Synthesis, tunable two and three-photon absorption properties of triazine derivatives by branches
Highlights
► Novel triphenylamine-based dendrimers were synthesized and characterized. ► Photophysical properties and two-photon induced fluorescence were investigated. ► Two and three-photon absorption cross-section of dendrimers were obtained. ► Two-photon induced excited-state absorption cross-section of dendrimers were obtained. ► We find strong structure-dependence relationship by tuning the number of branches in molecules.
Introduction
Organic nonlinear optical materials have attracted intensive research interest due to their possible applications in a number of fields, including two-photon fluorescence imaging [1], three-dimensional optical data storage [2], optical power limiting [3], and photodynamic therapy [4]. Especially, the flexible synthesis and tunable structures of electronic-states have made organic molecules promising candidates for applications based on nonlinear absorptions. In the past decade, much effort has been paid to the development highly efficient two-photon absorption (2PA) [5] and two-photon excited fluorescence (2PEF) materials [6]. Some effective strategies for molecular design to enhance two-photon absorption and emission have been found such as donor–bridge–acceptor (D–π–A) structures, donor–bridge–donor (D–π–D) quadruples structures, multi-branched molecules, and so on. However, the study for their higher-order nonlinear absorption including three-photon absorption (3PA), two-photon induced excited-state absorption (2PEA) and four-photon absorption (4PA), is still rather limited [7], [8]. Commonly, the higher-order nonlinear absorption strongly depends on molecular structures of electronic-states and so on parameters (wavelength, pulse duration and peak irradiance) of the incident laser beam. Up to now, the laser sources with ultrafast pulse durations offer the feasibility to observe different kinds of higher-order nonlinear absorption phenomena. However, to this day, organic molecules, showed 3PA or higher-order nonlinear optical properties, are still very a few. Therefore, organic materials with the higher-order nonlinear absorption or emission will be attracting extensively interests.
As an intriguing type of organic functional materials, triazine and its derivatives have recently received attention due to their special and significant properties [9], [10], [11]. Actually, triazine and its derivatives were extensively used as emitters in organic light-emitting materials, or as frequency converted fluorescent and nonlinear absorption in octupolar molecules [9]. Moreover, triazine can also be functionalized with a variety of substitutions and obtain excellently chemical and physical performance for the nonlinear optical application.
Triphenylamine and their derivatives, known as important chemical intermediates, are some of the typical aromatic amines and possess superior electron-donating properties [12], [13]. However, to the best of our knowledge, triazine derivatives containing triphenylamine unit, showing tunable 3PA and 2PEA with structure dependence in femotosecond timescales, have still been reported very a few during the past decade.
Here, we designed and synthesized several molecules with different branches containing triphenylamine and triazine units. (Scheme 1)Their nonlinear absorptive, and up-converted fluorescent properties were studied systematically using open Z-scan, frequency up-converted, and pump-probe methods by femtosecond laser pulses using 780 nm-wavelength as excitation.
Section snippets
Materials and characterization
4-(di-p-tolylamino)benzaldehyde were purchased from Tokyo Chemical Industry Co., LTD. All other reagents were used as received from commercial sources, unless otherwise stated. Melting points were determined using an X-4 apparatus and the thermometer was uncorrected. 1H NMR spectra were determined in CDCl3 with a Bruker DRX 400 MHz spectrometer. Chemical shifts (d) were given relative to tetramethylsilane (TMS). Elemental analyses were recorded with a Perkin–Elmer 2400 analyzer. MALDI-TOF-MS
Synthesis
2,4,6-trimethyl-1,3,5-triazine was synthesized based on Jung′s method [13]. In order to improve the yield of this reaction, we also investigated the inletting velocity and concentration of hydrogen chloride (HCl) gas. One can be concluded the HCl gas should be fast inlet at the beginning of the reaction by analyzing the results of many experiments. About 24 h later, the velocity should be reduced to a half of the beginnings. When the ratio of HCl and acetonitrile reaches about 1.2: 1, we can
Conclusions
In this paper, we have investigated systematically the nonlinear optical properties of three triazine derivatives with different numbers of branches using femtosecond Ti:Sapphire laser pulse. Their nonlinear optical studies are undertaken by performing open Z-scan, frequency up-converted and pump-probe measurements. Importantly, strong three-photon absorption was observed in this work with a value of 1.35 × 10−76 cm6 s2, which was firstly observed in femotosecond timescale using
Acknowledgement
Financial support from the National Natural Science Foundation of China (Nos 20671036, 2007A010500008 and 2008B010800030) and NIMTE Foundation (Y10821QF04), and the National University of Singapore are gratefully acknowledged.
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