2,5-Bis(azulenyl) pyrrolo[3,2-b] pyrroles - the key influence of the linkage position on the linear and nonlinear optical properties

The first route towards pyrrolo[3,2-b] pyrroles containing two azulene moieties at positions 2 and 5 was developed. The key step of this approach is the three-step transformation of pyridine scaffolds into azulene via sequential N-arylation followed by ring-opening and a reaction with cyclopentadiene. The resulting quadrupolar acceptor-donor-acceptor compounds possess interesting optical properties such as bathochromically shifted absorption with the magnitude of the red-shift strongly dependent on the linkage position. Two-photon absorption of these functional dyes is markedly different from that of previously described pyrrolo[3,2-b] pyrroles. The experimental optical spectra were rationalized using time-dependent density functional theory calculations of both the linear and nonlinear optical properties.