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The irradiation of β‐enaminones, generated in situ from cyclic 1,3‐diketones and activated alkenes leads to polyheterocyclic skeletons. The photoproduct chemoselectivity depends on the type of cyclic 1,3‐diketones employed viz., 2‐acetylcyclopentanone and 2‐acetylcyclohexanone. The observed chemoselectivity was rationalized based on the Dieckmann‐Kon rule. [ABSTRACT FROM AUTHOR]

[Display omitted] • 2-Thioxo-coumarins are established to show room temperature phosphorescence in aqueous media. • The utility of the organic cavitand octa acid in solubilizing and bringing out phosphorescence from 2-thioxo-coumarins in water at room temperature has been established. • 2-Thioxo-coumarins which undergoes intramolecular electron/charge transfer emits only fluorescence in water at room temperature. But, in presence of octa acid capsule it shows only phosphorescence. • Octa acid fails to induce room temperature phosphorescence from a 2-thioxo-coumarin that undergoes chemical reaction from the triplet state. • Self-quenching and oxygen quenching that competes with phosphorescence are suppressed by octa acid encapsulation. But it fails to overcome the photofragmentation reaction to favor phosphorescence. Unlike coumarins, the sulfur analogues, 2-thioxo-coumarins (TCs) have not attracted attention. Given the large energy gap between S 2 and S 1 and high spin–orbit coupling between S 1 and T 1 in these systems lack of attention is surprising. In this manuscript, we are concerned with the phosphorescence of seven TCs at room temperature. The TCs investigated here belong to two groups, one with the amino substitution at the 7-position and the other with an assorted collection of the parent, alkoxy and acetoxy substituted ones. All seven TCs show phosphorescence at 77 K while the ones with amino substitution exhibit both fluorescence and phosphorescence. The inability of phosphorescence to compete fully in this set is attributed to intramolecular electron/charge flow from the lone pair of the amino functionality. Occurrence of such a process opens opportunities to examine the solvation dynamics and TICT process in these systems. The most important result relates to the observation of room temperature phosphorescence (RTP) of these molecules in water with the help of octa acid host. TCs that are not soluble can be solubilized in water with OA capsule and this strategy suppresses self-quenching and oxygen quenching in favor of radiative process in the triplet state. When the chemical reaction is facile as in one system investigated here OA fails to bring about RTP. In our opinion, octa acid capsule can serve as an excellent medium to bring about RTP from organic molecules in water. [ABSTRACT FROM AUTHOR]

Hydrogen bonding mediated control of photochemical reactions is highlighted with an eye towards the development of Brønsted acid mediated photocatalysis. [ABSTRACT FROM AUTHOR]

Aryl‐maleimides undergo a novel [2+4]‐photodimerization instead of the expected [2+2]‐photodimerization under both direct irradiation with visible light and under sensitized energy transfer conditions. This new excited state reactivity in aryl‐maleimides is deciphered through photochemical, photophysical, and spectroscopic studies. The stereochemistry of the photodimer depends on the type of non‐bonding interactions prevalent during photodimerization which is in turn dictated by the substituents on the maleimide ring. More importantly, the stereochemistry of the photodimer formed is complementary to the product observed under thermal conditions. [ABSTRACT FROM AUTHOR]

Aryl‐maleimides undergo a novel [2+4]‐photodimerization instead of the expected [2+2]‐photodimerization under both direct irradiation with visible light and under sensitized energy transfer conditions. This new excited state reactivity in aryl‐maleimides is deciphered through photochemical, photophysical, and spectroscopic studies. The stereochemistry of the photodimer depends on the type of non‐bonding interactions prevalent during photodimerization which is in turn dictated by the substituents on the maleimide ring. More importantly, the stereochemistry of the photodimer formed is complementary to the product observed under thermal conditions. [ABSTRACT FROM AUTHOR]

This account highlights the role of restricted bond rotations in influencing the excited-state reactivity of organic molecules. It highlights the photochemical reactivity of various organic molecules and the design strategies that could be exploited by chemists to utilize restricted bond rotations to uncover new excited-state reactivity and to achieve selectivity. 1 Introduction. 2 NEER-Principle and Restricted Bond Rotations in the Excited State 3 [2+2]-Photocycloaddition of Enamides 4 [3+2]-Photocycloaddition vs. Paternò–Büchi Reaction of Enamides 5 Divergent Photoreactivity of Enones Dictated by Restricted Bond Rotations: Norrish–Yang reactions vs. 6π-Photocyclization 6 Divergent Photoreactivity of Imides with Alkenes: [2+2]-Photocycloaddition vs. Photoene Reaction 7 Summary and Outlook [ABSTRACT FROM AUTHOR]

Photoreaction of enamides tethered to a phenyl ketone leads to either [3+2]‐photocycloaddition or Paternò–Büchi reaction. This divergence in chemical reactivity originating from the same excited state was dependent on the reaction temperature. At low temperatures the Paternò–Büchi reaction was preferred, whereas at higher temperatures there was preference toward formation of [3+2]‐photoproduct. [ABSTRACT FROM AUTHOR]