INVESTIGATIONS OF CHL-A AGGREGATES CROSS-LINKED BY DIOXANE IN 3-METHYLPENTANE

In this work, dioxane-bound aggregates of chlorophyll a are prepared i n 3-methylpentane. The properties of the aggregates are studied by usi ng steady-state and time-resolved spectroscopies. The Q(y)-region abso rption spectrum of the chlorophyll a-dioxane aggregate shows four clea rly resolvable narrow bands with comparable intensities. The band maxi ma are located at 683, 689, 698 and 702 nm. The emission spectrum cons ists of two emission bands centred at 699 and 702 nm suggesting the pr esence of two types of aggregates. High degree of fluorescence polariz ation is detected yielding the angles between the absorption transitio n moments with respect to the 702 nm emission transition moment. The c ircular dichroism signal is strong and the sign sequence follows that of the polarization spectrum. psi-type effect is discovered due to lon g-range interactions within the sizable aggregate frame. The insensiti vity of the time-resolved emission kinetics to the temperature indicat e the excitation delocalization length to be short, The rapid loss of time-resolved anisotropy at certain wavelengths supports the energy tr ansfer to take place. The independence of the excitation spectrum over a broad range of emission wavelengths suggests that the two diverse s pectral types are found within the same macro-aggregate and that they an coupled together by the energy transfer. (C) 1997 Elsevier Science B.V.