High stability of photoinduced merocyanine in naphthopyran-doped polyvinylpyrrolidone electrospun nanofibers

Intramolecular excimers of open forms of 2H-benzopyran, 2H- and 3H-naphthopyrans in solution: TD-DFT/DFT analysis

We hypothesized that in the open forms of diphenyl-substituted photochromic compounds, immediately after the photoinduced cleavage of the C-O bond, one of the phenyl rings forms a stack with an aromatic system at the other end of the alkyl linker. The formation of these intramolecular stacking excimers is made possible by a known increase in the rotational mobility of the linker double bonds in the excited state. The flexibilities of linkers are analyzed in terms of changes in their dihedral angles. After internal conversion, stacking is partially broken but it still prevents the recovery of the C-O bond. This explains the significantly greater stability of the open forms of diphenyl-substituted benzopyran and naphthopyran compared to dimethyl-substituted ones. Open structures with residual stacking are intermediate and by thermal motion, they transform into stable trans-trans and cis-trans forms. However, in our opinion, they significantly affect the equilibrium of closed and open isomers in solution. In particular, the calculated stacking energy of 2H-naphthopyran turned out to be higher than that of 3H-naphthopyran, which partly explains the significantly greater stability of the open form of the former. For the DFT/TD-DFT calculations performed in this work, the following functionals were selected to provide reliable stacking for the open forms of all three studied compounds: APFD, M052X, M06HF, as well as B3LYP with D3(BJ) Grimme dispersion correction. The 6-311++G(d,p) basis set and cyclohexane solvent modeled using IEFPCM were also used.

Control of the Photo-Isomerization Mechanism in 3H-Naphthopyrans to Prevent Formation of Unwanted Long-Lived Photoproducts

In the photochromic reactions of 3H-naphthopyrans, two colored isomers TC (transoid-cis) and TT (transoid-trans) are formed. In terms of optimized photo-switchable materials, synthetic efforts are nowadays evolving toward developing 3H-naphthopyran derivatives that would not be able to photoproduce the long-living transoid-trans, TT, photoproduct. The substitution with a methoxy group at position 10 results in significant reduction of the TT isomer formation yield. The TC photophysics responsible for TT suppression were revealed here using a combination of multi-scale time resolved absorption UV-vis spectroscopy and ab initio calculations. The substitution changes the TC excited-state potential energy landscape, the bicycle-pedal isomerization path is favored over the rotation around a single double bond. The bicycle-pedal path is aborted in halfway to TT formation due to S₁→S₀ internal conversion populating back the TC species in the ground electronic state. This is validated by a shorter TC S₁ state lifetime for methoxy derivative in comparison to that of the parent-unsubstituted compound (0.47 ± 0.05 ps vs. 0.87 ± 0.09 ps) in cyclohexane.

Photochromic reaction in 3H-naphthopyrans studied by vibrational spectroscopy and quantum chemical calculations

Structural details on the species involved in the photochromic reaction of 3H-naphthopyrans in solution have been formerly determined using NMR spectroscopy. Herein we show that at room temperature time-resolved FT-IR spectroscopy is a simple and efficient tool for structural characterization of colored species generated upon continuous UV light irradiation of the model compound 3H-naphthopyran: 3,3-diphenyl-3H-naphtho[2,1-b]pyran. In solution and in the polymer matrix phase, a colored species transoid-cis is formed after a single-photon excitation process, while transoid-trans is a secondary long-lived photoproduct generated after two-step excitation involving two photons. Understanding the reaction mechanism leading to long-lived colored species can help with the design of new 3H-naphthopyran derivatives structurally optimized for making a photochromic reaction free from transoid-trans products, which is often important for applications. Ab initio calculations show that photoinduced ring-opening followed by isomerization occurs on a multidimensional potential-energy surface. The barriers separating the considered isomeric forms, both in the ground and in the excited state, help to interpret the step-by-step dynamics of the photoprocesses. The system is composed of a variety of ground state equilibrium forms. Each of them is characterized by fast excited-state deactivation pathways which may drive the system through different conical intersection regions.

Porous Aromatic Framework Modified Electrospun Fiber Membrane as a Highly Efficient and Reusable Adsorbent for Pharmaceuticals and Personal Care Products Removal

Water contamination by emerging organic pollutants, such as pharmaceuticals and personal care products (PPCPs), is becoming more and more serious. Porous aromatic frameworks (PAFs) are considered as promising adsorbents to remove the PPCPs. To overcome the limitation of PAFs in their powder forms for large-scale applications, herein, we proposed a strategy to covalently anchor PAFs onto electrospun polymer fiber membranes. Polyaniline (PANI) played the role of aromatic seed layer, which was coated on the electrospun polyacrylonitrile (PAN) fiber membrane first. Then, PAF-45 modification was in situ synthesized in the presence of the PANI-coated electrospun PAN fiber membrane. This study could make the PAF-based materials be handled more easily and improve the surface area of electrospun fiber membrane. The obtained composite adsorbent (PAF-45-PP FM) was applied for the adsorption of three PPCPs: ibuprofen (IBPF), chloroxylenol (CLXN), and N, N-diethyl-meta-toluamide (DEET), which exhibited high adsorption capacity and good recycling ability. According to the Langmuir model, the maximum adsorption capacities of PAF-45-PP FM toward IBPF, CLXN and DEET were 613.50, 429.18, and 384.61 mg/g, respectively. In addition, after ten adsorption-desorption cycles, the adsorption capacities toward the three PPCPs decreased slightly. Through an adsorption comparison test, the adsorption capacity of PAF-45-PP FM almost attributed to the loading PAF-45. The adsorption mechanism analysis illustrated that there were pore capture, hydrophobic interaction and π-π interaction between PPCPs and PAF-45-PP FM. Therefore, the PAF-45-PP FM can be potential adsorbents to purify water contaminated with PPCPs.

Preparation and photochromic properties of layer-by-layer self-assembly films and light-responsive micelles based on amphiphilic naphthopyran derivative

An amphiphilic naphthopyran derivative (DSNP) with negative-charged di-sulfatoethoxy was first designed and synthesized. DSNP was used to prepare the LBL self-assembly films with cationic quaternized poly(4-vinylpyridine) (P4VPQ). The multilayer assembly process was monitored via UV-Vis spectra and the DSNP displayed a significant slow fading rate in film. In addition, DSNP was also used as the light-responsive group to fabricate light-responsive micelles with polyethylene glycol-triethylamine bromide (PEG-NEt3). The photoisomerization of naphthopyran moieties can rapidly and reversibly tune the disassembly and re-assembly of the micelles. The changes of fluorescent spectra of Nile Red (NR) in water solution of polymeric micelles demonstrated that the polymeric micelle can be used as nanocarriers to encapsulate, release and re-encapsulate guest solutes on demand controlling of light irradiation. The results indicate that the amphiphilic naphthopyran can be used in both optical-switches and biomedical area for drug delivery.