TPA-active D–π–D fluorophores with rigid, planar cores from phenylene to indenofluorene and indolocarbazole
Introduction
Since the pioneering work of Parthenopoulos and Rentzepis [1] and Webb and co-workers [2] on three-dimensional optical data storage and two-photon laser scanning fluorescence microscopy, molecules with strong two-photon absorption (TPA) cross-sections (δ2) have attracted growing interest [3], [3](a), [3](b), [3](c), [3](d), [3](e). Many chromophores with a large variety of structures have been synthesized and had their TPA cross-sections measured, and fundamental efforts have been made to elucidate structure-property relationships in these systems. Among them, D–π–D and A–π–A one-dimensional quadrupolar systems have been successfully employed to enhance δ2 values. The quadrupolar intramolecular charge transfer taking place between either the electron-donating (D) or electron-accepting (A) end-groups and the π-conjugated cores is believed to be an important factor in this enhancement [4], [4](a), [4](b). Further efforts to introduce stronger acceptors or donors as well as elongated cores to enhance this charge transfer have confirmed this assumption [5]. Very large δ2 values have also been obtained with D–A–D, A–D–A, D–A–D–A–D, and A–D–A–D–A systems [3], [3](a), [3](b), [3](c), [3](d), [3](e), [6], [6](a), [7], [7], [8], [8](a). However, this success is often at the cost of reduced fluorescence quantum yield and/or pronounced red-shift of the absorption and emission bands. Blanchard-Desce and co-workers recently reported a systematic study on the structure-property relationships of various quadrupolar fluorophores with different types of π-conjugated cores. The results of this work indicated that increasing the connector length between D or A groups induces systematic and pronounced red-shifts and hyperchromic effects on the absorption bands [9].
Most applications of TPA and two-photon excited fluorescence (TPEF) require excitation in the near infrared region (700–900 nm), meaning that a molecular engineering approach to enhance the TPA cross section of the materials without inducing large shifts in their absorption bands would be highly desirable. Although D–π–D quadrupolar compound 1 and its fluorene analogue 2 have been exclusively examined as TPA-active compounds [10], [10](a), [10](b), [10](c), [11], little attention has been paid to their spectral similarities, particularly the linear absorption and single-photon excited fluorescence emission spectra in dilute solutions, which span nearly the same regions despite their molecular structural differences. Thus, in the current investigation, we have examined the structure-function relationship of D–π–D systems with respect to lengthening of the π-conjugated core, in particular focusing on extended fluorene and carbazole-based systems.
These systems, compounds 1–6 in Fig. 1, contain highly rigid and planar π-conjugated cores, with solubilizing alkyl groups. Diphenylamino moieties were chosen as the electron-donating end-groups with styryl bridging units to the varying π-conjugated cores. The linking units employed in this study were: 1,4-phenylene (1); 9,9-diethyl-9H-fluoren-2,7-yl (2); 9-n-pentyl-9H-carbazole-2,7-yl (3); 6,6,12,12-tetra-n-butyl-6,12-dihydroindeno[1,2-b]fluoren-2,8-yl (4); 6,12-di-n-octyl-6,12-dihydro-6,12-diazaindeno[1,2-b]fluoren-2,8-yl (5) and 6,12-di-n-octyl-6,12-dihydro-6,12-diazaindeno[1,2-b]fluoren-3,9-yl (6). Indenofluorene and indolocarbazole moieties have been widely employed in organic light emitting diode (OLED) and organic transistor materials, although there are few reports of their use in TPA chromophores [12], [13]. A compound similar to 4 with methyl rather than n-butyl substituents has recently been described in the patent literature and shown to exhibit high electro-luminescent efficiency [14]. Lin and co-workers also reported recently the synthesis and two-photon properties of a multipolar chromophore derived from triphenylamine with three diphenylaminoindenofluorenyl branches [15]. To understand the effects of positional substitution, if any, isomers 5 and 6 were also prepared.
Section snippets
Synthesis and characterization of compounds 1–6
All reactions were carried out under a nitrogen atmosphere using standard Schlenk techniques. NMR spectra were obtained by using Bruker Avance 400 spectrometers. All spectra were recorded in CDCl3. Chemical shifts are reported relative to tetramethylsilane and are referenced to residual proton or carbon resonances in CDCl3. Mass spectra were recorded on Applied Biosystems Voyager DE STR (MALDI-TOF) spectrometers. Elemental analyses were performed on a PE 2400 autoanalyser. The melting points
Synthesis
As shown in Fig. 1, compounds 1–6 were synthesized via cross-coupling of the appropriate dihalo-substituted arene with two equivalents of 4-N,N-diphenylaminostyrene under typical Heck reaction conditions, catalyzed by Pd(OAc)2 and P(o-tol)3. 1,4-diiodobenzene is commercially available. Indenofluorene was prepared starting from 1,4-dibromo-2,5-dimethylbenzene by a similar method to that reported by Wang and co-workers [20], [20] with a little revision. Normally, the direct halogenation of
Conclusion
In conclusion, we have synthesized a series of D–π–D systems with different cores, ranging from benzene to indenofluorene and indolocarbazole. Systematic comparison of their single- and two photophysical properties reveals that the systems with longer cores, such as indenofluorene and indolocarbazole have significantly enhanced one- and two-photon absorptions, but only small shifts in their absorption or emission bands.
Acknowledgement
Financial support from the National Natural Science Foundation of China (20802026, 50803033) Natural Science Foundation of Shandong Province (Q2008F02) Shandong Encouraging Fund for Excellent Scientists (BS2009CL013) is gratefully acknowledged. The authors thank Dr. Jonathan C. Collings of Durham University for his valuable suggestions.
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