Protein Induced Aggregation of Conjugated Polyelectrolytes Probed with Fluorescence Correlation Spectroscopy: Application to Protein Identification

The interaction of a series of water-soluble conjugated polyelectrolytes with varying backbone structure, charge type (cationic and anionic), and charge density with a set of seven different proteins is explored by using fluorescence correlation spectroscopy (FCS). The FCS method affords the diffusion time for a particular CPE/protein pair, and this diffusion time is a reflection of the aggregation state of the polymer/protein in the solution. The diffusion time is larger for oppositely charged CPE/protein combinations, reflecting the tendency toward the formation of CPE/protein aggregates in these systems. However, by careful analysis of the data, other factors emerge, including possible effects of hydrophobic interaction in specific CPE/protein systems. The final diffusion time for each CPE/protein mixture varies and the diffusion time response pattern created by the six-CPE array for a typical protein is unique, and this effect was leveraged to develop a sensor array for protein identification by using linear-discriminant analysis (LDA) methods. By application of multimode linear discrimination analysis, the unknown protein samples have been successfully identified with a total accuracy of 93%

Triplet Sensitization in an Anionic Poly(phenyleneethynylene) Conjugated Polyelectrolyte by Cationic Iridium Complexes

We describe a systematic study of triplet sensitization in a poly­(phenyleneethynylene) conjugated polyelectrolyte (CPE) in methanol solution by using a series of three cationic iridium complexes with varying triplet energy. The cationic iridium complexes bind to the anionic CPE by ion-pairing, leading to singlet state quenching of the polymer, and allowing for efficient back-transfer of triplet excitation energy to the polymer. Efficient (amplified quenching) of the polymer’s fluorescence is observed for each iridium complex, with Stern–Volmer quenching constants in excess of 10⁵ M^–1. Triplet sensitization is confirmed for two of the iridium complexes by monitoring the relative yield of the CPE triplet state by transient absorption spectroscopy. One of the iridium complexes does not sensitize the CPE triplet, and consideration of the energies of the three complexes allows us to bracket the triplet energy of the CPE within the range 1.95–2.26 eV

Photophysics and Nonlinear Absorption of Gold(I) and Platinum(II) Donor–Acceptor–Donor Chromophores

A series of Au­(I) and Pt­(II) acetylide complexes of a π-conjugated donor–acceptor–donor (D-A-D) chromophore were studied to develop quantitative structure–property relationships for their photophysical and nonlinear optical properties. The D-A-D chromophore consists of a “TBT” unit, where T = 3-hexyl-2,5-thienylene and BTD = 2,1,3-benzothiadiazole, capped with ethynylene groups. The D-A-D chromophore is functionalized with Au­(I)­PR₃ (R = −Me and −Ph) and <i>trans</i>-Pt­(II)­(PR₃)₂-CCPh (R = −Me and −Bu) “auxochromes”. All of the metal complexes were characterized by ground-state absorption, photoluminescence, nanosecond transient absorption, and two-photon absorption (2PA) spectroscopy. The experiments provided quantitative values of the photophysical parameters, including rates for radiative decay and intersystem crossing (ISC), triplet yields, and two-photon absorption cross sections. Pronounced solvatochromism in the fluorescence spectra suggests an enhanced dipole moment in the excited state of the complexes compared to the unmetalated TBT chromophore. The gold complexes feature larger fluorescence quantum yields and longer emission lifetimes compared to platinum. The Pt­(II) complexes exhibit enhanced triplet–triplet absorption, reduced triplet-state lifetimes, and larger singlet oxygen quantum yields, consistent with more efficient ISC compared to the Au­(I) complexes. When excited by 100 fs pulses, all of the D-A-D chromophores exhibit moderate two-photon absorption in the near-infrared between 700 and 900 nm. The 2PA cross section for the Au­(I) complexes is almost the same as the unmetalated D-A-D chromophore (∼100 GM). The Pt­(II) complexes exhibit significantly enhanced 2PA compared to the other chromophores, reaching 1000 GM at 750 nm. Taken together, the results indicate that the Pt­(II) center is considerably more effective in inducing singlet–triplet ISC and in enhancing the 2PA cross section. This result reveals the greater promise for Pt­(II) acetylides in chromophores for temporal and frequency agile nonlinear absorption

Biomimetic Light-Harvesting Antenna Based on the Self-Assembly of Conjugated Polyelectrolytes Embedded within Lipid Membranes

Here we report a biomimetic light-harvesting antenna based on negatively charged poly­(phenylene ethynylene) conjugated polyelectrolytes assembled within a positively charged lipid membrane scaffold constructed by the lipid 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP). Light harvested by the polymers was transferred <i>via</i> through-space mechanisms to a lipophilic energy acceptor (the cyanine dye DiI) whose effective molar absorption was enhanced by up to 18-fold due to the antenna effect. Absorption amplification of DiI was found to be due primarily to direct energy transfer from polymers. The efficiency of homoenergy transfer among polymers was next probed by the membrane embedding fullerene derivative phenyl-C₆₁-butryic acid methyl ester (PCBM) acting as an electron acceptor. PCBM was able to quench the emission of up to five polymers, consistent with a modest amount of homotransfer. The ability of the membrane to accommodate a high density of polymer donors without self-quenching was crucial to the success of electronic energy harvesting achieved. This work highlights the potential of lipid membranes as a platform to organize light-harvesting molecules on the nanoscale toward achieving efficient energy transfer to a target chromophore/trap

“Light Switch” Effect Upon Binding of Ru-dppz to Water-Soluble Conjugated Polyelectrolyte Dendrimers

We report the “light switch” effect of [Ru­(bpy)­₂­(dppz)]­²⁺ (where bpy = 2,2′-bipyridine and dppz = dipyrido­[3,2-a:2′,3′-c] phenazine, Ru-dppz) in the presence of anionic conjugated polyelectrolyte dendrimers (CPDs). The metal-to-ligand charge-transfer luminescence from Ru-dppz is efficient in the presence of CPD because the complex is shielded from water by binding to the hydrophobic dendrimer core

Effect of Oligomer Length on Photophysical Properties of Platinum Acetylide Donor–Acceptor–Donor Oligomers

We report a systematic study that explores how the triplet excited state is influenced by conjugation length in a series of benzothiadiazole units containing donor–acceptor–donor (DAD)-type platinum acetylide oligomers and polymer. The singlet and triplet excited states for the series were characterized by an array of photophysical methods including steady-state luminescence spectroscopy and femtosecond–nanosecond transient absorption spectroscopy. In addition to the experimental work, a computational study using density functional theory was conducted to gain more information about the structure, composition, and energies of the frontier molecular orbitals. It is observed that both the singlet and triplet excited states are mainly localized on a single donor–acceptor–donor unit in the oligomers. Interestingly, it is discovered that the intersystem crossing efficiency increases dramatically in the longer oligomers. The effect is attributed to an enhanced contribution of the heavy metal platinum in the frontier orbitals (HOMO and LUMO), an effect that leads to enhanced spin–orbit coupling

Intramolecular Triplet Energy Transfer in Anthracene-Based Platinum Acetylide Oligomers

Platinum acetylide oligomers that contain an anthracene moiety have been synthesized and subjected to photophysical characterization. Spectroscopic measurement and DFT calculations reveal that both the singlet and triplet energy levels of the anthracene segment are lower than those of the platinum acetylide segment. Thus, the platinum acetylide segment acts as a sensitizer to populate the triplet state of the anthrancene segment via intramolecular triplet–triplet energy transfer. The objective of this work is to understand the mechanisms of energy-transfer dynamics in these systems. Fluorescence quenching and the dominant triplet absorption that arises from the anthracene segment in the transient absorption spectrum of <b>Pt4An</b> give clear evidence that energy transfer adopts an indirect mechanism, which begins with singlet–triplet energy transfer from the anthracene segment to the platinum acetylide segment followed by triplet–triplet energy transfer to the anthracene segment

Triplet Energy Transport in Platinum-Acetylide Light Harvesting Arrays

Light harvesting and triplet energy transport is investigated in chromophore-functionalized polystyrene polymers featuring light harvesting and energy acceptor chromophores (traps) at varying loading. The series of precision polymers was constructed via reversible addition–fragmentation transfer polymerization and functionalized with platinum acetylide triplet chromophores by using an azide–alkyne “click” reaction. The polymers have narrow polydispersity and degree of polymerization ∼60. The chromophores have the general structure, <i>trans-</i>[−R–C₆H₄–CC–Pt­(PBu₃)₂–CC–Ar], where R is the attachment point to the polystyrene backbone and Ar is either –C₆H₄–CC–Ph or –pyrenyl (PE2-Pt and Py-Pt, respectively, with triplet energies of 2.35 and 1.88 eV). The polychromophores contain mainly the high-energy PE2-Pt units (light absorber and energy donor), with randomly distributed Py-Pt units (3–20% loading, energy acceptor). Photophysical methods are used to study the dynamics and efficiency of energy transport from the PE2-Pt to Py-Pt units in the polychromophores. The energy transfer efficiency is >90% for copolymers that contain 5% of the Py-Pt acceptor units. Time-resolved phosphorescence measurements combined with Monte Carlo exciton dynamics simulations suggest that the mechanism of exciton transport is exchange energy transfer hopping between PE2-Pt units

Conjugated Polyelectrolyte-Sensitized TiO₂ Solar Cells: Effects of Chain Length and Aggregation on Efficiency

Two sets of conjugated polyelectrolytes with different molecular weights (<i>M</i>_n) in each set were synthesized. All polymers feature the same conjugated backbone with alternating (1,4-phenylene) and (2,5-thienylene ethynylene) repeating units, but different linkages between the backbone and side chains, namely, oxy-methylene (-O-CH₂-) (P1-O-<i>n</i>, where <i>n</i> = 7, 9, and 14) and methylene (-CH₂-) (P2-C-<i>n</i>, <i>n</i> = 7, 12, and 18). They all bear carboxylic acid moieties as side chains, which bind strongly to titanium dioxide (TiO₂) nanoparticles. The two sets of polymers were used as light-harvesting materials in dye-sensitized solar cells. Despite the difference in molecular weight, polymers within each set have very similar light absorption properties. Interestingly, under the same working conditions, the overall cell efficiency of the P1-O-<i>n</i> series increases with a decreasing molecular weight while the efficiency of the P2-C-<i>n</i> series remains constant regardless of the molecular weight. Steady state photophysical measurements and dynamic light scattering investigation prove that P1-O-<i>n</i> polymers aggregate in solution while P2-C-<i>n</i> series are in the monomeric state. In P1-O-<i>n</i> series, a higher-molecular weight polymer results in a larger aggregate, which reduces the amount of polymers that are adsorbed onto TiO₂ films and overall cell efficiency

Phosphonium-Substituted Conjugated Polyelectrolytes Display Efficient Visible-Light-Induced Antibacterial Activity

We report the light-activated antibacterial activity of a new class of phosphonium (R-PMe3+)-substituted conjugated polyelectrolytes (CPEs). These polyelectrolytes feature a poly(phenylene ethynylene) (PPE) conjugated backbone substituted with side groups with the structure −O–(CH2)nPMe3+, where n = 3 or 6. The length of the side groups has an effect on the hydrophobic character of the CPEs and their propensity to interact with bacterial membranes. In a separate study, these phosphonium-substituted PPE CPEs were demonstrated to photosensitize singlet oxygen (1O2) and reactive oxygen species, a key factor for the photoinduced inactivation of bacteria. In this study, in vitro antibacterial assays against Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus were performed by employing the series of polyelectrolytes under both dark and illumination conditions. In general, the phosphonium-substituted CPEs displayed profound light-activated biocidal activity, with >99% colony forming unit (CFU) reduction after 15 min of light exposure (16 mW cm–2) at a ≤20 μM CPE concentration. Strong biocidal activity was also observed in the dark for a CPE concentration of 20 μM against S. aureus; however, higher concentrations (200 μM) were needed to enable dark inactivation of E. coli. The dark activity is ascribed to bacterial membrane disruption by the CPEs, supported by a correlation of dark biocidal activity with the chain length of the side groups. The light-activated biocidal activity is associated with the ability of the CPEs to sensitize ROS, which is cytotoxic to the microorganisms. Serial dilution bacterial plating experiments revealed that the series of CPEs was able to induce a >5-log kill versus E. coli with 15 min of exposure to a blue LED source (16 mW cm–2)