Novel Electron Acceptors Bearing a Heteroquinonoid System. Part 4. Syntheses, Properties, and Charge-Transfer Complexes of 2,7-Bis( dicyanomethylene)-2,7-dihydrobenzo(2,1-b:3,4-b′)dithiophene, 2,7-Bis( dicyanomethylene)-2,7-dihydroben