Strategies for optimising the second-order nonlinear optical response in zwitterionic merocyanine dyes

Synthesis, linear & non linear optical (NLO) properties of some indoline based chromophores

“…As a result 26 and 28 have beta values approximately 20% higher than 27. From our previous work we found that in some cases ring locking across the donor ring-polyene chain led to a 50% increase in b [16]. In the present work it is apparent that ring locking across the centre of the polyene chain does not lead to a corresponding increase in the NLO response, and that electronic factors probably play an important role.…”

“…This is a well documented phenomenon and, therefore, entirely expected [15,16]. The inclusion of ring locking in the centre of the conjugated p-system, along with either t-butyl or diphenyl substitution on the central cyclohexene ring (compounds 11, 17, 18, 24, 25) has a minimal effect on the second-order nonlinear response.…”

“…This should also translate to improved NLO responses in these compounds. A previous study by us demonstrated that configurational locking across the donor-polyene unit improved the linear optical response by up to 50% [16]. When the halogen on the cyclohexene ring is replaced by either a 2,6-di-tert-butyl-4-methylphenoxy (26), thiophenoxy (27) or pentafluorothiophenoxy (28) substituent a sizable red-shift in absorption maxima occurs, and this is to be expected as these auxochromes are in direct conjugation with the p-system.…”

“…While this approach allowed for ease of synthesis and for a controlled increase in the extent of conjugation in the molecules, the resultant "parent" merocyanines are prone to significant amounts of aggregation [16], and this is due to their highly polar, zwitterionic ground states. Evidence for H-aggregation in these compounds was routinely observed via the appearance of a high energy shoulder when UVeVis absorption spectra were obtained in solvents of low polarity such as dioxane and chloroform.…”

Synthesis and Properties of Zwitterionic Chromophores Containing Substituents for Shape Control

“…dioxane and THF) are characterised by the presence of high energy shoulders on the main absorption peak, and this is typical of the presence of H-aggregates. 3 However it can be seen from Figure 2 that the progressive introduction of bulky group 9 to the top (4), side (7) and both positions (8) of the basic pyridine chromophore skeleton (1) resulted in a steady decrease in the prominence of this shoulder, such that it was almost completely absent in 8. This therefore suggests that the introduction of bulky groups to zwitterionic chromophores such as these may be a valid strategy for overcoming deleterious aggregation.…”