The singlet–triplet energy gap in organic and Pt-containing phenylene ethynylene polymers and monomers

Bimetallic ethynylanthracenyl functionalised carbynes

Mono- and bimetallic anthracenes functionalised by alkynyl and alkylidynyl substituents are obtained via sequential cross-coupling reactions of the 9-bromoanthracenyl carbyne [W{CC(C₆H₄)₂CBr}(CO)₂(Tp*)].

Symmetric and non-symmetric anthracen-diyl bis(alkylidynes)

Three examples of 9-bromo-10-(alkylidynyl)anthracenes, [W{CC(C₆H₄)₂CBr}(CO)₂(L)] (L = hydrotris(dimethylpyrazol-1-yl)borate Tp*, hydrotris(pyrazol-1-yl)borate Tp, hydrotris(2-mercapto-N-methylimidazol-1-yl)borate Tm) were prepared via modified Fischer-Mayr acyl oxide-abstraction protocols. With a sufficiently bulky ancillary ligand (L = Tp*) the aryl bromide is ammenable to cross-coupling reactions that enable more elaborate derivatives to be prepared. These including symmetric bis(alkylidynyl)anthracenes as well as non-palindromic examples bearing disparate metals and/or co-ligands. In contrast, these couplings fail for smaller ligands (L = Tp, Tm) where it was found that Pd⁰ or Pt⁰ were instead able to coordinate across two WC bonds to give trimetallic bow-tie complexes, [W₂M{μ-CC(C₆H₄)₂CBr}₂(CO)₄(L)₂] (M = Pd, Pt; L = Tp, Tm).

Al-Balushi R, Rather JA, Munam A, Ilmi R, Raithby PR, Zhang Y, Fu Y, Xie Z, Chen S, Islam SM, Wong WY, Skelton JM, Khan MS. Synthesis, characterization, and optoelectronic properties of phenothiazine-based organic co-poly-ynes

We present the synthesis of seven new organic co-poly-ynes P1–P7 incorporating phenothiazine (PTZ) motif and evaluate their optoelectronic properties and performance in polymer light-emitting diodes (PLEDs) and polymer solar cells (PSCs).

Sensitizing TADF Absorption Using Variable Length Oligo(phenylene ethynylene) Antennae

Beyond their applications in organic light-emitting diodes (OLEDs), thermally activated delayed fluorescence (TADF) materials can also make good photonic markers. Time-gated measurement of their delayed emission enables “background-free” imaging in, for example, biological systems, because no naturally-occurring compounds exhibit such long-lived emission. Attaching a strongly-absorbing antenna, such as a phenylene ethynylene oligomer, to the TADF core would be of interest to increase their brightness as photonic markers. With this motivation, we study a sequence of TADF-oligomer conjugates with oligomers of varying length and show that, even when the absorption of the oligomer is almost resonant with the charge-transfer absorption of the TADF core, the antenna transfers energy to the TADF core. We study this series of compounds with time resolved emission and transient absorption spectroscopy and find that the delayed fluorescence is essentially turned-off for the longer antennae. Interestingly, we find that the turn-off of the delayed fluorescence is not caused by quenching of the TADF charge-transfer triplet state due to triplet energy transfer of the lower-lying triplet state to the antenna, but must be associated with a decrease in the reverse intersystem crossing rate. These results are of relevance for the further development of TADF “dyes” and also, in the broader context, for understanding the dynamics of TADF molecules in the vicinity of energy donors/acceptors (i.e., in fluorescent OLEDs wherein TADF molecules are used as an assistant dopant).

Polaron and Exciton Delocalization in Oligomers of High-Performance Polymer PTB7

A key characteristic of organic photovoltaic cells is the efficient charge separation in the active layer. Sufficient delocalization of the positive polaron in organic photovoltaics is considered essential for the effective separation of the opposite charges and the suppression of recombination. We use light-induced EPR and ENDOR spectroscopy combined with DFT calculations to determine the electronic structure of the positive polaron in PTB7-type oligomers. Utilizing the superior spectral resolution of high-frequency (130 GHz) D-band EPR, the principal components of the g tensors were determined. Pulsed ENDOR spectroscopy at X-band allowed the measurement of ¹H hyperfine coupling constants. A comparison of g tensors and ¹H hyperfine coupling constants of the PTB7-type oligomers with the high-performance PTB7 polymer revealed a delocalization of the positive polaron in the polymer over about four monomeric units, corresponding to about 45 Å in length. Our current study thus not only determines the polaron delocalization length in PTB7 but also validates the approach combining EPR/ENDOR spectroscopy with DFT-calculated magnetic resonance parameters. This is of importance in those cases where oligomers of defined length are not easily obtained. In addition, the delocalization of the neutral triplet exciton was also determined in the oligomers and compared with polymer PTB7. The analysis revealed that the neutral triplet exciton is substantially more delocalized than the positive polaron, exceeding 10 monomeric units.

Excitation wavelength dependent emission of silver(i) complexes with a pyrimidine ligand

Interplay of three emission mechanisms for silver(i) complexes leads to luminescence thermochromism and color tunable emission.

Stereochemical Effects on Platinum Acetylide Two-Photon Chromophores

A series of cis-platinum(II) acetylide complexes containing two-photon-absorbing chromophores have been synthesized and characterized to explore the effects of stereochemistry on the nonlinear absorption properties. The molecules feature 4-(phenylethynyl)phenylethynylene (PE2), diphenylaminofluorene (DPAF), and benzothiazolylfluorene (BTF) ligands. The photophysical properties were investigated under one- and two-photon conditions and compared to the known trans analogues via UV-visible absorption, photoluminescence, femtosecond and nanosecond transient absorption (TA), nanosecond z-scan, and femtosecond two-photon absorption (2PA). The bent cis complexes exhibit blue shifts in the absorption, emission, femtosecond, and nanosecond TA spectra along with lower molar extinction coefficients and lower phosphorescence yields relative to the trans complexes suggesting less efficient Pt-induced spin-orbit coupling and intersystem crossing in the cis configuration. The cis chromophores are noncentrosymmetric and therefore show dipolar behavior with a pronounced 2PA in the 0-0 transition of the S₀ → S₁ band, while the trans complexes show quadrupolar behavior with a forbidden 0-0 transition. In the S₀ → S_n region, both cis and trans complexes show intense two-photon-absorption bands (up to 3700 GM by the peak cross section for cis-BTF) which contain a significant contribution from the excited state absorption (S₁ → S_n). All six complexes exhibit comparable nonlinear absorption response with a significant contribution from triplet-triplet absorption that slightly favors trans complexes but is more strongly dependent upon the structure of the π-conjugated chromophore.

Limitations and Perspectives on Triplet-Material-Based Organic Photovoltaic Devices

Organic photovoltaic cells (OPVs) have attracted broad attention and become a very energetic field after the emergence of nonfullerene acceptors. Long-lifetime triplet excitons are expected to be good candidates for efficiently harvesting a photocurrent. Parallel with the development of OPVs based on singlet materials (S-OPVs), the potential of triplet materials as photoactive layers has been explored. However, so far, OPVs employing triplet materials in a bulk heterojunction have not exhibited better performance than S-OPVs. Here, the recent progress of representative OPVs based on triplet materials (T-OPVs) is briefly summarized. Based on that, the performance limitations of T-OPVs are analyzed. The shortage of desired triplet materials with favorable optoelectronic properties for OPVs, the tradeoff between long lifetime and high binding energy of triplet excitons, as well as the low charge mobility in most triplet materials are crucial issues restraining the efficiencies of T-OPVs. To overcome these limitations, first, novel materials with desired optoelectronic properties are urgently demanded; second, systematic investigation on the contribution and dynamics of triplet excitons in T-OPVs is necessary; third, close multidisciplinary collaboration is required, as proved by the development of S-OPVs.

Rise of Conjugated Poly-ynes and Poly(Metalla-ynes): From Design Through Synthesis to Structure-Property Relationships and Applications

Conjugated poly-ynes and poly(metalla-ynes) constitute an important class of new materials with potential application in various domains of science. The key factors responsible for the diverse usage of these materials is their intriguing and tunable chemical and photophysical properties. This review highlights fascinating advances made in the field of conjugated organic poly-ynes and poly(metalla-ynes) incorporating group 4-11 metals. This includes several important aspects of conjugated poly-ynes viz. synthetic protocols, bonding, electronic structure, nature of luminescence, structure-property relationships, diverse applications, and concluding remarks. Furthermore, we delineated the future directions and challenges in this particular area of research.

Multiply “trapped” 3[trans-Pt(PR3)2(CCC6H4X)2]* conformers in rigid media

The triplet emission exhibits multiple well-defined decays depending on the θ₁ and θ₂ values and environment in the crystal sites.

Computational study on thermally activated delayed fluorescence of donor–linker–acceptor network molecules

Using density functional theory we have investigated the structure–property relationships of organic molecules with a donor–linker–acceptor (DLA) framework, which can be used as precursors of OLED materials.

π-Conjugated Organometallic Isoindigo Oligomer and Polymer Chromophores: Singlet and Triplet Excited State Dynamics and Application in Polymer Solar Cells

An isoindigo based π-conjugated oligomer and polymer that contain cyclometalated platinum(II) "auxochrome" units were subjected to photophysical characterization, and application of the polymer in bulk heterojunction polymer solar cells with PCBM acceptor was examined. The objective of the study was to explore the effect of the heavy metal centers on the excited state properties, in particular, intersystem crossing to a triplet (exciton) state, and further how this would influence the performance of the organometallic polymer in solar cells. The materials were characterized by electrochemistry, ground state absorption, emission, and picosecond-nanosecond transient absorption spectroscopy. Electrochemical measurements indicate that the cyclometalated units have a significant impact on the HOMO energy level of the chromophores, but little effect on the LUMO, which is consistent with localization of the LUMO on the isoindigo acceptor unit. Picosecond-nanosecond transient absorption spectroscopy reveals a transient with ∼100 ns lifetime that is assigned to a triplet excited state that is produced by intersystem crossing from a singlet state on a time scale of ∼130 ps. This is the first time that a triplet state has been observed for isoindigo π-conjugated chromophores. The performance of the polymer in bulk heterojunction solar cells was explored with PC61BM as an acceptor. The performance of the cells was optimum at a relatively high PCBM loading (1:6, polymer:PCBM), but the overall efficiency was relatively low with power conversion efficiency (PCE) of 0.22%. Atomic force microscopy of blend films reveals that the length scale of the phase separation decreases with increasing PCBM content, suggesting a reason for the increase in PCE with acceptor loading. Energetic considerations show that the triplet state in the polymer is too low in energy to undergo charge separation with PCBM. Further, due to the relatively low LUMO energy of the polymer, charge transfer from the singlet to PCBM is only weakly exothermic, which is believed to be the reason that the photocurrent efficiency is relatively low.

Transient EPR Reveals Triplet State Delocalization in a Series of Cyclic and Linear π-Conjugated Porphyrin Oligomers

The photoexcited triplet states of a series of linear and cyclic butadiyne-linked porphyrin oligomers were investigated by transient Electron Paramagnetic Resonance (EPR) and Electron Nuclear DOuble Resonance (ENDOR). The spatial delocalization of the triplet state wave function in systems with different numbers of porphyrin units and different geometries was analyzed in terms of zero-field splitting parameters and proton hyperfine couplings. Even though no significant change in the zero-field splitting parameters (D and E) is observed for linear oligomers with two to six porphyrin units, the spin polarization of the transient EPR spectra is particularly sensitive to the number of porphyrin units, implying a change of the mechanism of intersystem crossing. Analysis of the proton hyperfine couplings in linear oligomers with more than two porphyrin units, in combination with density functional theory calculations, indicates that the spin density is localized mainly on two to three porphyrin units rather than being distributed evenly over the whole π-system. The sensitivity of the zero-field splitting parameters to changes in geometry was investigated by comparing free linear oligomers with oligomers bound to a hexapyridyl template. Significant changes in the zero-field splitting parameter D were observed, while the proton hyperfine couplings show no change in the extent of triplet state delocalization. The triplet state of the cyclic porphyrin hexamer has a much decreased zero-field splitting parameter D and much smaller proton hyperfine couplings with respect to the monomeric unit, indicating complete delocalization over six porphyrin units in this symmetric system. This surprising result provides the first evidence for extensive triplet state delocalization in an artificial supramolecular assembly of porphyrins.

Model for triplet state engineering in organic light emitting diodes

Engineering the position of the lowest triplet state (T1) relative to the first excited singlet state (S1) is of great importance in improving the efficiencies of organic light emitting diodes and organic photovoltaic cells. We have carried out model exact calculations of substituted polyene chains to understand the factors that affect the energy gap between S1 and T1. The factors studied are backbone dimerisation, different donor-acceptor substitutions, and twisted geometry. The largest system studied is an 18 carbon polyene which spans a Hilbert space of about 991 × 10(6). We show that for reverse intersystem crossing process, the best system involves substituting all carbon sites on one half of the polyene with donors and the other half with acceptors.

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 10(5) 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.

The role of C-H and C-C stretching modes in the intrinsic non-radiative decay of triplet states in a Pt-containing conjugated phenylene ethynylene

The intrinsic non-radiative decay (internal conversion) from the triplet excited state in phosphorescent dyes can be described by a multi-phonon emission process. Since non-radiative decay of triplet excitons can be a significant process in organic light-emitting diodes, a detailed understanding of this decay mechanism is important if the overall device efficiency is to be controlled. We compare a deuterated Pt(II)-containing phenylene ethynylene with its non-deuterated counterpart in order to investigate which phonon modes control to the non-radiative decay path. We observe that deuteration does not decrease the non-radiative decay rate. A Franck-Condon analysis of the phosphorescence spectra shows that the electronic excitation is coupled strongly to the breathing mode of the phenyl ring and the C≡C carbon stretching modes, while high-energy C-H or C-D stretching modes play an insignificant role. We, therefore, associate the internal conversion process with the carbon-carbon stretching vibrations.

A multiconfigurational time-dependent Hartree-Fock method for excited electronic states. II. Coulomb interaction effects in single conjugated polymer chains

Conjugated polymers have attracted considerable attention in the last few decades due to their potential for optoelectronic applications. A key step that needs optimisation is charge carrier separation following photoexcitation. To understand better the dynamics of the exciton prior to charge separation, we have performed simulations of the formation and dynamics of localised excitations in single conjugated polymer strands. We use a nonadiabatic molecular dynamics method which allows for the coupled evolution of the nuclear degrees of freedom and of multiconfigurational electronic wavefunctions. We show the relaxation of electron-hole pairs to form excitons and oppositely charged polaron pairs and discuss the modifications to the relaxation process predicted by the inclusion of the Coulomb interaction between the carriers. The issue of charge photogeneration in conjugated polymers in dilute solution is also addressed.

Synthesis and characterization of platinum(II) di-ynes and poly-ynes incorporating ethylenedioxythiophene (EDOT) spacers in the backbone

A series of trimethylsilyl-protected di-alkynes incorporating 3,4-ethylenedioxythiophene (EDOT) linker groups Me(3)Si-C≡C-R-C≡C-SiMe(3) (R = ethylenedioxythiophene-3,4-diyl 1a, 2,2'-bis-3,4-ethylenedioxythiophene-5,5'-diyl 2a, 2,2',5',2''-ter-3,4-ethylenedioxythiophene-5,5''-diyl 3a) and the corresponding terminal di-alkynes, H-C≡C-R-C≡C-H 1b-2b has been synthesized and characterized and the single crystal X-ray structure of 1a has been determined. CuI-catalyzed dehydrohalogenation reaction between trans-[(Ph)(Et(3)P)(2)PtCl] and the terminal di-alkynes 1b-2b in (i)Pr(2)NH/CH(2)Cl(2) (2:1 mole ratio) gives the Pt(II) di-ynes trans-[(Et(3)P)(2)(Ph)Pt-C≡C-R-C≡C-Pt(Ph)(Et(3)P)(2)] 1M-2M while the dehydrohalogenation polycondensation reaction between trans-[((n)Bu(3)P)(2)PtCl(2)] and 1b-2b (1:1 mole ratio) under similar reaction conditions affords the Pt(II) poly-ynes trans-[Pt(P(n)Bu(3))(2)-C≡C-R-C≡C-](n)1P-2P. The di-ynes and poly-ynes have been characterized spectroscopically and, for 1M and 2M, by single-crystal X-ray which confirms the "rigid rod" di-yne backbone. The materials possess excellent thermal stability, are soluble in common organic solvents and readily cast into thin films. Optical absorption spectroscopic measurements reveal that the EDOT spacers create stronger donor-acceptor interactions between the platinum(II) centres and conjugated ligands along the rigid backbone of the organometallic polymers compared to the related non-fused and fused oligothiophene spacers.

Negative polaron and triplet exciton diffusion in organometallic "molecular wires"

The dynamics of negative polaron and triplet exciton transport within a series of monodisperse platinum (Pt) acetylide oligomers is reported. The oligomers consist of Pt-acetylide repeats, [PtL(2)-C≡C-Ph-C≡C-](n) (where L = PBu(3) and Ph = 1,4-phenylene, n = 2, 3, 6, and 10), capped with naphthalene diimide (NDI) end groups. The Pt-acetylide segments are electro- and photoactive, and they serve as conduits for transport of electrons (negative polaron) and triplet excitons. The NDI end groups are relatively strong acceptors, serving as traps for the carriers. Negative polaron transport is studied by using pulse radiolysis/transient absorption at the Brookhaven National Laboratory Laser-Electron Accelerator Facility (LEAF). Electrons are rapidly attached to the oligomers, with some fraction initially residing upon the Pt-acetylide chains. The dynamics of transport are resolved by monitoring the spectral changes associated with transfer of electrons from the chain to the NDI end group. Triplet exciton transport is studied by femtosecond-picosecond transient absorption spectroscopy. Near-UV excitation leads to rapid production of triplet excitons localized on the Pt-acetylide chains. The excitons transport to the chain ends, where they are annihilated by charge separation with the NDI end group. The dynamics of triplet transport are resolved by transient absorption spectroscopy, taking advantage of the changes in spectra associated with decay of the triplet exciton and rise of the charge-separated state. The results indicate that negative polarons and excitons are transported rapidly, on average moving distances of ~3 nm in less than 200 ps. Analysis of the dynamics suggests diffusive transport by a site-to-site hopping mechanism with hopping times of ~27 ps for triplets and <10 ps for electrons.

Recent Progress on the Photonic Properties of Conjugated Organometallic Polymers Built Upon the trans-Bis(para-ethynylbenzene)bis(phosphine)platinum(II) Chromophore and Related Derivatives

This review article surveys the electronic and photophysical properties of conjugated organometallic polymers built upon the title compound and its related derivatives focussing primarily on systems investigated in our laboratories. The general structure of the polymers is (trans-bis(para-ethynylbenzene)bis(phosphine)platinum(II)-G)(n) where G is a conjugated group such as thiophene, fluorene, carbazole, substituted silole, quinone derivative, and metalloporphyrin residue, or a non-conjugated main-group moiety. Systems based on substituted phenylene units and other related fused rings are also discussed. The phosphine ligands are generally triethyl- or tri-n-butylphosphine groups. These trans-platinum(II) polymers and the corresponding model compounds are compared to the corresponding ortho-derivatives in the quinone series, and the newly prepared paracyclophane-containing polymers. For the porphyrin series, a comparison of fully conjugated oligomers exhibiting the general structure (trans-bis(para-ethynyl(zinc(porphyrin)))bis(phosphine)platinum(II))(n) (i.e., the C(6) H(4) group is absent from the main chain) will be made. This contribution also includes a description of the properties of the mononuclear chromophore itself, properties that define those of the polymers. Potential applications with regard to electronic and optical devices will be highlighted. These soluble and stable materials feature both the processing advantages of polymers and the functionality provided by the presence of metal centers. These multifunctional organometallic polyyne polymers exhibit convenient structural variability as well as optical and electronic properties, which renders them important for use in different research domains as chemical sensors and sensor protectors, as converters for light/electricity signals, and as patternable precursors to magnetic metal alloy nanoparticles.