BN Doping and the Photochemistry of Polyaromatic Hydrocarbons: Photocyclization of Hexaphenyl Benzene and Hexaphenyl Borazine

Photoredox Annulation of Polycyclic Aromatic Hydrocarbons

The rise of interest in using polycyclic aromatic hydrocarbons (PAHs) and molecular graphenoids in optoelectronics has recently stimulated the growth of modern synthetic methodologies giving access to intramolecular aryl-aryl couplings. Here, we show that a radical-based annulation protocol allows expansion of the planarization approaches to prepare functionalized molecular graphenoids. The enabler of this reaction is peri-xanthenoxanthene, the photocatalyst which undergoes photoinduced single electron transfer with an ortho-oligoarylenyl precursor bearing electron-withdrawing and nucleofuge groups. Dissociative electron transfer enables the formation of persistent aryl radical intermediates, the latter undergoing intramolecular C-C bond formation, allowing the planarization reaction to occur. The reaction conditions are mild and compatible with various electron-withdrawing and -donating substituents on the aryl rings as well as heterocycles and PAHs. The method could be applied to induce double annulation reactions, allowing the synthesis of π-extended scaffolds with different edge peripheries.

Multiphoton Control of 6π Photocyclization via State-Dependent Reactant-Product Correlations

Multiphoton excitation promises opportunities for opening new photochemical reaction pathways and controlling photoproduct distributions. We demonstrate photonic control of the 6π photocyclization of ortho-terphenyl to make 4a,4b-dihydrotriphenylene (DHT). Using pump-repump-probe spectroscopy we show that 1 + 1' excitation to a high-lying reactant electronic state generates a metastable species characterized by a red absorption feature that accompanies a repump-induced depletion in the one-photon trans-dihydro product (trans-DHT); signatures of the new photoproduct are clearer for a structural analogue of the reactant that is sterically inhibited against one-photon cyclization. Quantum-chemical computations support assignment of this species to cis-DHT, which is accessible photochemically along a disrotatory coordinate from high-lying electronic states reached by 1 + 1' excitation. We use time-resolved spectroscopy to track photochemical dynamics producing cis-DHT. In total, we demonstrate that selective multiphoton excitation opens a new photoreaction channel in these photocyclizing reactants by taking advantage of state-dependent correlations between reactant and product electronic states.

Computational Insights into the Role of External and Local Electric Fields in Macrocyclic Chemical and Biological Systems

The investigation of the role of the electric field in systems of widespread interest employing computational techniques is an emerging area of research. The outcome of applying an oriented external electric field (OEEF) on the geometric and electronic properties of the chemically unique π-conjugated cyclic carbon ring compounds has been explored with density functional theory (DFT). Distinct changes in the structural and electronic features of such ring compounds are observed upon the application of OEEFs. Importantly, the calculations indicate that a mixed aliphatic-aromatic conjugated ring converts from a singlet to a triplet after the application of an OEEF, suggesting potential applications in optoelectronics for such molecules, without the need for photochemically induced change in the spin state. Furthermore, the influence of built-in local electric fields (LEFs) present in naturally occurring macrocyclic systems such as valinomycin has also been explored. Static and ab initio molecular dynamics (AIMD) calculations indicate that LEFs are the primary driving factor in determining the energetically favoured position of counter anions such as chloride (Cl^- ) in the potassium (K⁺ ) and sodium (Na⁺ ) coordinated valinomycin macrocycle structures: they exist inside the cage in the case of K⁺ sequestration by valinomycin and outside for Na⁺ . This divergence has been proposed to be the determining factor for the selectivity of the valinomycin macrocycle for binding a K⁺ cation over Na⁺ .

Ultrafast Pump-Repump-Probe Photochemical Hole Burning as a Probe of Excited-State Reaction Pathway Branching

We demonstrate pump-repump-probe (PRP) transient hole burning as a spectroscopic tool for differentiating reactive from nonreactive deactivation of excited photochemical reactants observed by transient absorption spectroscopy (TAS). This method utilizes a time-delayed, wavelength-tunable ultrafast pulse to alter the excited reactant population, with the impact of "repumping" quantified through depletions in photoproduct absorption. We apply this approach to characterize dynamics affecting the nonadiabatic photocyclization efficiency to form S₀ dihydrotriphenylene (DHT) following 266 nm excitation of ortho-terphenyl (OTP). TAS studies revealed bimodal deactivation of OTP*, but neither relaxation time scale (700 fs and 3.0 ps) could be assigned unambiguously to DHT formation due to overlap of excited-state and product spectra. PRP studies reveal that S₁ OTP only cyclizes on the slower of these time scales, with the faster process attributable to nonreactive deactivation. We demonstrate that this method offers greater photochemical insights without assuming models to globally fit spectral transients collected by TAS.