Selective Triplet‐State Formation during Charge Recombination in a Fullerene/Bodipy Molecular Dyad (Bodipy=Borondipyrromethene)

Abstract

A conformationally restricted molecular dyad has been synthesized and subjected to detailed photophysical examination. The dyad comprises a borondipyrromethene (Bodipy) dye covalently linked to a buckminsterfullerene C60 residue, and is equipped with hexadecyne units at the boron centre in order to assist solubility. The linkage consists of a diphenyltolane, attached at the meso position of the Bodipy core and through an N-methylpyrrolidine ring at the C60 surface. Triplet states localised on the two terminals are essentially isoenergetic. Cyclic voltammetry indicates that light-induced electron transfer from Bodipy to C60 is thermodynamically favourable and could compete with intramolecular energy transfer in the same direction. The driving force for light-induced electron abstraction from Bodipy by the singlet excited state of C60 depends critically on the solvent polarity. Thus, in non-polar solvents, light-induced electron transfer is thermodynamically uphill, but fast excitation energy transfer occurs from Bodipy to C60 and is followed by intersystem crossing and subsequent equilibration of the two triplet excited states. Moving to a polar solvent switches on light-induced electron transfer. Now, in benzonitrile, the charge-transfer state (CTS) is positioned slightly below the triplet levels, such that charge recombination restores the ground state. However, in CH2Cl2 or methyltetrahydrofuran, the CTS is slightly higher in energy than the triplet levels, and decays, in part, to form the triplet state localized on the C60 residue. This step is highly specific and does not result in direct formation of the triplet excited state localized on the Bodipy unit. Subsequent equilibration of the two triplets takes place on a relatively slow timescale.

Visible‐Light‐Harvesting Triphenylamine Ethynyl C60‐BODIPY Dyads as Heavy‐Atom‐Free Organic Triplet Photosensitizers for Triplet‐Triplet Annihilation Upconversion

“…This quenched emission of the antenna can be assigned to the intramolecular energy transfer from the antenna to the C 60 unit. [47,50] This putative intramolecular energy transfer was confirmed by the enhanced C 60 emission at 710 nm (1.75 eV) in C-1, compared with the result of C 60 alone under the same experimental conditions (Figure 2 a, inset). [47] It should be pointed out that the antenna absorbs much more strongly than the C 60 itself at this excitation wavelength ( Figure 1).…”

“…[45][46][47] Nonsubstituted BODIPY absorbs at approximately 500 nm. To access longer absorption wavelengths, a triphenylamine moiety was linked to the BODIPY unit through an ethynylene bond (C C, dyads C-1 and C-2, Scheme 1 and Scheme 2).…”

“…Thus, we concluded that the electronic interaction between the C 60 and the antenna unit is negligible in the ground state. [47] A similar UV/vis absorption profile was obtained for dyad C-2 (Figure 1 b). We noted that the molar extinction coefficient of C-2 at 539 nm (118 800 m À1 cm À1 ) is approximately 2.5-fold larger than that of C-1 because of the bis-BODIPY unit in C-2.…”

Fullerene as Spin Converter

“…On the account of simple organic molecules, the triplet manifold is rarely reached by direct stimulation of the organic molecule, unless advertised spin-orbit coupling effect is populated by a heavy atom [68,69]. The triplet manifold may be reached via indirect secondary routes such as intersystem crossing from the first excited state (S 1 ) and charge separation and recombination between radical ions [70][71][72][73].…”

“…In nonpolar solvents, fast excitation energy transfer was declared, while electron transfer became laborious. Polar solvents play a critical role switching on light-induced electron transfer [68].…”