Experimental and theoretical ultraviolet spectra of haloindoles

The UV spectra of indole and the 4-, 5-, 6-, and 7-haloindoles (X=F, Cl, Br) have been recorded in methanol and calculated using time-dependent density functional theory. The lowest-energy UV absorption of each of the indoles is due to the HOMO–LUMO π–π∗ transition. This transition shifts to the red for the haloindoles relative to indole mainly as a result of the destabilization of the haloindole HOMOs.