title: Entropic changes control the charge separation process in triads mimicking photosynthetic charge separation.
abstract: Laser-induced optoacoustic spectroscopy (LIOAS) measurements with carotene-porphyrin-acceptor "supermolecular" triads (C-P-A, with A = C60, a naphthoquinone NQ, and a naphthoquinone derivative, Q) were carried out with the purpose of analyzing the thermodynamic parameters for the formation and decay of the respective long-lived charge separated state C*+-P-A*-. The novel procedure of inclusion of the benzonitrile solutions of the triads in Triton X-100 micelle nanoreactors suspended in water permitted the separation of the enthalpic and structural volume change contributions to the LIOAS signals, by performing the measurements in the range 4-20 degrees C. Contractions of 4.2, 5.7, and 4.2 mL mol-1 are concomitant with the formation of C*+-P-A*- for A = C60, Q and NQ, respectively. These contractions are mostly attributed to solvent movements and possible conformational changes upon photoinduced electron transfer, due to the attraction of the oppositely charged ends, as a consequence of the giant dipole moment developed in these compounds upon charge separation ( approximately 110 D). The estimations combining the calculated free energies and the LIOAS-derived enthalpy changes indicate that entropy changes, attributed to solvent movements, control the process of electron transfer for the three triads, especially for C-P-C60 and C-P-Q. The heat released during the decay of 1 mol of charge separated state (CS) is much smaller than the respective enthalpy content obtained from the LIOAS measurements for the CS formation. This is attributed to the production of long-lived energy storing species upon CS decay.
authors.affiliation: Max-Planck-Institut für Bioanorganische Chemie (formerly Strahlenchemie), Postfach 10 13 65, D-45413 Mülheim an der Ruhr, Germany.
authors.forename: Alberto C
authors.initials: AC
authors.lastname: Rizzi
authors.forename: Maurice van
authors.initials: Mv
authors.lastname: Gastel
authors.forename: Paul A
authors.initials: PA
authors.lastname: Liddell
authors.forename: Rodrigo E
authors.initials: RE
authors.lastname: Palacios
authors.forename: Gary F
authors.initials: GF
authors.lastname: Moore
authors.forename: Gerdenis
authors.initials: G
authors.lastname: Kodis
authors.forename: Ana L
authors.initials: AL
authors.lastname: Moore
authors.forename: Tom A
authors.initials: TA
authors.lastname: Moore
authors.forename: Devens
authors.initials: D
authors.lastname: Gust
authors.forename: Silvia E
authors.initials: SE
authors.lastname: Braslavsky
mesh_terms: D002338:Carotenoids / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D019277:Entropy; D005453:Fluorescence; D037741:Fullerenes / Q000737:chemistry*; D008823:Micelles; D036103:Nanotechnology; D009285:Naphthoquinones / Q000737:chemistry*; D010788:Photosynthesis; D011166:Porphyrins / Q000737:chemistry*; D012997:Solvents / Q000737:chemistry; D013696:Temperature
chemical_list: D037741:Fullerenes; D008823:Micelles; D009285:Naphthoquinones; D011166:Porphyrins; D012997:Solvents; D002338:Carotenoids; C069837:fullerene C60