Using non-bonding interactions to control photochemical reactions requires an understanding of not only thermodynamics and kinetics of ground state and excited state processes but also the intricate interactions that dictate the dynamics within the system of interest. This review is geared towards a conceptual understanding of how one can control the reactivity and selectivity in the excited state by employing confinement and non-covalent interactions. Photochemical reactivity of organic molecules within confined containers and organized assemblies as well as organic templates that interact through H-bonding and/or cation–carbonyl/cation–π interactions is reviewed with an eye towards understanding supramolecular effects and photocatalysis.

Atropisomeric maleimides were synthesized and employed for stereospecific [2 + 2] photocycloaddition. Efficient reaction was observed under direct irradiation, triplet-sensitized UV irradiation, and non-metal catalyzed visible-light irradiation, leading to two regioisomeric (exo/endo) photoproducts with complete chemoselectivity (exclusive [2 + 2] photoproduct). High enantioselectivity (ee > 98%) and diastereoselectivity (dr > 99:1%) were observed under the employed reaction conditions and were largely dependent on the substituent on the maleimide double bond but minimally affected by the substituents on the alkenyl tether. On the basis of detailed photophysical studies, the triplet energies of the maleimides were estimated. The triplet lifetimes appeared to be relatively short at room temperature as a result of fast [2 + 2] photocycloaddition. For the visible-light mediated reaction, triplet energy transfer occurred with a rate constant close to the diffusion-limited value. The mechanism was established by generation of singlet oxygen from the excited maleimides. The high selectivity in the photoproduct upon reaction from the triplet excited state was rationalized on the basis of conformational factors as well as the type of diradical intermediate that was preferred during the photoreaction.

Direct irradiation of atropisomeric α-substituted acrylanilides in the presence of alkali metal ions gave high ee values in the 3,4-dihydro-2-quinolin-2-one photoproduct, while in the absence of alkali metal ions, racemic photoproduct was observed. The heavy atom effect leading to enhanced triplet yields alters the reactive pathway leading to the observed enantioselectivity in the photoproduct.

Nonbiaryl atropisomeric acrylimides underwent facile [2 + 2] photocycloaddition leading to cross-cyclobutane adducts with very high stereospecificity (enantiomeric excess (ee): 99% and diastereomeric excess (de): 99%). The photoreactions proceeded smoothly in isotropic media for both direct and triplet sensitized irradiations. The reactions were also found to be very efficient in the solid state where the same cross-cyclobutane adduct was observed. Photophysical studies enabled us to understand the excited-state photochemistry of acrylimides. The triplet energy was found to be ∼63 kcal/mol. The reactions proceeded predominantly via a singlet excited state upon direct irradiation with very poor intersystem crossing that was ascertained by quantification of the generated singlet oxygen. The reactions progressed smoothly with triplet sensitization with UV or visible-light irradiations. Laser flash photolysis experiments established the triplet transient of atropisomeric acrylimides with a triplet lifetime at room temperature of ∼40 ns.

Atropisomeric α-oxoamides were synthesized and employed for intramolecular Paternò–Büchi reaction leading to very high enantio- and diastereoselectivity in the bicyclic oxetane photoproduct. A reversal of product selectivity was observed in solution and in the solid-state.

Atropisomeric 3,4-dihydro-2-pyridones undergo stereospecific [2+2]-photocycloaddition in solution with high stereoselectivity (ee > 98% and de > 96%) in the product. The chiral transfer during phototransformation was rationalized based on the stability/reactivity of the biradical.

6-Methylcoumarin (1) upon mechanical grinding with cucurbit[8]uril (CB[8]) or other additives (e.g. thiourea) showed enhanced photodimerization efficiency with exclusive formation of head-to-head photodimer. Mechanical grinding of 1 with CB[8] resulted in host–guest interaction with red-shift fluorescence in the solid-state. This red shift in fluorescence emission is similar to the red-shifted emission observed for CB[8]–1 host–guest complex in solution at 298 K. Phosphorescence emission profile in the solid state at 77 K for mechanically ground 1 with CB[8] was similar to the phosphorescence profile observed for CB[8]–1 host–guest complex at 77 K. Single crystal X-ray determination of 1 from water reveals that the coumarins are oriented in a head-to-head fashion with the distance between the double bonds being 4.695 Å, a distance that is greater than the required Schmidt distance of 4.2 Å for efficient photodimerization in the crystalline state. Enhancement of photoreactivity of 1 upon mechanical grinding with additives appears to be the result of a combination of factors, viz.: (a) crystalline imperfection/defects that are likely formed during mechanical grinding and/or; (b) formation of a new phase due to mechanical grinding (we were not able to observe any noticeable change in crystallinity, while the formation of an amorphous phase could not be ruled out completely).Graphical abstractHighlights► The photochemistry and photophysics of 6-methylcomarin 1 are influenced by mechanical grinding with cucurbit[8]uril. ► Bathochromic shift was observed in the room temperature fluorescence spectra of 1 upon mechanical grinding with CB[8]. ► A red shift in the phosphorescence spectra of 1 was observed at 77 K upon mechanical grinding with CB[8]. ► 6-Methylcoumarin 1 is not photoreactive in the solid-state but the photodimerization efficiency was enhanced upon mechanical grinding of 1 leading to the formation of head-to-head photoproduct.

Nonbiaryl axially chiral 2-pyridones were synthesized and employed for light-induced electrocyclic 4π ring closure leading to bicyclo-β-lactam photoproducts in solution. The enantioselectivity in the photoproducts varied from 22 to 95% depending on the reaction temperature and the ability of the axially chiral chromophore to form intramolecular and/or intermolecular H-bonds with the solvent. On the basis of the differential activation parameters, entropic control of the enantiospecificity was observed for 2-pyridones lacking the ability to form H-bonds. Conversely, enthalpy played a significant role for 2-pyridones having the ability to form H-bonds.

Guest induced shape change of the cucurbit[8]uril cavity is likely rate limiting in the supramolecular photocatalytic cycle for CB8 mediated photodimerization of 6-methylcoumarin.

The enantiomeric ratio (e.r.) in the 3,4-dihydroquinolin-2-one photoproduct during 6π-photocyclization of α-substituted axially chiral ortho-tert-butyl-acrylanilides depends on the nature of the reactive spin state (singlet or triplet), where the singlet-spin state reactivity gives a racemic mixture and the triplet reactivity gives an e.r. value >95∶5.

Atropisomeric benzoylformamides 1 undergo Type II reaction leading to cis-2 and trans-2 oxazolidin-4-one photoproducts. The N–C(Aryl) chiral axis is maintained during the course of the phototransformation in spite the reaction proceeding through a near planar intermediate(s). As the rotational barrier of the N–C(Aryl) chiral axis in the cis-2 and trans-2 photoproducts is lowered when compared with the reactant 1, the isolated optically pure trans-2 isomer is converted to the ent-cis-2 isomer without affecting the C-5 stereogenic center, resulting in resolution of the cis-2 enantiomers.

Cucurbit[8]uril (as low as 10 mol%) acts as a supramolecular catalytic nanoreaction vessel and facilitates the photodimerization of coumarins in water leading to syn dimers. Saturation kinetics shows a sigmoidal dependence with a turnover number of 3.4 min−1 and a Hill constant of 1.8 indicating a co-operative mechanism in the catalytic process.

α-Oxoamides 1 with o-tert-butyl substitution on the N-phenyl moiety were found to be stable axially chiral atropisomers. These axially chiral α-oxoamides undergo enantiospecific photochemical γ-Hydrogen abstraction in CHCl3 to yield β-lactams with high enantioselectivity (e.r. ∼90:10) in solution. The extent of enantioselectivity was found to be dependent on the reaction temperature.

O-tert-Butylacrylanilides with N–H substitution undergo 6π-photocyclization at the unsubstituted ortho carbon, whereas the corresponding N-methyl derivatives cyclize at the ortho carbon bearing the tert-butyl with the eventual loss of 2-methylpropene.

Photochirogenesis, the control of chirality in photoreactions, is one of the most challenging problems in stereocontrolled photochemistry, in which the stereodifferentiation has to be imprinted within the short lifetime of the electronically excited state. Singlet oxygen (1O2), an electronically excited molecule that is known to be sensitive to vibrational deactivation, has been selected as a model case for testing stereoselective control by vibrational deactivation. The stereoselectivity in the reaction of 1O2 with E/Z enecarbamates 1, equipped with the oxazolidinone chiral auxiliary, has been examined for the mode selectivity ([2 + 2]-cycloaddition versus ene-reaction) and the stereoselectivity in the oxidative cleavage of the alkenyl functionality to the methyldesoxybenzoin (MDB) product. Through the appropriate choice of substituents in the enecarbamate, the mode selectivity (ene versus [2 + 2]), which depends on the alkene geometry (E or Z), the steric bulk of the oxazolidinone substituent at the C-4 position, and the C-3′ configuration on the side chain, may be manipulated. Phenethyl substitution gives exclusively the [2 + 2]-cycloaddition product, irrespective of the alkene geometry. The stereoselection in the resulting methyldesoxybenzoin (MDB) product is examined in a variety of solvents as a function of temperature by using chiral GC analysis. The extent (% ee) as well as the sense (R versus S) of the stereoselectivity in the MDB formation for the E isomer depends significantly on solvent and temperature, whereas the corresponding Z isomers are not affected by such variations. The complex temperature and solvent effects are scrutinized in terms of the differential activation parameters (ΔΔS⧧, ΔΔH⧧) for the photooxygenation of E/Z-enecarbamates in various solvents at different temperatures. The enthalpy−entropy compensations provide a mechanistic understanding of the temperature dependence of the ee values for the MDB product and the difference in the behavior between the Z and E enecarbamates. The E enecarbamates show a relatively high contribution from the entropy term and an appreciable contribution from the enthalpy term; both terms possess the same sign. In contrast, the corresponding relative insensitivity of Z enecarbamates to temperature and solvent variation is convincingly explained by the near-zero ΔΔS‡ and ΔΔH‡. Such effects, associated with temperature- and solvent-dependent conformational factors, are most likely dictated by the stereogenic center at the C-3′ phenethyl substituent. The high stereocontrol during the photooxygenation of the chiral enecarbamates is shown to be independent of the steric demand of the oxazolidinone substituent at the C-4 position. In view of the reduced stereocontrol on deuteration of the oxazolidinone substituent at the C-4 position, we propose that the unusual stereoselective vibrational quenching of the attacking singlet oxygen (excited-state reactivity), a novel mechanistic concept, works in concert with the usual steric impositions (ground-state reactivity) exercised by the substituents to afford the high stereoselectivity observed in the dioxetane product during the [2 + 2] cycloaddition. Such synergistic interplay is held responsible for the highly stereoselective photooxidative cleavage of the chiral enecarbamates. The efficacy of stereocontrol in this photooxidation is demonstrated by kinetically resolving the epimers of the enecarbamate cleavage product (MDB) in essentially perfect stereoselectivity, a new methodology that we coin “photo-Pasteur-type kinetic resolution”.

Coumarin derivatives with non-polar substituents at the 6 or 7 position undergoes photodimerization in the presence of CB[8] in water to give the syn dimer as the major product. It is postulated that these neutral coumarins form dynamic complexes in the presence of CB[8] and the product selectivity is reflective of the type of complex, available volume in the CB[8] cavity and relative rate of photodimerization inside and outside the CB[8] cavity.

Atropisomeric α-oxoamides were synthesized and employed for intramolecular Paternò–Büchi reaction leading to very high enantio- and diastereoselectivity in the bicyclic oxetane photoproduct. A reversal of product selectivity was observed in solution and in the solid-state.

Guest induced shape change of the cucurbit[8]uril cavity is likely rate limiting in the supramolecular photocatalytic cycle for CB8 mediated photodimerization of 6-methylcoumarin.

Cucurbit[8]uril (as low as 10 mol%) acts as a supramolecular catalytic nanoreaction vessel and facilitates the photodimerization of coumarins in water leading to syn dimers. Saturation kinetics shows a sigmoidal dependence with a turnover number of 3.4 min−1 and a Hill constant of 1.8 indicating a co-operative mechanism in the catalytic process.

Using non-bonding interactions to control photochemical reactions requires an understanding of not only thermodynamics and kinetics of ground state and excited state processes but also the intricate interactions that dictate the dynamics within the system of interest. This review is geared towards a conceptual understanding of how one can control the reactivity and selectivity in the excited state by employing confinement and non-covalent interactions. Photochemical reactivity of organic molecules within confined containers and organized assemblies as well as organic templates that interact through H-bonding and/or cation–carbonyl/cation–π interactions is reviewed with an eye towards understanding supramolecular effects and photocatalysis.

Atropisomeric benzoylformamides 1 undergo Type II reaction leading to cis-2 and trans-2 oxazolidin-4-one photoproducts. The N–C(Aryl) chiral axis is maintained during the course of the phototransformation in spite the reaction proceeding through a near planar intermediate(s). As the rotational barrier of the N–C(Aryl) chiral axis in the cis-2 and trans-2 photoproducts is lowered when compared with the reactant 1, the isolated optically pure trans-2 isomer is converted to the ent-cis-2 isomer without affecting the C-5 stereogenic center, resulting in resolution of the cis-2 enantiomers.

Atropisomeric 3,4-dihydro-2-pyridones undergo stereospecific [2+2]-photocycloaddition in solution with high stereoselectivity (ee > 98% and de > 96%) in the product. The chiral transfer during phototransformation was rationalized based on the stability/reactivity of the biradical.

The enantiomeric ratio (e.r.) in the 3,4-dihydroquinolin-2-one photoproduct during 6π-photocyclization of α-substituted axially chiral ortho-tert-butyl-acrylanilides depends on the nature of the reactive spin state (singlet or triplet), where the singlet-spin state reactivity gives a racemic mixture and the triplet reactivity gives an e.r. value >95∶5.