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The ability to directly probe the adsorption configurations of organic regioisomeric molecules, specifically nonplanar isomers, on well-defined substrates holds promise to revolutionize fields dependent on nanoscale processes, such as catalysis, surface science, nanotechnology and modern day electronic applications. Herein, the adsorption configurations and surface sensitive interactions of two nonplanar regioisomer, trans - and cis -tetrakispentafluorophenylporphodilactone ( trans - and cis -H 2 F 20 TPPDL), molecules on (100) surfaces of Ag, Cu and Au were studied and investigated using high resolution scanning tunneling microscopy (STM), combined with ultrahigh vacuum tip-enhanced Raman spectroscopy (UHV-TERS). Depending on molecule–substrate interactions, similar “phenyl-up” configurations were observed for these molecules on Ag(100) and Au(100), while a “phenyl-flat” configuration was discovered on a Cu(100) surface. With the help of surface selection rules of TERS, we explain the spectral discrepancies recorded on the Ag and Cu substrate. Furthermore, the intermolecular interactions were addressed using STM analysis on these surfaces after the configurations were determined by TERS. This study sheds light on the distinct configurations of regioisomeric porphodilactone systems (at interfaces) for near-infrared (NIR) photosensitizers and molecular electronics in the near future. 

The photophysical properties of naturally occurring chlorophylls depend on the regioisomeric nature of the β-pyrrolic substituents. Such systems are the “gold standard” by which such effects are judged. However, simple extrapolations from what has been learned with chlorophylls may not be appropriate for other partially reduced porphyrinoids. Here we report the synthesis of a series of cis / trans -porphodilactones ( cis / trans -1) and related derivatives ( cis / trans 2–5) designed to probe the effect of regioisomeric substitution in porphyrinoids that incorporate degrees of unsaturation through the β-pyrrolic periphery that exceed those of chlorophyll. These test systems were obtained through β-pyrrolic modifications of the tetrapyrrolic core, which included reduction of β-diazalone to the corresponding dilactol moieties and 1,3-dipolar cycloadditions. In the case of cis - vs. trans -3 bearing two pyrrolidine-fused β-rings we found an unprecedented Δ Q L up to ca. 71 nm (2086 cm −1 ), where Δ Q L ( Q L means the lowest energy transfer band, also the S 0 → S 1 transition band, which is often assigned as Q y (0,0) band) refers to the transition energy difference between the corresponding cis / trans -isomers. The Δ Q L values for these and other systems reported here were found to depend on the differences in the HOMO–LUMO energy gap and to be tied to the degeneracy and energy level splitting of the FMOs, as inferred from a combination of MCD spectral studies and DFT calculations. The aromaticity, estimated from the chemical shifts of the N–H protons and supported by theoretical calculations ( e.g. , AICD plots and NICS(1) values), was found to correlate with the extent of porphyrin periphery saturation resulting from the specific β-modifications. The aromaticity proved inversely proportional to the degree to which the regioisomerism affected the photophysical properties as noted from plots of Δ Q L s in cm −1 vs. the average NICS(1) values for 1–5. Such a finding is not something that can be easily interpolated from prior work and thus reveals how aromaticity may be used to fine-tune photophysical effects in reduced porphyrinoids.