Application of Zn(II) complexes with π-conjugated Bis(terpyridine) ligands in light harvesting
Date of Issue2016
School of Materials Science and Engineering
Natural photosynthesis processes have as their first step light harvesting (LH) in which excitation energy transfer occurs from units with higher energy gaps to those with lower energy gaps, and as their second step coupled electron transfer (oxidation/reduction) reactions to produce organic compounds. The exquisite organization of multiple chromophores in LH complexes plays a crucial role in enhancing the efficiency of photosynthesis, and investigations of artificial light-harvesting systems have looked at both covalent and non-covalent assembly techniques for obtained such ordered arrangements of chromophores, while their photophysical investigation has been extended from linear one-photon (1P) excitation to multi-photon (MP) excitations. The features of Zn2+ and terpyridine complexation including facile preparation, and a highly directional, kinetically liable assembly of units are of interest as one could construct potential light harvesting prototypes by the insertion of luminescent units within such an assembly. Two carbazole bisterpyridyl ligands: 1-H (3,6-bis(4-([2,2':6',2''-terpyridin]-4'-yl)phenyl)-9H-carbazole) bearing a hydrogen atom, and 1-C12 (3,6-bis(4-([2,2':6',2''-terpyridin]-4'-yl)phenyl)-9-dodecyl-9H-carbazole) with a dodecyl chain at the 9-position, were synthesized for comparison of the effect of H-bonds on their assembly behavior with Zn2+. Ligand 1-H formed a single pentametric Zn2+ metallocycle, as confirmed by NMR, ESI-MS and X-ray photoelectron spectroscopy (XPS) (Zn-1-H), whereas 1-C12 produced a mixture of two metallocycles (Zn-1-C12). Molecular packing and assembly of Zn-1-H studied by microscopy showed it has a high propensity to form ordered layers. Spectroscopic studies revealed 1-H and its Zn2+ coordination complex (Zn-1-H) display rare excitation wavelength dependent dual blue or green emission either under both 1P and MP excitation circumstances, and multi-stimuli-response behaviours. The emission of the complex Zn-1-H could be facilely manipulated to obtain either pure blue or green emission by respectively changing the solution basicity through addition of triethylamine or tetraethylammonium hydroxide, or by changing the ion strength by adding anionic polyelectrolyte-polystyrene sulfonate (PSS), whereas the emission from the free ligand 1-H did not exhibit such complete transformations. The metal ion selectivity towards Fe2+, Cu2+, and Co2+ resulted in a superior binding through secondary amine of 1-H for Cu2+, and substitutional binding of Fe2+ through terpyridine units in Zn-1-H. A second bisterpyridine based on a red emitting dithienyl benzothiadiazole unit as a photonic acceptor was combined with two different donor bisterpyridine units (D, bisterpyridine linked respectively to carbazole at 3,6- (1-H), and 2,7- positions (2) (blue-greenish emission) to form statistical Zn2+-tpy copolymers P1 and P2. Spectroscopic studies revealed an approximately 4 fold enhancement of A emission for both P1 and P2. Transient fluorescence studies revealed decreased lifetime and quantum yield of D unit in P1 compared to 1-H, indicating a FRET mechanism in P1. By contrast, the fluorescence decay curve was unchanged and re-absorption was suggested as the energy transfer mechanism in P2. Moreover, the large multiphoton absorption cross-section in the donor part also resulted in obvious enhanced A emission under two- and three- photon circumstances. The discovery of TICT state facilitated one- to multi- photon energy transfer can be used to help design novel and efficient light harvesting systems.