Two-photon absorption properties of four new pentacoordinated diorganotin complexes derived from Schiff bases with fluorene
Graphical abstract
Introduction
The development of new materials for nonlinear optics (NLO) has increased during the last decades due to their potential applications in optoelectronics. In particular, materials based on organic compounds with extended conjugated π-systems between electron-donor and electron-withdrawing groups result of high interest due to their high molecular hyperpolarizability, efficient and tunable electronic response, chemical versatility and accessibility at low cost. Until the mid-90's, most of the efficient NLO candidates were based on the benchmark stilbazole skeleton [1],[2], nevertheless various heteroatoms and metals have gradually been incorporated into organic chromophores to develop molecules of greater electronic complexity, and enhanced capabilities [3]. Within this field, organotin complexes are well-known scaffolds that can be used to construct NLO compounds [4]. In addition to the well known biological activities [5], [6], [7], [8], the π-conjugated structure of organotin complexes have proved to be suitable for NLO properties [9], [10], [11], [12], [13], [14], [15], and optoelectronic applications such as OLEDS [16] (Organic Light Emitting Diodes).
The TPA phenomenon has become one of the most promising NLO properties, due to their applications in photolithography [17], microfabrication [18], 2D [19] or 3D [20] data storage and in biophotonics. Recently two-photon microscopy (TPM) has become an important tool in biomedical research [21], [22], [23], because it offers high spatial resolution and deep tissue penetration using laser radiation at wavelength within biomedical window (700–950 nm) [24], [25]. Kim and Cho [26] suggested that a suitable chromophore for TPM should have a two-photon brightness larger than 50 GM in order to avoid photodamage of the sample at laser powers commonly used in this technique, for which brightness [27] is defined as the product (σ′TPA = σTPA × ϕ) where ϕ is the fluorescent quantum yield.
Few examples of organotin complexes with TPA properties have been explored in the literature [9], [10], [11], [12]. They exhibit nonlinear absorption at the infrared wavelength of 750–800 nm with σTPA values ranging from 10 to 2500 GM (Fig. 1).
In continuation of our study of the third-order NLO properties in organotin complexes, and in a continuous effort aimed at extending their range of applications, herein we report the multicomponent synthesis and characterization of four novel pentacoordinated diorganotin complexes starting from 4-([9H-fluorene-2-yl]ethynyl)-2-hydroxybenzaldehyde, 2-amino-5-nitrophenol and diphenyl or dibutyl-tin oxides. Complexes were designed using Schiff-bases functionalized with ethynylfluorene in para position to the C=N group as ligands to increase the TPA response, fluorene was introduced because it is well known that it provides a better TPA response and favors efficient luminescence properties [28], [29], [30], [31], [32], [33], [34], [35], [36].
Section snippets
General considerations
Melting points were measured in a Barnstead Electrothermal 9300 apparatus and are uncorrected. ATR-FTIR experiments were registered in a PerkinElmer Spectrum 400 spectrometer. Absorption spectra were obtained with a Perkin Elmer spectrofluorometer Lambda 900 UV/Vis/NIR. Emission spectra were obtained with an Edinburgh Instruments spectrofluorometer FS5, with an excitation source of 405 nm; the fluorescence quantum yields were measured also with FS5 using an integrating sphere. The values of σTPA
Chemical characterization
The synthesis of diorganotin complexes 8–11 was corroborated through solution NMR, 1H, 13C, HSQC, HMBC and COSY experiments. The 1H NMR spectra show that the chemical shifts for H7 (HC=N) lie in the range between δ = 8.67–8.71 ppm; the 3J(119/117Sn-1H) coupling constants for H7 are between 41.7 Hz and 41.6 Hz for the dibutyl tin, and 52.4 Hz and 52.1 Hz for the diphenyl tin derivatives, which are in agreement with previous reports [43],[51]. The 13C NMR spectra show that the chemical shifts for
Conclusions
We synthesized four novel pentacoordinated diorganotin compounds which were fully characterized. A solvatochromic study showed no dependence on solvent polarity pointing out to the existence of specific interactions and a small change in the dipole moment. These complexes can be grouped depending on the substituent of the tin atom, either diphenyl or dibutyl, due to the small changes in their optical properties attributed only to the rigidity of the framework provided by the tin center, which
Acknowledgements
This work has been performed within the framework of the French-Mexican international laboratory (LIA-LCMMC). The authors thank CNRS (France), PAIP, PAPIIT IN216616 (UNAM) and CONACYT (Mexico) for financial supports, A.E.C. (270200) thanks CONACYT for the PhD grant, Violeta Álvarez. and Alberto Vega thanks UNAM-DGAPA.
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From February 01, 2013 to January 31, 2015 V.A.-V. had a postdoctoral fellowship at UNAM and from February 01, 2015 to July 31, 2016 at CIO.