The Emerging Mechanochemistry of Naphthopyran

Citation

McFadden, Molly Elizabeth (2023) The Emerging Mechanochemistry of Naphthopyran. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/r4gk-rn51. https://resolver.caltech.edu/CaltechTHESIS:05182023-034742629

Abstract

Asserting remote control over reactivity is a pervasive goal in modern chemistry. Several external stimuli can supply energy to facilitate productive chemical transformations. In recent years, that list has grown to include mechanical force. In the growing field of polymer mechanochemistry, privileged, mechanically sensitive molecules called mechanophores undergo desirable chemical reactions under force. Force is transduced to mechanophores through covalently attached polymer chains. Among many applications, mechanophores that react to produce colored species can be used for visual stress and damage detection in plastics. Naphthopyran is a highly modular molecular switch that can undergo ring-opening reaction to generates intensely colored merocyanine dyes. The studies described in this dissertation have thoroughly established the versatility and complexity of naphthopyran-based mechanochromic mechanophores.

In Chapter 1, the history of naphthopyran molecular switches is summarized with particular emphasis on the exhaustive and generalizable merocyanine structure-property relationships established for photochromic naphthopyrans. Naphthopyran mechanochemistry is reviewed and contextualized among other mechanochromic mechanophores. Mechanochromic structure-function relationships are highlighted, as well as a series of studies using the naphthopyran platform for illustration of the unusual reaction pathways accessible under force, and studies demonstrating that multimodal naphthopyrans are an ideal platform for multicolor mechanochromism.

As discussed extensively in Chapter 1, merocyanines are typically susceptible to thermal recyclization. However, Chapter 2 describes a scissile naphthopyran mechanophore that undergoes an unusual secondary mechanochemical ester cleavage after the ring-opening reaction. By revealing a β-hydroxy ketone group with a stable intramolecular hydrogen bonding interaction, mechanical force uniquely generates persistent merocyanine species.

The irreversible reaction sequence described in Chapter 2 enables determination of mechanochemical reaction kinetics under solution-phase ultrasound-mediated mechanical force. However, common methods for determining reaction rate constants under these conditions are time-intensive and convoluted by the competitive side reaction of nonspecific polymer backbone scission. In Chapter 3, through model studies on the highly efficient scissile naphthopyran and a comparatively inefficient coumarin dimer mechanophore, we validate a time-efficient and accurate initial rates method for determination of selective mechanophore reaction kinetics under ultrasonication.

The ability to colorimetrically report on the magnitude of applied stress is a grand challenge in the field of mechanochromism. Chapter 4 describes the first single mechanophore capable of such behavior. Mechanical force induces unexpectedly simultaneous ring-opening reactions from a bis-naphthopyran mechanophore, biasing a dynamic equilibrium between two distinct merocyanine states to effect gradient force-dependent multicolor mechanochromism.

In further studies seeking multicolor mechanochromic systems, it was discovered that mechanical force facilitates the first reported dual-ring-opening reaction of naphthodipyran. Chapter 5 describes the mechanochemical generation of an unusual dimerocyanine species with near-IR absorption that is not formed photochemically.

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