The cyclopentadienone-fused dihydropyrenes 46 and 47 were synthesized. The internal methyl resonances, the coupling constants, NICS calculations and X-ray results confirmed that the cyclopentadienone displays antiaromatic character resulting in bond localization in the annulene ring consistent with a 4n-pi fused system. The ring current of the dihydropyrene fragment is reduced by fusion of the antiaromatic system by about 80% of that caused by benzene. The syntheses of the methylfulvene fused dihydropyrene 56 and the phenylfulvene fused dihydropyrene 58 have been accomplished. The calculated and experimental NMR data and NICS calculations all demonstrated that the fulvenes had weak diatropic ring currents and caused bond localizations in the DHP rings, in which phenyl fulvene has a larger effect than that of methyl fulvene. A number of bis-dihydropyrene systems, bis-dihydropyrene ketone 117, bis-benzo[e]dihydropyrene ketone 119, benzo[e]dihydropyrene dihydropyrene ketone 122, bis-benzo[e]dihydropyrene methylene 124 and the benzo[e]dihydropyrene- dihydropyrene acetylene 130, have been synthesized, in which 117, 119 and 124 are homo-systems and 122 and 130 are hetero-systems. The multiple photoswitching properties study found that all of these systems except 130 showed multi-states during the photo opening and photo closing processes, which means that each end of the DHP units photo opens or closes separately rather than synchronously. In the homo switches 117, 119 and 124, the two DHP units act independently, but the relative differentiation is not very significant. On the other hand, the hetero-switch 122 showed fully differentiated photo opening process and almost a pure open-closed intermediate 122’ could be achieved. This is the first example which clearly showed four states in the UV closing process. The relative photo opening and closing rates compared to benzoDHP 36 have also been studied. It was found that while the carbonyl linker largely increased the relative photo opening rate (117, 119 and 122), the methylene linker only increased it slightly (124). The photo closing processes were always fast as usual. The studies of the thermal return reactions of these systems showed that while the carbonyl linker substantially slowed down the thermal return reactions of the DHP units (117, 119 and 122), the methylene linker speeded it up slightly (124). The mono-iron tricarbonyl benzo[e]dihydropyrene complex 152, the bis-iron tricarbonyl benzo[e]dihydropyrene complex 153 and the iron tetracarbonyl dihydropyrene complex 151 were synthesized. The structures of 152 and 153 were determined by X-ray crystallography. The coordinations of iron tricarbonyl moieties to the DHP rings caused a distortion of ca. 30 degree away from the central DHP plane. Coordination also increased bond alternation and reduced ring currents in the DHP rings. 1H-NMR and X-ray studies showed that 152 showed a weak paratropic ring current in the DHP ring. Iron coordination of the DHP completely stopped the photochromic properties of the dihydropyrenes.