Examining the Influence of Bilayer Structure on Energy Transfer and Molecular Photon Upconversion in Metal Ion Linked Multilayers

Abstract

Metal ion linked multilayers are a unique motif to spatially control and geometrically restrict molecules on a metal oxide surface, which is of interest in a number of promising applications. Here we use a bilayer composed of a metal oxide surface, an anthracene annihilator molecule, Zn(II) linking ion, and porphyrin sensitizers to probe the influence of the position of the metal ion binding site on energy transfer, photon upconversion, and photocurrent generation. Despite being energetically similar, varying the position of the carboxy metal ion binding group (i.e., ortho, meta, para) of the Pt(II) tetraphenyl porphyrin sensitizer had a large impact on energy transfer rates and upconverted photocurrent that can be attributed to differences in their geometries. From polarized attenuated total reflectance measurements of the bilayers on ITO, we found that the orientation of the first layer (anthracene) was largely unperturbed by subsequent layers. However, the tilt angle of the porphyrin plane varies dramatically from 41°to 64°to 57°for the p-, m-, and o-COOH substituted porphyrin molecules, which is likely responsible for the variation in energy transfer rates. We go on to show using molecular dynamics simulations that there is considerable flexibility in porphyrin orientation, indicating that an average structure is insufficient to predict the ensemble behavior. Instead, even a small subset of the population with highly favorable energy transfer rates can be the primary driver in increasing the likelihood of energy transfer. Gaining control of the orientation and its distribution will be a critical step in maximizing the potential of the metal ion linked structures.

Harnessing near-infrared lightviaS0to T1sensitizer excitation in a molecular photon upconversion solar cell

“…But previous results have shown that metal ion coordination and bilayer formation has minimal impact of the intrinsic photophysical properties of the first layer chromophore. 47 In the early time slices we see a distinct peak at B600 nm that is consistent with the excited state absorption from 1 A* in solution (red spectrum in Fig. 6).…”

“…But previous results have shown that metal ion coordination and bilayer formation has minimal impact of the intrinsic photophysical properties of the first layer chromophore. 47…”

“…This k ET is orders of magnitude larger than previously observed for diphenyl anthracene-Zn-Pt(II) porphyrin multilayer. 34,47 Presumably any decreases in rate due to increased spatial separation between the anthracene and osmium transition dipole moments 48 are offset by the near unity spectral overlap between emission from A and absorption of Os. This observation further emphasizes the utility of an absorption gap between the Soret and Q bands of metalloporphyrins and their continued popularity as sensitizers for TTA-UC.…”

Harnessing Sunlight via Molecular Photon Upconversion

“…Since the first report of utilizing TTA-UC for sunlight to electrical conversion in 2012, notable advancements have been made in harnessing the photocurrent contribution from UC ( J UC ), with the progress to date summarized in Figure . − This graph includes device relevance and device impact thresholds, which are roughly the minimum J UC to overcome PCE measurement uncertainties (0.1 mA/cm 2 ) and a >1 percentage point increase in PCE (1.0 mA/cm 2 ), respectively. , …”

“…To date, TTA-UC has been incorporated into both dye-sensitized solar cells (DSSCs) − ,− and organic photovoltaic (OPV) device architectures, as well as a proof-of-concept hybrid film . The DSSC motif is most common with heterogeneous, codeposited, and metal-ion-linked architectures shown in Figure .…”