Abstract
One- and two-photon characterizations of a series of hetero- and homoleptic [RuL3-n(bpy)n]2+ (n = 0, 1, 2) complexes carrying bipyridine π-extended ligands (L), have been carried out. These π-extended D−π−A−A−π−D-type ligands (L), where the electron donor units (D) are based on diphenylamine, carbazolyl, or fluorenyl units, have been designed to modulate the conjugation extension and the donating effect. Density functional theory calculations were performed in order to rationalize the observed spectra. Calculations show that the electronic structure of the π-extended ligands has a pronounced effect on the composition of HOMO and LUMO and on the metallic contribution to frontier MOs, resulting in strikingly different nonlinear properties. This work demonstrates that ILCT transitions are the keystone of one- and two-photon absorption bands in the studied systems and reveals how much MLCT and LLCT charge transfers play a decisive role on the two-photon properties of both hetero- and homoleptic ruthenium complexes through cooperative or suppressive effects.
Highlights
IntroductionOver the past two decades, increasing attention has been devoted to the design and study of molecular materials with optimized two-photon absorption (TPA, a list of acronyms can be found at the end of this article) efficiency [1,2,3,4,5], because of relevant applications from photonics to biology, such as microfabrication [6,7,8,9,10,11], micromachines [12], 3D optical data processing and storage [13,14,15,16,17,18], bio-imaging [18,19], and photodynamic therapy [20,21,22,23]
The analysis presented here is a step forward to promote two-photon absorption properties of homo- and hetero-leptic [RuL3-n(bpy)n]2+ polypyridine ruthenium complexes
Our studies demonstrated that the number of π-extended ligands around the metal does not fully account for the two-photon absorption (TPA) response of the complexes, which is not necessarily purely additive
Summary
Over the past two decades, increasing attention has been devoted to the design and study of molecular materials with optimized two-photon absorption (TPA, a list of acronyms can be found at the end of this article) efficiency [1,2,3,4,5], because of relevant applications from photonics to biology, such as microfabrication [6,7,8,9,10,11], micromachines [12], 3D optical data processing and storage [13,14,15,16,17,18], bio-imaging [18,19], and photodynamic therapy [20,21,22,23]. Attention progressively moved from asymmetric Donor–Acceptor systems (D−π−A) to symmetric (A−π−D−π−A or D−π−A−π−D) quadrupolar arrangements and, toward branched molecular structures built from the gathering of either dipolar or quadrupolar chromophores via a common conjugated core [32] Such design was expected to result at the same time in an increase of active TPA units per molecule and in “through bonds” or “through-space” interactions between these TPA units. The metal ion can act as a powerful template to gather organic TPA chromophores in a predetermined arrangement, while contributing in intramolecular charge transfers, such as intra-ligand (ILCT), metal-to-ligand (MLCT), ligand-to-metal (LMCT), or ligand-to-ligand (LLCT) charge transfers within the complexes [37,38,39,40]
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