Solvatochromic reversal: is it really possible?

Several theoretical and experimental studies have shown that organic dyes may show solvatochromic reversal in the visible region with respect to the solvent polarity. This controversial non-monotonic behavior is still not well understood. This has been analyzed here using the merocyanine of Brooker as the working example. Sometimes the solvatochromic reversal is attributed to self-aggregation.[1] In spite of this, some works discard the aggregation origin and point to structural changes of resonance hybrid between the neutral (quinoid) and the zwitterionic (benzenoid) canonical structures.[2,3,4,5,6]. Also it is argued that the flow direction of the charge transfer in the merocyanine dye plays an important role.[7] A better understanding of the real possibility of solvatochromic reversal is both timely and relevant. Considering an electric field to model the solvent polarity a solvatochromic reversal is obtained with a single solute without aggregation. Thus the reversal behavior is obtained as the result of a competition between structural changes and intramolecular charge transfer. [8] This change of regime, held responsible for the solvatochromic reversal, affects in a similar way the 13C magnetic shielding constant. [8] Key-words: Solvatochromic reversal, Merocyanine of Brooker, Absorption spectrum, Solvent effect. Support: This work has been supported by the Brazilian research agencies CNPq, CAPES, FAPESP, FAPEAL and the projects BioMol, INCT-FCx and NAP-FCx(USP). References: [1] C.T. Martins, M.S. Lima, E.L. Bastos, O.A. El Seoud, European J. Org. Chem. 2008 (2008) 1165– 1180. [2] N.A. Murugan, S. Chakrabarti, H. Ågren, J. Phys. Chem. B 115 (2011) 4025–4032. [3] I. Baraldi, G. Brancolini, F. Momicchioli, G. Ponterini, D. Vanossi, Chem. Phys. 288 (2003) 309– 325. [4] J.O. Morley, R.M. Morley, A.L. Fitton, J. Am. Chem. Soc. 120 (1998) 11479–11488. [5] J. Catalán, Dye. Pigment. 95 (2012) 180–187. [6] P. Jacques, J. Phys. Chem. 90 (1986) 5535–5539. [7] M.C. Rezende, A. Aracena, Chem. Phys. Lett. 542 (2012) 147–152. [8] V. Manzoni, K. Coutinho, S. Canuto, Chem. Phys. Lett. 655-656 (2016) 30–34.