Predicting the Outcome of Photocyclisation Reactions: A Joint Experimental and Computational Investigation

Photochemical oxidative cyclodehydrogenation reactions are a versatile class of aromatic ring‐forming reactions. They are tolerant to functional group substitution and heteroatom inclusion, so can be used to form a diverse range of extended polyaromatic systems by fusing existing ring substituents. However, despite their undoubted synthetic utility, there are no existing models—computational or heuristic—that predict the outcome of photocyclisation reactions across all possible classes of reactants. This can be traced back to the fact that “negative” results are rarely published in the synthetic literature and the lack of a general conceptual framework for understanding how photoexcitation affects reactivity. In this work, we address both of these issues. We present experimental data for a series of aromatically substituted pyrroles and indoles, and show that quantifying induced atomic forces upon photoexcitation provides a powerful predictive model for determining whether a given reactant will photoplanarise and hence proceed to photocyclised product under appropriate reaction conditions. The propensity of a molecule to photoplanarise is related to localised changes in charge distribution around the putative forming ring upon photoexcitation. This is promoted by asymmetry in molecular structures and/or charge distributions, inclusion of heteroatoms and ethylene bridging and well‐separated or isolated photocyclisation sites.     

Thermally Induced Synthesis of Anthracene-, Pyrene- and Naphthalene-Fused Porphyrins

The synthesis of π-extended porphyrins containing anthracenyl moieties still represents an important challenge. Here, we report on the synthesis of a series of unsubstituted naphthyl-, pyrenyl- and anthracenyl-fused zinc porphyrin derivatives. To this aim, meso-substitued porphyrins are synthesized and the fusion of the PAHs (Polycyclic Aromatic Hydrocarbon) on the β-positions are performed through thermally induced dehydro-aromatization. The fused zinc-porphyrin derivatives are fully characterized and their optical absorption and photoluminescence properties are reported. We also demonstrate that zinc can be removed from the porphyrin core, giving rise to pure C, H, N materials. This work constitutes the first step towards the synthesis of the fully-fused tetra-anthracenylporphyrin.     

矩形多环芳烃的合成

本文用3,4-二苯基-2,5-二(3,5-二溴苯基)环戊二烯酮(4a)与二苯乙炔(5a)通过Diels-Alder环加成反应得到1,2,4,5-四苯基-3,6-二(3,5-二溴苯基)苯(6a)。化合物6a通过经典的Suzuki偶联反应得到1,2,4,5-四苯基-3,6-二(3,5-二(4-十二烷基噻吩))苯基苯(8a),再利用Fe Cl3作为氧化剂发生Scholl氧化脱氢关环反应,得到目标化合物1a。采用类似合成方法,得到目标化合物1b。化合物的结构均通过1H NMR和MALDI-TOF MS表征,并对其光谱特征、热性能及电学性能进行了初步研究。     

Benzo‐Fused Double [7]Carbohelicene: Synthesis,Structures, and Physicochemical Properties

A benzo‐fused double [7]carbohelicene (D7H) was synthesized through a regioselective cyclodehydrogenation of a tetranaphthyl‐p ‐terphenyl‐based precursor. The twisted (D7H‐1) and anti ‐folded (D7H‐2) conformers of D7H were separated by recrystallization, and their double helicene structures with overlapping terminal benzene rings were unambiguously elucidated by X‐ray crystallography. A record‐high isomerization barrier (46.0 kcal mol⁻¹) in double helicenes was estimated based on density functional theory (DFT) calculation, which resulted in the excellent conformational stability of D7H. The physicochemical properties of D7H‐1 and D7H‐2 were investigated by UV/Vis absorption spectroscopy and cyclic voltammetry, displaying the variation of electronic structure upon conformational changes. The optical resolution of the racemic D7H‐1 was carried out by chiral HPLC, offering enantiopure D7H‐1‐(P ,P ) and D7H‐1‐(M ,M ), which were further characterized by circular dichroism spectroscopy.     

Current-voltage characteristics of a homologous series of polycyclic aromatic hydrocarbons

A novel alkyl-substituted polycyclic aromatic hydrocarbon (PAH) with D(2h) symmetry and 78 carbon atoms in the aromatic core (C78) was synthesized, thereby completing a homologous series of soluble PAH compounds with increasing size of the aromatic pi system (42, 60, and 78 carbon atoms). The optical band gaps were determined by UV/Vis and fluorescence spectroscopy in solution. Scanning tunneling microscopy (STM) and spectroscopy (STS) revealed diode-like current versus voltage (I-V) characteristics through individual aromatic cores in monolayers at the interface between the solution and the basal plane of graphite. The asymmetry of the current-voltage (I-V) characteristics increases with the increasing size of the aromatic core, and the concomitantly decreasing HOMO-LUMO gap. This is attributed to resonant tunneling through the HOMO of the adsorbed molecule, and an asymmetric position of the molecular species in the tunnel junction. Consistently, submolecularly resolved STM images at negative substrate bias are in good agreement with the calculated pattern for the electron densities of the HOMOs. The analysis provides the basis for tailoring rectification with a single molecule in an STM junction.     

Migratory Shift in Oxidative Cyclodehydrogenation Reaction of Tetraphenylethylenes Containing Electron‐Rich THDTAP Moiety

Four tetraphenylethylenes ( 2 a – d ) containing an electron‐rich 2,3,4,6‐tetrahydro‐1,6‐dithia‐3a‐azaphenalene (THDTAP) moiety have been synthesized. The 2 a – d show aggregation‐induced emission (AIE) with yellowish green photoluminescence (PL) in THF‐H₂O (v/v, 1:9) solution and in the solid state. Compounds 2 a – d undergo 1,2‐migratory shift in oxidative cyclodehydrogenation reactions to afford the unexpected products 3 a – d which display green PL in CH₂Cl₂ solution and are non‐emissive in the solid state. The PL intensities of 3 a – d are clearly enhanced in the presence of meta‐chloroperoxybenzoic acid (mCPBA) owing to the oxidation of the S‐atoms on the THDTAP moiety. In contrast, the PL of 2 a – d in THF‐H₂O (v/v, 1:9) solution is quenched by adding mCPBA, ascribable to the oxidation of the C=C bond on the ethylene moiety. It is found that the absorption of 3 a – d is distinctly red‐shifted from the UV/Vis region to the NIR region upon acidification, arising from the protonation of the N‐atom on the THDTAP moiety. Furthermore, 3 a – d display nonlinear optical response (NLO) and optical limiting (OL) behaviour which is superior to that of the well‐known OL material C₆₀.     

Structurally characterized hetero-oligopolyphenylenes: synthetic advances toward next-generation heterosuperbenzenes

The successful Diels-Alder [2+4] cycloaddition of dipyrimidyl acetylene and suitably substituted 2,3,4,5-tetraarylcyclopenta-2,4-dien-1-ones (3-7) generates a series of selectively functionalized hexaarylbenzenes. Each has two pairs of peripheral functional groups (R' and R=tert-butyl 8 and R=methyl 9, methoxy 10, bromo 11, triisopropylsilylethynyl 12) and four ortho-imine nitrogen atoms. The dibromo derivative 11 is a useful precursor for the formation of a mono ethynyl 13 and diethynyl 14 substituted polyphenylene. Changing the dienophile to di(2-thienyl)acetylene gives an S-heteroatom polyphenylene 15. The compounds were fully characterized by using (1)H, (13)C and a range of 2 D NMR spectroscopic techniques, elemental analysis, and mass spectrometry. Oxidative cyclodehydrogenation of dimethoxy hexaphenylbenzene 10 by using iron(III) chloride results in the formation of a spirocyclic dienone 16, which in a separate reaction undergoes dienone/phenol rearrangement to give the first 4-fused-ring, N-heterosuperbenzene (HSB) 17. Six single crystal molecular structures reveal the commonality of unidirectional twisting of the external aromatic rings in these heteroatom polyphenylenes. The twist angles and any H-bonding or interdigitation in these structures are discussed.