Molecular panels for energy transduction in C60-based conjugates

Article abstract of DOI:10.1021/ol060515g

Light-harvesting C₆₀-based dyads endowed with a truxene fluorophore unit have been synthesized. Their photophysical studies in solution reveal a singlet-singlet energy transfer deactivation mechanism that confirms the actuation of the latter as

Full text of DOI:10.1021/ol060515g

4026
J. Phys. Chem. B 2007, 111, 4026-4035
Electronic and Molecular Structures of Trigonal Truxene-Core Systems Conjugated to
Peripheral Fluorene Branches. Spectroscopic and Theoretical Study
Mar´ıa Moreno Oliva, Juan Casado, and Juan T. Lo´pez Navarrete*
Departamento de Qu´ımica F´ısica, UniVersidad de Ma´laga, Campus de Teatinos s/n, Ma´laga 29071, Spain
Rory Berridge
Department of Chemistry, UniVersity of Manchester, Manchester M1 9PL, United Kingdom
Peter J. Skabara and Alexander L. Kanibolotsky
WestCHEM, Department of Pure and Applied Chemistry, UniVersity of Strathclyde, Glasgow G1 1XL, United
Kingdom
Igor F. Perepichka
L. M. LitVinenko Institute of Physical Organic and Coal Chemistry, National Academy of Sciences of Ukraine,
Donetsk 83114, Ukraine
ReceiVed: August 15, 2006; In Final Form: February 15, 2007
An analysis is performed on the molecular and electronic features in a series of trigonal molecules constituted
by a central truxene core which is ramified with three oligofluorene moieties of different lengths. Arms and
core are studied independently and upon threefold unification. Special emphasis is paid to the modulation of
the conjugational properties in relation to substitution, molecular dimension, ring aromaticity, intermolecular
forces, oxidation state, etc. Raman and optical absorption/emission spectroscopies in conjunction with
computational theoretical results are combined for this purpose. The evolution of some key intensity ratios in
the Raman spectra (i.e., I1300/I1235) is followed as an indication of electronic interaction between the core and
the branches. The changes of the electronic delocalization upon solvation, with varying temperature in the
solid state, with the nature of the aromatic unit (bithiophene/fluorene) or after electrochemical oxidation are
interpreted. The modulation of the optical properties on the basis of the structure and energetics of the orbital
around the gap is also addressed. Density functional theory was used to assign the vibrational and electronic
spectra.
I. Introduction
homologues (i.e., oligofluorenes) serve as good models for
photophysical studies and offer a greater degree of chemical
purity, structural uniformity, or resistance to degradation, crucial
factors for the performance of their LED devices.9 Oligofluorene
units have recently been used as arms in star-shaped architec-
tures with various cores.2h,3b-d,h,10 10,15-Dihydro-5H-diindeno-
[1,2-a;1′,2′-c]fluorene (truxene), a polycyclic aromatic system
with C3 symmetry, has been used as a potential starting material
for the construction of bowl-shaped fragments of fullerenes, C3
tripodal materials in chiral recognition, and liquid crystalline
compounds.11 Recently truxene has been recognized as a
promising building block in conjugated star-shaped and dendritic
architectures and potentially interesting for optoelectronic
applications.2e,5b,10,12 Truxene can be viewed as three “over-
lapped” fluorene fragments, which is an excellent choice as a
core for the visualization of star-shaped oligofluorenes.
The field of well-defined π-conjugated oligomers is today
recognized as a prominent prospect in materials science. In this
field, linearly (i.e., one-dimensional) conjugated molecules
constitute a fruitful area of research.1 Recently, star-shaped
conjugated oligomers have attracted tremendous interest,2
particularly because they expand the potential of linear analogues
in electronic, optoelectronic, and photonic applications [i.e.,
light-emitting diodes (LED),3 photovoltaics,4 field-effect transis-
tors (FET),5 third-order nonlinear optics,6 etc.]. The possibility
of extending the electronic structure in two dimensions leads
to the realization of organic molecules built with peripheral
antennae converging to a central core for energy conversion
(i.e., light-harvesting systems).4 Conversely, these new star-
shaped or dendrimer-like molecules, if constructed with fluo-
rescent blocks, can be viewed as condensates of more efficient
or highly emitting systems.3 In this context, the precise
elucidation of the interplay between the energy and electronic
states of the core and of the external branches is of relevance
for improved designs.
In this work, we describe the molecular and electronic
structure of a series of trigonal star-shaped oligofluorenes having
truxene as the central core and up to quaterfluorene arms (see
Figure 1), resulting in nanosized macromolecules with ca. 4
nm radius.10 The 9-positions of the fluorene units are substituted
with hexyl chains for improving solubility and processability.
A combination of molecular spectroscopies (i.e., vibrational
Polyfluorenes have emerged as leading electroluminescent
materials with bright emission, high hole mobility, and ease of
functionalization for tuning properties.7,8 Their oligomeric
10.1021/jp065271w CCC: $37.00 © 2007 American Chemical Society
Published on Web 04/03/2007

Truxene-Core Systems with Fluorene Branches
J. Phys. Chem. B, Vol. 111, No. 16, 2007 4027
Figure 1. Chemical structures of the compounds studied: T0-T4, F1-F4, and Tth2. In theoretical calculations of truxene and fluorene derivatives,
methyl (R ) CH3) instead of hexyl (R ) C6H13) groups have been considered. In addition, calculations for fluorene series F1, F2, and F4 have been
performed for structures with a hydrogen atom instead of bromine.
Raman and absorption/emission) and theoretical calculations
(density functional theory) is used to explore the electronic and
molecular properties as a function of (i) substitution of truxene,
(ii) core-to-arms electronic delocalization, (iii) length of the
branches, (iv) intermolecular interactions, and (v) solid-state
packing by means of thermospectroscopy.
electrode. The experiments were carried out using Voltalab40
electrochemical equipment from Radiometer.
Absorption and emission spectra were obtained in dichlo-
romethane solution in concentrations (ca. 10-6-10-7 M) not
exceeding 0.1 au of absorbance (1 cm path length cell). Under
these conditions, the absorption intensity was found to have
linear dependence with concentration. There was no shift in the
absorption wavelengths, indicating no aggregation in the system.
UV-vis-NIR absorption spectra were recorded on an Agilent
8453 instrument equipped with a diode array detection system.
Emission spectra were measured using a JASCO FP-750
spectrofluorometer. No fluorescent contaminants were detected
upon excitation in the wavelength region of experimental
interest. FT-Raman spectra were measured using an FT-Raman
accessory kit (FRA/106-S) of a Bruker Equinox 55 FT-IR
interferometer. A continuous-wave Nd:YAG laser working at
1064 nm was employed for excitation. A germanium detector
operating at liquid nitrogen temperature was used. Raman
scattering radiation was collected in a backscattering configu-
ration with a standard spectral resolution of 4 cm-1. To avoid
possible damage to the oxidized samples upon laser radiation,
its power was kept at a level lower than 20 mW and 1000-
3000 scans were averaged for each spectrum. Raman spectra
in solution were obtained in analytical grade CH2Cl2. A variable-
temperature cell Specac P/N 21525 was used to record the FT-
Raman spectra at different temperatures. This cell consists of a
surrounding vacuum jacket (0.5 Torr) and combines a refrigerant
Dewar and a heating block as the sample holder, allowing
achievement of temperatures between -170 and +150 °C.
Samples were inserted into the heating block part or the Dewar/
cell holder assembly as a solid in a quartz cell and Raman
spectra were recorded after waiting for thermal equilibrium
of the sample, which required 20 min for every increment of
10 °C.
Vibrational Raman spectroscopy is well suited for studying
the electronic coupling between covalently connected conjugated
moieties. This suitability lies in the important enhancement of
the Raman activity for those vibrational modes that vibronically
couple the electronic structure of the ground electronic state
and the first accessible dipole-allowed excited state.13 The
coupling between electronic and vibrational degrees of freedom
might strongly affect the nature of the photoexcitations; hence
the rationalization of this behavior would help to optimize
photophysical properties. In this sense Raman spectra (i.e., peak
position and intensity of the relevant lines) as a function of the
substitution pattern might be useful in estimating electronic
delocalization (i.e., to what extent different moieties π-interact).
Electronic spectra, on the other hand, constitute the means for
investigating the electronic structure, emphasizing the relative
disposition of the electronic levels and energy gap upon
molecular trigonalization. The understanding of the underlying
structure-property relationships is the driving force for execut-
ing this work, with the aim of offering information for the design
of new conjugated systems.
II. Experimental and Computational Details
The syntheses of all the compounds studied have been
reported previously.10 Electrochemical oxidation was performed
on thin solid films at room temperature in a 0.1 M tetrabuty-
lammonium hexafluorophosphate solution in dry and oxygen-
free acetonitrile. Platinum disk and platinum foil were used as
working and auxiliary electrodes, and Ag wire was used as a
pseudoreference electrode. Electrolysis conditions were tested
against the Fc/Fc+ couple as an internal reference. The procedure
involved the immersion of the oligomer film (drop-cast on the
Pt working electrode) in the electrolyte solution followed by
application of an anodic potential according to previously
reported cyclic voltammetry data.10 Upon full oxidation, the
Raman spectra were directly recorded in situ on the coated Pt
Density functional theory (DFT) methods have become very
popular since the inclusion of electron correlation effects with
accuracies comparable to those of correlated ab initio proce-
dures, such as MP2, while reducing substantially the compu-
tational costs of the latter.14 Hence, the use of low-cost
computational DFT procedures has demonstrated very satisfac-
tory results for conjugated molecules where electron correlation

4028 J. Phys. Chem. B, Vol. 111, No. 16, 2007
Moreno Oliva et al.
Figure 3. B3LYP/3-21G* theoretical eigenvector for some representa-
tive bands of T0.
Figure 2. Comparison of (a) B3LYP/3-21G* theoretical and (b) solid-
state Raman spectra of truxene T0 and (c) B3LYP/3-21G* theoretical
and (d) solid-state Raman spectrum of fluorene F1. For the theoretical
spectra, methyl groups instead of hexyl groups have been considered.
plays a decisive role. Ground-state total energies, equilibrium
geometries, eigenfrequencies, and normal coordinates were
calculated using density functional theory by means of the
Gaussian 03 package of programs.15 The Becke’s three param-
eter (B3) gradient-corrected exchange functional combined with
the correlation Lee-Yang-Parr (LYP) functional was utilized.16
The 3-21G* basis set was used.17 Details of the modeling
experiments are as follows: (i) Optimal geometries were
determined on isolated entities, and neither solvent nor coun-
teranion effects were considered. (ii) All the geometrical
parameters were left to vary independently during the optimiza-
tion. In the case of the conformational study, the dihedral angle
between the fluorene units in F2 was kept constant. Eventually,
methyl groups instead of hexyl chains were considered, assum-
ing that this substitution has a negligible affect on conjugation
and the energies of the frontier orbitals. For F2 Raman spectra
calculations, the entire molecular structure was considered. (iii)
For the theoretical Raman spectra of the resulting ground-state
optimized geometries, harmonic vibrational frequencies and
Raman intensities were calculated numerically. (iv) Ab initio
calculations can be expected to yield vibrational frequencies
with an accuracy of about 10% compared to the respective
experimental values.18 To improve the numerical comparison,
calculated harmonic vibrational frequencies are uniformly scaled
down by a random factor (0.97) to best fit the experimental
quantities. All quoted theoretical vibrational frequencies reported
are thus scaled values.
Figure 4. B3LYP/3-21G* theoretical eigenvector for some representa-
tive bands of F1.
and electronic structures.13 For the 1600-1200 cm-1 region, a
relatively good experimental/theoretical correlation is noticed
for both compounds, which allows us to investigate and assign
the principal experimental bands of the Raman spectra of truxene
and fluorene. Figures 3 and 4 sketch the molecular normal
modes of the main theoretical bands. In the following section
Raman spectra of the arms are considered for their bromine
derivatives (see Figure 1), assuming that one Br atom occupying
one of the two terminal positions of these arms has a negligible
influence over the whole molecular conjugation, especially the
largest members.
The strongest Raman band of truxene T0 is measured at 1603
cm-1 (calculated: 1600 cm-1) and arises from a CC stretching
mode (i.e., mode 8a of benzene20) involving exclusively the
external benzene rings. In the case of fluorene, this mode appears
at 1608 cm-1 (calculated: 1602 cm-1). The Raman band of
truxene T0 at 1568 cm-1 (calculated: 1558 cm-1) is owing to
the same CC stretching mode but located in the innermost
benzene ring (i.e., this mode is typical of hexa-substituted
benzenes).20 The doublets at 1608/1580 cm-1 in fluorene F1
and at 1603/1589 cm-1 in truxene T0 display much similarity
to the doublet found in oligophenyl oligomers at 1600/1580
cm-1 (i.e., Fermi resonance feature), whose I1600/I1580 intensity
ratio increase is related to efficient conjugation.21,22 According
to this criterion, it is established that fluorene offers better
conditions for interannular conjugation than biphenyl (i.e., two
phenyl rings connected through a single bond).22 As for truxene,
its I1603/I1589 intensity ratio is slightly less than the I1600/I1580
one in fluorene. Note that the consideration of the I1603/I1568
ratio in truxene is not likely to be a valid argument since it
does not correspond to a fundamental/Fermi resonance pair.
The time-dependent DFT (TD-DFT) approach is now being
widely applied in both chemistry and condensed matter physics
to describe electronic excitations.19 While it is not as accurate
for excitations as the ordinary DFT is for ground-state properties,
this theory has a considerable predictive power and is compu-
tationally quite tractable. At least 30 lowest energy electronic
excited states were computed for all the molecules. TD-DFT
calculations were performed using the same functional (B3LYP)
and basis set (3-21G*).
III. Molecular Structures. Raman Spectra:
Structure-Spectra Correlations
III.1. Raman Spectrum of Truxene and Fluorene Building
Blocks. Figure 2 compares the FT-Raman spectra of T0 and
F1 together with their B3LYP/3-21G* theoretical data. The
focus of this analysis concerns CC skeletal vibrations because
of their direct implication in the coupling between the molecular

Truxene-Core Systems with Fluorene Branches
J. Phys. Chem. B, Vol. 111, No. 16, 2007 4029
Figure 6. B3LYP/3-21G* theoretical eigenvector for some representa-
tive bands of F2.
etries with inter-fluorene dihedral angles of 38°);9e and finally
(iii) a conformer with the two fluorene moieties forced to a
dihedral angle of 30°. Figure 6 displays the vibrational eigen-
vectors associated with the bands that are now discussed. The
theoretical feature at 1248 cm-1 (i.e., experimental at 1239-
1235 cm-1) corresponds to an in-plane deformation vibration
of the aromatic CH bonds. For this particular mode the CC
stretches do not take part at all, a result of importance in the
following discussion since this band is taken as a reference for
the relative changes of the intensity of the CC modes. A band
calculated at 1276 cm-1 cm-1 a band is calculated (correspond-
ing to the experimental feature at 1274 cm-1), is represented
by the stretching of the C-C bond connecting the two fluorene
units. As for the predicted band at 1290 cm-1 and its weak
shoulder at 1305 cm-1 (i.e., experimental lines at 1298 and 1317
cm-1), these are due to similar vibrational modes that couple
the interfluorene C-C and the intrafluorene C-C stretching
bonds (aromatic rings). The intrafluorene C-C stretch of the
bond connecting the two benzene rings in fluorene is calculated
at 1347 cm-1 (experimentally at 1344 cm-1). As can be seen
in the eigenvectors, all these vibrations involve, to some extent,
the deformation modes of the hexyl chains. However, these alkyl
modes are not expected to contribute appreciably to the Raman
activity of the C-C/CdC conjugated modes.
Of the calculated spectra for the different conformers, the
planar system better reproduces the experimental Raman
spectrum recorded for the solid F2. This finding can be
explained by assuming that solid-state packing could partially
planarize the molecule, a fact supported by the relatively low
rotation energy barrier around the C-C inter-fluorene single
bond (the total energy difference between the planar and the
optimized conformation is 3.35 kcal/mol). The most noticeable
feature upon successive lowering of the inter-fluorene distortion
angle (i.e., planarization) is the increased intensity for the inter-
fluorene ν(C-C) band. Accordingly, one would derive the
following: (i) Decreasing/increasing solid-state forces would
have an incidence on the molecular spectra, a hypothesis that
Figure 5. DFT/B3LYP/3-21G* Raman spectrum of the planar
conformer of F2 (bold line, bottom), of the molecule in its optimized
geometry (dotted, red lines) and of the conformer with a forced dihedral
conformation of 30° (dotted, blue lines). The inset corresponds to the
case with the spectra normalized with respect to the â(CH) Raman
band, while the normal spectra are displayed as calculated. Arrows in
the inset show the evolution of intensity upon planarization.
The group of weak and overlapped bands at 1480-1420 cm-1
is likely arising to alkyl CH deformation modes, thus lacking
further interest. The most important and distinctive Raman band
of the innermost benzene ring of truxene T0 is detected at 1377
cm-1 (calculated: 1378 cm-1) belonging to a symmetric CC
stretching mode wherein consecutive CC bonds lengthen and
shorten simultaneously. The medium-intensity band at 1295
cm-1 (calculated: 1256 cm-1) involves the stretching of the
C-C bonds between the benzene rings coupled with the CCC
bending modes. As for fluorene F1, the medium-weak band at
1342 cm-1 (calculated: 1349 cm-1) and the medium-intense
line at 1294 cm-1 (calculated: 1317 cm-1) might be equally
related to combined intra- and inter-ring CC stretches. The weak
bands at 1242/1214 cm-1 in truxene might be correlated with
the weak theoretical band at 1211 cm-1 that corresponds to in-
plane CH deformation vibrations of the aromatic rings. In
fluorene these â(CH) deformations are measured at 1247/1217
cm-1 and calculated at 1256/1226 cm-1 (see Figures 3 and 4
for the vibrational eigenvectors).
Figure 5 compares the solid-state Raman spectrum of F2 with
that calculated at the B3LYP/3-21G* level (with hexyl groups)
for the following cases: (i) the fully planar structure in which
the dihedral angle between fluorene groups is forced to 0°
(fluorene moieties are disposed in anti conformation whereas
the hexyl chains are perpendicular to the fluorene plane and
pointing in opposite directions); (ii) the fully optimized geom-
etry, which results in an inter-fluorene distortion angle of 42°
(calculations for other fluorene oligomers, i.e., the trimer and
pentamer, at the B3LYP/6-31G* level gave optimized geom-
TABLE 1: Raman Frequency (cm-1) Correlations and Assignment of the Main Bands Discussed in the Text
F1
F2
assignment
ν(CdC)
T0
T1
T2
T3
T4
assignment
1608
1580
1607
1575
1603
1589
1568
1377
1608
1584
1563
1377
1343
1607
1584
1562
1377
1340
1607
1585
1560
1377
1344
1607
1586
ν(CdC)
1377
1345
ν(CC)
1342
1344
1317
ν
intra(CC) + νinter(CC)
ν
inter(CC)
1311
1294
1268
1235
1309
1292
1271
1235
1307
1291
1274
1235
1306
1291
1277
1235
ν
ν
ν
inter(CC)
inter(CC)
inter(CC)
1294
1298
1274
1239
ν
inter(CC) + νintra(CC)
1295
ν
inter(CC)
1247
1217
â(CH)
1242
1214
â(CH)

4030 J. Phys. Chem. B, Vol. 111, No. 16, 2007
Moreno Oliva et al.
will be addressed in the following sections by discussing the
spectra in solution and as a function of temperature. (ii)
Increasing conjugation (i.e., planarization) between the fluorene
units leads to a selective intensification of the Raman intensity
of the inter-fluorene ν(C-C) band. Thus, changes of the
molecular structure that would modify inter-fluorene π-conjuga-
tion could be monitored by changes of the Raman intensity of
this band. Note, furthermore, that in the case of the largest
molecules several inter-fluorene stretching modes exist depend-
ing on the distance to the core, such that their relative Raman
intensities are related to the electronic conjugation between the
fluorene units as a function of the branching. This could also
be the case of the Raman line associated with the C-C
stretching mode of the bond connecting the central core and
arms. This interpretation has been already used in the parent
oligophenylenes to estimate the efficiency of π-conjugation upon
successive planarization of the oligomeric backbone. The main
conclusion is that the intensity ratio between the Raman lines
of the ν(C-C) around 1280 cm-1 and of the â(C-H) at 1220
Figure 7. FT-Raman spectra of (upper plot) F2 in dotted line, T2 in
solid line; and (lower plot) truxene T0 in dotted line, T2 in solid line.
Spectra are normalized with respect to the bands at 1235-1239 cm-1
(top) and 1377 cm-1 (bottom).
cm-1 can be used as a measure of the planarity, with large I1280
/
I1220 ratios corresponding to effective conjugation.21,22 Conse-
quently, the relative intensity between the Raman bands
involving the C-C stretching of the interfluorene units and the
â(C-H) fluorene modes can be taken as a conjugation
parameter.
Our next step is to attempt a reliable assignment of the bands
around 1300 cm-1 in the larger samples (T1-T4 in Figure 9),
for which Raman spectra calculations cannot be afforded
(computationally very expensive). Table 1 summarizes the
measured frequencies and establishes an assignment for both
the fluorene and truxene series. This correlation is consistent
with the whole analysis in the paper, and takes into account
the following: (i) It assumes that fluorene units do interact in
between and with the core, and likely provoking changes in
the relative intensities as a function of the molecular size. This
discards the hypothesis of a simple addition of the intensity of
individual oscillators to account for the whole of the Raman
intensity. (ii) It agrees with the conclusions derived from
electronic spectroscopy which show a concomitant red shift of
the lowest energy absorption band on going from T1 to T4,
indicating sizable electronic interactions among the aromatic
subunits. Finally, (iii) it allows a reliable interpretation of the
experimental data in the following sections, which show a tuning
of the relative intensity between some Raman lines.
Figure 8. FT-Raman spectra of F1 (black line), F2 (dotted line), and
F4 (red line). Inset shows the spectra normalized relative to the 1230-
1240 cm-1 â(CH) vibration.
III.2. Raman Spectra and the Effect of Branching. Figure
7 displays the FT-Raman spectra of the truxene T0, bifluorene
F2, and T2. The spectrum of T2 is dominated by the Raman
scattering of the arms (1607 cm-1 in both F2 and T2).
An undistinguished variation of the I1600/I1580 ratios is noticed
on passing to the branched compound. This agrees with the
constant peak position and intensity for the innermost benzene
band of the core at 1377 cm-1 in truxene T0 and in T2, showing
that the interaction is rather limited on the basis of this argument.
Conversely, the signal at 1340 cm-1 in T2 is intensified relative
to its homologue at 1344 cm-1 in bifluorene F2. The C-C
stretching modes of the bonds connecting the arms to the core
are likely involved in this Raman line; therefore, this feature
represents the possible channel for electronic interaction of the
periphery with the central truxene which might account for the
enhanced Raman activity on F2 f T2 (1344f1340 cm-1).
We now examine this interaction by considering the relative
intensity ratios of the following Raman features: I1340/I1235 in
T2 (I1344/I1239 in F2), I1307/I1235 in T2 (I1317/I1239 in F2), and
If one assumes that the molecular symmetry and vibrational
dynamics are essentially unaltered in the truxene series, our
argument is that the Raman bands recorded in the 1280-1350
cm-1 interval mainly emerge from inter-fluorene ν(C-C) modes
of the bonds connecting the successive units and the core. The
reassignment of these bands is based on the following: (i) They
are particularly intense under conditions of efficient inter-
fluorene π-interactions, which is probably the case of the solid
state, as observed in F2 by varying the dihedral angle. (ii) The
intra-fluorene modes, also appearing in this interval (see
assignment of F2) significantly decrease their Raman activity
upon planarization. (iii) In experiments where π-electron
delocalization between the fluorenes and the truxene is a priori
favored, a net intensification of these bands is observed. From
another point of view, our hypothesis stems on the existence of
different domains for π-electron delocalization depending on
the distance to the core. In the following sections, the intensity
ratios of the different Raman lines are considered as the quotient
between the intensities of each band, measured as the height at
the maximum peak wavenumber.
I1292/I1235 in T2 (I1298/I1239 in F2). The ratios are always greater
in T2 than in F2 for the three lines at higher energies. These
differences decrease steadily on going to lower frequencies from
1340 to 1298 cm-1. However, for the I1271/I1235 ratio in T2 and
I1274/I1239 in F2, the intensity ratio is smaller for the truxene
derivative. According to the description in section III.1, this
latter result can be due to the increased distortion on the
outermost periphery of the C3 molecule compared to F2 and

Truxene-Core Systems with Fluorene Branches
J. Phys. Chem. B, Vol. 111, No. 16, 2007 4031
Figure 11. FT-Raman spectra of (a) T2 in the solid state and (b) T2
in dichloromethane solution (blue line; the spectrum of the solvent has
been subtracted) and in the solid state (red line). These spectra are
normalized with respect to the 1235 cm-1 band. FT-Raman spectra of
(c) pure dichloromethane (dashed line) and (d) T2 in dichloromethane
(the spectrum of the solvent has not been subtracted).
Figure 9. FT-Raman spectra of (a) T1, (b) T2, (c) T3, and (d) T4.
Figure 10. Raman spectra of the trigonal truxene-fluorenes T1-T4
normalized at 1235 cm-1
.
the limited degree of conjugation in these external parts of the
molecule. Similar behavior is observed upon increasing the chain
length within the arms (see Figure 8), namely from fluorene
F1 to bifluorene F2 and quaterfluorene F4. It is well established
that, within large linearly conjugated oligomers, π-conjugation
takes place more efficiently in the central part of the molecules
and decreases toward the ends (terminal or boundary effect).21,22
From another point of view, the similarity between the spectra
of F4 and T4 (section III.3) implies a dominant role of the
branches in the description of the molecular features of the
trigonal compounds for the longest members of the series (i.e.,
antenna behavior).
Figure 12. FT-Raman spectra of T4 as a function of the temperature
in solid state.
favored. This behavior reaches the inflection point for the 1294,
1292, 1291, and 1291 cm-1 bands in T1, T2, T3, and T4,
respectively. Again, this is due to the involvement of ν(C-C)
modes of the outermost periphery (I1268/I1235 in T1, I1271/I1235
in T2, I1274/I1235 in T3, and I1277/I1235 in T4).
III.4. Solid-Solution Raman Spectra. Solid-State Packing
Forces. Figure 11 compares the Raman spectra of T2 in
dichloromethane and in the solid state. The strongest peak is
observed at the same frequency in both spectra. Typical of
organic molecules is the displacement of the Raman bands by
4-5 cm-1 upon solvation; however, the constant peak in both
spectra is in line with a pronounced glassy or amorphous
character of these molecules in the solid or thin film states.23
III.3. Raman Spectra of the Trigonal Compounds: In-
creasing the Arm Length. Figure 9 displays the FT-Raman
spectra of the threefold molecules T0-T4 in the whole spectral
range, while Figure 10 expands the 1200-1400 cm-1 region
for normalized spectra. The analysis of the evolution of the I1600
/
I1580 ratio in this series is obscured by the broadening of the
strongest band that overlaps both features. As for the two bands
at 1563 and 1377 cm-1 in T1, these are weakened on increasing
branching in response to the decreasing statistical weight of the
central core.
However, extra conformational effects due to the removal of
packing forces cannot be excluded. If these effects do indeed
occur, a reduction of the inter-fluorene and fluorene-truxene
interactions and molecular π-electron delocalization might be
expected due to increased distortion around C-C single bonds
in the liquid phase. This reasoning is highlighted by the
The intensities of the Raman bands at 1343/1311 cm-1 in
T1, 1340/1309 cm-1 in T2, 1344/1307 cm-1 in T3, and 1345/
1306 cm-1 in T4 greatly increase relative to the intensity band
at 1235 cm-1 of each compound with increasing molecular size.
This increment is, however, more significant for the higher
frequency band which, as already described, could contain an
important contribution from ν(C-C) modes located close to the
core wherein π-electron delocalization might be particularly
observation of a decrease of the I1310/I1235, I1294/I1235, and I1270
/
I1235 ratios upon solvation. A similar effect might be caused by
the variation of temperature, since its lowering would promote
intermolecular forces (i.e., more contacts) and increased pla-
narization. Figure 12 displays the Raman spectra of T4 as a
function of temperature. It is observed that, upon cooling of
the solid material, the ν(C-C) bands involving the innermost

4032 J. Phys. Chem. B, Vol. 111, No. 16, 2007
Moreno Oliva et al.
consists of their partial aromatic f quinoidal evolution. Oxida-
tion of the quaterfluorene arm leads to a double 1604/1591 cm-1
scattering. The differences between these two spectra might
highlight the delocalization of the charge defects through the
central truxene and only slightly influence the peripheral
branches in T4.
Assuming that the band around 1235 cm-1 is not altered very
much upon oxidation (as this band is of â(C-H) nature, while
electron extraction alters the CC skeleton), a general weakening
of the Raman activity of the single C-C bond skeletal modes
is noticed with respect to the neutral species. Oxidation leading
to a more quinoidal structure mainly impacts the inter-fluorene
and fluorene-truxene dihedral angles due to their planarization.
As a result, these C-C bonds should be strengthened and their
corresponding Raman frequencies increased. The new oxidized
band at 1323 cm-1, likely proceeding from the 1310-1290 cm-1
ν(C-C) modes, might corroborate this prediction. The new band
at 1361 cm-1 can be tentatively originated by the 1345 cm-1
ν(C-C) band of the neutral molecule.
Figure 13. Comparison of the FT-Raman spectra of (a) Tth2 and (b)
T1. Dotted line corresponds to the spectrum of 2,2′-bithiophene.
IV. Electronic Structures. Frontier Orbitals
It is useful to examine the highest/lowest occupied/virtual
molecular orbitals for these compounds because of their relative
ordering as a function of the substitution pattern. Such a study
might provide a reasonably good indication of the electronic
structure in relation to optical properties. To inspect these
characteristics, DFT B3LYP/3-21G* calculations of the opti-
mized geometries and of the orbital topologies for representative
compounds have been carried out; the results are shown in
Figure 15.
IV.1. Electronic Spectra of Truxene and of the Fluorene
Building Blocks. Figure 16 compares the UV-vis absorption
and emission spectra recorded in dichloromethane for the
compounds calculated in Figure 15, which allow us to interpret
experimental/theoretical comparisons of energy levels and
HOMO-LUMO gaps (i.e., related to the redox gaps). Fluorene
derivatives without bromine atoms were not studied experi-
mentally, and although this limits a comparison of the precise
wavelength in branched arms, the trends on increasing lengths
are comparable between both series.
For truxene, TD-DFT reproduces the appearance of one
theoretical transition at 279 nm, with an oscillator strength of
0.41, corresponding to one-electron promotions involving the
pairs of π degenerate orbitals (i.e., H - 1 f L, H - 1 f L +
1, H f L, and H f L + 1). These excitations consist of electron
transitions among the fluorene moieties of truxene, with the
central ring being the overlapping center. In this regard, the
electronic absorption spectrum of truxene can be viewed as that
of a fluorene, laterally conjugated with two more fused benzenes.
This extraconjugation is represented by the red shift of 9 nm
from F1 to T0, likely due to the reduction of the HOMO-
LUMO gap (i.e., HOMO/LUMO destabilization/stabilization).
Both spectra display vibronic resolution resulting from their
molecular rigidity. The spectrum of F2 already lacks this
vibronic structure due to the rotation around the single inter-
fluorene C-C bond. However, its π-π* band is further shifted
to the red by 25 nm, resulting from the symmetric reduction of
the HOMO-LUMO gap on going from F1 to F2. This finding,
also outlined in the previous section of the Raman data, is typical
of incremental conjugation upon linear chain elongation within
the same series of monomers.
Figure 14. FT-Raman spectra of (a) neutral T4, (b) electrochemically
oxidized T4, (c) neutral F4, and (d) electrochemically oxidized F4.
Each two spectra are normalized relative to the 1235 cm-1 band.
connections between the fluorenes and with truxene are
particularly intensified.
III.5. Raman Spectra of the Trigonal Compounds: Chang-
ing the Type of Arm. Figure 13 shows the FT-Raman spectra
of T1 and a molecule constituted by a central truxene with linear
bithiophene arms (Tth2), which, similar to T1, has two (hetero)-
aromatic rings connected to the truxene core. The 1608/1377
cm-1 truxene bands are not altered when fluorene arms are
replaced by bithiophene, showing the electronic pinning effect
due to the aromatization of the central benzene rings.
Comparing the spectrum of Tth2 to those of the isolated units,
no spectral changes due to the interaction between bithiophene
and truxene in the 1400-1200 cm-1 region are found. The rather
different aromatic natures of benzene and thiophene likely
restrict electronic interaction relative to the case of interaction
between equal units (i.e., thiophene/thiophene or benzene/
benzene), such as in the case of fluorene and truxene which
both constitute aromatic six-membered rings.
III.6. Raman Spectra of the Trigonal Compounds: Oxi-
dized Species. Figure 14 shows the Raman spectra of T4 in
neutral and oxidized states together with the respective neutral
and oxidized spectra of the quaterfluorene derivative F4. As
for T4, oxidation produces the disappearance of the scatterings
of the neutral system; this is clearly evidenced for the strongest
Raman band that undergoes a 1607f1595 cm-1 displacement
upon electrochemical treatment. This finding is typical of
oligophenylene compounds upon oxidation and relates to the
general softening of the molecular structure of the six-membered
benzene rings upon electron extraction. This C-C softening
IV.2. Electronic Spectra and the Effect of Branching. TD-
DFT calculations for T1 predict the first dipole-allowed
electronic excitation to be S0 f S2 at 354 nm with a very large

Truxene-Core Systems with Fluorene Branches
J. Phys. Chem. B, Vol. 111, No. 16, 2007 4033
Figure 15. B3LYP/3-21G* energies and topologies of the frontier orbitals around the gap for some representative molecules.
trum of T1 does not show vibronic resolution, which is
explained by the incorporation of single C-C bonds, around
which rotation is permitted, leading to the appearance of
rotamers, especially in solution. This is consistent with the
Raman discussion in section III.4.
At the level of the frontier orbitals, the structure of T1 is
reminiscent of that of truxene T0 with the first two occupied
and virtual terms degenerated. The HOMO/HOMO - 1 and
LUMO/LUMO + 1 of T1 spread, in similar extent, over the
arms and the outer/central benzene rings of truxene. This
situation already changes for the HOMO - 2/LUMO + 2
orbitals mainly located on peripheral arms. This vision is
interesting from the point of view of antenna systems operating
with gradient energy. Higher energy excitations involving the
absorbing arms find a deactivation channel (i.e., Sn f S2) that
collects the energy toward the core. This representation is very
relevant for light-harvesting materials to which highly branched
star-shaped or dendrimeric molecules are also conceived.
Figure 16. UV-vis absorption and emission spectra in CH2Cl2 of some
representative molecules. Solutions are prepared in concentrations
(10-6-10-7 M) giving an absorbance not exceeding 0.1 au.
IV.3. Electronic Spectra of the Trigonal Compounds:
Increasing the Arm Length. The wave functions of the orbitals
around the gap are increasingly determined by the arms on going
from T1 to T2: this is clearly seen in the existence of
degeneration in HOMO/HOMO - 1 and LUMO/LUMO + 1
terms in T1, which is reminiscent of truxene, while these terms
in T2 are not degenerated anymore, such as in F2. Now the
topologies of these orbitals in T2 resemble those of fluorene
F2. The optical absorption spectrum is red-shifted by 18 nm
on T1 f T2, which can be predicted on the basis of the
narrowing of the HOMO-LUMO gap with arm elongation.
Compared to the F1 f F2 evolution (bathochromic shift of 25
nm), branching of the central truxene with the same length arms
limits the gap reduction. This is clearly noticed for the
destabilization/stabilization of the HOMO/LUMO levels by
0.38/0.46 eV for F1 f F2 and 0.17/0.25 eV for T1 f T2. The
origin of this behavior is the limiting efficiency of meta
1.85 oscillator strength (S0 f S1 is a prohibited transition at
364 nm), and that would correspond to the experimental band
at 343 nm in T1. This S0 f S2 excitation corresponds to the
sum of several one-electron promotions (i.e., H - 2 f L, H -
1 f L, H - 1 fL + 1, H f L, H f L + 1 and H f L + 2).
According to this theoretical description, fluorescence in these
compounds might correspond to the S2 f S0 deactivation
channel that, due to its strong coupling with the ground
electronic state, might justify their strong fluorescence acti-
vity. Threefold substitution of truxene with fluorene arms
leads to a 34 nm bathochromic shift of the lowest lying
electronic absorption and is accounted for by the stabilization
of the LUMO (by 0.55 eV) and destabilization of the HOMO
(by 0.35 eV) on T0 f T1. This theoretical result correlates
well with the observation of electrochemical reductions and
oxidations in these truxene derivatives.10 The absorption spec-

4034 J. Phys. Chem. B, Vol. 111, No. 16, 2007
Moreno Oliva et al.
conjugation in this optical excitation, which proceeds between
two arms that are connected by meta links through the truxene
unit (in contrast to isolated fluorene arms, having para links
between the fluorene moieties). Supporting this hypothesis, meta
conjugation blocks electron delocalization, especially in the
ground-aromatic electronic state (see HOMO/HOMO - 1 wave
functions), while in the excited-quinoidal electronic state (see
the LUMO/LUMO + 1 wave functions) interaction is favored
due to the bonding nature of the C-C bond between the fluorene
units and between truxene and fluorene. This bonding feature
rigidifies the system, leading to more efficient electron molecular
delocalization. As a result, the virtual terms are always more
stabilized than the occupied ones.
IV.4. Joint Consideration of Vibrational Raman and
Electronic Spectra. In the whole set of compounds, vibrational
Raman frequencies are affected only slightly by branching, arm
elongation, or substitution (see section III). The origin of this
observation probably lies in the minimal changes of their
molecular geometries. It is well documented for oligophenylenes
that their molecular structures are almost independent of the
oligomer size due to electron pinning or strong aromaticity
within the six-membered rings (optimized geometries can be
provided upon request to the authors).22 In turn, the evolution
of the conjugational properties in these molecules has been
assessed based on the changes of the relative Raman intensities
(from I1300/I1235 ratio, for example). Assuming that Raman
activity in these conjugated molecules is mainly dictated by
vibronic coupling (i.e., term of pure electronic contribution of
the Albrecht theory),24 the Raman intensity is distributed among
the normal modes that better mimic the geometrical changes
experienced by the molecules on passing from the aromatic-
like structure of the ground electronic state to the quinoidal-
like structure of the first dipole accessible excited state. This
excitation carries out the largest variation of the dipole moment
transitions due to the large oscillator strength, and of molecular
polarizability in the presence of an external electric field at the
origin of the Raman activity. Part of the important structural
reorganization on passing to the quinoidal state occurs within
the inter-unit C-C bonds due to the antibonding f bonding
change of the charge density. This phenomenon might be more
pronounced with increasing number of C-C connecting units
as a consequence of (i) the conjugation increment with increas-
ing molecular size and (ii) the Raman enhancement of the ν-
(C-C) lines as discussed in section III.
is the elucidation of these properties as they evolve from the
building block units. Special emphasis is paid to the modulation
of conjugation in relation to substitution, molecular dimension
and nature, intermolecular forces, oxidation state, etc. Raman
spectroscopy and optical absorption/emission spectra in con-
junction with computational DFT theory are combined to this
end.
The identification of truxene and fluorene Raman bands (i.e.,
through band assignment guided by DFT calculations), and the
estimation of the evolution of the I1340-1270/I1235 ratios, as
dictated by the electronic interactions among the fluorene units
and with the truxene unit, allows us to make a conclusion about
the extent of the electronic connection between the core and
arms. This interaction is restricted to the outer rings of the
truxene core. Conjugation between fluorene units is also found
to increase for each two connected units with increasing number
of fluorenes, though there is seemingly a saturation of this
conjugation due to the increasing propensity toward conforma-
tional distortions. Spectroscopic arguments outline the glassy
amorphous character of these threefold molecules in the solid
state, although the electronic properties are affected by removing
solid-state forces or weakening these by increasing temperature.
Raman spectra are interpreted on the basis of quinoidization of
the molecular structures upon electrochemical oxidation. The
introduction of different aromatic rings (fluorene vs thiophene)
leads to a very slight change in the ground-state electronic
properties (i.e., minimal changes of the Raman spectra and
moderate shifts of the HOMO energies). This substitution,
however, has a greater effect on the features of the excited states
due to the distinct role of the sulfur atoms.
The modulation of optical properties as a function of structure
and near-gap energy levels is also interpreted. These experimental/
theoretical data are understood in line with the application of
these materials: (i) high fluorescence activity and (ii) the design
of star-shaped macromolecules pursuing antenna systems. TD-
DFT theory is used to assign the main electronic bands to one-
electron interorbital promotions. The inversion of the dominance
of the truxene features versus those of the fluorene branches is
located at the T1 f T2 step. It is interesting how certain
molecular properties are dominated by the arms or the core,
depending on the molecular architecture.
Acknowledgment. J.C. is grateful to the Ministerio de
Educacio´n y Ciencia (MEC) of Spain for a I3 research position
of Chemistry at the University of Ma´laga and for funding my
stay at the National Research Council of Canada through the
fellowship PR2006-0253 of the Programa Nacional de Ayudas
a la Movilidad de Profesores e Investigadores en el Extranjero.
Research at the University of Ma´laga was supported by the
Ministerio de Educacio´n y Ciencia (MEC) of Spain through
the project CTQ2006-14987-C02 and by the Junta de Andaluc´ıa
(Grant P06-FQM-01678). A.K. thanks the EPRSC (GR/T28379)
and R.B. thanks The Leverhulme Trust (F/00120/U) for funding.
We thank Professor Jian Pei from the College of Chemistry
and Molecular Engineering, Peking University, for providing
sample Tth2.
IV.5. Electronic Spectra of the Trigonal Compounds:
Changing the Type of Arm. Comparing T1 and Tth2 a
displacement of 20 nm of the strongest absorption band is
observed, which is consistent with the reduction of the energy
gap predicted by theory. The orbital degeneracy is still
maintained in Tth2, similar to the case of T1. The predicted
0.12/24 eV destabilization/stabilization of the HOMO/LUMO
orbitals can be explained by the reduced aromatic character of
thiophene, which might lead to a favored electronic delocal-
ization in the LUMO and further facilitated by the electron-
donating effect of the sulfur atoms. Indeed, the presence of
electron-rich sulfur heteroatoms might disfavor cathodic pro-
cesses in the truxene-thiophene derivatives in contrast to the
truxene-fluorene molecules.
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