Hydrogen-Bonded Complexes of Fluorophenylacetylenes: To Fluoresce or Not?

The hydrogen-bonded complexes of fluorophenylacetylenesexhibit unusual and interesting fluorescence turn ON/OFF behaviour following excitation to ¹ ππ* (S₁ ) state. The fluorescence switching behaviour can be realized by (i) "change in the intermolecular structure, (ii) change in the position of fluorine substitution and (iii) change in the hydrogen bonding partner or a combination thereof. Experiments indicate that the ≡C-H⋅⋅⋅X (X=O, N) hydrogen bonding with the acetylenic group plays a pivotal role in this switching behaviour. Intriguingly, weaker ≡C-H⋅⋅⋅X hydrogen bonding leads to fluorescence OFF state, which is turned ON by stronger hydrogen bonding. The observed fluorescence this switching behaviour is rationalized on the basis of a phenomenological model which suggests a coupling between the initially excited S₁ state and a dark S_n state in the Franck-Condon region with limited window controlled by the ≡C-H⋅⋅⋅X hydrogen bonding as a crucial parameter. Such fluorescence switching behaviour in hydrogen-bonded complexes is unprecedented and these intriguing results hopefully will stimulate theoreticians to test 'state of the art' theories to explain these observations in a consistent manner.