Intrachain Exciton Research Articles

Conjugation Enhancement of Intramolecular Exciton Migration in Poly(p-phenylene ethynylene)s

Efficient energy migration in conjugated polymers is critical to their performance in photovoltaic, display, and sensor devices. The ability to precisely control the polymer conformation is a key issue for the experimental investigations and deeper understanding of the nature of this process. We make use of specially designed iptycene-containing poly(p-phenylene ethynylene)s that display chain-extended conformations when dissolved in nematic liquid crystalline solvents. In these solutions, the polymers show a substantial enhancement in the intrachain exciton migration rate, which is attributed to their increased conjugation length and better alignment. The organizational enhancement of the energy transfer efficiency, as determined by site-selective emission from lower energy traps at the polymer termini, is accompanied by a significant increase of the fluorescence quantum yield. The liquid crystalline phase is a necessary requirement for these phenomena to occur, and when the temperature was increased above the nematic-isotropic transition, we observed a dramatic reduction of the energy transfer efficiency and fluorescence quantum yield. The ability to improve the exciton migration efficiency through precise control of the polymer structure with liquid crystalline solutions demonstrates the importance of a polymer's conformation for energy transfer, and provides a way to improve the energy transporting performance of conjugated polymers.

A Platinum Acetylide Polymer with Sterically Demanding Substituents: Effect of Aggregation on the Triplet Excited State

The effect of interchain interaction on the triplet excited state is explored in two Pt-acetylide polymers of the type [-trans-Pt(PBu(3))(2)-C triple bond C-Ar-C triple bond C-](n), where Ar is either 1,4-phenylene or is based on the pentiptycene unit (polymers 2 and 3, respectively). To explore the effect of interchain interaction in Pt-acetylide materials, the optical properties of parent polymer 2 are compared with those of polymer 3 in which interchain interaction is precluded by the sterically bulky pentiptycene moiety. Insight into the effect of the pentiptycene unit on packing in the solid state comes from the X-ray structure of monomer 1b, Ph-C triple bond C-[trans-Pt(PBu(3))(2)]-C triple bond C-Ar-C triple bond C-[trans-Pt(PBu(3))(2)]-C triple bond C-Ph. Spectroscopic studies indicate that weak phosphorescence emission from an interchain aggregate is observed from parent polymer 2, both in solution and in the solid state. By contrast, the photophysics of 3 is dominated by the intrachain triplet exciton. Interestingly, the phosphorescence emission of polymer 3 in the solid state is nearly superimposable with that of a single crystal of monomer 1b, suggesting that the solid polymer experiences an environment that is similar to that of the monomer in the crystal.

Intramolecular fluorescence quenching in luminescent copolymers containing fluorenone and fluorene units: A direct measurement of intrachain exciton hopping rate

The quenching process of fluorescence emission in polyfluorene (PF) due to the presence of intramolecular 9-fluorenone (9 FL) moieties is studied in dilute toluene solution as a function of 9 FL content in eight copolymers containing both fluorene and fluorenone units (PF/FL(x)). The absorption spectrum of PF/FL(x) copolymers clearly shows a new absorption band, redshifted relatively to the PF and 9-fluorenone absorption, which increases in intensity when the fluorenone fraction increases and also decreases with solvent polarity. Fluorescence emission spectra of PF/FL(x) show that this redshifted and unstructured emission does not coincide with the 9-fluorenone emission and, with increasing solvent polarity, it further redshifts and decreases in intensity. An isoemissive point is clearly observed on the fluorescence emission spectra of PF/FL(x) as a function of fluorenone content, showing that the new emission band is formed at the expense of PF. We propose the formation of an intramolecular charge transfer complex (ICTC) between PF units and 9-fluorenone to explain the appearance of the new emission band. Global analysis of time resolved fluorescence decays collected at 415 nm (PF emission) and 580 nm (the ICTC emission) show that three exponentials are generally needed to achieve excellent fits. Two of the components (420 ps and 6.5 ns) are independent of 9-fluorenone fraction. A further fast component is strongly dependent on fluorenone fraction and ranges between 280 and 70 ps. This component appears as a decay time at 415 nm and as a rise time at 580 nm and is ascribed to the migration of exciton to quenching sites (formation of intramolecular CT complex or exciton ionization at CT complex). A kinetic mechanism involving three different kinetic species, quenched PF units kinetically coupled with the ICTC complex, and unquenched PF units is proposed to explain the experimental data and the quenching rate constant is obtained, k(1) congruent with 10(11) s(-1). This is an experimental measurement of the intrachain exciton hopping rate.

Photoluminescence and Relaxation Processes in MEH−PPV

A detailed description of the thermal relaxation processes in MEH−PPV is reported. Bulk methods such as DMTA were employed in conjunction with other techniques that probe molecular motions, such as fluorescence spectroscopy, thermal stimulated current, and 13C NMR. From the two main transitions observed (glass transition process at 340 K and β-relaxation between 200 and 220 K), it was demonstrated that the first is strongly correlated with the dissociation of a fluorescent emissive interchain complex and that the second relaxation involves movements of the lateral substituents of the polymer backbone and, more specifically, their CH2 groups. NMR dipolar chemical shift correlation experiments pointed an increasing gain in mobility through the side chain, the lateral carbons close to the aromatic ring being more rigid than those located more distant from the main polymer chain. A kinetic model involving the dissociation of interchains to re-form intrachain excitons was proposed to explain the profiles of the photoluminescence spectra at higher temperatures.

Luminescence Properties of Organometallic/Coordination Oligomers and Polymers Containing Diphosphine and Diisocyanide Assembling Ligands: Comparison Between Mononuclear Model Complexes and Polymers

AbstractFor Abstract see ChemInform Abstract in Full Text.

Mechanisms of excited-state enhancement of optical non-linearity in poly(distyrybenzene-co-triethylene glycol)

We present femtosecond optical Kerr effect (OKE) studies of the third-order non-linear optical response of poly (distyrybenzene-co-triethylene glycol) (PDTG) under both resonance and off-resonance conditions. The excited-state enhancement of OKE signal was observed under resonance excitation. The temporal behavior of resonant OKE signal was found to be characterized by a fast component with time constant of about 250 fs and a slow component with lifetime of about 420 ps, which are attributed to delocalized π-electron and intra-chain singlet exciton migration among the backbone of the polymer, respectively. The off-resonance OKE signal only was seen to consist of an ultrafast response due entirely to delocalized electrons contribution.

Luminescence properties of organometallic/coordination oligomers and polymers containing diphosphine and diisocyanide assembling ligands: comparison between mononuclear model complexes and polymers

AbstractThis paper describes the luminescence properties of selected examples of homo‐ and mixed‐bridging ligand‐containing organometallic/coordination oligomers and polymers of copper(I), silver(I), palladium(I) and −(0.5), and platinum(0.5). The bridging ligands are the 1,8‐diisocyano‐p‐menthane (dmb) and bis(diphenylphosphino)methane (dppm), ‐ethane (dppe), ‐propane (dppp), ‐butane (dppb), ‐pentane (dpppen), and −hexane (dpph), as well as bis(diphenyl‐phosphino)acetylene (dpa) and bis(dimethylphosphino)methane (dmpm). The comparison between the mononuclear model complexes and the polymers of the emission maxima (λmax) and emission lifetimes (τe) will be made, and interpreted by the presence of intrachain interactions and exciton phenomena.

Exciton-exciton annihilation and the production of interchain species in conjugated polymer films: Comparing the ultrafast stimulated emission and photoluminescence dynamics of MEH-PPV

Despite the enormous versatility of conjugated polymers for use in optoelectronic devices and the correspondingly large number of research studies on their photophysics, understanding of the fundamental nature of the primary photoexcitations remains highly controversial. Part of the reason for this controversy stems from the fact that the photophysics of conjugated polymer films depends sensitively on the excitation intensity, making it difficult to compare the results of different experiments such as pump-probe stimulated emission (SE) and time-resolved photoluminescence (PL), which usually are performed at excitation intensities that differ by orders of magnitude. In this paper, we present a detailed exploration of the pump-probe SE and time-resolved PL dynamics of poly(2-methoxy 5-$[{2}^{\ensuremath{'}}$-ethylhexyloxy]-p-phenylene vinylene) (MEH-PPV) films under identical excitation conditions. Using an optically triggered streak camera, we are able to simultaneously measure the PL and SE dynamics as a function of both excitation intensity and emission wavelength. Although the SE and PL dynamics of dilute MEH-PPV solutions are identical, we find that even at relatively low excitation intensities, the PL and SE dynamics of MEH-PPV films are different, uncovering the presence of an interchain excited-state absorbing species that has dynamics distinct from the intrachain exciton. The number of interchain absorbing species increases nonlinearly with excitation intensity, suggesting that interchain species are formed both directly upon photoexcitation and as a by-product of exciton-exciton annihilation (E-EA). A comparison of the emission dynamics of pristine and intentionally oxidized MEH-PPV films leads us to conclude that there are multiple types of interchain species; we propose that the directly excited interchain species are likely aggregates or excimers, while the interchain species produced by E-EA are best assigned to polaron pairs. We also find that the wavelength dependence of the SE and PL from MEH-PPV films is complex, both because energy migration leads to a dynamic redshift of the exciton emission and because the rate of E-EA is higher for hot (blue-emitting) excitons than for thermalized (red-emitting) excitons. All the results are compared in detail to previous work, providing a means to resolve many of the apparently contradictory ideas in the literature concerning the photophysics of conjugated polymer films.

On mechanisms of population of spin substates of polaron pairs

Two mechanisms of formation of polaron pairs at photoexcitation of organic polymeric semiconductors are considered. The first one consists in charge-transfer process from excited singlet precursor and the second assumes formation of polaron pairs at recombination of free charge carriers, which are generated first. Model of population of polaron pairs is critical for estimation of recombination rates of the pairs on the basis of experimental data. Picosecond time-resolved experiments performed by the two-correlated-pulses technique are considered, which are in favor of the first model. Consideration is given to the problem of intra-chain triplet exciton formation at geminate recombination of triplet polaron pairs in a DC magnetic field taking into account a dependence of the annihilation rate on the population of magnetic sublevels of excitons. Recombination rates of polaron pairs obtained in the frameworks of two models are compared giving opposite relation between the rates for singlet and triplet polaron pairs.

Effect of interchain interactions on the absorption and emission of poly(3-hexylthiophene)

The absorption spectrum of polythiophene and its derivative poly(3-hexylthiophene) (P3HT) is usually described in terms of an intrachain exciton coupled to a single phonon mode. We show that this model is too simplistic for highly ordered, regioregular P3HT and that, analogous to the case of charged polarons in this material, interchain interactions must be taken into account to correctly describe the absorption spectrum. We show that the lowest energy feature in the $\ensuremath{\pi}\ensuremath{-}{\ensuremath{\pi}}^{*}$ region of the absorption spectrum is associated with an interchain absorption, the intensity of which is correlated with the degree of order in the polymer. Correspondingly, we show that the emission from P3HT also exhibits contributions from both interchain and intrachain states, in a manner similar to that recently shown for poly(phenylenevinylene). Having reinterpreted the physical origin of the features in the absorption and emission spectra of P3HT, we then model these spectra and show how they evolve as the degree of order in the polymer is changed by varying several physical parameters including temperature and regioregularity of the polymer.

Spectroscopic Studies of Photoexcitations in Regioregular and Regiorandom Polythiophene Films

Using a variety of optical probe techniques we studied the steady state and transient dynamics of charged and neutral photoexcitations in thin films of poly-3-alkyl thiophene with regioregular order, which forms self-assembled lamellae structures with increased interchain interaction, as well as regiorandom order that keeps a chain-like morphology. In regiorandom polythiophene films we found that intrachain excitons with correlated photoinduced absorption and stimulated emission bands are the primary photoexcitations; they give rise to a moderately strong photoluminescence band, and long-lived triplet excitons and intrachain charged polarons. In regioregular polythiophene films, on the contrary we found that the primary photoexcitations are excitons with much larger interchain component; this results in lack of stimulated emission, vanishing intersystem crossing, and a very weak photoluminescence band. The long-lived photoexcitations in regioregular polythiophene films are interchain excitons and delocalized polarons (DP) within the lamellae, with very small relaxation energy. The characteristic properties of the DP species are thoroughly investigated as a function of the alkyl side group of the polymer backbone, film deposition conditions and solvents used, as well as at high hydrostatic pressure. The quantum interference between the low energy absorption band of the DP species and a series of photoinduced infrared active vibrations, which give rise to antiresonances that are superimposed on the electronic absorption band is studied and explained by a Fano-type interference mechanism, using the amplitude mode model.

Photophysics of pristine andC60-doped disubstituted polyacetylene

Using a variety of steady-state spectroscopies we studied the long-lived photoexcitations and electronic excited states of poly disubstituted acetylene $(\mathrm{PDPA}\ensuremath{-}n\mathrm{Bu}),$ the backbone structure of which is a disubstituted trans-polyacetylene, as well as $\mathrm{PDPA}\ensuremath{-}n\mathrm{Bu}/{\mathrm{C}}_{60}$ blends. The cw spectroscopies include absorption, photoluminescence (PL), photoinduced absorption (PA), and PA-detected magnetic resonance (PADMR). Although $\mathrm{PDPA}\ensuremath{-}n\mathrm{Bu}$ is a degenerate ground-state polymer, nevertheless, and in contrast to trans-polyacetylene, we found that it has a strong PL band with quantum efficiency larger than 60%. From polarized PL studies on stretched oriented films we conclude that the PL emission originates from intrachain excitons rather than the polymer side groups. This shows that the lowest-lying exciton in $\mathrm{PDPA}\ensuremath{-}n\mathrm{Bu}$ is a ${B}_{u}$ state rather than an ${A}_{g}$ state, in contrast to the order of the lowest-lying excitonic states in trans-polyacetylene. The polarized absorption in $\mathrm{PDPA}\ensuremath{-}n\mathrm{Bu}$ contains three distinctly different bands with different polarization properties, which are interpreted according to the model of Shukla and Mazumdar. The PA spectra of pristine and photo-oxidized $\mathrm{PDPA}\ensuremath{-}n\mathrm{Bu}$ films show neutral and charged solitons as well as polaron excitations, whereas the long-lived photoexcitations in $\mathrm{PDPA}\ensuremath{-}n\mathrm{Bu}$ in solution are mainly polarons. This may be due to destabilization of the soliton-antisoliton pairs in the polymer chains in solution caused by short conjugation length. From the PA and PADMR spectroscopies of $\mathrm{PDPA}\ensuremath{-}n\mathrm{Bu}/{\mathrm{C}}_{60}$ blends we conclude that a photoinduced charge-transfer reaction takes place, again in contrast to blends of other nonluminescent polymers. Interestingly the PA spectrum of $\mathrm{PDPA}\ensuremath{-}n\mathrm{Bu}/{\mathrm{C}}_{60}$ blends shows both charged polarons and charged solitons that are correlated with the PA band of ${\mathrm{C}}_{60}^{\ensuremath{-}}.$ We found that the ratio between charged solitons and polaron excitations depends on the ${\mathrm{C}}_{60}$ concentration in the blend, the film morphology, and temperature. It is shown that the mechanism by which two polarons on the same $\mathrm{PDPA}\ensuremath{-}n\mathrm{Bu}$ chain recombine into two charged solitons is the dominant process that determines the soliton/polaron ratio in the blends.

Interaction of intermediate excited states and formation of free charge carriers under laser excitation of a conjugated polymer

Picosecond time-resolved technique, based on correlated action of periodically repeated pairs of femtosecond laser pulses of 400 nm wavelength and recording of average photocurrent, was applied for studying the mechanism of free charge carrier formation in ladder type poly( p -phenylene) (MeLPPP). The technique distinguishes selectively components of the photoconductivity caused by second-order processes between intermediate species formed under the light absorption and which are responsible for cw photoconductivity. It was shown that though intra-chain excitons take part in those processes, an essential part of charge carriers is generated very quickly (within 1 ps). These primarily formed charge carriers are however not free, belonging to polaron pairs. The latter are demonstrated to be precursors of free polarons. It is because of their ability to suffer electric-field-assisted dissociation and to take part in random recombination reactions that generate free polarons though at a price of destroying more polarons inside the pairs. General scheme of free charge carrier formation, based on the results obtained, is presented.

Intrachain exciton quenching analysis in conjugated polymers by two-photon spectroscopy

Intrachain exciton quenching in ladder-type polyparaphenylene has been quantified for the first time. Two-phonon absorption and subsequent upconverted photoluminescence of ladder-type polyparaphenylene and polyfluorene has been measured and analyzed with a new theoretical framework for the two-photon absorption saturation. This microscopic analysis determines the number of singlet excitons as a function of the excitation intensity, which is compared to the photoluminescence intensity. The absorption saturation alone perfectly describes the photoluminescence saturation in polyfluorene, whereas in ladder-type polyparaphenylene, exciton–exciton quenching occurs. By measuring in solution, the formation of stacked polymer chains is prevented. Therefore, the quenching is due to intrachain interaction.

Photoexcitation dynamics in regioregular and regiorandom polythiophene films

Using a variety of optical probes techniques we studied the photoexcitation dynamics in thin films of poly-3-hexyl thiophene with regioregular order that forms lamellae structures with increased interchain interaction, as well as regiorandom order that keeps a chainlike morphology. In regiorandom films we found that intrachain excitons with correlated induced absorption and stimulated emission bands are the primary excitations; they give rise to a moderately strong photoluminescence band. In regioregular films, on the contrary, we found that the primary excitations are excitons with a much larger interchain component; this results in lack of stimulated emission, vanishing intersystem crossing, and very weak photoluminescence band. In regioregular films we also measured photogenerated geminate polaron pairs with ultrafast dynamics that are precursor to long-lived polaron excitations.

Controlling the optical properties of a conjugated co-polymer through variation of backbone isomerism and the introduction of carbon nanotubes

The need to control the formation of weakly emitting species in polymers such as aggregates and excimers, which are normally detrimental to device performance, is illustrated for the example of the polymer poly( m-phenylenevinylene-co-2,5-dioctyloxy- p-phenylenevinylene), using the model compound, 2,5-dioctyloxy- p-distyrylbenzene as a comparison. Two different methods, namely a Horner–Emmons polycondensation in dimethylformamide (DMF) and a Wittig polycondensation in dry toluene, have been used during synthesis resulting in a polymer with a predominantly trans-vinylene backbone and a polymer with a predominantly cis-vinylene backbone, respectively. Photoluminescence and absorption spectroscopy indicate that the polymer forms aggregate species in solution with spectra that are distinctly red-shifted from those associated with the intra-chain exciton. Concentration dependent optical studies were used to probe the evolution of aggregation in solution for both polymers. The results indicate that inter-chain coupling in the predominantly cis-polymer is prominent at lower concentrations than in the case of the trans-counterpart. These results are supported by pico-second pump and probe transient absorption measurements where, in dilute solutions, the polymer in a cis-configuration exhibits highly complex excited state dynamics, whereas the polymer in a trans-configuration behaves similarly to the model compound. It is proposed therefore that the degree of backbone isomerism has a profound impact on the morphology of the polymeric solid and control over it is a route towards optimising the performance of the material in thin film form. Another method to inhibit inter-chain effects using multi walled carbon nanotubes (MWNT) as nano-spacers in the polymer solutions is proposed. By comparison to spectroscopic analysis, aggregation effects are shown to be reduced by the introduction of nanotubes. Electron microscopy and computer simulation suggest a well-defined interaction between the polymer backbone and the lattice of the nanotube.

Intra- and Interchain Luminescence in Amorphous and Semicrystalline Films of Phenyl-Substituted Polythiophene

We report time-resolved photoluminescence (PL) measurements of spin-cast amorphous films of a regioregular polythiophene derivative poly[3-(4-octylphenyl)thiophene] and of annealed semicrystalline films of the same polymer with an interchain stacking distance of 5 Å. Red-shifted PL appears at long delay times in both pristine and annealed films, which we assign to interchain aggregates populated by excitation energy migration. Aggregate luminescence in annealed films exhibits a pronounced vibronic structure indicating the coupling to a CC bond stretch with a Huang−Rhys factor S = 2. Two types of aggregates are distinguished in annealed films: in a few picoseconds most excitations are trapped by aggregates with a large energy gap (about 1.83 eV) between the lowest excited singlet state and the ground state. Excitation energy is transferred or the aggregates relax structurally with a time constant of about 200 ps to aggregates with a smaller energy gap (about 1.75 eV). The radiative lifetime of aggregates with smaller energy gap is estimated to be about 35 ns, two times longer than that of aggregates with the larger energy gap. In spin-cast amorphous films, emission from intrachain singlet excitons has a longer lifetime than in annealed films and only high-energy-gap aggregates are populated.

Singlet Exciton Migration in a ConjugatedPolymer by Picosecond Time-ResolvedPhotoluminescence

The transient photoluminescence (PL) of DO-PPV(poly-(2,5-dioctyloxy-1,4-phenylene vinylene)) solution in chloroform wasinvestigated by picosecond time-resolved PLspectroscopy. An ultrafast rise of PL and the followingsingle exponential decay with a time constant of about 400 ps were assigned tothe formation of the intrachain exciton and its decay process, respectively. Theredshift of the PL emission spectrum with time was caused by the subsequentexciton migration among the different conjugated segments in the DO-PPVpolymer.

Luminescence from inter-chain aggregates in polythiophene films

We report time-resolved photoluminescence of two polythiophene derivatives with different chain packing. Emission of inter-chain aggregates is distinguished from that of intra-chain singlet excitons. The aggregate luminescence is shifted towards lower energies relative to that of intra-chain exciton by 0.17 eV, and the Huang-Rhys factor of the coupling to the intra-chain C=C vibration is S = 1.5. The excitonic coupling in aggregates is of intermediate strength.

Interchain photoluminescence in substituted polyfluorenes

We have characterised the interchain emitting states of two substituted polyfluorenes with steady-state and time-resolved photoluminescence (PL). Site selective excitation of the interchain state shows that physical dimers are formed in the film. These dimers have low concentration in the films but exciton diffusion to the dimers is reducing the intrachain exciton lifetime. With asymmetrical substituents, the PL quantum yield is higher and the PL lifetime is longer due to lower concentration of dimers. Slow cooling from the melt, gives a new red shifted emission band and less intrachain emission. A discrepancy in the calculated radiative lifetimes suggest a fast formation of this new band.