Synthesis and Properties of a
Fluorene-Capped Isotruxene: A New
Unsymmetrical Star-Shaped
π
-System
Jye-Shane Yang,* Yar-Ru Lee,
†Jyu-Lun Yan,
†and Ming-Chun Lu
†Department of Chemistry, National Taiwan UniVersity, Taipei, Taiwan 10617, and Department of Chemistry, National Central UniVersity, Chungli, Taiwan 32054 [email protected]
Received September 30, 2006
ABSTRACT
An unsymmetrical star-shapedπ-system (1) with an isotruxene core and three fluorene arms has been synthesized, and its photophysical, electrochemical, and thermochemical properties have been investigated and compared with the corresponding symmetrical truxene derivatives 2a−d (Kanibolotsky, A. L.; Berridge, R.; Skabara, P. J.; Perepichka, I. F.; Bradley, D. D. C.; Koeberg, M.J. Am. Chem. Soc. 2004, 126, 13695− 13702). Stronger electronic couplings between the arms in 1 vs 2a−d lead to lower optical band gap, larger splitting in oxidation potentials, and superior thermostability.
The differences in electronic properties (e.g., photolumines-cence, conductivity, and nonlinear optics) between p- and
m-phenylene (p-Ph and m-Ph)-derivedπ-conjugated systems
have recently attracted much attention.1-7Both theoretical
and experimental studies have shown that electronic coupling between the m-Ph-linked segments is nonnegligible but the size strongly depends on the nature of segments.1-5For pure hydrocarbon π-segments, the coupling mediated by m-Ph linkers is generally weak, which is particularly evident in polymeric systems such as poly(p-phenylenevinylene)s (PPVs), poly(p-phenylene)s (PPPs), and poly(p-phenyleneethy-nylene)s (PPEs), where the electronic spectra are substantially blue-shifted when m-Ph linkers are introduced into the conjugated backbones.2,3 In addition to the disubstituted †National Central University.
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ORGANIC
LETTERS
2006
Vol. 8, No. 25
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phenylene linkers, trisubstituted phenylene branching groups have been largely employed in constructing star-shaped or dendritic π-systems.6 However, most examples adopt the 3-fold symmetrical m,m-Ph branching units, and much less attention was paid to systems with the unsymmetrical p,m-Ph (or p,o-p,m-Ph) ones. Recent reports by Peng et al. have shown that switching of the m,m-Ph to p,m-Ph branching units in phenylacetylene monodendrons can significantly enhance the light-harvesting properties.7We report herein the synthesis of a new star-shaped conjugated system with the p,m-Ph linker in the core: namely, the fluorene-capped isotruxene
1. When compared with the corresponding m,m-Ph-derived
truxene analogs 2a-d recently reported by Kanibolotsky et al.,8 the photoluminescent, electrochemical, and thermal properties of 1 are more like those of 2d than those of
2a-c, indicating the presence of strong electronic couplings
among the fluorene arms in 1.
Isotruxene (3) can be considered as three “overlapping” fluorene moieties, which arrange in a way different from the symmetrical truxene isomer. Although the synthesis of isotruxene has been known for more than 45 years9and many truxene derivatives have recently been investigated as light-emitting and light-harvesting materials,10,11the chemistry of isotruxene is essentially an unexplored area. One of the possible reasons is the lack of facile methods for the preparation of isotruxene. The currently known synthetic method for isotruxene is a one-step reaction from indene, but it requires high pressure (20 atm) and high temperature (350°C).9Thus, improvement of the reaction conditions will be an important step toward the development of isotruxene-based materials. In the following, we will show that in the
presence of appropriate catalysts (I2and carbonate salts) the reaction can be carried out at much lower temperatures and under ambient atmosphere (Scheme 1).
As shown in Table 1, the conversion of indene to isotruxene is very sensitive to the reaction conditions, including the amount of iodine, the nature of base and
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Scheme 1
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solvent, and the reaction temperature. Preliminary studies indicate that the condition of 2 M of indene in DMF along with 0.4 equiv of iodine and 1 equiv of cesium carbonate at 160 °C provided the best yield (18%, entry 8 in Table 1). Although the isolated yield for isotruxene is unsatisfactory, it appears to be comparable to that from the harsh reaction conditions.12A possible mechanism for this reaction, which is initiated by abstraction of the benzylic hydrogn of indene by elemental iodines, is proposed in the Supporting Informa-tion (Scheme S1). However, more concrete evidence is required to draw firmer conclusions on the reaction mech-anism.
The synthesis of compound 1 was conducted by following the same methodology for compound 2a8 by replacing truxene with isotruxene as the starting material (Scheme 1). It should be noted that the use of shorter alkyl chains, the ethyl groups, for 1 instead of the longer hexyl groups adopted by 2a reflects the fact that isotruxene is more soluble than truxene in organic solvents,9 presumably due to its lower molecular symmetry.
The absorption and fluorescence spectra for 1 in toluene are shown in Figure 1. Unlike the single intense absorption bands (λmax) 343-372 nm) observed for all four cases of
2a-d,8the absorption spectrum for 1 displays a maximum at 348 nm and a shoulder at 380 nm. The fluorescence spectrum for 1 shows vibronic bands, but the overall spectrum is less structured than those for 2a-d.8 It is interesting to note that the maximum of the 0-0 fluorescence band (427 nm) for 1 is red-shifted relative to those of 2a-d (375-411 nm). The fluorescence quantum yield for 1 is high but slightly lower than that for 2a-d. All these spectroscopic data along with the corresponding data for 2a-d8 are reported in Table 2.
The spectral features clearly indicate that the exciton is delocalized in all 10 phenyl rings of 1 but essentially localized in the individual arms of 2a-d. More specifically, compound 2d possesses 10 phenyl rings in each arm so that it is more comparable than 2a-c to 1 in terms of the peak maxima. With the same 10 phenyl rings for exciton delo-calization, the deviations in peak maxima between 1 and 2d
reflect the difference in the phenyl ring connections and thus the electronic couplings. In the case of 1, the presence of two absorption bands is a characteristic of coupling-induced band splitting in o-Ph- or m-Ph-linkedπ-systems.7b,13 The absence of band splitting in the m,m-Ph-linked systems 2a-d indicates weak exciton couplings between the arms. Accord-ingly, the band splitting observed for 1 should be attributed to the ortho conjugated effect, and the central phenyl ring in isotruxene is better described as a p,o-Ph rather than a p,m-Ph branching group. Since band splitting often reduces the oscillator strength for the S0f S1transition, correspond-ing to the slightly lower log value for 1 (4.82) vs 2a-d (4.97-5.67) (Table 2), this might account for the somewhat lower fluorescence quantum yields for 1 vs 2a-d.
The electronic conjugation interactions among the three arms of 1 are also revealed by its electrochemical behavior. As shown in Figure 2, compound 1 displays two reversible
oxidation processes with similar current values. Although the values of ∆Epa-pc (85-100 mV) are somewhat larger
(12) The reaction in ref 9 generated a mixture of isotruxene and truxene with a yield of 40%, but no isolated yield was reported for isotruxene.
(13) Lewis, F. D.; Kalgutkar, R. S.; Yang, J.-S. J. Am. Chem. Soc. 1999,
121, 12045-12053. Table 1. Comparison of Yields of Different Reaction
Conditions for Isotruxene Formation from Indene
entry I2(equiv) basea solvent T (°C) yieldb(%)
1 0.4 K2CO3 DMF 140 12 2 0 K2CO3 DMF 140 0 3 0.6 K2CO3 DMF 140 7 4 0.4 KtBuO DMF 140 0 5 0.4 pyridine DMF 140 0 6 0.4 Cs2CO3 DMF 140 12 7 0.4 K2CO3 DMF 160 7 8 0.4 Cs2CO3 DMF 160 18 9 0.4 Cs2CO3 mesitylene 160 0
a1 equiv of indene.bReaction time is 24 h.
Figure 1. Normalized absorption and fluorescence spectra of 1 in toluene.
Figure 2. Cyclic voltammogram of 1 in (a) THF (for reduction) and (b) dichloromethane (for oxidation); electrode Pt for reduction and glassy carbon for oxidation; electrolyte 0.1 M Bu4NPF6, scan
rate 100 mV s-1.
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than the theoretical value of 59 mV, both oxidation processes are essentially one-electron steps, corresponding to the formation of radical cation and dication species.8,14 The separation between the two oxidation peaks (∆Eox) is ca. 0.33 eV. More than one cyclic voltammetric oxidation peaks are also observed for 2a-d, but the separations between the peaks are all less than 0.3 eV (0.08-0.25).8A larger∆E
ox value for 1 vs 2a-d suggests a stronger electronic coupling among the arms, which is consistent with the observations in electronic spectra.
The thermal and morphological stabilities of 1 have also been investigated and compared with 2a-d. As shown in Table 2, both the glass transition temperature (Tg) and the TGA 5% decomposition temperature for 1 are larger than those for 2a-2d, indicating that isotruxene-based star-shaped π-systems are promising candidates for optoelectronic ap-plications.
In summary, we have synthesized the first isotrxuene-based star-shaped π-system 1, which displays promising
light-emitting properties and thermostabilities. In addition, com-parison of 1 and the corresponding truxene derivatives 2a-d provide a new example toward a better understanding of the
p,o-Ph vs m,m-Ph effect on the properties ofπ-conjugated
systems. Further studies on the synthesis and properties of new isotruxene derivatives are in progress.
Acknowledgment. Financial support for this research was
provided by the National Science Council of Taiwan, ROC. We are grateful to Professor C. K. Lai (NCU) for use of his DSC apparatus and Professor Y.-M. Tsai (NTU) for helpful discussions.
Supporting Information Available: Experimental
meth-ods, synthetic procedures, characterization data, a proposed mechanism for isotruxene formation, and1H and13C NMR spectra for all new compounds and DSC and TGA scans for compound 1. This material is available free of charge via the Internet at http://pubs.acs.org.
OL062408S
(14) Choi, J.-P.; Wong, K.-T.; Chen, Y.-M.; Yu, J.-K.; Chou, P.-T.; Bard, A. J. J. Phys. Chem. B 2003, 107, 14407-14413.
Table 2. Spectral, Electrochemcial, and Thermal Data for 1 and 2a-da
compd log b λabs,bnm λfl,bnm Φflb E
1/2oxd,cV E1/2red,dV Tg, °C Td, °C 1 5.01, 4.82 348, 380 427, 450 0.65 0.61, 0.94 -2.74 160 415 2aa 4.97 343 375sh, 396, 416sh 0.70 0.80, 1.05 -2.80 63 401 2ba 5.50 360 399, 422, 443sh 0.83 0.76, 0.84, 1.03 -2.74 86 408 2ca 5.61 370 408, 431, 460sh 0.83 0.76, 0.94 -2.70 101 410 2da 5.67 374 411, 436, 460sh 0.86 0.74, 0.87 -2.66 116 413
aData for 2a-d are from ref 8.bIn toluene solutions.cIn dichloromethane solution.dIn THF solution.
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