Tetrapyrrole singlet excited state quenching by carotenoids in an artificial photosynthetic antenna.
Two artificial photosynthetic antenna models consisting of a Si phthalocyanine (Pc) bearing two axially attached carotenoid moieties having either 9 or 10 conjugated double bonds are used to illustrate some of the function of carotenoids in photosynthetic membranes. Both models studied in toluene, methyltetrahydrofuran, and benzonitrile exhibited charge separated states of the type C*+-Pc*- confirming that the quenching of the Pc S1 state is due to photoinduced electron transfer. In hexane, the Pc S1 state of the 10 double bond carotenoid-Pc model was slightly quenched but the C*+-Pc*- transient was not spectroscopically detected. A semiclassical analysis of the data in hexane at temperatures ranging from 180 to 320 K was used to demonstrate that photoinduced electron transfer could occur. The model bearing the 10 double bond carotenoids exhibits biexponential fluorescence decay in toluene and in hexane, which is interpreted in terms of an equilibrium mixture of two isomers comprising s-cis and s-trans conformers of the carotenoid. The shorter fluorescence lifetime is associated with an s-cis carotenoid conformer where the close approach between the donor and acceptor moieties provides through-space electronic coupling in addition to the through-bond component.
Palacios
Rodrigo E
RE
The Center for the Study of Early Events in Photosynthesis, Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287-1604, USA.
Kodis
Gerdenis
G
Herrero
Christian
C
Ochoa
Ernesto Mariño
EM
Gervaldo
Miguel
M
Gould
Stephanie L
SL
Kennis
John T M
JT
Gust
Devens
D
Moore
Thomas A
TA
Moore
Ana L
AL
D002338:Carotenoids / Q000737:chemistry*; D004563:Electrochemistry; D007211:Indoles / Q000737:chemistry* / Q000528:radiation effects; D054833:Isoindoles; D008567:Membranes, Artificial; D008954:Models, Biological; D015394:Molecular Structure; D017646:Organosilicon Compounds / Q000737:chemistry* / Q000528:radiation effects; D010788:Photosynthesis; D013237:Stereoisomerism; D013696:Temperature; D045725:Tetrapyrroles / Q000737:chemistry*; D013997:Time Factors
D007211:Indoles; D054833:Isoindoles; D008567:Membranes, Artificial; D017646:Organosilicon Compounds; D045725:Tetrapyrroles; D002338:Carotenoids; C013647:phthalocyanine