Based on rotational dynamics measurements carried out with isomorphic co-crystals formed by halogen-bonding (XB) between tritylacetylene halides (TrX) and diazabicyclo[2.2.2]octane (dabco), we were able to distinguish the sources of the enthalpic and entropic components in the rotational free energy barrier. We describe the formation of the 1:1 co-crystals (TrX···N(R)N) obtained from 1 equiv of dabco and 1 equiv of either TrI or TrBr, respectively, to give 4a and 4b instead of the potential 2:1 complexes. The co-crystals were prepared by solvent evaporation and mechanochemical synthesis. No co-crystal with TrCl was obtained, reflecting the weaker nature of the TrCl···NR interaction. Single-crystal X-ray diffraction confirmed structures that resemble a spinning top on a tripod and revealed that the two XB co-crystals are isomorphous, with slightly different C-X···NR (X = I, Br) distances and packing interactions. Quadrupolar-echo H NMR experiments withH-labeled samples showed that fast rotation of dabco in these co-crystals follows a six-fold potential energy surface with three lowest energy minima. Variable-temperature H NMR spin-lattice relaxation (VTH T) data revealed rotational dynamics with indistinguishable pre-exponential factors and small but distinguishable activation energies. The activation energy of 4b (E = 0.71 kcal mol) is the lowest reported in the field of amphidynamic crystals. Using the Eyring equation, we established that their activation entropy for rotation is small but negative (ΔS = -3.0 cal mol K), while there is almost a 2-fold difference in activation enthalpies, with 4a having a higher barrier (ΔH = 0.95 kcal mol) than 4b (ΔH = 0.54 kcal mol). Analysis of the rotator cavity in the two co-crystals revealed subtle differences in steric interactions that account for their different activation energies.

It is recognized that metal organic complexes that serve as sensitizers can present various degrees of challenges viz. synthesis and stability for photonic applications such as triplet-triplet annihilation based photon upconversion (TTA-PUC). Presently, researchers, including our group, are turning their attention toward purely organic triplet sensitizers, which can be handled more easily for photon management science. In this review, we surveyed recently developed all-organic chromophoric systems that were devised and used for TTA-PUC research. Knowing that TTA-PUC research has mainly been focused on the design and synthesis of the triplet sensitizers, we detailed the underlying photophysics and thermodynamics that served as the starting point for the synthesis of the purely organic chromophores in question. Accordingly, this review details triplet sensitizers that operate on (i) spin-orbit coupling or heavy atom effect, (ii) Baird-type aromaticity and antiaromaticity, (iii) open-shell characteristics or doublet excited state and (iv) thermally activated delayed fluorescence.

We report a novel reductive desulfurization reaction involving π-acidic naphthalene diimides (NDI) 1 using thionating agents such as Lawesson's reagent. Along with the expected thionated NDI derivatives 2-6, new heterocyclic naphtho-p-quinodimethane compounds 7 depicting broken/reduced symmetry were successfully isolated and fully characterized. Empirical studies and theoretical modeling suggest that 7 was formed via a six-membered ring oxathiaphosphenine intermediate rather than the usual four-membered ring oxathiaphosphetane of 2-6. Aside from the reduced symmetry in 7 as confirmed by single-crystal XRD analysis, we established that the ground state UV-vis absorption of 7 is red-shifted in comparison to the parent NDI 1. This result was expected in the case of thionated polycyclic diimides. However, unusual low energy transitions originate from Baird 4nπ aromaticity of compounds 7 in lieu of the intrinsic Hückel (4n + 2)π aromaticity as encountered in NDI 1. Moreover, complementary theoretical modeling results also corroborate this change in aromaticity of 7. Consequently, photophysical investigations show that, compared to parent NDI 1, 7 can easily access and emit from its T state with a phosphorescence (7a)* lifetime of τ = 395 μs at 77 K indicative of the formation of the corresponding "aromatic triplet" species according to the Baird's rule of aromaticity.

Molecularly chiral o-tert-butylacrylanilides undergo enantiospecific 6pi-photocyclization to yield 3,4-dihydroquinolin-2-ones with very high enantioselectivity (>90%) in solution. The photocyclization results in the removal of the ortho tert-butyl substituent, presumably via a zwitterionic intermediate. Beta-substitution in the alkene is found to be critical for the transfer of molecular chirality (axial chirality) in the reactant to point chirality in the photoproduct(s).

Alpha-oxoamides 1 with o-tert-butyl substitution on the N-phenyl moiety were found to be stable axially chiral atropisomers. These axially chiral alpha-oxoamides undergo enantiospecific photochemical gamma-Hydrogen abstraction in CHCl(3) to yield beta-lactams with high enantioselectivity (e.r. approximately 90:10) in solution. The extent of enantioselectivity was found to be dependent on the reaction temperature.