Luminescence Quenching of a Novel Phosphorescent Ir(III) Complex/MWCNT Hybrid

Abstract

Electron donor-acceptor hybrid systems have received great interest in photovoltaic applications, in which photoinduced electrons of photosensitizers convert to electrical energy via electron transfer to electron acceptors. The electron transfer efficiency is determined by the competition between recombination of the excited electrons to the ground state and electron transfer of the excited electrons to acceptors. Consequently, the photosensitizer with a slow rate of recombination, which means a long lifetime for the excited electrons, could be a good candidate for achieving high electron transfer efficiency in donor-acceptor hybrids. Phosphorescence, emissive decay of the excited electrons to the ground state through intersystem crossing between singlet and triplet states by a strong spin-orbit coupling, shows a longer lifetime for the excited electrons than fluorescence, emissive decay between singlet states. Here, we present the synthesis of a novel Ir-based phosphorescent complex and demonstrate its excellent luminescence quenching when it forms a hybrid complex with multiwall carbon nanotubes (MWCNTs). The system shows an almost 100% quenching efficiency of the excited electrons. To the best of our knowledge, this is the first investigation of a Ir-based phosphorescent photosensitizer for possible photovoltaic applications. Carbon nanotubes (CNTs) composed of an extended πelectron system with one dimensional structure have been used for photovoltaic applications as an electron acceptor of donor-acceptor hybrid systems because of their high electron mobility. CNTs have often showed excellent electron transfer efficiency in hybrid systems containing fluorescent photosensitizers, such as porphyrins, pyrenes, polymers, and organometallic compounds. Consequently, we thought that hybrid systems of the phosphorescent d Ir(III) complex with a long lifetime and CNTs could show excellent performance for photovoltaic applications. In the CNT-based hybrid systems, the systems containing covalently tethered photosensitizers on CNTs have frequently showed good performances, such as good stability and high electron transfer efficiency. Thus, we designed a novel tris-cyclometalated iridium(III) complex [Ir(ppy)2(hppy)] (1), bis(2-phenylpyridyl)-4-(4'-hydroxyphenyl)-2-phenylpyridyliridium(III), that has one –OH end group and synthesized by heating dichloro-bridged dimer [(ppy)2Ir(μCl)2(ppy)2] with two-fold amount of 4-hydroxyphenyl-2phenylpyridine (hppy) (see the experimental section and supporting information (SI)). Such –OH group is useful for covalent attachment of the Ir complex on –COOH groups of functionalized MWCNTs via esterification (Figure 1). Complex 1 showed a long excited-state lifetime (500 ± 6 ns) studied by time-resolved measurement when it was excited at 450 nm, confirming that it is a phosphorescent complex (inset in Figure 2(b)). On the other hand, fluorescent materials usually have a shorter lifetime, below several tens of nanoseconds. Carboxyl acid groups on the MWCNTs were activated by reaction with SOCl2 and then mixed with complex 1 in toluene containing triethylamine with reflux for 1 day, resulting in Ir complex/MWCNT hybrids ([Ir(ppy)2(hppy)]ng-MWCNTs, 2) via covalent attachment. The esterification between carboxyl and hydroxyl groups is well known to generate covalent bonding. The products were filtered through a membrane filter (0.1 μm pore size), and the residual complex 1 was thoroughly removed by washing with dichloromethane and tetrahydrofuran 5 times each and then dried in a vacuum. The resulting product was obtained as black powder. The UV-vis absorption spectrum (Figure 2(a)) of the ethanol solution of the complex 1 shows a strong band at 280 nm corresponding to the ligand-centered (LC) transition, that is, a spin-allowed (π-π) transition of the phenylpyridyl