2-[(1E)-3-(1,3-diethyl-1,3-dihydro-2H-imidazo[4,5-b]quinoxalin-2-ylidene)prop-1-en-1-yl]-1,3-diethyl-2,3-dihydro-1H-imidazo[4,5-b]quinoxalin-4-ium chloride