Photophysical properties and structural analysis of modified methylene blues as near infrared dyes

[Display omitted] •The UV–vis absorption spectra for MBI, MBPF6, and MBBPh4 are identical to the original MB and exhibit peaks at 655nm.•The solubility in the media could be controlled by the counter anion without change of absorption spectrum.•3,7-Bis(diphenylamino)phenothiazinium chloride 3 (MB 3) displays red-shifted absorption in which the diphenylamine moieties play as a donor.•Curiously MB 3 in CH2I2 illustrates the absorption peak at 729nm and the edge reaches to 900nm despite its anthracene-sized compound at the most.•Hence, this work demonstrates the utility of MB dye as an efficient NIR light harvesting material. To study and improve methylene blue (MB) on its physical properties, structural modifications have been implemented. The counter ion exchanged or 3,7-substituted MB derivatives were prepared and the structural investigation by single crystal X-ray analysis has been performed. The positively-charged symmetric structures for MBI, MBPF6, and MBBPh4 were confirmed. The UV–vis absorption spectra for these products are identical to the original MB and exhibit peaks at 655nm, indicating negligible interactions between the phenothiazine core and the counter anions. According to this behavior, the solubility in the media could be controlled by the counter anion without change of absorption spectrum. On the other hand, 3,7-bis(diphenylamino)phenothiazinium chloride 3 (MB 3) displays remarkably red-shifted absorption bands in which the diphenylamine moieties play as a donor part. The absorption peaks strongly depend on the solvent polarity. Curiously MB 3 in halogenated solvents exhibits the largest red-shift of the absorption and negative solvatochromism. For instance, MB 3 in CH2I2 illustrates the absorption spectral peak at 729nm and the edge reaches to 900nm despite its anthracene-sized compound at the most. Importantly no solvent dependency was observed for the absorption of normal MB so that this is characteristic for MB 3. Hence, this work demonstrates the utility of MB dye as an efficient NIR light harvesting material.