Dendritic Porphyrin–Fullerene Conjugates: Efficient Light‐Harvesting and Charge‐Transfer Events

Abstract

A novel dendritic C(60)-H(2)P-(ZnP)(3) (P=porphyrin) conjugate gives rise to the successful mimicry of the primary events in photosynthesis, that is, light harvesting, unidirectional energy transfer, charge transfer, and charge-shift reactions. Owing, however, to the flexibility of the linkers that connect the C(60), H(2)P, and ZnP units, the outcome depends strongly on the rigidity/viscosity of the environment. In an agar matrix or Triton X-100, time-resolved transient absorption spectroscopic analysis and fluorescence-lifetime measurements confirm the following sequence. Initially, light harvesting is seen by the peripheral C(60)-H(2)P- *(ZnP)(3) conjugate. Once photoexcited, a unidirectional energy transfer funnels the singlet excited-state energy to H(2)P to form C(60)-*(H(2)P)-(ZnP)(3), which powers an intramolecular charge transfer that oxidizes the photoexcited H(2)P and reduces the adjacent C(60) species. In the correspondingly formed (C(60))(*-)-(H(2)P)(*+)-(ZnP)(3) conjugate, an intramolecular charge-shift reaction generates (C(60))(*-)-H(2)P-(ZnP)(3) (.+), in which the radical cation resides on one of the three ZnP moieties, and for which lifetimes of up to 460 ns are found. On the other hand, investigations in organic media (i.e., toluene, THF, and benzonitrile) reveal a short cut, that is, the peripheral ZnP unit reacts directly with C(60) to form (C(60))(*-)-H(2)P-(ZnP)(3) (*+). Substantial configurational rearrangements- placing ZnP and C(60) in proximity to each other-are, however, necessary to ensure the required through space interactions (i.e., close approach). Consequently, the lifetime of (C(60))(*-)-H(2)P-(ZnP)(3) (*+) is as short as 100 ps in benzonitrile.

Synthesis of New Porphyrin–Fullerene Dyads Capable of Forming Charge‐Separated States on a Microsecond Lifetime Scale

“…All measurements were performed at room temperature (22 AE 2 8C). [30] and 8 d [13] were obtained according to known procedures. Porphyrins 2 b-d were obtained analogously to compound 2 a.…”

“…[6] However, the vast amount of literature published throughout the last two decades devoted to porphyrin-fullerene dyads allows some empirical rules to be formulated concerning the relationship between the structure of a given compound and the lifetime of the charge-separated state it may form. Thus, the effects of the inter-chromophoric distance, [1c, 7] the relative orientation of chromophores, [1c, 7a-c, 8] the nature of the linker, [7d-i, 9] the role of the microenvironment of the porphyrin core, including the nature of central metal atom [9b, 10] and ligands bound to this atom, [11] the porphyrin substitution pattern, [12] and the dendritic environment [13] on the CS state lifetime have been studied and discussed in recent years.…”

Strong electronic coupling and electron transfer in a Ce2@Ih-C80–H2P electron donor acceptor conjugate

“…Fluorescence lifetimes were detected to be solvent dependent and are as short as 0.46, 0.54, and 0.79 ns in toluene, THF, and benzonitrile respectively, while the intrinsic fluorescence lifetime of H 2 P is 9.8 ns. [20][21][22][23] To further explore the excited state interactions between Ce 2 @I h -C 80 and H 2 P in Ce 2 @I h -C 80 -H 2 P, time-resolved pump probe experiments following femto-and nanosecond excitation were performed. 420 nm photoexcitation of the H 2 P reference (Fig.…”

Photochemical Transformations Involving Porphyrins and Phthalocyanines

“…Photosynthetic models containing dendritic C 60 -H 2 P-(ZnP) 3 conjugates were analyzed in relation to the primary events in photosynthesis, i.e., light harvesting, unidirectional energy transfer, charge transfer, and charge-shift reactions. (Schlundt et al 2009). Similar approaches have been used with zinc(II) phthalocyanines (Guldi et al 2002) .…”