Studies of Some Strained organic Molecules: Alkylidenecycloproparenes

Abstract

The alkylidenecycloproparenes, e.g. 54, have surprising stability when the double bond carries aryl substituents. Theoretical calculations reveal that these compounds have an inherent ability to minimize strain and charge separation is the likely reason. In the present study the alkylidenecycloproparenes 50a-50d, and 73a-73d were synthesized and systematic investigations of their physical and spectroscopic properties made. Experimental evidence has been collected from changes in the characteristic infrared stretching band in the range 1740-1795 cm-1, unusual solvent effects upon the ultraviolet absorption maxima' and correlations between 13C ,n.m.r. chemical shifts and the appropriate Hammett substituent constant σ p +. The results suggest that the cycloproparene component of these molecules is ambiphilic and will sustain both positive and negative character depending upon the specific nature of the substituents. Several of the alkylidene derivatives are found to fluoresce strongly with quantum efficiency close to unity. Twisted internal charge transfer states are proposed to account for this behaviour. Furthermore, these compounds were found to absorb and lase with exceptional large "Stokes shifts" and are classified as unusual tunable laser dyes. The measurement of dipole moments and of crystallographic parameters have also been employed to unambiguously establish charge separation in the alkylidenecycloproparenes. The dipole measurements show that the alkylidene compounds are moderately polar molecules (0.4-3.2 Debye) with mesomerism dominating. Interesting geometric predictions made from the dipole results have been confirmed by x-ray crystallography. 1H, 4H-Dicyclopropalb, glnaphthalene 96 is one of the most highly strained organic compounds thus far isolated. The synthesis of this compound has been improved upon and its methylenation studied in an attempt to extend the charge separation character of the alkylidenecycloproparenes into the long range “push-pull” system, e.g. 144.