Invited feature articlePhotophysical properties and structural analysis of modified methylene blues as near infrared dyes
Graphical abstract
Introduction
Considerable attention has been focused on the development of near-infrared (NIR) absorption and emission dyes in light of their potential applications in optoelectronic devices, sensors, photovoltaic devices, non-linear optical materials, and light-harvesting antenna models [1]. In general, in order to create the molecules having small HOMO-LUMO gap, expansion of π-conjugated system is one of the most straightforward strategy. However, large π-conjugated molecules such as higher acenes are often chemically unstable and difficult to handle because of their intrinsic high reactivity and insolubility. Thus, developing NIR dyes with smaller π-system (molecular size) is highly desired. We have reported the synthesis and photophysical properties of new dibenzo[b,i]fluorone dye framework FBX [2]. FBX having a pentacene skeleton displayed NIR sharp absorption 817 nm and emission at 857 nm in DMSO upon the addition of base due to the achievement of efficient π-conjugation based on the small bond length alternation (BLA) strategy.
Methylene blue (MB) is one of the most commonly used redox indicators, owing to its high solubility in water, large molar absorption coefficient, and vivid color change upon reduction [3]. Recently it has also been used as an optical probe of biophysical systems [4], an intercalator in nanoporous materials [5] and a redox mediator [6]. MB has absorption maximum at around λabs = 650 nm in solution despite its relatively small molecular size. These are achieved by effective delocalization of positive-charge on the nitrogen atoms originated from the contributions of resonance structures (Scheme 1). The cumulenic structures in the resonance contributions A and B could cause red-shift of its absorption spectrum. To impart MB with superior functionality, structural modifications have been implemented so far. The simple approaches are the substitution of amino groups and the exchange of the counter anion. For instance, the replacement of the amino groups in the phenothiazinium skeleton with various amines, successfully furnished a variety of useful small probes with characteristic properties, including a red-shifted absorption compared to the parent phenothiazine dyes [7]. Thus, diversity in the functionalization of phenothiazine [8] would provide not only a deeper understanding of the relationship between the molecular structures and their photophysical properties but also the effective design of the functionalized π molecule towards organic electronic devices. Considering that the contributions of resonance structures should provide red-shifted absorption and emission, the modified MB would be a good candidate to exhibit efficient visible and NIR light harvesting properties due to well π-conjugated planar structure. However, although several MB derivatives were known so far [8], the systematic structural investigation on the 3,7-substituted or counter ion exchanged MB by single crystal X-ray analysis has not been reported. Here, we have prepared MB derivatives and determined their crystal structures for the first time to study their physical properties (Scheme 2).
Section snippets
Experimental
1H NMR (400 MHz or 300 MHz) and 13C NMR (100 MHz) spectra were recorded with JEOL JNM-ECX 400 and 300 spectrometers at ambient temperature by using tetramethylsilane as an internal standard. The high-resolution MS were measured by a JEOL JMS-700 MStation spectrometer and a BRUKER Autoflex II MALDI TOF MS. The high-resolution APCI MS were performed on a BRUKER DALTONICS micrOTOF using positive and negative ion modes. UV/Vis absorption spectra were measured with a JASCO UV/Vis/NIR spectrophotometer
Results and discussion
Scheme 2 summarizes the preparation of MB derivatives from MB or phenothiazine. First, we exchanged the counter anion Cl− of MB with I−, PF6−, and BPh4−. The aqueous solution of MB was shaken with KI aq, AgPF6, and NaBPh4, giving MBI in 51%, quantitative MBPF6, and MBBPh4 in 41% yield, respectively. To the best of our knowledge, this is the first systematic study of the effect of counter anion of MB [9].
The positively-charged symmetric structures for MBI, MBPF6, and MBBPh4 were confirmed by
Conclusions
In summary, the counter ion exchanged and 3,7-substituted MB derivatives were prepared and characterized. The structures for MBI, MBPF6, and MBBPh4 were confirmed by single crystal X-ray analysis and we found that the bond lengths for the phenothiazinium core are essentially same with the original MB regardless of the counter anions. Therefore, in principal, it would be possible to use MB absorption in any media by exchanging the counter anion appropriately. Owing to the flat and π-extended
Acknowledgements
This work was supported by CREST JST (JPMJCR15F1) and JSPS KAKENHI Grant Numbers JP17H03042, JP16H02286, JP26105004 and JP15H00876 ‘AnApple’, and the program for promoting the enhancement of research universities in NAIST supported by MEXT. We thank Ms. Y. Nishikawa, and Mr. S. Katao, NAIST, for the mass spectroscopy measurements and X-ray analysis, respectively.
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