Ultrafast Photodynamics of Exciplex Formation and Photoinduced Electron Transfer in Porphyrin−Fullerene Dyads Linked at Close Proximity

Abstract

The ultrafast photodynamics of porphyrin-fullerene dyads in which the distance between the porphyrin and C 60 moieties is varied systematically at close proximity has been examined using fluorescence up-conversion and pump-probe transient absorption techniques with time resolutions of ca. 100 fs. The porphyrin-fullerene dyads examined are MP-D-C 60 (M ) Zn and 2H) in which the C 60 moiety is directly connected with the porphyrin ring at the meso position and MP-O-C 60 , MP-M-C 60 , and MP-P-C 60 in which the C 60 moiety is linked with porphyrin moieties through the benzene ring at the ortho, meta, and para positions, respectively. The charge transfer (CT) bands are observed for MP-D-C 60 and MP-O-C 60 , whereas no CT band is seen for MP-M-C 60 and MP-P-C 60 . Time-resolved absorption spectral measurements indicate that the photoexcitation of ZnP-D-C 60 in benzonitrile (PhCN) results in formation of the exciplex, which decays to the ground state without forming the charge-separated state. The strong interaction between the ZnP and the C 60 moieties due to the short linkage distance in ZnP-D-C 60 as indicated by the observation of the strongest CT band at the ground state results in formation of the exciplex. The energy of the exciplex is lower than that of the chargeseparated state even in a polar solvent such as PhCN. In contrast, the photoexcitation of the dyad with longer linkage, ZnP-O-C 60 , in PhCN results in formation of the charge-separated state via the exciplex formation, which is higher in energy than the charge-separated state. The photodynamics of exciplex formation of porphyrin-C 60 -linked dyads with a short linkage is characterized by the extremely fast formation rate from the singlet excited states of porphyrins involving both the second and first excited states due to the interaction between the porphyrin and C 60 moieties, which are placed at close proximity. In the case of MP-D-C 60 , the exciplex formation from the first singlet excited state of MP occurs at an ultrafast time scale with a time constant of 160 fs and that from the second singlet excited state occurs faster with a time constant less than 50 fs.

Remarkable Dependence of the Final Charge Separation Efficiency on the Donor–Acceptor Interaction in Photoinduced Electron Transfer

“…Nevertheless,o wing to the stronger D-A electronic coupling,a fter IC from ZnP S2 to ZnP S1 (t = 0.15 ps), the decay of ZnP S1 (t = 1.6 ps) generates the singlet exciplex state,asseen in analogous ZnP-C 60 linked molecules with one phenylene bridge. [10] TheZ nP emission decays with t = 2.2 ps,which is close to the TA decay rate.The exciplex state is transformed with t = 12 ps into the singlet CS state and then seems to decay with t = 132 ps to the ground ( Figure 4d). This profile is well consistent with the nanosecond to microsecond TRTA decay ( Figure 2d and Table 1) of the CS state.Thus,the EPR signal is assigned to the triplet CS states generated after spinlattice relaxations in the spin sublevels,r esulting in the absorptive thermal equilibrium spin populations (see the Supporting Information for details).…”

Creation of Fullerene-Based Artificial Photosynthetic Systems

“…64,66,72,87,101 We determined various ET parameters, including V and , for porphyrin-fullerene systems from the CT absorption and emission for the first time. 64,66,72,87,101 Such CT absorption and emission were also reported for other fullerene-porphyrin-linked [102][103][104][105] and fullerene-phthalocyanine-linked 106 systems. Furthermore, the specific -interaction has been successfully used in dye-sensitized bulk heterojunction solar cells consisting of porphyrin and fullerene (vide infra).…”

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).…”