Active infrared absorption by photosynthetic intermediates. Photosynthesis as a biochemical Partridge test: can a prepared reaction-centre state control a neighbour's energy loss?

When an emitter and an absorber are coupled, the rate at which the emitter loses energy depends on the absorber's prepared state. Photosynthesis shows this every day: closing photosystem-II reaction centres lengthens antenna fluorescence four- to sixfold, closed purple-bacterial centres quench their LH1 antenna, and the oxidized P700+ of photosystem I quenches the long-wavelength emission of its red chlorophylls. We ask whether such prepared states can also accept mid-infrared energy from a neighbour, and whether that energy can be used rather than wasted. The long-lived P700+ (about 0.1 s, broad mid-infrared absorption) and a W(CO)6 membrane donor (120 ps at 1980 cm^-1) form a concrete first pair, with conditional Förster radii of 1.2–1.75 nm; band strength and donor geometry are the decisive missing inputs (a measured/assumed/missing table is given). A branching and turnover model shows that infrared can change reaction pathways before it thermalizes, as infrared push pulses already do in solar cells, but that higher yield per photon is not higher throughput or efficiency. Thermal energy demonstrably assists productive photochemistry (far-red oxygen evolution with an activation energy close to the uphill gap, 0.13–0.15 eV, i.e. 8–10 µm quanta inside the atmospheric window), and repeated thermally assisted visits explain how sparsely populated upper states still trap efficiently. An energy ledger from public photo-calorespirometry data (independently re-integrated) shows net chemical storage of 1.8–2.9% of the optical input and large net heat release: local thermal uptake can coexist with overall heating, and no refrigeration is established. The paper gives experiments A (infrared Förster test near a switched P700), B (a later-state test in chlorophyll fluorescence with a fixed, hashed record before a fresh random choice — a recorded-choice test at rung 4-C of the CPT ladder), C (an infrared action spectrum of terminal yield), and D/D' (infrared spectrum at fixed leaf temperature; sky versus cold plate), with controls, reviewer upgrades and a tiered plan. Claims are graded: prepared IR-active intermediates (supported); prepared acceptors changing neighbouring emission and thermally assisted photochemistry (observed); local donor cooling (plausible, to be demonstrated); improved chemistry or whole-system cooling (unestablished). Companion to the Partridge-test series and the two-boundary programme.