Wurster's Blue-type cation radicals framed in a 5,10-dihydrobenzo[a]indolo[2,3-c]carbazole (BIC) skeleton: dual electrochromism with drastic changes in UV/Vis/NIR and fluorescence.

Electron-donating dihydrobenzindolocarbazoles (BICs) 1 a-c, which adopt planar disk-shaped geometries, were prepared by gold(I)-catalyzed cyclization as a key step. Due to the presence of a 1,4-phenylenediamine (PD) moiety in the framework, they undergo reversible one-electron oxidation to the corresponding Wurster's Blue (WB)-type species that exhibits NIR absorptions up to λ=1200 nm. In the case of the N,N'-dimethyl derivative, cation radical 1 c(+.) is stable enough to be isolated as a salt and X-ray analysis indicated paraquinoid-type bond alternation in the WB core unit, whereas the bond lengths in the peripheral benzene rings are identical to those in the neutral donor. Upon electrochemical interconversion, the redox pairs of 1 a-c and 1 a-c(+.) exhibited an electrochromic response in the UV/Vis/NIR region, which was accompanied by a drastic change in the fluorescence spectrum because only neutral donors 1 a-c are highly emissive (Φ(F) : 0.7-0.8).