Chih-Hsien Chen

2,3,4,5-Tetraphenylsilole-based conjugated polymers: synthesis, optical properties, and as sensors for explosive compounds.

A series of linear 2,5-tetraphenylsilole-vinylene-type polymers were successfully synthesized for the first time. The tetraphenylsilole moieties were linked at their 2,5-positions through a vinylene bridge with p-dialkoxybenzenes to obtain polymer PSVB and with 3,6-carbazole to obtain polymer PSVC. For comparison, 2,5-tetraphenylsilole-ethyne-type polymer PSEB was also synthesized, in which the vinylene bridge of PSVB was replaced with an ethyne bridge. Very interestingly, the bridging group (vinylene or ethyne) had a significant effect on the photophysical properties of the corresponding polymers. The fluorescence peak of PSEB at 504 nm in solution originated from the emission of its silole moieties, whereas PSVB and PSVC emitted yellow light and no blueish-green emission from the silole moieties was observed, thus demonstrating that the emissions of PSVB and PSVC were due to their polymer backbones. More importantly, the 2,5-tetraphenylsilole-ethyne polymer exhibited a pronounced aggregation-enhanced emission (AEE) effect but the 2,5-tetraphenylsilole-vinylene polymer was AEE-inactive. Moreover, both AEE-active 2,5-tetraphenylsilole-ethyne polymer and AEE-inactive 2,5-tetraphenylsilole-vinylene polymers were successfully applied as fluorescent chemosensors for the detection of explosive compounds.

Folding of Alternating Dialkylsilylene-Spaced Donor-Acceptor Copolymers: The Oligomer Approach

A series of oligmers with donor-acceptor pairs separated by diisopropylsilylene (iPr(2)Si) spacers, composed of monomer 4b, dimer 5, trimer 6, and tetramer 7, were synthesized to scrutinize the folding behavior. Monomer 4a with a dimethylsilylene (Me(2)Si) spacer was also prepared for comparison. The 4-aminostyrene moiety was used as the donor and the stilbene moiety as the acceptor. Both steady-state and time-resolved fluorescence spectroscopic measurement were made. Regardless of the substituents on the silicon atom, the emission spectra of 4a and 4b exhibit both local excited (LE) emission of the acceptor chromophore and emission from the charge-separated state (CT emission), which are similar to that of the corresponding Me(2)Si-spaced copolymer 2a with the same donor and acceptor chromophores, but different from that of the copolymer with the iPr(2)Si spacer 2b. Dimer 5 behaves like 4 and 2a. As the chain length of the oligomers increases, the emission properties of the higher homologues become prone to that of 2b. Thus, tetramer 7 exhibits emission from the charge-transfer complex, which is essentially same as that of 2b. Moreover, charge-transfer absorptions emerge in 6 and 7. These results suggest that the folding nature of oligomers approaches that of the corresponding polymer, as the degree of oligomerization increases, and the electronic interactions between adjacent donor-acceptor pairs are controlled by the steric effect of the substituents on the silicon atoms and concomitant amplification of the stabilizing energy by extending the distance of the folding structure.

Hydrogen-Bonding Induced Cooperative Effect on the Energy Transfer in Helical Polynorbornenes Appended with Porphyrin-Containing Amidic Alanine Linkers

Polynorbornenes appended with porphyrins containing a range of different linkers are synthesized. The use of bisamidic chiral alanine linkers between the pending porphyrins and the polymeric backbone has been shown to bring the adjacent porphyrin chromophores to more suitable orientation for exciton coupling owing to hydrogen bonding between the adjacent linkers. The hydrogen bonding between the adjacent pendants in these polymers may induce a cooperative effect and therefore render single-handed helical structures for these polymers. Such a cooperative effect is reflected in the enhancement of FRET efficiencies between zinc-porphyrin and free base porphyrin in random copolymers.

Coherently Aligned Porphyrin‐Appended Polynorbornenes

Six different kinds of coherently aligned porphyrin-appended polynorbornenes derived from 5,6-endo-fused N-arylpyrrolidenonorbornenes have been synthesized. Pi-pi interactions between the pendant groups are essential for dictating the photophysical properties of the polymers and the mechanism for the stereoselective formation of polymers. Splitting of the Soret band of polymers 2a-c, which have alkyl-substituted porphyrin pendant groups, suggests strong exciton coupling between chromophores. No splitting of the Soret band is observed for polymers 2d-f, which have tetraaryl substituents on the porphyrin moiety. Significant fluorescence quenching is found in polymers 2a-e, whereas only slightly reduced quantum yield is observed for 2f. Time-resolved fluorescence measurements also indicate a similar trend. The AFM image of 2d on graphite shows aggregation to form a two-dimensional, ordered pattern.

Controlling Conformations in Alternating Dialkylsilylene‐Spaced Donor–Acceptor Copolymers by a Cooperative Thorpe–Ingold Effect and Polymer Folding

A series of dialkylsilylene-spaced copolymers 6 and 7, which contain Me(2)Si and iPr(2)Si spacer groups, respectively, and have alternating donor and acceptor chromophores, have been designed and regioselectively synthesized by hydrosilylation. The ratio of the donor and acceptor chromophores for each repeat unit is 2:1, and the two donor chromophores are linked by a trimethylene bridge. A 4-aminostyrene moiety is used as the donor and a series of acceptor chromophores with different reduction potentials are employed. Both steady-state and kinetic measurements reveal that the photoinduced electron transfer (PET) in 6 obeyed the Marcus theory in which normal and inverted regions are observed. On the other hand, the iPr(2)Si-spaced copolymers 7 exhibit absorption and emission from the charge-transfer complexes exclusively due to ground-state interactions between the donor and acceptor chromophores. The discrepancy in photophysical behavior may have arisen from the difference in distance between the adjacent donor and acceptor chromophores. The bulkiness of the substituents on the silicon atom (i.e., Me versus iPr) may exert the Thorpe-Ingold effect on the local conformation around the silicon atom. The differences in the small energetic barriers for each of the conformational states may be amplified by extending the distance of the folding structure, which results in perturbing the conformation of the polymers. These results suggest that the electronic interactions between adjacent donor-acceptor pairs in these copolymers are controlled by the synchronization of the substitution effect and corresponding polymeric structures.

Supramolecular Porphyrin–DABCO Array in Single- and Double-Stranded Polynorbornenes†

Zinc-porphyrin-appended norbornene derivative 6, the corresponding dimer 7, coherently aligned single-stranded polynorbornene 5, and zinc-porphyrin-linked double-stranded polymeric ladderphane 3 have been used for complexation with 1,4-diazabicyclo[2.2.2]octane (DABCO) to generate an array of porphyrin-DABCO supramolecular scaffolds. The stoichiometries of the complex formation are analyzed in detail by (1)H NMR spectroscopy as well as absorption and emission spectroscopic methods. Bidentate ligand DABCO is found to form with 7, 5, and 3 to give 2-to-1 porphyrin-DABCO sandwich structures. Monomer 6, on the other hand, forms a 2-to-1 porphyrin-DABCO sandwich structure at high concentration (20 mM), but a 1-to-1 porphyrin-DABCO complex at low concentration (10(-6) M). Time-resolved fluorescence decays revealed a similar behavior for the 2-to-1 porphyrin-DABCO sandwich structures that are formed from 3, 5, and 7.

Influence of polymer conformations on the aggregation behaviour of alternating dialkylsilylene-[4,4′-divinyl(cyanostilbene)] copolymers

The influence of the substituents at silicon on the aggregation enhanced emission (AEE) of alternating dialkylsilylene-[4,4′-divinyl(cyanostilbene)] copolymers 3 and 4 is examined. The presence of the bulky isopropyl group in 3b significantly enhances the quantum yield upon aggregation. On the other hand, the emission intensity of the methyl-substituted polymer 3a is only slightly enhanced under the same conditions. The bulky isopropyl substituents on silicon may exert the Thorpe–Ingold effect on these copolymers resulting in conformation difference, which may dictate the nature of the aggregation between polymers, and hence, the photophysical behaviour of these polymers.

Combining light harvesting and electron transfer in silica-titania-based organic-inorganic hybrid materials.

A convenient protocol to fabricate an organic-inorganic hybrid system with covalently bound light-harvesting chromophores (stilbene and terphenylene-divinylene) and an electron acceptor (titanium oxide) is described. Efficient energy- and electron-transfer processes may take place in these systems. Covalent bonding between the acceptor chromophores and the titania/silica matrix would be important for electron transfer, whereas fluorescence resonant energy transfer (FRET) would strongly depend on the ratio of donor to acceptor chromophores. Time-resolved spectroscopy was employed to elucidate the detailed photophysical processes. The coupling of FRET and electron transfer was shown to work coherently to lead to photocurrent enhancement. The photocurrent responses reached a maximum when the hybrid-material thin film contained 60 % acceptor and 40 % donor.

Excimer Formation in a Confined Space: Photophysics of Ladderphanes with Tetraarylethylene Linkers

Communication between chromophores is vital for both natural and non-natural photophysical processes. Spatial confinements offer unique conditions to scrutinize such interactions. Polynorbornene- and polycyclobutene-based ladderphanes are ideal model compounds in which all tetraarylethylene (TAE) linkers are aligned coherently. The spans for each of the monomeric units in these ladderphanes are 4.5-5.5 Å. Monomers do not exhibit emission, because bond rotation in TAE can quench the excited-state energy. However, polymers emit at 493 nm (Φ=0.015) with large Stokes shift under ambient conditions and exhibit dual emission at 450 and 493 nm at 150 K. When the temperature is lowered, the emission intensity at 450 nm increases, whereas that at 493 nm decreases. At 100 K, both monomers and polymers emit only at 450 nm. This shorter-wavelength emission arises from the intrinsic emission of TAE chromophore, and the emission at 493 nm could be attributed to the excimer emission in the confined space of ladderphanes. The fast kinetics suggest diffusion-controlled formation of the excimer.

Database for retaining wall design

Abstract This paper provides details pertaining to computer integrated design systems for low height cantilever retaining walls. A database is created by modifying an existing design chart using numerical analysis. This database then represents a central pool for all the information involved in the modelling of a complete design. All of the operation information is under interactive graphic control. Using this system, one can obtain optimum dimensions for designs quickly, satisfying any predefined safety factors. Viewing the profile of a design at any stage in the process is another benefit to this system. In the case of an existing retaining wall structure, in order to meet assigned safety factors, this system can also evaluate the force required on anchors to reinforce the structure. Discussions on a sloped base design of a retaining wall are also included in this study.