Article pubs.acs.org/IC
Triplet Energy Transfers in Well-Defined Host−Guest Porphyrin− Carboxylate/Cluster Assemblies Peng Luo,† Paul-Ludovic Karsenti,† Benoit Marsan,*,‡ and Pierre D. Harvey*,† †
Département de chimie, Université de Sherbrooke, Sherbrooke, Québec J1K 2R1, Canada Département de chimie, Université du Québec à Montréal, Montréal, Québec H2X 2J6, Canada
‡
S Supporting Information *
ABSTRACT: The dyes (5-(4-carboxylphenyl)-10,15,20tritolylporphyrinato)zinc(II) (MCP) and (5,15-bis(4-carboxylphenyl)15,20-ditolylporphyrinato)zinc(II) (DCP), as their sodium salts, were used to form assemblies with the unsaturated cluster Pd3(dppm)3(CO)2+ ([Pd32+], dppm = (Ph2P)2CH2) via ionic CO2−···Pd32+ interactions. The photophysical properties in their triplet states were studied. The position of the T1 state of [Pd32+] (∼8190 cm−1) has been proposed using DFT computations and was corroborated by the presence of a Tn → S0 delayed emission at 680−700 nm arising from a T1−T1 annihilation process at 77 K. The static quenching of the near-IR phosphorescence of the dyes at 785 nm (T1 → S0) was observed. Thermodynamically poor reductive and oxidative driving forces render the photoinduced electron transfer quenching process either inoperative or very slow in the T1 states. Instead, slow to medium T1−T1 energy transfer (3dye*···[Pd32+] → dye···3[Pd32+]*) operates through a Förster mechanism exclusively with kET values of ∼1 × 105 s−1 on the basis of transient absorption measurements at 298 K.
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INTRODUCTION Triplet energy transfers (TETs) in porphyrin chromophores play a crucial role in their efficiency in their upconversion1 and two-photon absorption processes2 and in the generation of singlet oxygen for potential applications in photodynamic therapy.3 This topic has also attracted some attention from a theoretical standpoint.4 Moreover, multiple investigations of TETs have been reported for dyad models in recent years, including dyads containing covalently attached porphyrin− porphyrin5 and porphyrin−organic acceptor,6 whether these models are simple molecules5,6 or are embedded inside polymers7 or covalently incorporated within dendrimers (or exhibiting dendritic structures).8 Concurrently, dyads and polyads constructed with porphyrin and coordination complexes were also occasionally studied for their TET properties, specifically focusing on ruthenium(II), 9 osmium(II), 10 platinum(II),11 iridium,12 and aluminum(III)13 chromophores. Conversely, dyads containing a porphyrin interacting via purely electrostatic interactions with an inorganic component for TET studies are very scarce and to the best of our knowledge focus only on the unsaturated M3(dppm)3(CO)2+ clusters (Chart 1: M = Pd ([Pd32+]), Pt ([Pt32+]); dppm = (Ph2P)2CH2) via CO2−···Pd32+ interactions.14 In this respect, we recently studied the fluorescence quenching of the two carboxyl-containing zinc(II)porphyrin chromophores (5-(4-carboxylphenyl)10,15,20-tritolylporphyrinato)zinc(II) (MCP) and (5,15-bis(4-carboxylphenyl)-15,20-ditolylporphyrinato)zinc(II) (DCP), as their sodium salts, by [Pd32+].15 In these cases, femtosecond © XXXX American Chemical Society
transient absorption spectroscopy clearly indicated ultrafast electron transfer processes from the MCP and DCP dyes to the reducible [Pd32+] cluster (
