J. Linnanto et al., EXCITON INTERACTIONS AND FEMTOSECOND RELAXATION IN CHLOROPHYLL-A WATER AND CHLOROPHYLL-A DIOXANE AGGREGATES, The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory, 102(23), 1998, pp. 4337-4349
Chlorophyll a (Chl a) in hydrocarbon solution with a small amount of d
ioxane or water shows red-shifted absorption bands at 686 nm and at 70
0 nm (dioxane) and at 745 nm (water), indicative of self-organized agg
regate structures in solution. To study the relationship between the s
tructure and spectral properties of the aggregates, several one-dimens
ional model structures of Chi a-dioxane and Chi a-water aggregates wer
e computed by the molecular mechanics method. Three overall structures
ranging from stick to a ring shape were energetically favored for the
dioxane system. All these structures contain structural heterogeneity
that consists of repeating dimers that further form tetramer substruc
tures. For the Chi a-water system a one-dimensional homogenous helical
structure was obtained. By using the model structures, the transition
energies and fluorescence excitation polarizations were computed. Exc
iton theory with dipole-dipole interaction approximation and semiempir
ical quantum mechanical CI calculations were used. Excitonic splitting
s of the aggregate transition energies were calculated by diagonalizin
g 10 x 10 interaction matrixes. For the Chi a-dioxane dimers, trimers,
and tetramers the exciton theory with dipole-dipole interaction appro
ximation produced blue-shifted exciton transitions of the computed str
uctures, while the semiempirical calculation gave red-shifted transiti
ons for all these species, the tetramer shifts being closest to the ex
perimental shifts. The quantum chemical calculations of the two tetram
ers appearing in the computed one-dimensional model structure predict
the quartet structure of the absorption spectrum. The calculations als
o produce fluorescence excitation polarizations that are very similar
to the values observed at low temperatures for the Chi a-dioxane aggre
gate. In the case of the Chi a-water aggregate both dipole approximati
on and semiempirical exciton shifts were only one-half of the observed
spectral shift. It is suggested that the remainder is due to the envi
ronmental effects not included in the calculations, like two-dimension
al chromophore-chromophore interactions and solvent effects in the Chi
a-water aggregate. Calculations on the Chi a-water and Chf a-dioxane
model aggregates demonstrate that at close chromophore-chromophore dis
tances the dipole-dipole approximation and the semiempirical calculati
on give very different results. The results from the model calculation
s are compared with available spectral data of each aggregate together
with new femtosecond results for the Chi a-water aggregate. In one-co
lor absorption recovery experiments the Chi a-water aggregate shows an
obvious wavelength dependence of electric relaxation. The decay has a
strong femtosecond component (about 300 fs) in the blue side of the Q
(y) band that is not present in the red side. No rise time could be ob
served when the Q(y) band was pumped in the blue side and probed in th
e red side with 350 fs time resolution. Our results suggest that therm
alization of the excitation energy of the Chi a-water aggregate takes
place in less than 350 fs.