Effect of Varying Thiophene Units on Charge‐Transport and Photovoltaic Properties of Poly(phenylene ethynylene)‐alt‐poly(phenylene vinylene) Polymers

Poly(phenylene ethynylene)‐alt‐poly(phenylene vinylene)s (PPE‐PPVs) with various thiophene units (thiophene, bithiophene, and 3,4‐ethylenedioxythiophene) at the X position, with the general backbone design (Ph? C?C? X ? C?C? Ph? CH?CH? Ph? CH?CH? ), bearing identical solubilizing side chains at the phenylene rings of the polymers, are synthesized to study the effect of this structural alteration on the properties such as the photophysics, the electrochemical properties, the charge‐carrier mobility, and the morphology of the materials and its impact on their photovoltaic performance. The polymers are obtained in good yields with reasonable molecular weights and show solubility in ordinary organic solvents required for solution‐processing applications. The polymer with a basic thiophene ring at the X positions shows the highest open‐circuit voltage (V_OC of 930 mV) and the polymer with a bithiophene unit at the X position shows the highest short‐circuit current density and charge‐carrier mobility, whereas the polymer with 3,4‐ethylenedioxythiophene shows the lowest photovoltaic performance.

     

Charge separation and charge delocalization identified in long-living states of photoexcited DNA

Base stacking in DNA is related to long-living excited states whose molecular nature is still under debate. To elucidate the molecular background we study well-defined oligonucleotides with natural bases, which allow selective UV excitation of one single base in the strand. IR probing in the picosecond regime enables us to dissect the contribution of different single bases to the excited state. All investigated oligonucleotides show long-living states on the 100-ps time scale, which are not observable in a mixture of single bases. The fraction of these states is well correlated with the stacking probabilities and reaches values up to 0.4. The long-living states show characteristic absorbance bands that can be assigned to charge-transfer states by comparing them to marker bands of radical cation and anion spectra. The charge separation is directed by the redox potential of the involved bases and thus controlled by the sequence. The spatial dimension of this charge separation was investigated in longer oligonucleotides, where bridging sequences separate the excited base from a sensor base with a characteristic marker band. After excitation we observe a bleach of all involved bases. The contribution of the sensor base is observable even if the bridge is composed of several bases. This result can be explained by a charge delocalization along a well-stacked domain in the strand. The presence of charged radicals in DNA strands after light absorption may cause reactions—oxidative or reductive damage—currently not considered in DNA photochemistry.DNA photophysics is crucial for the understanding of light-induced damage of the genetic code (1). The excited state of single DNA bases is known to decay extremely fast on the subpicosecond time scale, predominantly via internal conversion (2, 3). This ultrafast decay is assumed to suppress destructive decay channels, thereby protecting the DNA from photodamage and avoiding disintegration of the genetic information. In contrast to this ultrafast deactivation of single nucleobases, the biological relevant DNA strands show further long-living states (4, 5). Several explanations for these long-living states and the size of their spatial extent have been discussed in the literature (5–9). Delocalized excitons (9); excitons that decay to charge-separated states or neutral excimer states (10, 11); exciplexes located on two neighboring bases (5, 8, 12, 13); or even excited single bases, where steric interactions in the DNA strand impedes the ultrafast decay (14), have been proposed. Further computations suggest a decay of an initially populated delocalized exciton to localized neutral or charged excimer states (15–17). However, to our knowledge, a final understanding of the nature of these long-living states has not been reached. Related experiments were performed in the last decade to investigate charge transport processes in DNA, motivated by DNA electronics and oxidative damage (18, 19). Charge transport was initiated by photoexcitation of modified DNA bases or chromophores and followed by transient absorption (20–23). The transport mechanism was described by charge-hopping, superexchange, or transfer of charge along delocalized domains in DNA (18).Until now, most experimental investigations of the long-living state were performed with transient absorption spectroscopy in the UV-visible (UV/Vis) regime (5, 9, 12) or with time-resolved fluorescence (10, 24, 25). Due to the broad, featureless, and overlapping absorption bands of the different DNA bases in this spectral region, it is difficult to investigate the molecular origin of the long-living states using these methods. A further drawback is the unselective and simultaneous excitation of several bases used in most experiments. To circumvent these problems, we used for the present study well-defined oligonucleotides, which enable selective excitation of one single base. Observation of the long-living excited states was performed via time-resolved IR spectroscopy, which can profit from the many “fingerprint” vibrational bands (26, 27). IR spectroscopy is able to distinguish between different DNA bases and their molecular states. It can also reveal changes in the electronic structure and identify charge-separated states.In this study we used single-stranded DNA, in which π stacking between neighboring bases leads to structured domains, similar to the structure in a double helix (28). This interaction is known to be crucial for the long-living states (5). The investigation of single-stranded DNA enables us to construct special sequences, where only one base can be selectively excited. We used the natural bases 2′-deoxyuridine (U), 2′-deoxyadenosine (A), 5-methyl-2′-deoxycytidine (mC), and 2′-deoxyguanosine (G). The nucleobase U occurs naturally in RNA and is similar to the DNA base thymine but shows a blue-shifted absorbance spectrum. mC occurs with a frequency of 4–5% in mammalian DNA (29) and plays an important role as an epigenetic marker (30). The UV/Vis absorbance of mC and G are red-shifted in comparison with A and U, which allows selective excitation at 295 nm in oligonucleotides consisting of mC, A, and U (Fig. 1 A and B) or G and A. This selectivity can only be obtained in single-stranded DNA because G and its complementary base mC have overlapping absorbance bands in the UV range (Fig. S1). Selectivity in probing is based on the significant differences in the IR-absorption spectra of these bases, which display distinct marker bands for each base (Fig. 1 A and E).Open in a separate windowFig. 1.Selective excitation of mC in mCUA and probing of characteristic A, U, and mC marker bands in the IR. (A, B, and E) Picosecond UV light pulses at 295 nm allow selective excitation of mC (shown in bold) in mixed DNA sequences consisting of mC, U, and A. (B) Absorbance spectra of 2′-deoxyadenosine monophosphate (A), 2′-deoxy-5-methylcytidine (mC), and uridine monophosphate (U). (C) Time-resolved absorption difference (color-coded) plotted vs. wavenumber and delay time for mCUA and (D) for a mixture of the corresponding monomers. (E) Probing the individual contribution of each base is possible in the IR at 1,625 cm⁻¹ (A), 1,655 cm⁻¹ (U), and 1,667 cm⁻¹ (mC) (marked by dashed lines). (F) Transients at 1,667 cm⁻¹ for mCUA and the mixture of monomers.With the combination of selective excitation and selective probing we are able to elucidate the nature of the long-living states in DNA strands. Investigation of dinucleotides clearly shows that light absorption in DNA leads to charge separation between stacked neighboring bases, which recombine on the 100-ps time scale. In longer oligonucleotides we observe simultaneous bleach of several bases, which points to a delocalization of the charges along the strand. Our results show that charge transfer in DNA is a natural process, induced by UV-light absorption of DNA.     

Refractive Index Mismatch Can Misindicate Anomalous Diffusion in Single‐Focus Fluorescence Correlation Spectroscopy

Fluorescence correlation spectroscopy (FCS) is often used to study diffusion in complex media, wherein the refractive index usually differs from that of the immersion medium. This paper assesses the effect of this refractive index mismatch. Confocal single‐focus and two‐focus fluorescence correlation spectroscopy (2fFCS) is used to probe the diffusion of tagged dextran tracers in water, dilute dextran solutions, acrylamide monomer solutions, polyacrylamide polymer solutions, and a cross linked polyacrylamide hydrogel. In these experiments, both the refractive index and the potential topological constraint and thermodynamic interaction to the probe‐diffusion are varied, and pairs of samples with same refractive indexes but different compositions are compared. Whereas 2fFCS shows no anomalous diffusion in any of them, single‐focus FCS indicates anomalous diffusion. In particular, the values of the stretching exponent of the fluorescence autocorrelation function, which is often interpreted to reflect the extent of anomaly of diffusion, do not vary systematically with the extent of topological or thermodynamic complexity of the different matrixes, but with their refractive index. This shows that apparent anomalous diffusion in FCS is at risk to be the result of refractive index mismatch rather than reflecting truly complex diffusion.

     

Synthesis,Characterization and Field‐Effect Transistor Performance of Poly[2,6‐bis(3‐alkylthiophen‐2‐yl)benzo[1,2‐b;4,5‐b′]diselenophene]s

Benzo[1,2‐b:4,5‐b′]diselenophene (BDS) has been incorporated for the first time in a polymer. bis(Stannyl)‐functionalized BDS was copolymerized with 3,3′‐bis(alkyl)‐5,5′‐bithiophenes (dodecyl and tetradecyl side chains) through Stille copolymerization, to yield p‐type polymer semiconductors for organic field‐effect transistor application. The electronic and structural effect of the selenium atoms, compared to sulphur atoms in analogous copolymers, is described. The molecular weight has a decisive influence on the photophysical properties and supramolecular ordering, expressed in field‐effect transistor measurements. Saturation mobilities around 10^?2 cm² · V^?1s^?1 are obtained on standard silicon substrates.

     

Novel Alternating Dioctyloxyphenylene Vinylene‐Benzothiadiazole Copolymer: Synthesis and Photovoltaic Performance

Summary: A novel alternating conjugated copolymer (poly[(dioctyloxyphenylene vinylene)‐alt‐(2,1,3‐benzothiadiazole)], C8‐PPV‐BT) consisting of electron‐rich dioctyloxybenzene and electron‐deficient 2,1,3‐benzothiadiazole repeat units was synthesized via a Pd‐catalyzed Heck cross‐coupling polycondensation. The thermal, electrochemical, optical properties, and photophysics of C8‐PPV‐BT blended with [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) have been investigated. Photovoltaic devices of ITO/PEDOT‐PSS/C8‐PPV‐BT + PCBM (1/4, w/w)/Ba/Al were fabricated and their photovoltaic parameters were measured. The results showed the energy conversion efficiency (η_e) and external quantum efficiency (EQE) were 0.335 and 11.7% (451 nm), respectively, under simulated solar light illumination having light intensity of 78.2 mW · (cm²)^?1 (AM1.5), which were both higher than those of devices based on poly[(dioctyloxyphenylene ethynylene)‐alt‐(2,1,3‐benzothiadiazole)] (C8‐PPE‐BT) and PCBM blend under the same conditions. Optical, electrochemical, and photovoltaic properties of C8‐PPV‐BT and C8‐PPE‐BT copolymers were also compared and discussed.

     

Chain Length Dependence in Diketopyrrolopyrrole/Thiophene Oligomers

Diketopyrrolopyrrole‐based polymers have shown good performance in polymer solar cells. Monodisperse short oligomers, monomer up to tetramer, of one of these polymers, pBBTDPP1, have been prepared and the chain length dependence of their optical and electrochemical properties has been investigated. The optical and electrochemical data obtained in solution lead to the conclusion that conjugation in this system is very limited. The most probable reason for this is a twist in the backbone, caused by steric hindrance of the dodecyl side chains with the sulfur atoms. In thin films, the chain length dependence of the optical properties is much stronger, due to planarization of the oligomer chains upon aggregation.

     

Poly[2,7‐(9,9‐dihexylfluorene)]‐block‐Poly(2‐vinylpyridine) Rod–Coil Star‐block Copolymers: Synthesis,Micellar Structures,and Photophysical Properties

The first successful synthesis of conjugated rod–coil star block copolymer, (PF‐b‐P2VP)_n, containing conjugated poly[2,7‐(9,9‐dihexylfluorene)] (PF), and coil‐like poly(2‐vinylpyridine) (P2VP) by combining a Suzuki coupling reaction and living anionic polymerization is reported. With increasing methanol content in THF/methanol mixtures (PF‐b‐P2VP)_n symmetric star‐block copolymers maintain spherical micelles, but PF‐b‐P2VP asymmetric diblock copolymers vary from spherical micelles to vesicles. Both the absorption and emission spectra of PF‐b‐P2VP blue shift with increasing methanol content, suggesting an “H‐type” aggregation. However, (PF‐b‐P2VP)_n star‐block exhibits no shift in absorption but a red shift in the emission spectra, indicating a different type of aggregation. These results suggest the significance of polymer architectures on microphase‐separated morphologies and photophysical properties.

     

pH-Dependent Photophysical Properties of Metallic Phase MoSe2 Quantum Dots

Fluorescence properties of quantum dots (QDs) are critically affected by their redox states, which is important for practical applications. In this study, we investigated the optical properties of MoSe₂-metallic phase quantum-dots (MoSe₂-mQDs) depending on the pH variation, in which the MoSe₂-mQDs were dispersed in water with two sizes (Φ~3 nm and 12 nm). The larger MoSe₂-mQDs exhibited a large red-shift and broadening of photoluminescence (PL) peak with a constant UV absorption spectra as varying the pH, while the smaller ones showed a small red-shift and peak broadening, but discrete absorption bands in the acidic solution. The excitation wavelength-dependent photoluminescence shows that the PL properties of smaller MoSe₂-mQDs are more sensitive to the pH change compared to those of larger ones. From the time-resolved PL spectroscopy, the excitons dominantly decaying with an energy of ~3 eV in pH 2 clearly show the shift of PL peak to the lower energy (~2.6 eV) as the pH increases to 7 and 11 in the smaller MoSe₂-mQDs. On the other hand, in the larger MoSe₂-mQDs, the exciton decay is less sensitive to the redox states compared to those of the smaller ones. This result shows that the pH variation is more critical to the change of photophysical properties than the size effect in MoSe₂-mQDs.     

Efficient UV-induced charge separation and recombination in an 8-oxoguanine-containing dinucleotide

During the early evolution of life, 8-oxo-7,8-dihydro-2′-deoxyguanosine (O) may have functioned as a proto-flavin capable of repairing cyclobutane pyrimidine dimers in DNA or RNA by photoinduced electron transfer using longer wavelength UVB radiation. To investigate the ability of O to act as an excited-state electron donor, a dinucleotide mimic of the FADH₂ cofactor containing O at the 5′-end and 2′-deoxyadenosine at the 3′-end was studied by femtosecond transient absorption spectroscopy in aqueous solution. Following excitation with a UV pulse, a broadband mid-IR pulse probed vibrational modes of ground-state and electronically excited molecules in the double-bond stretching region. Global analysis of time- and frequency-resolved transient absorption data coupled with ab initio quantum mechanical calculations reveal vibrational marker bands of nucleobase radical ions formed by electron transfer from O to 2′-deoxyadenosine. The quantum yield of charge separation is 0.4 at 265 nm, but decreases to 0.1 at 295 nm. Charge recombination occurs in 60 ps before the O radical cation can lose a deuteron to water. Kinetic and thermodynamic considerations strongly suggest that all nucleobases can undergo ultrafast charge separation when π-stacked in DNA or RNA. Interbase charge transfer is proposed to be a major decay pathway for UV excited states of nucleic acids of great importance for photostability as well as photoredox activity.The RNA world hypothesis suggests that ancient life originated from RNA-based oligomers due to their ability to both store genetic information and catalyze reactions in a manner similar to protein-based enzymes (1). In proteins, the 20 canonical amino acids are not versatile enough in their redox activity for many purposes, and special redox cofactors, such as the dinucleotides FADH₂ and NAD(P)H, are often recruited to facilitate a desired transformation. Similarly, facile oxidation or reduction reactions involving the canonical nucleobases could put the integrity of the genome at risk. Recent work has shown that 8-oxo-7,8-dihydroguanine (8-oxo-G), an oxidatively damaged form of guanine (G), is a redox-active base capable of photoinduced reversal of thymine dimers in DNA oligomers (2, 3). In particular, continuous irradiation of substrates containing a thymine dimer and a nearby 8-oxo-G using a UVB lamp decreases the amount of dimers over time (2, 3), but direct evidence of photoinduced electron transfer (ET) has been lacking.The ability of 8-oxo-G to act as an excited-state electron donor is plausible, but by no means assured in light of conflicting indications. On the one hand, 8-oxo-G is easier to oxidize in its electronic ground state by ∼30 kJ⋅mol^–1 compared with G (4), the most easily oxidized of the canonical bases, but this advantage may be lost for excited-state oxidation due to the lower energy of the first excited singlet state of 8-oxo-G. On the other hand, the 2′-deoxynucleoside of 8-oxo-G (8-oxo-dGuo, or O) has an excited-state lifetime of just 0.9 ± 0.1 ps at physiological pH (5)—a value that is similar to the subpicosecond lifetimes of the undamaged bases (6). Rapid nonradiative decay could thus frustrate ET despite favorable thermodynamics. Time-resolved spectroscopy can resolve this puzzle by detecting any short-lived radicals produced by photoinduced ET. Here, we use femtosecond time-resolved infrared (TRIR) spectroscopy to definitively show that UV excitation of the dinucleotide d(OA) (Fig. 1) transfers an electron from O to 2′-deoxyadenosine (A) on a subpicosecond timescale to form a contact radical ion pair (exciplex) that can be unambiguously identified by comparison with density functional theory (DFT) calculations.Open in a separate windowFig. 1.UV-visible (A) and FTIR spectra (B) for d(OA) at neutral pH. The spectra of monomeric 8-oxo-dGuo (red dashed curves) and AMP (green dotted curves) are shown for comparison. The excitation wavelengths used in the pump-probe experiments are indicated in A by arrows.The dinucleotide d(OA) was chosen as a crude mimic of FADH₂ in which 8-oxo-G replaces the dihydroflavin moiety of the cofactor that serves as the electron source for photoinitiated ET to a cyclobutane pyrimidine dimer (CPD) (7). Combining O with A offers the further advantage that the steady-state UV-visible and IR absorption spectra have several nonoverlapping transitions that arise from just one of the two chromophores (Fig. 1 and Fig. S1). These spectral characteristics permit selective excitation of O and selective detection of the localization site of an excited state via mid-IR probing of either O or A vibrations. A vibrational spectrum with its comparatively narrow resonances is frequently easier to assign than overlapping electronic absorption spectra. Consequently, TRIR spectroscopy can often differentiate between charge transfer (CT) states and other excited states that may have similar electronic absorption spectra (8, 9).Besides its interest as a mimic of a redox cofactor, the d(OA) dinucleotide also provides valuable insights into the role of CT states in DNA. As shown below, the CT state of d(OA) decays in ∼60 ps by charge recombination (CR). This decay is much longer than the excited-state lifetimes of either A or O as monomers. Similarly long-lived excited states are formed in high yields whenever DNA bases are stacked with one another both in single- and double-stranded forms (10–14). The identity of these long-lived states has been one of the most debated issues in the photophysics and photochemistry of nucleic acids during the past decade. The results from this study suggest that a primary decay channel for excited states of stacked nucleobases in DNA, whether modified or not, is ultrafast interbase ET.     

Semiconductor Nanocomposites of Emissive Flexible Random Copolymers and CdTe Nanocrystals: Preparation,Characterization, and Optoelectronic Properties

Synthesis and characterization of a series of new emissive flexible copolymers composed of two kinds of special side‐chain functional groups is demonstrated. One of the side‐chain groups provides emissive properties, while the other serves as the ligand unit. The copolymers are used to prepare a new kind of CdTe nanocrystal‐based semiconductor nanocomposite. Their structures and morphologies are characterized and their optoelectronic properties are studied. The results reveal that an energy‐transfer process from the emissive random copolymers to the CdTe nanocrystals occurs, while a photoinduced charge transfer is estimated to be thermodynamically forbidden.