observed for R5 excited at 400 nm is likely
approaching the limiting anisotropy in this work. Note that the
excitation corresponds to the 0ꢀ0 S0 f S1 transition (Figure S1
in SI) and transitions to higher singlet excited states are predicted
to be forbidden for R5 (Table S2 in SI). The r value for S10
excited at 440 nm is also high (0.34), consistent with a nearly
pure S0 f S1 transition by inspecting its absorption spectrum
(Figure 2). Assuming that the other minor transitions are
negligible at the excitation wavelengths, the relative fraction of
contributions of the two allowed transitions (f1 and f2) can be
estimated by the determined r values at any excitation wave-
length λ based on eqs 4ꢀ6.
Figure 9. Fluorescence and anisotropy decay curves for (a) R5 and
(b) S19 excited at 400 nm and for (c) S19 excited at 440 nm. The solvent
is CH2Cl2. Solid line denotes the measured polarized fluorescence decay
curves in blue (I ), red (I^), and green (IMA). Dotted black line denotes
the constructed population decay curve (Ipop), while magenta dashed
line denotes the fitting curve. Anisotropy decay curve is shown as a solid
black line with estimated error in red shadow area.
fluorescence anisotropy r based on eq 3:41
I ꢀ I^
2
5
r ¼
¼
ÆP2ðcos θÞæ
ð3Þ
I þ 2I^
where P2(cos(θ)) is the second Legendre Polynomial (P2(x) =
0.5(3x2 ꢀ 1)) of the cosine of the angle between the absorption
and emission dipole moments. A constructed population decay
rðλÞ ¼ f1ðλÞr1 þ f2ðλÞr2
ð4Þ
ð5Þ
ð6Þ
curve, Ipop = 1/3 (I þ 2I^) of R5 (Figure 9a), which is
3
physically as meaningful as IMA, is nearly identical to the
polarized decay curve monitored at magic angle (IMA) after
normalization. This further supports proper setup and alignment
for the polarizer and the reliability of our anisotropy measure-
ments. The resulting anisotropy decay curves are also shown in
Figure 9, and the calculated timeꢀzero anisotropy (r) values are
depicted in Figure 10. Evidently, the anisotropy of Sn depends on
both the n value and the excitation wavelength. At 440 nm, the
anisotropy r values are all positive and in the order S10 > S13 >
S22 > S16 ≈ S19mb ≈ S19. The anisotropy decreases for them
on going from 440 to 400 nm, and the sign becomes negative for
f1ðλÞ ¼ ðrðλÞ þ 0:12Þ=0:47
f2ðλÞ ¼ ð0:35 ꢀ rðλÞÞ=0:47 ¼ 1 ꢀ f1ðλÞ
where r1 and r2 are the limiting anisotropy of the S0 f S1 and
S0 f S2 or S3 transitions. With the assumption of r1 = 0.35 based
on the data for R5 and S10, the value of r2 is predicted to be
ꢀ0.12 for θ = 69ꢀ (i.e., the average θ value for the second allowed
transition with respect to the S0 f S1 transition in Sn). The
calculated f1 values for Sn are shown in Figure 10, and they agree
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reasonably well with the appearance of absorption spectra shown
in Figure 2 (marked with blue lines). More detailed data for θ, r,
f1, and f2 are shown in Table S3 (Supporting Information).
6-31G** basis set. The electronic nature of the absorption bands was
investigated by TD-DFT calculations at the B3LYP/6-31G** level.
’ ASSOCIATED CONTENT
’ CONCLUSION
S
Supporting Information. Detailed synthetic procedures
b
Our systematic studies on the ortho-branched ladder-type
MCO Sn show that each Sn is a molecule-like single chromophore
covering all the n phenylene rings in the backbone, including the
multibranched case (S19mb). This in turn leads to the following
implications. First, exciton delocalization through an ortho-
phenylene bridge is as efficient as that through a para-phenylene
bridge provided that the π-conjugated backbone is free from large
torsion angles and torsional motions. In this context, isotruxene is
a useful building block for constructing ortho-branched systems of
effective two-dimensional conjugation. Second, backbone planar-
ity plays the most important role in determining the size of
electron coupling among the oligomeric π-segments than the
substituted patterns of solubilizers. Third, exciton delocalization is
multidimensional (or nondirectional) in nature, and the effective
conjugation plane for a two-dimensional system possesses a
diameter equivalent to the effective conjugation length defined
by linear MCO analogues. This information should be valuable for
developing novel multidimensional π-conjugated systems of de-
sired electronic properties such as low bandgap, large two-photon
absorption cross section,42 and high charge-carrier mobility.43
and characterization data for new compounds, electronic spectra
in methylcyclohexane at 77 and 300 K, alternative correlation
plots, DFT-calculated molecular orbitals, Cartesian coordinates,
orientations of allowed transition dipoles, and full fluorescence
anisotropy spectra and data for Sn and Rn, TD-DFT-calculated
absorption descriptions for Rn, and complete ref 39. This
material is available free of charge via the Internet at http://
pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
jsyang@ntu.edu.tw; chop@ntu.edu.tw
’ ACKNOWLEDGMENT
We thank the National Science Council of Taiwan for financial
support. We appreciate Prof. Yuan-Chung Cheng of NTU for
sharing his computers and software and Prof. Chung-Hsuan
Chen, Zhen-Yu Xie, Ting-An Liu, and Hung-Ta Chu for the
support on compound characterization. The computing time
granted by the Computing Center of NTU is also acknowledged.
’ EXPERIMENTAL SECTION
Materials. All commercially available materials were used as re-
ceived. Solvents for photochemical and electrochemical measurements
were HPLC grade. CH2Cl2 was dried by calcium hydride and distilled
before use. Detailed synthetic procedures and structural characterization
data for new compounds are provided as Supporting Information.
Methods. All the spectral and electrochemical data were collected at
room temperature (23 ( 1 ꢀC). UVꢀvisible spectra were measured on a
Cary300 double beam spectrophotometer. Fluorescence spectra were
recorded on a PTI QuantaMaster C-60 spectrometer and corrected for
instrumental nonlinearity. The optical density (OD) of all solutions was
about 0.1 at the wavelength of excitation. Fluorescence quantum yields
were determined using an integrating sphere (150 mm diameter, BaSO4
coating) of Edinburgh Instruments by the Edinburgh FLS920 spectro-
meter. Fluorescence decays were also measured at room temperature
with the use of the Edinburgh FLS920 spectrometer with a gated
hydrogen arc lamp using a scatter solution to profile the instrument
response function. The goodness of the nonlinear least-squares fit was
judged by the reduced χ2 value (<1.2 in all cases), the randomness of the
residuals, and the autocorrelation function. The excitation and fluores-
cence spectra at 77 K were measured with sample solutions in an Oxford
OptistatDN cryostat with an ITC502 temperature controller. Frequency-
doubled femtosecond pulses of 400 and 440 nm generated from the
Tsunami (Spectra-Physics, U.S.A.) along with an Edinburgh OB900-L
spectrometer were used to carry out the time-resolved fluorescence
anisotropy measurements. A polarizer was settled in front of the detector
and could be tuned to select fluorescence of designated polarization. The
cyclic voltammetry (CV) and differential pulse voltammetry (DPV)
were recorded on a CHI 612B electrochemical analyzer, and the
electrochemical cells adopted a glassy carbon as the working electrode,
a Pt wire as a counter electrode, an Ag wire as a reference electrode, and
0.1 M Bu4NPF6 as electrolyte. The substrates are ∼1 mM in CH2Cl2. All
reported potentials were calibrated using ferrocene as an internal
standard. DFT and TD-DFT calculations were performed with the
Gaussian 09 program.38,39 The gas-phase conformations of Sn and Rn
were derived from DFT calculations with B3LYP level of theory and
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Journal of the American Chemical Society
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(32) It should be noted that donor-acceptor substituted π-systems
allow charge-transfer from the donor subunit to the acceptor subunit
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dx.doi.org/10.1021/ja202144v |J. Am. Chem. Soc. 2011, 133, 8028–8039