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A Computational Study of Organic Molecular Crystals for Photocatalytic Water Splitting

James D. Green, Daniel G. Medranda, Hong Wang, Andrew I. Cooper, Jenny Nelson, Kim E. Jelfs

Abstract

Organic crystalline materials are potential candidates for photocatalytic overall water splitting (OWS). Although organic crystals have been heavily investigated for application in organic electronics, such as organic light-emitting diodes (OLEDs) and solar cells, there have been comparatively fewer studies into OWS in these materials. A major challenge is the large number of electronic and structural criteria that must be met for a material to make a viable OWS photocatalyst. Optical absorption, reduction and oxidation potentials and charge-transport properties are among the key considerations, and these are influenced both by molecular properties and the solid-state packing arrangement, making computational modelling challenging. Here, we investigate a series of known organic electronic materials that have published crystal structures using periodic density functional theory (DFT) and compare their calculated electronic properties of optical absorption and reduction and oxidation potentials with literature experimental data. Furthermore we perform a series of gas-phase molecular calculations which show a good agreement with literature data and periodic DFT for the optoelectronic properties of the organic molecular crystals studied, showing that gas-phase molecular calculations could be used to screen organic crystals for OWS at a reduced computational cost.

A Computational Study of Organic Molecular Crystals for Photocatalytic Water Splitting

Abstract

Organic crystalline materials are potential candidates for photocatalytic overall water splitting (OWS). Although organic crystals have been heavily investigated for application in organic electronics, such as organic light-emitting diodes (OLEDs) and solar cells, there have been comparatively fewer studies into OWS in these materials. A major challenge is the large number of electronic and structural criteria that must be met for a material to make a viable OWS photocatalyst. Optical absorption, reduction and oxidation potentials and charge-transport properties are among the key considerations, and these are influenced both by molecular properties and the solid-state packing arrangement, making computational modelling challenging. Here, we investigate a series of known organic electronic materials that have published crystal structures using periodic density functional theory (DFT) and compare their calculated electronic properties of optical absorption and reduction and oxidation potentials with literature experimental data. Furthermore we perform a series of gas-phase molecular calculations which show a good agreement with literature data and periodic DFT for the optoelectronic properties of the organic molecular crystals studied, showing that gas-phase molecular calculations could be used to screen organic crystals for OWS at a reduced computational cost.
Paper Structure (4 sections, 5 figures, 3 tables)

This paper contains 4 sections, 5 figures, 3 tables.

Table of Contents

  1. Optical absorption:
  2. Reduction and oxidation potentials:
  3. Charge transport:
  4. Aim of this article

Figures (5)

  • Figure 1: The five molecular crystals studied here: rubrene, 4,4',4",4"'-(Pyrene-1,3,6,8-tetrayl)tetrabenzoic acid (TBAP), perylenetetracarboxylic dianhydride (PTCDA), dibutyl-perylenetetracarboxylic diimide (sometimes referred to as C4-PTCDI and referred to here as simply PTCDI) and 5,10,15,20-tetra(4-pyridyl)-porphine (TPyP).
  • Figure 2: DFT relaxed crystal structures of five organic crystals: rubrene (viewed perpendicular to $c$ axis), b) TBAP (viewed perpendicular to $b$ axis), c) PTCDA (viewed perpendicular to $b$ axis), d) PTCDI (viewed perpendicular to $c$ axis), e) TPyP (viewed perpendicular to $b$ axis). C atoms coloured in grey, H in white, N in blue and O in red. Figure produced using Mercury 2024.1.0.mac20a
  • Figure 3: Comparison of the $S_1$ energies for five organic molecular crystals calculated by periodic TD-DFPT (M06-2X and $\omega$B97x in CP2Kcp2k) with molecular TD-DFT (using four functionals: B3LYP, $\omega$B97x, LC-PBE and M06-2X in Orcaorcaorca5). The experimental optical gap deduced from thin-film UV-Visible absorption spectra is also plotted for reference.irk12ayam12afer06amak19a
  • Figure 4: The ionisation potentials for five organic molecular crystals calculated by HSE06 PAW compared with values calculated by molecular DFT (using four functionals: B3LYP, $\omega$B97x, LC-PBE and M06-2X). The experimental IP is also plotted (where available). The water oxidation potential is also shown for reference.nak08asch11abal20a
  • Figure 5: The electron affinites for five organic molecular crystals calculated by HSE06 PAW compared with values calculated by molecular DFT (using four functionals: B3LYP, $\omega$B97x, LC-PBE and M06-2X). The experimental EA is also plotted (where available). The proton reduction potential is also shown for reference.