Substituent effects in stilbene-based merocyanine dyes

Students in organic chemistry are taught a number of fundamental principles concerning the factors which govern the properties of organic compounds. Among these principles is the effect of substituents on the properties of aromatic rings, especially the benzene ring. For instance, we teach the students that the reactivity of the benzene molecule towards electrophilic substitution reactions is enhanced by the presence of electron-donating groups (eg OR, NR,Rz) on the ring,' and that the degree to which an atom or group of atoms feeds electrons into the aromatic ring determines the level of ring activation. It is well known that the order of decreasing activation power is given by: 0> NR,R2 > OH > OR; where R, and R2 H, alkyl; and R = alkyl.2 It is also well known that electron-withdrawing groups placed para or ortho enhance the ability of the above types of groups to feed electrons into the ring.-' In related courses concerning the chemistry of dyes and colour, we teach textile chemistry undergraduates that resonance and extended conjugation in an organic compound are essential to the production of colour no resonance, no colour; and no extended conjugation, no colour. We also emphasise the importance of coplanarity and axis of polarisability to generating colour in organic molecules,4 and normally do so with the aid of molecular models. Several years ago Minch and Shah' published a paper which described the synthesis and unusual properties of the merocyanine dye 1 -methyl4{(oxocyclohexadienylidene) ethylidenel1 ,Cdihydropyridine (I, MOED). The authors noted that the dye possesses a colour in solution which is deDendent uDon the Dolaritv of the solvent employed (cf Table 1). Colours ranging from yellow in H 2 0 to blue in pyridine were reported. Several of the high school students in our summer research programme have repeated the synthesis of dye 1, and more recently we have extended work in the area to include dyes 2-9. These new merocyanine dyes provide a chemistry instructor with an additional tool to use as a visual reinforcement of some of the concepts outlined above. It has been