Thermo‐Solvatochromism of Merocyanine Polarity Probes – What Are the Consequences of Increasing Probe Lipophilicity through Annelation?

Abstract

The question raised in the title has been answered by comparing the solvatochromism of two series of polarity probes, the lipophilicities of which were increased either by increasing the length of an alkyl group (R) attached to a fixed pyridine-based structure or through annelation (i.e., by fusing benzene rings onto a central pyridine-based structure). The following novel solvatochromic probes were synthesized:. The solvatochromic behavior of these probes, along with that of 2,6-dibromo-4-[(E)-2-(1-methylpyridinium-4-yl)ethenyl]phenolate (MePMBr 2 ) was analyzed in terms of increasing probe lipophilicity, through annelation. Values of the empirical solvent polarity scale [E T (MePMBr 2 )] in kcal mol -1 correlated linearly with E T (30), the corresponding values for the extensively employed probe 2,6-diphenyl-4-(2,4,6-triphenylpyridinium-1-yl)phenolate (RB). On the other hand, the nonlinear correlations of E T (MeQMBr 2 ) or E T (MeAMBr 2 ) with E T (30) are described by second-order polynomials. Possible reasons for this behavior include: i) self-aggregation of the probe, ii) photoinduced cis/trans isomerization of the dye, and iii) probe structure-and solvent-dependent contributions of the quinonoid and zwitterionic limiting formulas to the ground and excited states of the probe. We show that mechanisms (i) and (ii) are not operative under the experimental conditions employed; experimental evidence (NMR) and theoretical calculations are presented to support the conjec-

Solvation in aqueous binary mixtures: consequences of the hydrophobic character of the ionic liquids and the solvatochromic probes

“…16 The probes MePMBr 2 , OcPMBr 2 , MeQMBr 2 , and ILs AlMeImCl, AlBuImCl and AlHxImCl were available from previous studies. 8,9,17 AlBuImCl and AlHxImCl are liquids at room temperature, AlMeImCl is a solid (mp 52-53 1C), once melted it crystallizes only over a period of several days. 3 Binary mixtures (16 per set) of IL and W were prepared by weight at 25 1C.…”

Estudos dos efeitos de solventes no espectro de absorção eletrônica da merocianina de Brooker e derivados

“…Experimentalmente, foi obtido o espectro de absorção das sondas em 38 solventes próticos e apróticos [30], e foram observadas correlações não lineares entre as escalas empíricas de polaridade, (30). De maneira resumida observou-se que a escala E T (M eP M Br 2 ) apresenta uma relação linear com a escala E T (30), já as escalas E T (M eQM Br 2 ) e E T (M eAM Br 2 ) apresentam relações não lineares com a escala E T (30), ver gura 1.4.…”

“…Figura 1.4: Correlação entre E T (sonda) e E T (30) para os 38 solventes estudados experimentalmente. Valores para E T foram obtidos da referência [30]. Os solventes que estão numerados nos grácos são: água (1), metanol (2), DMSO (3), 1,4 dioxano (4), acetato de etila (5), e benzeno (6).…”

“…De acordo com a gura 1.4 o processo de anelação das sondas provoca o comportamento reverso para o solvatocromismo [30]. Desta forma, neste trabalho, estudamos quais são as mudanças nas interações soluto-solvente devido a mudança estrutural da sonda provocado pelo acréscimo de 1 ou 2 anéis aromáticos.…”

“…Geometria otimizada no nível B3LYP/6-311+G** e momento de dipolo obtido com MP2/6-311+G**. Figura 6.5: Energia de máxima absorção experimental, E max , para a sonda MePMBr 2 versus a escala de polaridade de Reichardt normalizada, E N T [30]. Os solventes considerados foram: 1,4 Dioxano(1); (2) ; Acetonitrila(3); Cicloexano(4); 2-Metil-1-butanol(5); 2-Metoxietanol(6); Metanol(7) e Água(8).…”