Searching for a near-infrared fluorescence band supporting a hypothetical double proton transfer in ESIPT reaction of 2,5-bis(2′-benzoxazolyl) hydroquinone (BBHQ), as requested by theoreticians

•Successive search of the phototautomeric fluorescence of 2,5-bis(2'-benzoxazolyl)hydroquinone (BBHQ) in the near-infrared spectral range.•The double proton transfer in the excited BBHQ, as predicted by theory, experimentally documented.•The selected derivatives, as a help in search of the diketotautomer – a product of the double PT in the electronically excited BBHQ. New near-infrared broad-band emission with λmax~920 nm was detected in 2,5-bis(2′-benzoxazolyl)hydroquinone (BBHQ), a molecule known to undergo the single proton transfer in the ESIPT reaction and to emit the fluorescence with λmax ~600 nm. The new emission was attributed to the fluorescence of the product of double proton transfer in the excited BBHQ. The attribution was based on the qualitative agreement verifying the theoretical predictions and on the behaviour of the two properly chosen derivatives: the diethyl-BBHQ independently showing the same broad-band IR fluorescence, and the monomethoxy-BBHQ, a “negative witness”- a structure with one of two ESIPT reactivity sites eliminated, showing no trace of the new emission. [Display omitted]