Structural Control of Nonadiabatic Bond Formation: The Photochemical Formation and Stability of Substituted 4a,4b-Dihydrotriphenylenes

An AIEE-active Triphenylethylene Derivative with Photoresponsive Character for Latent Fingerprints Detection via a Simple Soaking Method

Latent fingerprints (LFPs) is one of the most important physical evidence in the criminal scene, playing an important role in forensic investigations. Therefore, developing highly sensitive and convenient materials for the visualization of LFPs is of great significance. We designed and synthesized an organic fluorescent molecule TP-PH with aggregation-induced enhanced emission (AIEE) activity. By simply soaking, blue fluorescent images with high contrast and resolution are readily developed on various surfaces including tinfoil, steel, glass and plastic. Remarkably, LFPs can be visualized within 5 min including the first-, second- and tertiary-level details. In addition, TP-PH exhibits interesting photoactivated fluorescence enhancement properties. Under irradiation of 365 nm UV light with a power density of 382 mW/cm2, the fluorescence quantum yield displays approximately 21.5-fold enhancement. Mechanism studies reveals that the photoactivated fluorescence is attributed to the irreversible cyclodehydrogenation reactions under UV irradiation. This work provides a guideline for the design of multifunctional AIEE fluorescent materials.

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.

Aerobic and Ligand-Free Manganese-Catalyzed Homocoupling of Arenes or Aryl Halides via in Situ Formation of Aryllithiums

Ligand-free manganese-catalyzed homocoupling of arenes or aryl halides can be carried out under aerobic conditions via the in situ formation of the corresponding aryllithiums. A wide range of biaryls and derivatives has been obtained, and a mechanism involving monomeric manganese-oxo complexes has been proposed on the basis of DFT calculations.

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.

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

Boron-nitrogen doping of polyaromatic hydrocarbon (PAH) materials can be used to tune their electronic properties while preserving the structural characteristics of pure hydrocarbons. Many multicycle PAHs can be synthesized photochemically; in contrast, very little is known about the photochemistry of their BN-doped counterparts. We present results of fs, ns, and μs time-resolved spectroscopic studies on the photoinduced dynamics of hexaphenyl benzene and hexaphenyl borazine in order to examine how BN doping alters photochemical C-C bond formation via 6π electrocyclization as well as the stability of resulting cyclized structures. Ultrafast measurements reveal photoinduced behaviors reflecting differences in excited-state decay pathways for the two molecules, with hexaphenyl borazine relaxing from its excited state with a rate that is 2 orders of magnitude faster than that of hexaphenyl benzene (3.0 vs 428 ps). Tetraphenyl dihydrotriphenylene generated from hexaphenyl benzene is observed to reopen with a ∼2 μs lifetime controlled by entropic stabilization of the cyclized structure; in contrast, photoinduced dynamics appear to be complete within 100 ps after excitation of hexaphenyl borazine. This significant difference in photochemical dynamics is reflected in the cyclodehydrogentation yields obtained for the two reactants (25 vs 0% for hexaphenyl benzene and borazine, respectively). Quantum-chemical computations predict that BN doping gives rise to energetic destabilization and increased singlet diradical character in cyclized structures. These findings indicate that the polarized BN bonds of the borazine core adversely impact photochemical bond formation relative to analogous hydrocarbons.

Excited-State Deactivation Pathways and the Photocyclization of BN-Doped Polyaromatics

Boron-nitrogen doping of polyaromatic hydrocarbons (PAH), such as borazine-core hexabenzocoronene, presents possibilities for tuning the properties of organic electronics and nanographene materials while preserving structural characteristics of pure hydrocarbons. Previous photochemical studies have demonstrated extension of a borazine-core PAH network (1,2:3,4:5,6-tris(o,o'-biphenylylene)borazine, 1) by photoinduced cyclodehydrogenation. We present steady-state and femtosecond-to-microsecond resolved spectroscopic studies of the photophysics of 1 and a related borazine-core PAH in order to characterize competing excited-state relaxation pathways that determine the efficacy of bond formation by photocyclization. Transient spectra evolve on time scales consistent with S₁ fluorescence lifetimes (1-3 ns) to features that persist onto microsecond time scales. Nanosecond-resolved oxygen-quenching measurements reveal that long-lived metastable states are triplets rather than cyclized products. Determination of fluorescence and triplet quantum yields reveal that photochemical bond formation is a minor channel in the relaxation of 1 (∼5% or less), whereas highly efficient fluorescence and intersystem crossing result in negligible photoinduced bond formation in more extended borazine-core networks. Results of computational investigations at the RICC2 level reveal sizable barriers to cyclization on the S₁ potential energy surfaces consistent with quantum yields deduced from experiment. Together these barriers and competing photophysical pathways limit the efficiency of photochemical synthesis of BN-doped polyaromatics.

Excited-State Decay Paths in Tetraphenylethene Derivatives

The photophysical properties of tetraphenylethene (TPE) compounds may differ widely depending on the substitution pattern, for example, with regard to the fluorescence quantum yield ϕ_f and the propensity to exhibit aggregation-induced emission (AIE). We report combined electronic structure calculations and nonadiabatic dynamics simulations to study the excited-state decay mechanisms of two TPE derivatives with four methyl substituents, either in the meta position (TPE-4mM, ϕ_f = 0.1%) or in the ortho position (TPE-4oM, ϕ_f = 64.3%). In both cases, two excited-state decay pathways may be relevant, namely, photoisomerization around the central ethylenic double bond and photocyclization involving two adjacent phenyl rings. In TPE-4mM, the barrierless S₁ cyclization is favored; it is responsible for the ultralow fluorescence quantum yield observed experimentally. In TPE-4oM, both the S₁ photocyclization and photoisomerization paths are blocked by non-negligible barriers, and fluorescence is thus feasible. Nonadiabatic dynamics simulations with more than 1000 surface hopping trajectories show ultrafast cyclization upon photoexcitation of TPE-4mM, whereas TPE-4oM remains unreactive during the 1 ps simulations. We discuss the chances for spectroscopic detection of the postulated cyclic photoproduct of TPE-4mM and the relevance of our findings for the AIE process.

Combined aggregation induced emission (AIE), photochromism and photoresponsive wettability in simple dichloro-substituted triphenylethylene derivatives

A dichloro-substituted triphenylethylene derivative (TrPECl2) with aggregation-induced emission (AIE), photochromism and photoresponsive wettability has been synthesized. The new compound shows fast-response photochromic behaviour with good ON/OFF repeatability by utilizing a proposed stilbene-type intramolecular photocyclization in the solid state. Compared with the more usual diphenylethylene derivatives, the photochromic properties of the triphenylethylene derivative are much more striking and easier to achieve. The triphenylethylene derivative also displays AIE properties leading to strong fluorescence in the solid state. Therefore, both the ultraviolet-visible absorption and fluorescence emission are drastically changed during the photochromic processes. Furthermore, the morphology of the TrPECl2 microcrystalline surface could be controlled by irradiation. The wettability of the surface could be drastically decreased with contact angles of a water droplet changing from 73° to 118°. The triphenylethylene derivative with a simple molecular structure is, therefore, attractive for multifunctional materials.