In ground-state chemistry, such as metal-catalyzed processes, exquisite levels of reaction control can often be achieved by subtle changes to the catalyst or reaction conditions. In photochemistry the situation is much more complex, as fates of excited states are governed by competing pathways for reaction, coupling to other electronic states, possible further photoexcitation, and relaxation to the ground state; thus, the outcomes of such processes are difficult to control once initiated. Some useful selectivity through singlet or triplet states can be achieved by a number of means, including use of triplet quenchers or sensitizers, [1][2][3] irradiation at user-selected wavelengths, [4] through higher exited states from multiphoton absorption, [5, 6] and feedback-based optical control using evolutionary algorithms and pulse shapers. [7] Herein, we describe how photon flux [8] can be used to switch cleanly between Norrish type I and type II pathways [1, 5, 6f-l,9] in the photochemical rearrangement of 1,3-indandiones.[10] In particular, the study demonstrates that photon flux, not average power or wavelength, is the controlling factor in product selection.As part of an ongoing study of the [5+2] photocycloaddition of maleimides, [11,12] we investigated the photochemistry of 2-ethyl-1,3-indandione 1 (Scheme 1). Unfocused [13] Nd:YAG laser irradiation of 1 at 266 nm [14] gave rise to the volatile g-alkylidinephthalide lactones 2 as an E/Z mixture (18 %). Using the higher power output of the laser at 355 nm (350 mW), a yield of 60 % of isolated 2 was obtained after 18 h. Addition of isoprene as a triplet quencher had no effect, suggesting a singlet mechanism. Quantum-yield (F) measurements using tunable lasers at five specific wavelengths between 222 and 355 nm indicated an inefficient photochemical reaction (F = 0.5-3.3 %).At this point we attempted to scale up the formation of 2 by irradiating 1 with higher (average) power UV lamps. To our surprise, however, irradiation of 1 with a low-pressure phosphor-coated mercury lamp (312 nm) gave only the diones 3 and 4 (43 %, 2:1). Similar results were obtained using medium-pressure lamps (125-600 W). Irradiation of 1 in CD 3 CN and direct analysis of the photosylate by 1 H NMR spectroscopy showed greater than 99 % of 3 and 4 and a trace of 2 (less than 1 %). These results indicate that the pathway leading to 2 is effectively closed for lamp irradiation. Significantly, addition of isoprene (one equivalent) resulted in quenching of the reaction and recovery of 1, thus indicating that 3 and 4 are formed from a different (triplet) pathway on lamp irradiation.This stark, source-dependent difference in behavior can be explained by the mechanisms in Scheme 2. Initial excitation of 1 with either source leads to the singlet state (S 1 ), from which two separate pathways can evolve. With the laser, S 1 undergoes a cleavage to the diradical 5 and subsequent recombination to the lactones 2 through a Norrish type I pathway. A singlet manifold is supported by the lack of quenchin...
