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) 1a-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 lambda=1200 nm. In the case of the N,N'-dimethyl derivative, cation radical 1c(+center dot) is stable enough to be isolated as a salt and Xray 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 1a-c and 1a-c(+center dot) 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 1a-c are highly emissive (Phi(F): 0.7-0.8).