Abstract
N 1s Resonant Inelastic X-ray Scattering (RIXS) was used to probe the molecular electronic structure of the ruthenium photosensitizer complex cis-bis(isothiocyanato) bis(2,2'-bipyridyl-4,4'-dicarboxylato) ruthenium(II), known as "N3." In order to interpret these data, crystalline powder samples of the bipyridine-dicarboxylic acid ligand ("bi-isonicotinic acid") and the single ring analog "isonicotinic acid" were studied separately using the same method. Clear evidence for intermolecular hydrogen bonding is observed for each of these crystalline powders, along with clear vibronic coupling features. For bi-isonicotinic acid, these results are compared to those of a physisorbed multilayer, where no hydrogen bonding is observed. The RIXS of the "N3" dye, again prepared as a bulk powder sample, is interpreted in terms of the orbital contributions of the bi-isonicotinic acid and thiocyanate ligands by considering the two different nitrogen species. This allows direct comparison with the isolated ligand molecules where we highlight the impact of the central Ru atom on the electronic structure of the ligand. Further interpretation is provided through complementary resonant photoemission spectroscopy and density functional theory calculations. This combination of techniques allows us to confirm the localization and relative coupling of the frontier orbitals and associated vibrational losses.
Highlights
Ruthenium centred organometallic complexes have a range of applications in light harvesting devices such as dye sensitised solar cells (DSSCs)[1] and photoelectrochemical water splitting devices.[2]
Further interpretation is provided through complementary resonant photoemission spectroscopy (RPES) and density functional theory calculations
This study is focussed on the “N3” dye, cis-bis(isothiocyanato)bis(2,2’bipyridyl-4,4’-dicarboxylic acid)ruthenium(II) known as Ru535, which has become a benchmark molecule for use in DSSCs and has been previously studied using photoelectron spectroscopies on rutile titanium dioxide[3], gold[4,5] and aluminium oxide surfaces[6]
Summary
Ruthenium centred organometallic complexes have a range of applications in light harvesting devices such as dye sensitised solar cells (DSSCs)[1] and photoelectrochemical water splitting devices.[2]. N3 bonds to oxide surfaces primarily through the carboxyl groups of the bi-isonicotinic acid (2,2’-bipyridyl-4,4’dicarboxylic acid) ligands, which in turn is made up of two isonicotinic acid molecules (pyridine-4-carboxylic acid). Resonant inelastic x-ray scattering (RIXS) can provide atom-specific information about the occupied and unoccupied valence orbitals of a molecule and the coupling between them.[16] It is an ideal probe of molecular electronic structure, which underpins electron dynamics and solar energy conversion mechanisms in molecular systems.[17] RIXS provides a sensitive probe of molecular interactions such as hydrogen-bonding.[18,19] In RIXS the unoccupied molecular orbitals are populated by excitation of a core-electron through the absorption of a photon tuned to a specific resonance producing a core-excited state.
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