Photochemical and Electrochemical Properties of Zinc Chlorin−C₆₀Dyad as Compared to Corresponding Free-Base Chlorin−C₆₀, Free-Base Porphyrin−C₆₀, and Zinc Porphyrin−C₆₀Dyads

Abstract

The photochemical and electrochemical properties of four chlorin-C60 or porphyrin-C60 dyads having the same short spacer between the macrocycle and the fullerene are examined. In contrast with all the previous results on porphyrin-fullerene dyads, the photoexcitation of a zinc chlorin-C60 dyad results in an unusually long-lived radical ion pair which decays via first-order kinetics with a decay rate constant of 9.1 x 10(3) x s(-1). This value is 2-6 orders of magnitude smaller than values reported for all other porphyrin or chlorin donor-acceptor of the molecule dyad systems. The formation of radical cations of the donor part and the radical anion of the acceptor part was also confirmed by ESR measurements under photoirradiation at low temperature. The photoexcitation of other dyads (free-base chlorin-C60, zinc porphyrin-C60, and free-base porphyrin-C60 dyads) results in formation of the ion pairs which decay quickly to the triplet excited states of the chlorin or porphyrin moiety via the higher lying radical ion pair states as is expected from the redox potentials.

Photochemical Transformations Involving Porphyrins and Phthalocyanines

“…Fullerenes (mostly C 60 but also C 70 (Kesti et al 2003)) have been incorporated into multicomponent systems with step-wise charge separation (Imahori 2004, Imahori et al 2002, Imahori and Fukuzumi 2004, Kesti et al 2003, Imahori, et al 2002b, Bell et al 2002) and used as core groups for dendrimers with zinc(II) porphyrin donors (Camps et al 1999). By now C 60 has also been linked to chlorins and phytochlorin derivatives (Fukuzumi et al 2001, Helaja et al 1999) and carotenoid-porphyrin-C 60 systems are under investigation ). Many of these systems utilize a pyrrolidine linkage, but bislactone (Montforts et al 2003), or Diels-Alder derived cyclohexene-linked systems (Bell et al 2002, Fukuzumi et al 2001 have also found wide applications.…”

“…By now C 60 has also been linked to chlorins and phytochlorin derivatives (Fukuzumi et al 2001, Helaja et al 1999) and carotenoid-porphyrin-C 60 systems are under investigation ). Many of these systems utilize a pyrrolidine linkage, but bislactone (Montforts et al 2003), or Diels-Alder derived cyclohexene-linked systems (Bell et al 2002, Fukuzumi et al 2001 have also found wide applications. Currently, nanostructured systems are emerging (Yamaguchi et al 2003) and new attention has been focused on combining the utility of fullerene acceptor groups with rotaxane systems (Watanabe et al 2003, Schuster et al 2004, Li et al 2004b.…”

Bioinspired Electron-Transfer Systems and Applications

“…3) affords the longer CS lifetime as compared with other zinc chlorin-fullerene dyads with the longer spacers. [36][37][38][39][40] A deoxygenated PhCN solution containing ZnCh-C 60 gives rise upon a 388 nm laser pulse to a transient absorption maximum at 460 nm due to the singlet excited state of ZnCh. 36 The decay rate constant was determined as 1:0 Â 10 11 s À1 , which agrees with the value determined from the fluorescence lifetime measurements.…”

“…27 A number of simple donor-acceptor dyads have been designed and synthesized to attain a long-lived CS state, where the donor and acceptor molecules are linked with a short spacer to minimize the solvent reorganization energy. [34][35][36][37][38][39] Efficient photoinduced electron transfer occurs in a zinc imidazoporphyrin-C 60 dyad (ZnImP-C 60 ) with a short linkage to form the CS state (ZnImP þ -C 60 À ) with the rate constant of 1:4 Â 10 10 s À1 (Scheme 2). 34 The CS state (1.34 eV) is lower in energy than both the triplet excited states of C 60 (1.50 eV) and ZnImP (1.36 eV).…”

New Paradigm of Electron Transfer Chemistry in Organic Reactions

“…The BET rate was determined from the disappearance of the absorption band at 790 nm due to ZnCh'+ of 4 as 9.1 X 103 s-1, which corresponds to the CS lifetime of 110 ps. 25 The CS lifetime is highly sensitive to the substituents on the porphyrin ring and the type of linkage between the electron donor and acceptor ( Figure 2). Comparison of the CS lifetime of 6 with 4 is particularly intriguing.…”