Relaxation of photo-excitations in films of oligo- and poly-(para-phenylene vinylene) derivatives

The photo-luminescence from solid films of poly(para-phenylene vinylene) polymers and an oligomeric model system, consisting of seven repeat units, are investigated at low temperature (8 K) using time-resolved spectroscopic techniques. Results are compared to those for the materials in solution. In the case of the oligomer, the shape of the visible absorption band observed for the film is quite different from the band shape for the polymer in frozen solution and is characteristic of H-type aggregates. Theoretical models are presented describing the dependence of the band shape of absorption and emission spectra on intermolecular excited state interactions, electron-vibration coupling and disorder represented by distributions of the molecular excitation and intermolecular interaction energies. Using these models, it is concluded that intermolecular interactions in the film of the oligomer are strong (1400 cm⁻¹), and the disorder low, implying delocalization of the excitation over several molecules. In accordance with these models the fluorescence lifetime for the film (2 ns) is considerably longer than for isolated molecules in solution (0.45 ns). The emission spectra of the film, taken early after excitation, are consistent with delocalization of the excitation over several molecules. A time-dependent red shift of the fluorescence band is observed and interpreted in terms of migration of localized excitations between disorder induced trap sites, which exist in the low energy tail of the density of excited states. For the polymers, differences between the shape of the absorption bands of solid film and frozen solution are smaller than for the oligomer indicating that interchain interactions that are, on average, weaker than for the oligomer. For the polymer films, a time-dependent red shift of the emission is observed and fluorescence depolarization measurements provide direct evidence for migration of the photo-excitations between trap sites. For one polymer, a time dependent change in the band shape of the fluorescence after pulsed excitation is observed with the band shape of the long-lived emission being compatible with that expected for an excitation delocalized over at least two, nearly parallel aligned, chains. For a second polymer, the emission band shape and its time evolution indicate that the major part of the fluorescence originates from disorder induced luminescent sites. These results indicate that the spectroscopic properties of films of π-conjugated polymer critically depend on parameters such as density of defects and excited state interchain interaction energy.     

Influence of aggregates and solvent aromaticity on the emission of conjugated polymers

The influence of aggregates and solvent aromaticity on the photophysics and fluorescence dynamics of two conjugated polymers is studied. The two polymers are derivatives of poly(p-phenylene vinylene) (PPV) containing different kinked moieties along the main chain. The polymers contain 2,6-diphenylpyridine and m-terphenyl kinked moieties and they are abbreviated as PN and PC, respectively. The insertion of kinked segments along the main chain shifts the emission spectrum from the yellow-orange spectral region, common to PPV derivatives, to the blue-green spectral region. The results show that in dilute solutions the polymers decay monoexponentially, while in concentrated ones the fluorescence decays biexponentially, indicating fluorescence quenching. This is attributed to an energy transfer process from polymer chains to aggregates that occurs within a few tens of picoseconds. By comparing the photophysics and fluorescence dynamics of polymer PN in a nonaromatic and an aromatic solvent, we conclude that the polymer conformation adopted in the aromatic solvent leads to a higher fluorescence quantum yield and a longer fluorescence lifetime. Furthermore, the fluorescence quenching of PN because of aggregates is faster and more efficient in the aromatic than in the nonaromatic solvent. These results can be explained through a more extended chain conformation of PN in the aromatic solvent.     

Insights into the AIEE of 1,8‐Naphthalimides (NPIs): Inverse Effects of Intermolecular Interactions in Solution and Aggregates

Systematic structural perturbation has been used to fine‐tune and understand the luminescence properties of three new 1,8‐naphthalimides (NPIs) in solution and aggregates. The NPIs show blue emission in the solution state and their fluorescence quantum yields are dependent upon their molecular rigidity. In concentrated solutions of the NPIs, intermolecular interactions were found to quench the fluorescence due to the formation of excimers. In contrast, upon aggregation (in THF / H₂O mixtures), the NPIs show aggregation‐induced emission enhancement (AIEE). The NPIs also show moderately high solid‐state emission quantum yields (ca. 10–12.7 %). The AIEE behaviour of the NPIs depends on their molecular rigidity and the nature of their intermolecular interactions. The NPIs 1 – 3 show different extents of intermolecular (π–π and C?H???O) interactions in their solid‐state crystal structures depending on their substituents. Detailed photophysical, computational and structural investigations suggest that an optimal balance of structural flexibility and intermolecular communication is necessary for achieving AIEE characteristics in these NPIs.     

Synthesis and properties of perylenetetracarboxylic diimide dimers linked at the bay position with conjugated chain of different length

Three perylenetetracarboxylic diimide (PDI) dimers linked with a conjugated chain of different lengths have been designed and prepared. The UV-Vis absorption and fluorescence spectra of these three dimers revealed different photophysical properties owing to the different length of the linkage. The intermolecular π-π interactions were found to be enhanced significantly with the increase in the length of the linkage and therefore induced different aggregation behaviors of these molecules. The structure of the molecular aggregates was investigated by X-ray diffraction (XRD), and the morphology of the aggregates was examined by atomic force microscopy (AFM). One-dimensional fibers were observed for the aggregates of compounds 2 and 3, and thin solid films were observed for the aggregates of compound 1.     

Well-packed chains and aggregates in the emission mechanism of conjugated polymers

We synthesized dialkoxy-substituted poly[phenylene vinylene]s (dROPPV-1/1, 0.2/1, and 0/1) consisting of two repeating units with different side-chain lengths (methoxy and 3,7-dimethyloctyloxy). These polymers can serve as a model system to clarify roles of aggregates (the sites with ground-state interchain interactions) and the independent chain segments in the well-packed chains (the chain segments that are compactly packed without interaction) in the emission mechanism of conjugated polymers. Due to the packing of polymer chains, films of all of these polymers are accessible to interchain excitations, after which excitons can re-form to result in delayed luminescence. Besides, some chains form aggregates so that the delayed luminescence is no more the ordinary single-chain emission but red-shifted and less structured. Not only the re-formation of these indirect excitons but also the aggregation of chains are facilitated in the polymers with short methoxy side groups, revealing that both packing and aggregation of chain segments require a short spacing between polymer chains. However, the incorporation of other side chains such as the 3,7-dimethyloctyloxy group to dROPPVs is necessary for the formation of aggregates because these long branched side chains can reduce the intrachain order imposed by the short methoxy groups, which accounts for the absence of aggregate emission in the well-studied poly[2,5-dimethoxy-1,4-phenylene vinylene]. This study reveals that the well-packed chains do not necessarily form aggregates. We also show that the photophysical properties and the film morphology of conjugated polymers can be deliberately controlled by fine-tuning of the copolymer compositions, without altering the optical properties of single polymer chains (e.g., as in dilute solutions).     

On the angular dependence of the optical polarization anisotropy in ladder-type polymers

Two ladder-type polymers have been examined using polarized optical absorption and fluorescence spectroscopy in solution and solid state. The fluorescence anisotropy excitation spectra of ladder-type methyl-substituted poly(para-phenylene) and poly(naphthylene-phenylene) follow their absorption profiles, contrary to the typical behavior in other conjugated polymers, which show excitation wavelength dependent anisotropy until the onset of the typical red edge effect. Using Raman microscopy and linear dichroism of aligned stretched polyethylene films containing the ladder polymer, it has been found that the orientation of the absorption transition dipole moment is dependent on the conjugation length. The transition dipole of the shorter conjugation lengths of the distribution is localized approximately 7 degrees off axis, and that of the longer conjugation lengths is delocalized along the chain.     

Blue light‐emitting poly(p‐phenylenevinylene) derivatives containing alternating conjugated segments and aliphatic spacers

Two new poly(p‐phenylenevinylene) derivatives were prepared by Heck coupling. They contained alternating conjugated segments on the basis of p‐distyrylbenzene and flexible nonconjugated spacers. The synthesized polymers P1 and P2 carried two m‐terphenyl of four tertbutyl pendants, respectively, per repeat unit. Both polymers were amorphous and exhibited satisfactory thermal stability. Polymer P1 displayed a limited solubility in common organic solvents, whereas P2 dissolved readily in these solvents. The glass‐transition temperature values were 128 °C for P1 and 37 °C for P2 . The polymers emitted blue or violet‐blue light with photoluminescent maxima at about 445 and 460 nm for solutions and thin films, respectively. The bulky pendants reduced their tendency to form aggregates. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1091–1098, 2003     

Single-molecule studies of a model fluorenone.

Single-molecule fluorescence measurements of 2,7-bis(3,4,5-trimethoxyphenylethenyl)fluorenone (OFOPV) reveal narrow emission spectra concentrated around 540 nm, with weak emission at longer wavelengths. The wide scattering of emission-maximum wavelengths is attributed to varying molecular environments, with dimers or higher-order aggregates contributing to the low-energy emission. This spectral distribution indicates that emission from monomers of this model fluorenone is mostly green, which is consistent with contaminant emission (g-bands) often observed in fluorene- and polyfluorene-based organic light emitting diode (OLED) devices. A histogram of center wavelengths from 118 single-molecule spectra shows good agreement with the green emission previously observed in thermally stressed 2,7-bis(3,4,5-trimethoxyphenylethenyl)-9,9-diethylfluorene (OFPV). Whereas bulk OFPV exhibits blue fluorescence at about 480 nm, OFOPV bulk thin film measurements reveal red luminescence shifted to 630 nm. This unexpected peak position for bulk OFOPV shifts to higher energies (ca. 540 nm) upon dilution in a solid-state matrix, suggesting that the bulk red emission finds its origins in interactions between fluorenone molecules. Explanations for this red emission include aggregate or excimer formation or intermolecular energy transfer between fluorenone molecules.     

Luminescence studies in polymers-IX. Polarization of the fluorescence of copolymer films

Energy migration has been directly demonstrated to be very efficient in solid polymers by studying the depolarization of the fluorescence of copolymer films as a function of composition. The results are compared with those obtained for the same copolymers in glassy solution. They indicate that migration is more important in bulk polymers than in dilute glassy solutions owing to intermolecular interactions in agreement with the structural models proposed for amorphous polymers.     

Solid-state optical properties of linear polyconjugated molecules: pi-stack contra herringbone

The intermolecular arrangement in the solid state and the consequences on the optical and photophysical properties are studied on different derivatives of oligophenylenevinylenes by UV/VIS absorption and angular-resolved polarized fluorescence spectroscopy. Unsubstituted distyrylbenzene (DSB) organizes in a herringbone manner, with the long axes of the molecules oriented in parallel, but the short axes almost perpendicular to each other. Fluorinated distyrylbenzene (F(12)DSB) as well as the DSB:F(12)DSB cocrystals prefer cofacial pi-stacking in the solid state. For all structures, the consequence of the parallel alignment of the transition moments is a strongly blueshifted H-type absorption spectrum and a low radiative rate constant k(F). Significant differences are observed for the emission spectra: the perpendicular arrangement of the short axes in DSB crystals leads to only very weak intermolecular vibronic coupling. Hence the emission spectrum is well structured, very similar to the one in solution. For F(12)DSB and DSB:F(12)DSB, the cofacial arrangement of the adjacent molecules enables strong intermolecular vibronic coupling of adjacent molecules. Thus, an unstructured and strongly redshifted excimerlike emission spectrum is observed. The differences in the electronic nature of the excited states are highlighted by quantum-chemical calculations, revealing the contribution of interchain excitations to the electronic transitions.     

Highly emissive conjugated polymer excimers

Conjugated polymers often display a decrease of fluorescence efficiency upon aggregation due in large part to enhanced interpolymer interactions that produce weakly emissive species generally described as having excimer-like character. We have found that poly(phenylene ethynylene)s with fused pendant [2.2.2] ring structures having alkene bridges substituted with two ester groups function to give highly emissive, broad, and red-shifted emission spectra in the solid state. To best understand the origin of this new solid-state emissive species, we have performed photophysical studies of a series of different materials in solution, spin-coated thin films, solid solutions, and Langmuir films. We conclude that the new, red-shifted, emissive species originate from excimers produced by interchain interactions being mediated by the particular [2.2.2] ring system employed. The ability to design structures that can reliably produce highly emissive conjugated polymer excimers offers new opportunities in the emission tailoring of electroluminescence and sensory devices.     

Flexible Alkylene Bridges as a Tool To Engineer Crystal Distyrylbenzene Structures Enabling Highly Fluorescent Monomeric Emission

To design ultrabright fluorescent solid dyes, a crystal engineering strategy that enables monomeric emission by blocking intermolecular electronic interactions is required. We introduced propylene moieties to distyrylbenzene (DSB) as bridges between the phenyl rings either side of its C=C bonds. The bridged DSB derivatives formed compact crystals that emit colors similar to those of the same molecules in dilute solution, with high quantum yields. The introduction of flexible seven-membered rings to the DSB core produced moderate distortion and steric hindrance in the DSB π-plane. However, owing to this strategy, it was possible to control the molecular arrangement with almost no decrease in the crystal density, and intermolecular electronic interactions were suppressed. The bridged DSB crystal structure differs from other DSB derivative structures; thus, bridging affords access to novel crystalline systems. This design strategy has important implications in many fields and is more effective than the conventional photofunctional molecular crystal design strategies.     

Metal-Enhanced S(2) Fluorescence from Azulene

In this paper, we report the first observation of metal-enhanced S(2) emission at room and low temperature (77K). The S(2) emission intensity of Azulene is enhanced by close proximity to Silver island films (SiFs). In this regard, a ≈ 2-fold higher S(2) fluorescence intensity of Azulene was observed from SiFs as compared to a glass control sample. This suggests that S(2) excited states can couple to surface plasmons and enhance S(2) fluorescence yields, a helpful observation in our understanding the interactions between plasmons and lumophores, and our continued efforts to develop a unified plasmon-lumophore/fluorophore theory.     

Spectroscopy of conjugated polymers: phosphorescence and delayed fluorescence

We report on the observation of delayed fluorescence (DF) and phosphorescence (P) from films and dilute frozen solutions of various conjugated polymers of the PPP‐type. The materials differ with respect to the rotational freedom along the polymer backbone. Upon pulsed optical excitation into the S₁←S₀ transition of the materials, delayed emission occurs on a time scale of μs to ms in solid films at 80 K. The phosphorescence in dilute frozen solution decays monoexponentially with a radiative lifetime on the order of one second. The data analysis reveals that the DF is caused by recombination of geminate electron hole pairs rather than triplet‐triplet annihilation. This conclusion is supported by investigations of the response of the DF to an applied electric field.     

Non-Covalent Interactions between Polyvinyl Chloride and Conjugated Polymers Enable Excellent Mechanical Properties and High Stability in Organic Solar Cells

The incorporation of insulating polymers into conjugated polymers has been widely explored as a strategy to improve mechanical properties of flexible organic electronics. However, phase separation due to the immiscibility of these polymers has limited their effectiveness. In this study, we report the discovery of multiple non-covalent interactions that enhances the miscibility between insulating and conjugated polymers, resulting in improved mechanical properties. Specifically, we have added polyvinyl chloride (PVC) into the conjugated polymer PM6 and observed a significant increase in solution viscosity, indicative of favorable miscibility between these two polymers. This phenomenon has been rarely observed in other insulating/conjugated polymer composites. Thin films of PM6/PVC exhibit a much-improved crack-onset strain of 19.35 %, compared to 10.12 % for pristine PM6 films. Analysis reveal that a “cyclohexyl-like” structure formed through dipole-dipole interactions and hydrogen bonding between PVC and PM6 acted as a cross-linking site in the thin films, leading to improved mechanical properties. Moreover, PM6/PVC blend films have demonstrated excellent thermal and bending stability when applied as an electron donor in organic solar cells. These findings provide new insights into non-covalent interactions that can be utilized to enhance the properties of conjugated polymers and may have potential applications in flexible organic electronics.     

Photophysics and X-ray structure of crystalline 2-aminopurine.

To explore the effect of intermolecular interactions on the photophysics of 2-aminopurine (2AP) in a well-defined environment, we have investigated the fluorescence properties of single 2AP crystals, having determined their X-ray structure. In the crystal, 2AP is subject to base-stacking and hydrogen-bonding interactions similar to those found in DNA. The crystal shows dual fluorescence: pi-stacked molecules in the bulk of the lattice have redshifted excitation and emission spectra, while molecules at defect sites have spectra similar to those of 2AP in solution or in DNA. Heterogeneous intermolecular interactions in the crystal give rise to multiexponential fluorescence decay characteristics similar to those observed for 2AP-labelled DNA. The presence of about 13 % of the 7H tautomer in the crystal confirms that 9H-7H tautomerisation of 2AP occurs in the ground state. Long-wavelength excitation of a 2AP-labelled oligonucleotide duplex produced redshifted emission similar to that observed in the crystal, indicating that pi-stacking interaction of 2AP with nucleobases gives rise to a low energy excited state.     

The growth of highly ordered textures in planar poly(γ-benzylglutamate) liquid crystals

We have observed the growth of highly ordered textures in homogeneously aligned poly(γ-benzyl-glutamate) (PBG) liquid crystals as the polymer concentration increases. The textures are similar to the hexagonal columnar textures exhibited by recent studies on highly concentrated solutions of helical biological polymers. In a geometry of homeotropic alignment, no evidence of such texture growth was seen, which is probably due to strong binding interactions between PBG polymers and the substrates.     

Fluorescence Study of Energy Transfer in PMMA Polymers with Pendant Oligo‐Phenylene‐Ethynylenes

A spectroscopic characterization of polymers containing rigid π‐conjugated oligo(phenyleneethynylene) chromophores as well as oligo(phenyleneethynylene) and methyl methacrylate is presented. The polymers exhibit molar masses of up to 15 000 g mol^?1 and a degree of polymerization between 22 and 80. Emission measurements of the monomeric and polymeric species show that radiative as well as nonradiative rates are influenced by the degree of polymerization due to intramolecular interactions of chromophores pendant to the polymer backbone. Time‐resolved emission anisotropy measurements suggest that energy migrates within the polymers. Steady‐state emission anisotropy measurements also point to energy migration. Additionally, two oligo(phenyleneethynylene)s with different sizes of the conjugated system are copolymerized in order to enable energy trapping due to energy transfer. The shortened energy‐donor fluorescence lifetime within the donor–acceptor copolymers suggest energy transfer. Depending on the degree of polymerization, dispersion of the donor fluorescence lifetime is observed.     

Femtosecond time resolved fluorescence dynamics of a cationic water-soluble poly(fluorenevinylene-co-phenylenevinylene)

A recently synthesized cationic water-soluble poly(fluorenevinylene-co-phenylenevinylene) was studied by means of steady state and femtosecond time resolved upconversion spectroscopy in aqueous and EtOH solutions. Steady state spectroscopic measurements showed that the polymer emits at the blue-green spectral region and that aggregates are formed in concentrated polymer solutions. The fluorescence dynamics of the polymer in concentrated solutions, studied at a range of emission wavelengths, exhibited a wavelength dependent and multiexponential decay, indicating the existence of various decay mechanisms. Specifically, a rapid decay at short emission wavelengths and a slow rise at long wavelengths were observed. Both features reveal an energy transfer process from isolated to aggregated chains. The contribution of the energy transfer process as well as of the isolated chains and the aggregates on the overall fluorescence decay of the polymer was determined. The dependence of the energy transfer rate and efficiency on polymer concentration was also examined.     

Modeling disorder in polymer aggregates: the optical spectroscopy of regioregular poly(3-hexylthiophene) thin films

Absorption and emission in polymer aggregates is studied theoretically, taking into account excitonic (intermolecular) coupling, exciton-phonon (EP) coupling, and disorder, all treated on equal footing within a generalized Holstein Hamiltonian with numerically generated eigenmodes and energies. The disorder is modeled as a Gaussian distribution of molecular transition frequency offsets of width sigma and spatial correlation length l(0). Both herringbone (HB) and lamellar aggregate morphologies are considered. The emission spectral line shape is shown to undergo marked changes in response to increasing disorder, with the intensity of the ac-polarized 0-0 emission peak generally increasing relative to the replica intensities (0-1,0-2,[ellipsis (horizontal)]) as sigma increases and/or as l(0) decreases. This is contrary to the behavior of the b-polarized component of the 0-0 intensity, which, in HB aggregates, decreases with increasing disorder. Comparisons are made to analogous trends in oligomer aggregates. Analytical results are obtained in the strong EP coupling regime appropriate for conjugated polymers while treating the disorder perturbatively. A method for uniquely determining sigma and l(0) from the emission and absorption spectra is presented. Applications are made to absorption and low-temperature emission in thin films of regioregular poly(3-hexylthiophene), with excellent agreement between theory and experiment obtained for a spatial correlation length of only 3-4 molecules.