The role of triplet states in light-induced reactions: spectroscopic insights into energy and light conversion pathways
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Abstract
Triplet–triplet energy transfer-driven photoreactions offer distinct advantages over those induced by
excited singlet states, including longer excited state lifetimes and lower excitation energies. These
features make triplet–triplet energy transfer (TTET) highly attractive for applications in catalysis, light
harvesting, and energy conversion. However, the mechanistic understanding of many triplet-driven
photoreactions remains limited due to the non-emissive nature of most organic chromophore triplet states
and the complexity of photochemical pathways. This thesis addresses these challenges by using
advanced spectroscopic techniques to resolve TTET mechanisms across a broad range of systems,
organized in three thematic parts: mechanistic studies, photoisomerization, and photon upconversion. The
first part focuses on the direct observation of interactions between energy donors (photosensitizers) and
acceptors, aiming at understanding the initial step of energy transfer. In the first project, TTET is
demonstrated between chromophores embedded within DNA helices. The observed strong distance
dependence and low efficiency of long-range TTET, possibly via a tunneling mechanism, in this system
support the hypothesis that DNA has evolved as a photoprotective scaffold. This work offers rare
spectroscopic insights into DNA-mediated energy transfer and contributes to the topic of photosensitized
DNA damage. Subsequent projects shift the focus toward TTET-driven photocatalysis. A polyazahelicene
was found to operate dually as a photoredox catalyst and as a triplet sensitizer for E→Z alkene
isomerization. In two other synthetically valuable reactions, TTET was identified as the reaction pathway in
reactions previously not achievable by energy transfer photocatalysis. This conclusion was primarily
drawn by excluding competing pathways and identifying key intermediate species. In the second part, the
focus is on the triplet-sensitized isomerization of olefins, an emerging strategy for stereodivergent
synthesis and energy storage systems. In particular, the E→Z isomerization of alkenylboronates –
valuable intermediates in organic synthesis due to their versatility and the traceless nature of the boron
group – was spectroscopically investigated. Mapping triplet state energies and lifetimes provided a
detailed picture of the isomerization dynamics. Mechanistic insights translated into practical reaction
improvements. This concept was extended to fluorinated β-borylacrylates. Their selective isomerization
was harnessed as a key step in a multistep synthesis route toward biologically relevant targets. Further,
this chapter explores norbornadiene–quadricyclane photoswitches, promising candidates for molecular
solar thermal energy storage (MOST). The inherent limitation of direct excitation, stemming from its poor
absorption of visible light and low photoisomerization quantum yield, was overcome using TTET. A set of
aryl-substituted norbornadienes was evaluated in combination with both metal-based and organic
sensitizers. The quantum yield of norbornadiene-to-quadricyclane conversion approached unity under
optimized conditions. The best system had a hitherto unreported solar-to-chemical energy storage
efficiency of 5.8%. In a follow-up study, norbornadiene derivatives extended with acetylene were
investigated, expanding the understanding of structure–property relationships and clarifying design criteria
for future generations of energy-storing photoswitches. The final part addresses photon upconversion via
triplet–triplet annihilation (TTA–UC), a method for generating a high-energy photon from a singlet-excited
state by combining two lower energy triplet states. A novel BODIPY-derived sensitizer with excellent
photostability was introduced and used in a green-to-blue upconversion system. The singlet-excited state
of the annihilator was exploited in photocatalytic applications for challenging substrate reductions that
would otherwise not be achieved under green light irradiation. The following projects pursued the design
of annihilators capable of upconversion into the UVB (280–315 nm) and UVC (<280 nm) regions, where
conventional light sources are inefficient or unsustainable. A biphenyl-based annihilator was synthesized
and shown to produce UVB emission (S1 = 4.04 eV) for the first time under blue light excitation. Following
this, a benzene-based annihilator was developed that further pushed emission energies (S1 = 4.15 eV),
with most photons emitted in the UVB range. This annihilator was applied in a UC–FRET scheme to
activate inert aliphatic carbonyls. Such substrates typically require harsh conditions, demonstrating the
practical potential of these systems. A maximum emission (S1 = 4.33 eV) approaching the UVB/UVC
boundary (~280 nm), was achieved with another benzene-derived annihilator, thus setting a new
benchmark for TTA–UC systems. Key to this success was the identification of a high-triplet-energy
sensitizer capable of efficient TTET to the annihilator. Limitations for visible-to-UV TTA-UC were
evaluated, laying the groundwork for future improvements. Lastly, recent advances in visible-to-UV photon
upconversion were reviewed and summarized, highlighting applications in photocatalysis, bond activation,
and photopolymerization. This comprehensive review provides a practical framework for developing
upconversion-driven photoreactions.
Overall, this thesis demonstrates how mechanistic insight – gained through state-of-the-art spectroscopic
techniques – can guide the design of efficient energy donors and acceptors, improve reaction conditions,
and enable practical applications. The findings advance both fundamental understanding and
development in TTET-driven catalysis, solar energy storage, and photon upconversion.