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The journal of chemical physics 155(23), 234304 - (2021). doi:10.1063/5.0072785, Pyrene-based molecules are inclined to form excimers through self-association upon photoexcitation. In this work, the pyrene core is functionalized with the N-methylacetamide group at the position 1 or 2 to develop pyren-1-methylacetamide (PyMA1) and pyren-2-methylacetamide (PyMA2), respectively. Upon photoexcitation with 345 nm, a portion of molecules in PyMA1 and PyMA2 solutions at ���1.0 mM have formed static excimers. The steady state spectroscopic measurements suggest that, whether it is the dimerization of molecules in the ground state (GS) or in excimer formation, characteristic signs are more pronounced in PyMA1 than its isomeric counterpart, PyMA2. The shift of the excimer band in their respective emission spectra suggests that the extent of overlap in $�������$ stacking is greater for PyMA1 than for PyMA2 in the excited state. The optimized geometry of dimers in toluene shows that the overlapping area between the pyrene moieties in $�������$ stacking between the dimers is greater for PyMA1 than for PyMA2 in GS. The natural bond orbital analysis with the optimized GS geometries shows that the stabilization/interaction energy between the dimers in $�������$ stacking is higher in PyMA1 compared to PyMA2 in toluene. The transient absorption (TA) measurements in toluene over the fs���ps regime (fs-TA) showed that the formation of static excimers with pre-associated dimers in PyMA1 happens in ���700 fs whereas the excimers for the pre-associated dimers in PyMA2 have formed in slightly slower time scale (���1.95 ps). Contrary to what was observed in solution, the extent of overlap in $�������$ stacking is lower for PyMA1 dimers (���17%) than for PyMA2 dimers (���37%) in single crystals., Published by American Institute of Physics, Melville, NY

The preparation and crystal structure of six molecular compounds consist of light-stable host molecules [(1-cyclohexyl-4-hydroxybenzene, 1,1,6,6-tetraphenyl-2,4-hexadiyne-1,6-diol, 4-((10-[4-(ethoxycarbonyl)phenoxy]decyl)oxy)benzene, 1,2,4,5-benzenetetracarboxylic acid, resorcinol] and light-sensitive guest molecules [(5-chloro-2(1H)-pyridone or 5-nitro-2(1H)-pyridone] are described in light of their failure to undergo [4 + 4] photodimerization in the solid-state upon irradiation with UV light. Unlike in many cases were pyridone and its derivatives undergo such dimerization in the solid-state due to the packing arrangement in the crystalline form in which the geometric requirement for photodimerization meet, the compounds described here failed to pack in ways to enable such photodimerization. The distances between the potentially reactive atoms ranged from 4.620 to 8.408 A which are too long to react.

[4 + 4] Photodimerization of isoquinolin-3(2H)-one (isoquinolinone) (1) may yield 12 different isomers depending on the relative geometry of the two monomers prior to the reaction. Irradiation of the solid neat compound shows that of all possible dimers only the dimer between the two hetero-rings having an inversion center is produced. On the other hand, exposure to UV light of solid molecular compounds composed of isoquinolinone as the guest molecule and different host molecules shows to produce few other isomers of the dimer. The use of different host molecules affects the packing of the molecules in the crystal. As a result, the relative geometries between two monomer molecules are varied enabling photochemical dimerization to yield different isomers. When 1,1,6,6-tetraphenyl-2,4-hexadiyne-1,6-diol (I) is used as the host molecule, two polymorphic forms of the molecular compound are obtained. Irradiation of single-crystal of the two polymorphs yields a mixture of isomers of the dimer. When the host molecule is cyclohexanol-1,2,4,5-tetracarboxylic acid (II) three isomers are expected, and when the host molecule is 1,3-benzenediol (III) a single isomer is formed which is identical to that obtained from the neat isoquinolinone.

The [4 + 4] photodimerization of 1,4-dimethyl-2-pyridinone (A) in its molecular compound with 1,1,6,6-tetraphenyl-2,4-hexadiyne-1,6-diol (I) was investigated by irradiation of a single crystal of the compound. The conversion to the product in a single-crystal to single-crystal transformation enabled the determination of its crystal structure after each exposure cycle of 10-20 min with a pulsed 355 nm laser light. The unit cell changes as a function of the conversion were monitored and showed monotonic changes. The kinetics of the reaction was studied at two different temperatures (230 and 280 K) and revealed a sigmoidal behavior that could be explained by the JMAK model for crystal growth with a mechanism that is intermediate between random distribution of products in the crystal and the existence of growing nuclei. The Arrhenius plot provided calculated activation energy of 32.5 kJ/mol.

The Cambridge Crystallographic Data Center provides valuable data for analysis and detection of compounds with potential to undergo photodimerization in the solid-state. Such analysis is presented for anthracene derivatives, and showed that there are many compounds that their packing in the crystal meets the requirement needed for [4 + 4] photodimerization in the solid-state. The crystal structures of various neat acyl anthracenes before and after exposure to UV light are described and the molecular structures of the dimers are compared with those obtained by exposing molecular compounds possessing a light-stable host (1,1,6,6-tetraphenyl-2,4-hexadiyne-1,6-diol) and light-sensitive acyl anthracenes. While exposing neat compounds to UV light causes disintegration of the single crystal, the reaction in the molecular compounds proceeds in a single-crystal to single-crystal mode until a certain rate of conversion, and then disintegrates.