How molecular architecture defines quantum yields

The study addresses how molecular architecture—specifically the spacing and topology of photoreactive groups—governs the quantum efficiency of intramolecular [2+2] photocycloadditions and how this, in turn, dictates network formation and performance in two-photon microprinting. Photochemical processes are central to sustainable synthesis and spatiotemporal control but remain limited in industrial settings by selectivity, yields, heat management, and scalability. Quantum yield is a key descriptor of photochemical efficiency and can display significant wavelength dependence. Prior work highlights that designed molecular environments and confinement can enhance reaction rates and selectivity, including in photochemical systems. However, a systematic solution-phase exploration of how macromolecular architecture alone governs bond-forming photoreactions has been lacking. The authors propose that an optimal balance between steric hindrance and entropic limitations (a goldilocks regime) maximizes intramolecular cyclisation efficiency and that this efficiency impacts macroscopic printability and material properties in additive manufacturing.

The paper surveys factors influencing photochemical efficiency and luminescence quantum yields (solvent, temperature, excitation wavelength) and highlights wavelength-dependent action plots where photochemical activity can be red-shifted relative to absorption. It reviews catalytic confinement and nanoreactors that raise local reactant concentrations, enabling efficient photochemical transformations in dilute bulk conditions. Solid-state templating (hydrogen bonding, metal coordination) has facilitated otherwise challenging photodimerisations and complex architectures (e.g., ladderenes). Peptide-driven self-assemblies enable pH-gated [2+2] cycloadditions in solution, and [2+2] geometry has been used to stabilize macromolecular conformations. Prior studies on excimer fluorescence formation have quantified spatial arrangement in chromophore-bearing systems. Collectively, these works suggest architecture-reactivity relationships but lack a systematic solution-phase study connecting macromolecular spacing to intramolecular photocycloaddition quantum yields and downstream material performance.

• Synthesis: Developed an iterative sequential growth route to monodisperse, trifunctional macromolecules bearing three pyrene–chalcone (PyChal) units separated by tunable ε-caprolactone spacers. A PyChal-bearing difunctional alcohol initiator was extended by coupling with protected alcohol-bearing acids; deprotections used tetrabutylammonium fluoride. Final macromolecules T0, T1, T3, and T5 were obtained by coupling a carboxylic acid–functionalised PyChal at the chain end; the numeral denotes the number of caprolactone units between PyChal groups.
• Photochemistry and quantum yield: Intramolecular [2+2] photodimer formation confirmed by NMR via cyclobutane proton resonances. Quantum yields were determined in acetonitrile at low concentration (typically 25 µM) using a monochromatic tunable pulsed laser while recording absorbance to track conversion vs photons. The linear regime was fit to extract intramolecular quantum yield using a provided relation involving conversion, number of photons, concentration, volume, extinction at 445 nm, and Avogadro’s number. Additional dilute experiments (12.5 µM) used a 10 W 445 nm LED to suppress intermolecular reactions and promote intramolecular cyclisation.
• Isomer identification: Size exclusion chromatography (SEC) before/after irradiation showed shifts to lower apparent molecular weight indicating intramolecular folding. SEC traces displayed bimodality due to P (center–end) and Q (end–end) isomers; deconvolution used fitting with two monodisperse peaks. Tandem SEC–ESI–(MS)2 identified isomers via diagnostic fragments and elution behavior, assigning the larger hydrodynamic volume peak to the P isomer.
• Molecular dynamics simulations: All four macromolecules (T0, T1, T3, T5) were simulated for 1500 ns each in explicit acetonitrile to sample conformational ensembles. Analyses included RMSD for flexibility, cluster analyses revealing π–π stacking motifs, and pairwise distances between photoactive double bonds to assess sampling of geometries fulfilling Schmidt’s topochemical postulate for [2+2] cycloaddition. Distances for P-promoting (center–end) and Q-promoting (end–end) interactions were compared.
• Two-photon microprinting: Four single-component photoresists were formulated by dissolving each macromolecule in propylene carbonate:acetophenone (3:2 v:v) at 93.9 µmol L⁻1. Using a Nanoscribe two-photon printer, arrays of 10 µm micro-cubes were printed across a matrix of laser powers (5–100%) and scan speeds (0.5–12.5 mm s⁻1). SEM assessed print quality. Larger 3D structures (e.g., 100 µm-tall duck models) were printed to demonstrate scalability and feature resolution.
• Nanoindentation: To assess material properties, blocks (25 × 25 × 20 µm) of T3 and T5 were printed under identical parameters (laser power 15, scan speed 1.5 mm s⁻1). Displacement-controlled nanoindentation to 1000 µm depth measured reduced modulus and hardness; n = 7 per condition.

• Architecture–quantum yield relationship: Quantum yield of intramolecular cyclisation exhibits a non-monotonic dependence on spacer length, revealing a goldilocks regime. T0 (no caprolactone spacers) shows low quantum yield due to steric hindrance preventing fulfillment of Schmidt’s postulate (required 0.35–0.42 nm approach). Introducing a single spacer unit (T1) increases the intramolecular quantum yield by more than sevenfold versus T0, attributed to increased flexibility allowing favorable approach. Further increasing spacing (T3, T5) reduces efficiency due to decreased local concentration and entropic penalties, approaching bulk behavior.
• Isomer outcomes: Intramolecular cyclisation yields two isomers: P (center–end) and Q (end–end). SEC deconvolution and SEC–ESI–(MS)2 assign the larger hydrodynamic volume isomer to P. Higher fractions of P correlate with higher intramolecular quantum yields across T1–T5. T0 is an exception, favoring Q despite short center–end distances.
• MD simulation insights: Flexibility (RMSD mean ± SD): T0 0.69 ± 0.11 nm, T1 0.80 ± 0.12 nm, T3 1.17 ± 0.22 nm, T5 1.18 ± 0.21 nm. All systems favor conformations with π–π stacking for much of the trajectory (T0 86.6%, T1 93.6%, T3 91.8%, T5 95.6%). P-promoting average distances (center–end) were 1.50 ± 0.52 nm (T0), 1.68 ± 0.62 nm (T1), 2.13 ± 0.80 nm (T3), 2.21 ± 0.96 nm (T5), indicating reactive-aligned conformations are rare events. Parallel π–π stacking reduces distances between reactive double bonds; antiparallel stacking increases them. For T0, a distinct π-stacking network (edge-to-face plus antiparallel face-to-face) persists ~1.4% of time, favoring Q and hindering P-type photocycloaddition.
• Translation to microprinting: Print quality improves with increasing PyChal–PyChal distance for both low- and high-exposure matrices. Higher intramolecular quantum yields (notably T1) reduce available sites for intermolecular crosslinking, degrading printability. T0 prints poorly despite low quantum yield due to steric shielding from compact π-stacked conformations hindering intermolecular attack. Smaller spacings exhibit more frequent micro-explosions (holes/lumps), especially at higher laser powers (e.g., T3). At high power/low speed, thermal swelling causes structure fusion and bridging.
• Mechanics of printed parts: Under identical printing parameters, T3 shows higher reduced modulus and hardness than T5, attributed to shorter linkers generating denser networks. Despite low resin concentration, large 3D structures (100 µm) with pronounced overhangs were printed rapidly (≈8 min 15 s each) with good resolution, indicating promise for [2+2]-based single-component resists.

The findings demonstrate that molecular architecture tightly controls intramolecular [2+2] photocycloaddition efficiency via a balance between steric constraints and entropic sampling. Very short spacings (T0) constrain reactive group approach and orientational alignment, lowering quantum yield and, via π–π-stacked shielding, also suppressing intermolecular crosslinking during printing. Introducing modest flexibility (T1) optimizes access to reactive geometries, maximizing intramolecular quantum yield and favoring the P isomer; however, this enhanced intramolecular folding consumes reactive sites, diminishing network formation and print quality. Longer linkers (T3, T5) increase entropy and average separation, lowering intramolecular cyclisation efficiency yet improving availability for intermolecular crosslinking, thereby enhancing printability. MD results rationalize these trends, showing that parallel π–π interactions promote shorter reactive distances and that reactive alignments are sampled infrequently, with architecture modulating their frequency and orientation (P vs Q propensity). The interplay between intramolecular efficiency and macroscopic processing outcomes underscores that optimal resist design must balance reactive group spacing to achieve both print fidelity and desired mechanical properties; denser networks (T3) yield higher modulus/hardness than longer-linker systems (T5) at the expense of some print robustness at extreme exposures.

The study establishes that precise molecular architecture dictates the quantum yield of intramolecular [2+2] photocycloadditions in trifunctional macromolecules, revealing a goldilocks regime between steric hindrance and entropic limitations. This architectural control translates directly to two-photon microprinting performance: higher intramolecular quantum yields generally impair print quality by depleting sites for intermolecular crosslinking, whereas lower intramolecular yields facilitate robust network formation and high-fidelity printing. While longer linkers improve printability, they can reduce material hardness and modulus, highlighting a design trade-off between print quality and mechanical performance. These insights provide a foundation for designing single-component photoresists with tailored crosslinking pathways and suggest broader relevance to fields such as photocatalysis and medicinal chemistry, where careful spatial positioning of functional groups could maximize reaction efficiency. Future work may expand solvent systems, linker chemistries, and chromophore types, and integrate wavelength-resolved action plots to further optimize architectural parameters for targeted photochemical and additive manufacturing outcomes.