Thiophene-Containing Pechmann Dyes and Related Compounds: Synthesis, and Experimental and DFT Characterisation

Eric Assen B. Kantchev, Tyler B. Norsten, Marilyn L. Y. Tan, Joey J. Y. Ng, Michael B. Sullivan

Received: June 21, 2011 Published online: December 8, 2011

Chem. Eur. J. 2012, 18, 695-708.

ABSTRACT:

Attaching 2-thienyl residues to the Pechmann dye core chromophore (5,5-exo-dilactone situated around a C-C double bond) results in a novel magenta-coloured compound (UV/Vis spectroscopy λ_max=570 nm in CHCl₃), which can be rearranged to a yellow 6,6-endo-dilactone (λ_max=462 nm in CHCl₃). Single and double amidation results in pronounced redshift in the 5,5-exo series (violet, λ_max=570 nm and blue, λ_max=606 nm in CHCl₃, respectively) but pronounced blueshift in the 6,6-endo series (yellow, λ_max=424 nm and pale yellow bordering on colourless, λ_max=395 nm in CHCl₃, respectively). Incorporation of a 3-alkyl substituent on the thiophene ring allows for sharp increase of solubility in organic solvents concomitant with fine-tuning of the colour: a redshift in 5,5-exo-dilactones but a blueshift in 5,5-exo-dilactams. DFT computations demonstrate that both lactone classes are planar regardless of the presence of a 3-alkyl group. The lactam derivatives are non-planar: the thiophene-core chromophore dihedral angles increase on going from 5,5-exo to 6,6-endo and from thiophene to 3-alkyl thiophene. Depending on the core heteroatom (O vs. N-alkyl), ring junction (5,5-exo vs. 6,6-endo) and 3-thiophene substituent (H vs. alkyl), two, three, four or six conformers are possible. All of these conformers were characterised by DFT and were found to be very close in energy at both IEFPCM/B3LYP/DGDZVP and SMD/M06/DGDZVP levels of theory. Within each conformer set, the HOMO and LUMO energies were within 0.05 eV and the predicted λ_max values (TD-DFT) within 10 nm, and this implies low sensitivity of the optical and electronic properties to conformation. Cyclic voltammetry measurements of selected compounds demonstrated good matching to the HOMO and LUMO energies from IEFPCM/B3LYP/DGDZVP computations. M06-2X was the best DFT functional for TD-DFT, giving predicted λ_max values within about 20 nm.

DOI: 10.1002/chem.201101903