Response to Comment “On the Existence of Excitonic Signatures in the Optical Response of Metal–Organic Frameworks”

Abstract

behavior is due to the synergistic combination of the well-defined organic-inorganic nature of 1 and its anisotropic layered structure that gives rise to two different types of robust excitons (interlayer and intralayer). The assignment of the observed optical features to the excitons was done based on the fundamental principles of molecular excitons extensively described in the last 50 years in books, [2] reviews [3a-c] and research papers; [3d-l] and supported by complex transmission, reflectance, and photoluminescence (PL) spectroscopy with polarization and time resolution.Polozkov et al. [4] made a comment to dispute our assignment of the optical features to excitons, based on the results of electronic structure calculations carried out using the generalized gradient approximation Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional on a simplified 2D periodic model of a single layer of 1 (Figure 1). Despite the severe oversimplification of the actual system in the model and the known limited accuracy of pure exchange-correlation functionals for describing excitations in molecular systems, [5] the authors obtained a nearly quantitative agreement between the computed and experimentally observed features in the absorption spectrum. Given the intrinsic limitation of the time-dependent density functional theory (DFT) method to single-particle excitations and its fundamental inability to describe excitonic states, these results motivated the authors to question our interpretation of the experimental data and propose an alternative model that is solely based on a single-particle approximation. We do not agree with the new proposal and identify a number of critical flaws in the logics underlying the comment. Below we provide extensive argumentation supporting our viewpoint.First of all, we believe that the coincidence between a computed parameter and an experimental one cannot be used as a justified ground for the construction of a physical or chemical hypothesis without a critical assessment of the potential limitations related to the accuracy of both the model and the method employed in the calculations. The method accuracy in this context can be better viewed as the precision of the calculation -that is, how well a particular quantum chemical method describes the electronic structure and basic parameters of chemical systems; and it is commonly assessed against a back-drop of a desired well-defined outcome. [6] Modern quantum-chemical methodologies are suitable for resolving ground-state chemical phenomena with <4 kJ mol −1 absolute accuracy, which is often referred to as the "chemical accuracy". [6c] However, when dealing with such complex systems as the metal-organic frameworks, this high This is a response to a comment on the interpretation of the origin of the nonlinear changes of optical properties of van der Waals' metal-organic frameworks (MOFs). The concerns are addressed by clarifying potential pitfalls in density functional theory (DFT) simulations, careful analysis of prior literature, and...

Computational Approach to Molecular Catalysis by 3d Transition Metals: Challenges and Opportunities

“…206−208 Similar to conventional DFT, there is a large dependence of the results of TD-DFT computations on the actual choice of the exchange-correlation functional, 209 which may result in erroneous agreements between spectroscopic data and computational results on simplified models. 210…”

“…The accuracy of TD-DFT is significantly increased when it is combined with functionals with 100% HF exchange or range-separated functionals. TD-DFT has been also applied successfully on the computation of electronic spectra of transition metal complexes. − Similar to conventional DFT, there is a large dependence of the results of TD-DFT computations on the actual choice of the exchange-correlation functional, which may result in erroneous agreements between spectroscopic data and computational results on simplified models …”

“…206−208 Similar to conventional DFT, there is a large dependence of the results of TD-DFT computations on the actual choice of the exchange-correlation functional, 209 which may result in erroneous agreements between spectroscopic data and computational results on simplified models. 210 Green's functions with the Bethe-Salpeter equation can be considered an alternative methodology that can reach TD-DFT's popularity in the near future. 211 Two recent benchmark studies of GW and Bethe-Salpeter equation on the excited states of small molecules with transition metals show some promise for the application of those methods.…”

Metal–Organic Frameworks in Modern Physics: Highlights and Perspectives

“…An ability to generate excitons by light, and electrical control of these excitons, also supports MOF application in optoelectronics in the near future, and we believe that MOFs can compete with materials such as inorganic 2D vdW crystals and heterostructures, organic polymers, and crystals for micro‐ and nano‐electronic devices that operate based on the Bose–Einstein condensation effect . However, there are problems to be solved prior to this application, as an increase in the monocrystallinity and size of single MOF crystals, increases the binding energy, rates of exciton hopping and energy transfer.…”

“…[47] Moreover, the rate and efficiency of the energy migration for MOFs such as Tb 0.95 Eu 0.05 HL (HL = 5-hydroxy-1,2,4-benzenetricarboxylic acid) can be significantly improved by mixing different ions (multivalence). [58,59] However, there are problems to be solved prior to this application, as an increase in the monocrystallinity [60] and size of single MOF crystals, increases the binding energy, rates of exciton hopping and energy transfer. [45] The existence of excitons at room temperature, and their ability to transfer energy within the MOFs, is important for solar energy and excitonic devices.…”