Browsing by Author "Salzner, Ulrike"

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A Twist in biphthalimide-based chromophores enables thermally activated delayed fluorescence
(American Chemical Society, 2024-04-18) Debnath, S.; Ramkissoon, P.; Vonder Haar, A.L.; Salzner, Ulrike; Smith, T.A.; Musser, A.J.; Patil, S.
Thermally activated delayed fluorescence (TADF) emitters, which convert nonemissive triplets into emissive singlets, have garnered tremendous impetus as next-generation organic electroluminescent materials. Employing donor−acceptor (D−A) designs to produce intramolecular charge transfer (ICT) states is considered an attractive strategy to effectively reduce the singlet− triplet ($ΔE_{ST}$) gap, thereby enhancing reverse intersystem crossing (rISC) in TADF emitters. Herein, we report two ICT chromophores (BP-1TPA and BP-2TPA) utilizing a rational design strategy based on a twisted biphthalimide acceptor core integrated with varying triphenylamine donors. We accomplish efficient TADF emission with a high photoluminescence quantum yield (PLQY) of ∼80% at ambient conditions from poly(methyl methacrylate)-doped films of these chromophores. Twisting the acceptor core ensures the separation of natural transition orbitals, leading to small $ΔE_{ST}$ and generates an intermediate triplet excited state to facilitate rISC. The present study, therefore, sheds light on how delayed fluorescence can be realized from a simple twisted phthalimide core by rational molecular engineering and enables new insights toward exploring the aromatic imide class of molecules as potential organic light-emitting materials.
Open Access
Conjugated organic polymers: from bulk to molecular wire
(American Scientific Publishers , 2006) Salzner, Ulrike; Rieth, M.; Schommers, W.
Open Access
Deciphering intramolecular charge transfer in fluoranthene derivatives
(Royal Society of Chemistry, 2024-05-24) Debnath, S.; Mohanty, A.; Naik, P.; Salzner, Ulrike; Dasgupta, J.; Patil, S.
An intramolecular charge transfer (ICT) state evolves via excited state structural change and solvent reorganization, where the charge distribution in the excited state is markedly different from that in the ground state. Due to its ubiquitous nature, this intriguing photophysical phenomenon offers promising applications in the realm of optoelectronics. Judicious choice of donor-acceptor-based (D-A) push-pull chromophores is the most fundamental strategy to achieve ICT state formation. In the present work, we introduce a unique class of fluoranthene-based chromophores that do not belong to the conventional D-A design principle, as the fluoranthene core is seldom used as an acceptor. Nevertheless, we observe ICT state formation upon attaching strong donor triphenylamine to the fluoranthene backbone (TPF-2TPA). Theoretical studies demonstrate that the hole and electron densities are localized over the triphenylamine and fluoranthene core, respectively, which induces ICT character in the lowest energy transition of TPF-2TPA. Solvent polarity-dependent steady-state and time-resolved spectroscopic studies confirm the formation of the ICT state. Furthermore, viscosity-dependent study of TPF-2TPA reveals the involvement of a structural relaxation during ICT state formation. The present study sheds light on the rational design of unconventional ICT chromophores based on fluoranthene, thereby widening the applications of fluoranthene-based molecular systems in optoelectronic devices.
Open Access
Electrochemical polymerization of ambipolar carbonyl-functionalized indenofluorene with memristive properties
(Elsevier, 2019) Figà, V.; Usta, H.; Macaluso, R.; Salzner, Ulrike; Özdemir, M.; Kulyk, B.; Krupka, O.; Bruno, M.
Carbonyl-functionalized indenofluorene was electropolymerized with a high faradaic efficiency of 85% and the solid state properties of the resulting polymeric thin films were investigated. They displayed modular optical properties depending on their oxidation state. The approach used for inorganic semiconductors was applied to polyindeonofluorene derivative. Mott-Schottky analysis evidenced a switching from p-type to n-type electrical conduction, suggesting an ambipolar behaviour of the polymer. As an application, flexible organic memristors were fabricated and resistive switching properties were observed.
Open Access
Electrochemical stability and ambipolar charge transport in diketopyrrolopyrrole-based organic materials
(ACS, 2019) Ray, S.; Panidi, J.; Mukhopadhyay, T.; Salzner, Ulrike; Anthopoulos, T. D.; Patil, S.
An important strategy for realizing flexible complementary circuits with organic semiconductors is to achieve balanced ambipolar charge transport properties with reduced anisotropy. Here, we present a series of star-shaped diketopyrrolopyrrole (DPP)-based organic materials synthesized for improved intermolecular charge transport while retaining the ambipolar charge transport properties of their linear counterparts. Steady-state UV−visible spectroscopic studies confirm that the oligomers are highly aggregated in the thin film as evidenced from appearance of prominent vibronic features and red-shifted absorption bands. Ambipolar transport properties of these materials were verified in organic field-effect transistors (OFETs). The results show that the star-shaped DPP systems have the potential to outperform their linear counterparts in devices
Open Access
Fermi level pinning ınduced by doping in air stable n type organic semiconductor
(American Chemical Society, 2020) Sharma, S.; Ghosh, S.; Ahmed, T.; Ray, S.; Islam, S.; Salzner, Ulrike; Ghosh, A.; Seki, S.; Patil, S.
Doping of organic semiconductors enhances the performance of optoelectronic devices. Although p-type doping is well studied and successfully deployed in optoelectronic devices, air stable ntype doping was still elusive. We succeeded with n-type doping of organic semiconductors using molecular dopant N-DMBI under ambient conditions. Strikingly, n-type doping accounts for a gigantic increase of the photoconductivity of doped thin films. Electrical and optical properties of the n-doped molecular semiconductor were investigated by temperature dependent conductivity, electron paramagnetic resonance (EPR), and flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements. A significant reduction and saturation in activation energy with increasing doping level clearly suggests the formation of an impurity band and enhancement in carrier density. Computational studies reveal the formation of a charge transfer complex mediated by hydrogen abstraction as the rate-determining step for the doping mechanism. The colossal enhancement of photoconductivity induced by n-doping is a significant step toward optoelectronic devices made of molecular semiconductors.
Open Access
Optoelectronic properties of diketopyrrolopyrrole homopolymers compared to donor–acceptor copolymers
(A I P Publishing LLC, 2021-02-04) Salzner, Ulrike
Diketopyrrolopyrrole (DPP) is a component of a large number of materials used for optoelectronic applications. As it is exclusively used in combination with aromatic donors, the properties of its homopolymers are unknown. Because donor–acceptor character has been shown for other systems to reduce bandwidths, DPP homopolymers should have even larger conduction bands and better n-type conductivity than the thiophene-flanked systems, which have exceptional n-type conductivity and ambipolar character. Therefore, a theoretical study was carried out to elucidate the properties of the unknown DPP homopolymer. Calculations were done with density functional theory and with the complete active space self-consistent field method plus n-electron valence state perturbation theory for the dynamic correlation. Poly-DPP is predicted to have radical character and an extremely wide low-lying conduction band. If it were possible to produce this material, it should have unprecedented n-type conductivity and might be a synthetic metal. A comparison with various unknown donor–acceptor systems containing vinyl groups and thienyl rings with a higher concentration of DPP than the known copolymers reveals how donor–acceptor substitution reduces bandwidths and decreases electron affinities.
Open Access
Spin density encodes intramolecular singlet exciton fission in pentacene dimers
(Nature Publishing Group, 2019) Krishnapriya, K. C.; Roy, P.; Puttaraju, B.; Salzner, Ulrike; Musser, A. J.; Jain, M.; Dasgupta, J.; Patil, S.
The formation of two triplet excitons at the cost of one photon via singlet exciton fission in organic semiconductors can potentially enhance the photocurrent in photovoltaic devices. However, the role of spin density distribution in driving this photophysical process has been unclear until now. Here we present the significance of electronic spin density distribution in facilitating efficient intramolecular singlet exciton fission (iSEF) in π-bridged pentacene dimers. We synthetically modulate the spin density distribution in a series of pentacene dimers using phenyl-, thienyl- and selenyl- flanked diketopyrrolopyrrole (DPP) derivatives as π-bridges. Using femtosecond transient absorption spectroscopy, we find that efficient iSEF is only observed for the phenyl-derivative in ~2.4 ps while absent in the other two dimers. Electronic structure calculations reveal that phenyl-DPP bridge localizes α- and β-spin densities on distinct terminal pentacenes. Upon photoexcitation, a spin exchange mechanism enables iSEF from a singlet state which has an innate triplet pair character.