Intramolecular Down‐ and Up‐Conversion in Dimeric Tetracene Complexes Centered via Platinum(II) and Palladium(II)

Abstract Two dimeric tetracene complexes, Pt(L _tc ) ₂ Cl ₂ and Pd(L _tc ) ₂ Cl ₂ , have been synthesized by coordination of a pyridyl substituted acene‐ to a Pt(II) and Pd(II) metal center. The photophysical properties of the two dimers are characterized and compared with the free tetracene ligand L _tc , as well as the corresponding monomeric complexes ( Pt(L _tc )( L _ref )Cl ₂ and Pd(L _tc )(L _ref )Cl ₂ ) to shed light on the processes of intramolecular up‐ and down‐conversion, namely triplet‐triplet annihilation and singlet fission, respectively. Both the free ligand L _tc and Pt(L _tc )(L _ref )Cl ₂ show high fluorescence quantum yields, while emission is heavily reduced in the Pt(L _tc ) ₂ Cl ₂ and Pd(II) complexes. Dimer Pt(L _tc ) ₂ Cl ₂ exhibits efficient singlet fission within 1.6 ns to form a correlated triplet pair (T ₁ T ₁ ), confirming that coupling is mediated through Pt(II) over remarkably long inter‐chromophore distances. Subsequently, (T ₁ T ₁ ) dissociates into free triplets (T ₁ + T ₁ ) with yields of ca. 50%. Moreover, due to the isoenergetic level of the singlet excited state and (T ₁ T ₁ ) for tetracene dimers, some (T ₁ T ₁ ) in Pt(L _tc ) ₂ Cl ₂ is observed to undergo intramolecular up‐conversion and return to the singlet excited state, followed by upconverted fluorescence. On the other hand, photoexcitation of Pd(L _tc ) ₂ Cl ₂ gives a kinetic behavior that is similar to the monomer Pd(L _tc )(L _ref )Cl ₂ , namely no singlet fission but fast intersystem crossing within hundreds of picoseconds. It is noted that triplet formation in the Pd(II) monomer and dimer complexes is low, and that non‐radiative decay of the singlet excitons to the ground state dominates in both cases. The singlet excited state of all metal–organic complexes exhibits subtle intrachromophore charge‐transfer characteristics in polar solvents and kinetic schemes that are similar to that observed in apolar solvents.