Spiropyran photoswitching: From molecules to functional materials

Cerasale, Daniel 2024. Spiropyran photoswitching: From molecules to functional materials. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

This thesis explores the synthesis and photoswitching properties of functionalised spiropyrans, along with their incorporation into photoresponsive materials. Chapter 1 provides a summary of the fundamental concepts underpinning the work in this thesis, such as photophysics, photochemistry and coordination chemistry. Photochromism is described with reference to archetypal organic photochromic systems such as azobenzene, diarylethene and spiropyran (SP), the latter of which is the subject of this thesis. A brief historical perspective on the field of metal-organic frameworks (MOFs) is provided, along with recent advances in the field of stimuli-responsive MOFs. The spiropyran ⇋ merocyanine (MC) photoconversion pathway is discussed, as well as the physicochemical differences between the two photoisomers. The influence of chemical functionalisation on spiropyran properties is briefly described using the -NO2 group as an example. This concept of chemical functionalisation is discussed further within the context of photoresponsive MOF design, whereby spiropyran modification with metal-binding groups permits its incorporation into photoresponsive MOFs. Chapter 2 describes the synthesis of thirty-three spiropyran molecules through three key types of chemical transformation: (i) precursor synthesis, (ii) spiropyran formation and (iii) post-synthetic modification. Pertinent to Chapters 5 and 6, the synthesis of spiromolecules with substituents relevant to photoresponsive materials fabrication is reported. The electronic effects of substituents on reaction outcomes are quantitatively explored, with a positive correlation observed between spiropyran yield and salicylaldehyde substituent electron-withdrawing group strength. The structural characterisation of fourteen spiropyrans is performed by single-crystal X-ray diffraction (SCXRD). Some qualitative links between the types of functional groups present and spiropyran molecular properties are present, however, quantitative correlations were generally not observed. Individual spiromolecules generally occupy relatively isolated positions in space, however, when substituents contain hydrogen bond donor or acceptor atoms, SP-SP dimerization occurs. If multiple hydrogen bond donor or acceptor atoms are present, long-range hydrogen-bonded networks may form. iii Chapter 3 describes the systematic photophysical characterisation of twenty-eight spiropyrans in solution and the solid-state. In solution, correlations between the Hammett parameters of substituent groups and the MC peak position, SP → MC photoconversion efficiency and MC → SP back-conversion rate were observed. The effects of chromene and indoline substituents appear orthogonal; photoconversion efficiency is enhanced by strong electron-withdrawing groups on the chromene moiety and the absence of electron-withdrawing groups on the indoline moiety. A difference in the substituent electronic effects across the molecule was found to be required for observed photoswitching in MeOH solutions. These correlations are not all-inclusive, however, since the influence of singlet- versus triplet-state excitation is not fully understood. High conversion is observed for SP3 and SP6, which contain substituents capable of facilitating singlet → triplet intersystem crossing. In all molecules except these two, the MC → SP back-conversion rate fits well to a biexponential function, implying the formation of two MC isomers or an MC-MeOH hydrogen-bonded complex upon irradiation. MC → SP back-conversion is significantly enhanced at elevated temperatures or under visible irradiation, and the solvent was found to play a significant role in the observed photophysical behaviour of spiropyrans. In the solid-state, correlations of spectroscopic properties with the Hammett parameters of substituents were poor, likely due to the influence of steric effects. SP → MC photoconversion was also generally poor, although some molecules exhibited photoswitching despite not doing so in solution (SP17, SP26 and SP27). The solid-state MC → SP back-conversion rate was lower than that measured in solution due to confinement effects. The implications of crystal structure features on solid-state photoswitching are explored through void space and Hirshfeld surface calculations. Both analyses strongly suggest a connection between the amount of free space within the crystal structure and solid-state photoswitching ability. There appears to be a link between the presence of bulky, aromatic substituents, such as pyrimidyl groups and isophthalate rings, and solid-state photoswitching. The absence of specific interactions, such as hydrogen bonds, also appears to be advantageous. Photocrystallography showed that photoconversion likely occurs at the surface, with few bulk spiropyran molecules converted. Chapter 4 focuses on using DFT calculations to aid in the interpretation of experimental data as well as in the potential identification of novel relationships iv between structural features and photophysical properties. Geometry-optimised spiropyran structures showed good agreement with those from SCXRD measurements. The differences that are observed appear to be correlated with the extent of solid-state confinement effects present. Calculated structural parameters, such as Cspiro-O and Cspiro-N bond lengths and indoline-chromene dihedral angles, tend to correlate better with solution-state photophysical properties than SCXRD measurements do, however, all relationships are weak. Solution-state photoconversion is correlated with both a long calculated Cspiro-O bond and a short calculated Cspiro-N bond in the ground state structures. Quantitative analysis of substituent effects on the molecular orbitals was not permitted through orbital composition analysis, although qualitative assessments could be made. TDDFT analysis shows that the dominant excitations are composed of π → π* transitions between orbitals of the indoline and chromene rings, respectively. Comparably rarer are orthogonal transitions between occupied indoline orbitals and unoccupied chromene orbitals, which are expected to lead to efficient photoconversion. High-lying occupied orbitals localised on -NO2/-CHO chromene substituents were present in molecules exhibiting the highest photoconversion efficiencies. The presence of low lying π*-states, such as those present in spiropyrans with extended aromatic substituents, appears to negatively affect the observation of SP → MC photoconversion. Chapter 5 reports the synthesis and characterisation of expected type IV linkers, SP29, SP30 and TrSP1, for subsequent incorporation into photoresponsive MOFs. The crystal structure of SP30 was shown to be a 2D hydrogen-bonded network composed of hydrogen-bonded layers of SP30 molecules stacked together via π-π interactions and solvent-mediated interactions. The photophysical properties of these linkers were characterised in solution and the solid-state. SP29 and SP30 exhibited limited observed photoswitching capabilities in MeOH solutions and the solid-state, likely due to substituent effects in the former and MC/MCH+ presence in the latter. TDDFT calculations are consistent with the experimental data, and molecular orbital analysis supports a detrimental effect of low-lying π*-orbitals on photoswitching efficiency. TrSP1 demonstrates efficient and reversible photoconversion in DMSO solutions. Kinetic analyses suggest both TTC-MC and TTT-MC conformers form upon irradiation and decay to the ground state at different rates. v Chapter 6 details the attempted incorporation of spiropyran linkers, SP29, SP30 and TrSP1, into photoresponsive MOFs. In most MOF reactions, an amorphous red solid presumed to be a coordination network or organic polymer involving MC molecules is observed. This suggests a strong tendency to favour amorphous network formation over crystalline solids. Some PXRD patterns did display promising diffraction peaks, and coloured single crystals were also observed, however, the data collected was insufficient to conclude whether framework formation has occurred. The synthesis and photophysical characterisation of photoresponsive SP-functionalised silica nanoparticles are reported, along with their applications in light-triggered on/off release of antibiotics.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Chemistry
Date of First Compliant Deposit:	10 February 2025
Last Modified:	10 Feb 2025 16:39
URI:	https://orca.cardiff.ac.uk/id/eprint/176064

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