Performance of Electronic Structure Methods for the Description of Hückel-Möbius Interconversions in Extended π-Systems.

Tatiana Woller,Mercedes Alonso,Xavier Deraet,Golokesh Santra,Nitai Sylvetsky,Frank De Proft,Jan M L Martin,Ambar Banerjee

doi:10.1021/acs.jpca.9b10880

Abstract

Expanded porphyrins provide a versatile route to molecular switching devices due to their ability to shift between several π-conjugation topologies encoding distinct properties. DFT remains the workhorse for modeling such extended macrocycles, when taking into account their size and huge conformational flexibility. Nevertheless, the stability of Hückel and Möbius conformers depends on a complex interplay of different factors, such as hydrogen bonding, π···π stacking, steric effects, ring strain, and electron delocalization. As a consequence, the selection of an exchange–correlation functional for describing the energy profile of topological switches is very difficult. For these reasons, we have examined the performance of a variety of wave function methods and density functionals for describing the thermochemistry and kinetics of topology interconversions across a wide range of macrocycles. Especially for hexa- and heptaphyrins, the Möbius structures have a stronger degree of static correlation than the Hückel and twisted-Hückel structures, and as a result the relative energies of singly twisted structures are a challenging test for electronic structure methods. Comparison of limited orbital space full CI calculations with CCSD(T) calculations within the same active spaces shows that post-CCSD(T) correlation contributions to relative energies are very minor. At the same time, relative energies are weakly sensitive to further basis set expansion, as proven by the minor energy differences between the extrapolated MP2/CBS energies estimated from cc-pV{T,Q}Z, diffuse-augmented heavy-aug-cc-pV{T,Q}Z and explicitly correlated MP2-F12/cc-pVDZ-F12 calculations. Hence, our CCSD(T) reference values are reasonably well-converged in both 1-particle and n-particle spaces. While conventional MP2 and MP3 yield very poor results, SCS-MP2 and particularly SOS-MP2 and SCS-MP3 agree to better than 1 kcal mol–1 with the CCSD(T) relative energies. Regarding DFT methods, the range-separated double hybrids, such as ωB97M(2) and B2GP-PLYP, outperform other functionals with RMSDs of 0.6 and 0.8 kcal mol–1, respectively. While the original DSD-PBEP86 double hybrid performs fairly poorly for these extended π-systems, the errors drop down to 1.9 kcal mol–1 for the revised revDOD-PBEP86-NL, which eliminates the same-spin correlation energy. Minnesota meta-GGA functionals with high fractions of exact exchange (M06-2X and M08-HX) also perform reasonably well, outperforming more robust and significantly less empirically parametrized functionals like SCAN0-D3.

Highlights

Expanded porphyrins have attracted considerable attention over the past few decades in view of their large conformational flexibility, facile redox interconversions, novel metal coordination behaviors, and versatile electronic states.[1]
We introduce a representative database of topological switches based on expanded porphyrins with varying ring size
The ultimate goal of this study is to identify affordable wave function methods and density functionals for the prediction of accurate relative energies for Hückel-Möbius topology interconversions in expanded porphyrins

Summary

Introduction

Expanded porphyrins have attracted considerable attention over the past few decades in view of their large conformational flexibility, facile redox interconversions, novel metal coordination behaviors, and versatile electronic states.[1] The rich chemistry of these extended macrocycles has led to diverse applications including near-infrared dyes,[2] nonlinear optical materials,[3] magnetic resonance imaging contrast agents,[4] and molecular switches.[5]. In contrast to the regular porphyrin, expanded porphyrins are flexible enough to switch between different π-conjugation topologies (Hückel, Möbius, and twisted-Hückel), each with different properties (Figure 1).[6−8] Such a change of topology involves a Hückel−Möbius aromaticity switch in a single molecule and it can be induced by solvent, pH, and metalation, among others.[9,10] These Hückel−Möbius aromaticity switches combine both mechanical and π-electron switching, providing a new route to molecular optoelectronic devices.[11] we have recently demonstrated the applicability of these unique. An illustrative example is the potential energy curve for the Hückel−Möbius interconversion of neutral [32]heptaphyrin computed with different density functionals (Figure 2)

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Results

Conclusion