Dynamics of guest molecules in PHTP inclusion compounds as probed by solid-state NMR and fluorescence spectroscopy.

Partially deuterated 1,4-distyrylbenzene () is included into the pseudohexagonal nanochannels of perhydrotriphenylene (PHTP). The overall and intramolecular mobility of is investigated over a wide temperature range by (13)C, (2)H NMR as well as fluorescence spectroscopy. Simulations of the (2)H NMR spectral shapes reveal an overall wobble motion of in the channels with an amplitude of about 4 degrees at T = 220 K and 10 degrees at T = 410 K. Above T = 320 K the wobble motion is superimposed by localized 180 degrees flips of the terminal phenyl rings with a frequency of 10(6) Hz at T = 340 K. The activation energies of both types of motions are around 40 kJ mol(-1) which imply a strong steri…

Combined Spectroscopic and TD-DFT Analysis to Elucidate Substituent and Acidochromic Effects in Organic Dyes: A Case Study on Amino- versus Nitro-Substituted 2,4-Diphenylquinolines

Made available in DSpace on 2020-12-12T01:33:30Z (GMT). No. of bitstreams: 0 Previous issue date: 2020-01-01 A combined spectroscopic and TD-DFT case study was performed, to identify a robust method to calculate the complex near UV/Vis absorption spectra of various amino- vs. nitro-substituted 2,4-diphenylquinolines, which vary strongly under neutral and successively acidic conditions. For this, different DFT functionals were tested for geometry optimization and the TD part to calculate the neutral and different protonated species in a fast screening approach, i. e. using single point calculations in an implicit solvent. Offset-corrected M06HF, hitherto only applied to polymers, was identif…

Twist-Elasticity-Controlled Crystal Emission in Highly Luminescent Polymorphs of Cyano-Substituted Distyrylbenzene (βDCS)

The occurrence of polymorphs in crystals of luminescent π-conjugated organic materials is an intriguing matter, particularly because polymorphs can give rise to widely different broadband and amplified emission properties. Here, a new, highly luminescent polymorph of cyano-substituted distyrylbenzene, being a prominent example of “twist elasticity”—the ability of a molecule to react to external constraints with substantial changes in the torsional coordinates, is investigated. The twist elasticity concept is fully explored here through molecular modeling by comparison of the new polymorph with two known ones exhibiting largely different intra- and intermolecular coordinates. In a second ste…

Luminescence in Crystalline Organic Materials: From Molecules to Molecular Solids

Excited state absorption spectra of dissolved and aggregated distyrylbenzene: A TD-DFT state and vibronic analysis

Made available in DSpace on 2018-12-11T17:33:14Z (GMT). No. of bitstreams: 0 Previous issue date: 2017-07-21 Federación Española de Enfermedades Raras Ministerio de Economía y Competitividad A time-dependent density functional theory study is performed to reveal the excited state absorption (ESA) features of distyrylbenzene (DSB), a prototype π-conjugated organic oligomer. Starting with a didactic insight to ESA based on simple molecular orbital and configuration considerations, the performance of various density functional theory functionals is tested to reveal the full vibronic ESA features of DSB at short and long probe delay times. UNESP-Universidade Estadual Paulista POSMAT-Graduate Pr…

Counterion-Mediated Crossing of the Cyanine Limit in Crystals and Fluid Solution: Bond Length Alternation and Spectral Broadening Unveiled by Quantum Chemistry

Absorption spectra of cyanine⊕·Br⊖ salts show a remarkable solvent dependence in non/polar solvents, exhibiting narrow, sharp band shapes in dichloromethane but broad features in toluene; this chan...

Independent tuning of electronic levels in pentacene by site-specific substitution.

Effective conjugation in conjugated polymers with strongly twisted backbones: A case study on fluorinated MEHPPV

Conjugated polymers with strongly twisted backbones, such as MEHPPV with fluorinated vinylene units (F-MEHPPV), demand a redefinition of the all-important ‘effective conjugation length’ ECL, which we extract here by a facile graphical method. In MEHPPV (being essentially planar), the ECL coincides with the ‘maximum conducive chain length’ MCC and extends over about n ≈ 9 repetition units (RU). In F-MEHPPV, the MCC is similarly long with n ≈ 8, but the ECL localizes on just one RU. The strong twist in F-MEHPPV persists in the excited state, broadening the emission spectrum and quenching the fluorescence with reduced radiative and enhanced non-radiative rates.

Oligophenylenevinylenes in Spatially Confined Nanochannels: Monitoring Intermolecular Interactions by UV/Vis and Raman Spectroscopy

Perhydrotriphenylene-based channel-forming inclusion compounds (ICs) and thin films made of polyphenylenevinylene (PPV)-type oligomers with terminal alkoxy groups are investigated and compared in a combined experimental and theoretical approach. Interchromophore interactions and host-guest interactions are elucidated by UV/Vis and Raman spectroscopy. The impact of the local environment of the chromophore on the optical and photophysical properties is discussed in light of quantum-chemical calculations. In stark contrast to thin films where preferential side-by-side orientation leads to quenching of photoluminescence (PL) via non-emissive traps, the ICs are found to be attractive materials f…

¿Conjugated? Copolymers from a Pechmann Dye Derivative

"Though It Be but Little, It Is Fierce": Excited State Engineering of Conjugated Organic Materials by Fluorination.

Fluorination is frequently used to significantly change the properties of conjugated organic materials due to fluorine’s exceptional properties; well-known is its impact on electronic structure, but it also impacts the geometry despite fluorine’s small size. Less known, the changes in the electronic and geometrical properties may provoke drastic changes of the excited state properties like batho- and hypsochromic shifts of absorption and emission bands (inter alia leading to excited state switching), hypo- and hyperchromic effects, spectral broadening, and changes of the nonradiative deactivation pathways. The state of the art on these issues is summarized in the current Perspective to stim…

Calculation of low bandgap homopolymers: Comparison of TD-DFT methods with experimental oligomer series

Made available in DSpace on 2018-12-11T17:26:45Z (GMT). No. of bitstreams: 0 Previous issue date: 2016-02-01 The performance of different DFT functionals (B3LYP, BHLYP, CAM-B3LYP, M06HF) on the prediction of vertical transition energies Evert of low bandgap homopolymers is tested against the experimentally available oligomer series (thienopyrazines and thienothiophenes). This allows for a detailed and accurate comparison on the consistency of DFT methods for chainlength evolution and polymer limit prediction, and for an understanding of geometry and time-dependent contributions to Evert by combinatorial analysis. Together with former studies on wide/medium bandgap polymers and low bandgap c…

Tuning of the electronic and photophysical properties of ladder-type quaterphenyl by selective methylene-bridge fluorination

The photophysics (spectral positions, band shapes, fluorescence quantum yields and lifetimes) of a series of fluorinated ladder type quaterphenyls L4P and L4P-Fn (n = 2, 4, 6) depend strongly on the degree and position of fluorine, despite the fact that substitution is not performed in the rings but only in methylene-bridges. This is driven by subtle differences in the molecular orbitals (MOs) participating in the electronic transitions, and in the vibronic pattern of the S0 and S1 electronic states as revealed by (TD)DFT calculations. Solid state spectra for n = 0, 2, 4 are similar to those of solution due to small intermolecular interactions as revealed by combined X-ray and (TD)DFT analy…

Assembly-Induced Bright-Light Emission from Solution-Processed Platinum(II) Inorganic Polymers

Synthesis, processing, and characterization are reported for a series of tetracyanoplatinate Magnus' salt (TCN-MS) derivatives-soluble derivatives of the generally intractable Magnus' green salt-that feature the general structure [Pt(NH2R)(4)] [Pt(CN)(4)] where R is a branched alkyl group or a w-phenylalkyl group. In solutions, these coordination compounds generally dissolve on the level of individual ion pairs as shown by X-ray diffraction analysis. To enable the formation of quasi-one-dimensional linear stacks of Pt(II) atoms in thin films, the matrix-assisted assembly is employed, whereby the compounds are codissolved with poly(ethylene oxide) (PEO), followed by film casting, thermally a…

Efficient deep-red light-emitting electrochemical cells based on a perylenediimide-iridium-complex dyad

A two-layer light-emitting electrochemical cell device based on a new perylenediimide-iridium-complex dyad is presented emitting in the deep-red region with high external quantum efficiencies (3.27%). Costa Riquelme, Ruben Dario, Ruben.Costa@uv.es ; Orti Guillen, Enrique, Enrique.Orti@uv.es ; Bolink, Henk, Henk.Bolink@uv.es ; Gierschner, Johannes, Johannes.Gierschner@uv.es

Spectroscopic signatures for planar equilibrium geometries in methyl-substituted oligothiophenes

In recent studies it was demonstrated that temperature-dependent optical spectroscopy is a valuable tool for revealing the differences in the geometries of flexible molecules like oligothiophenes (OTs) in the ground (S0) and first excited (S1) electronic states, by examining the symmetry relations between the absorption and emission spectra: while at low temperature the spectra show mirror symmetry, pointing to planar geometries in S0 and S1, the symmetry relation breaks down at ambient temperature due to thermal population of torsional modes. In the present joint spectroscopic and theoretical study, we demonstrate that this behavior is also observed for di- and tetramethyl-substituted OTs,…

Solid State Luminescence Enhancement in π-Conjugated Materials: Unraveling the Mechanism beyond the Framework of AIE/AIEE

Solid state luminescence enhancement (SLE) of conjugated organic materials has had a great impact in materials science, but a deep understanding has been rather limited to date. Here, we investigate a prototype example of SLE materials, cyano-substituted distyrylbenzene (DCS), by varying systematically and subtly the substitution pattern (inter alia of the position of the cyano-substituent) to give largely different photoresponse in fluid and solid solution as well in the crystalline state. The combination of quantitative (ultra)fast optical spectroscopic techniques, appropriate quantum-chemical methods, and structural (X-ray) data allows us to elucidate and rationalize all details of the S…

Inverted energy gap law for the nonradiative decay in fluorescent floppy molecules: larger fluorescence quantum yields for smaller energy gaps

A data survey on experimental fluorescence quantum yields of (multi)substituted dicyano-distyrylbenzenes in fluid solution evidences that non-radiative decay increases with the Franck-Condon energy (E-FC), being opposite to the conventional energy gap law. Quantum-chemistry indicates that this is controlled by access to the conical intersection (CI) following the Bell-Evans-Polanyi principle as a first-step approximation for this family of molecules; the variations in E-FC among the different compounds are found to be decisive, while those of E-CI are estimated to be weaker or even enhancing the effect. The current findings may have significant consequences for the design of molecules for o…

Bent-core liquid crystalline cyanostilbenes: fluorescence switching and thermochromism

et al.

Excited-state non-radiative decay in stilbenoid compounds: An: Ab initio quantum-chemistry study on size and substituent effects

In the framework of optoelectronic luminescent materials, non-radiative decay mechanisms are relevant to interpret efficiency losses. These radiationless processes are herein studied theoretically for a series of stilbenoid derivatives, including distyrylbenzene (DSB) and cyano-substituted distyrylbenzene (DCS) molecules in vacuo. Given the difficulties of excited-state reaction path determinations, a simplified computational strategy is defined based on the exploration of the potential energy surfaces (PES) along the elongation, twisting, and pyramidalization of the vinyl bonds. For such exploration, density functional theory (DFT), time-dependent (TD)DFT, and complete-active-space self-co…

Oligothienoacenes versus oligothiophenes: Impact of ring fusion on the optical properties

The impact of backbone rigidity on the optical properties of thiophene-based compounds is studied by analyzing in detail the geometrical, electronic, optical and vibronic features of a family of oligothienoacenes (nnTAs) in comparison to non-fused α-oligothiophenes (nnTs) by means of quantum-chemical calculations. Ring fusion in nnTAs provokes a greater conjugation in the ground state. However, the change in the bond length alternation upon electronic excitation is very similar in both systems, which is also reflected in a similar evolution of the first optical transition energy with increasing oligomer size. Larger transition energies in nnTAsvs.nnTs arise from an electronic effect rather …

Crossed 2D versus Slipped 1D π-Stacking in Polymorphs of Crystalline Organic Thin Films

Polymorphs of organic semiconductors are of great interest as they shed light to structure-property relationships. The full X-ray thin film structure analysis of two polymorphs (B, G) of an important n-type semiconducting dicyano-distyrylbenzene based small molecule (CN-TFPA) is reported. Drastically different structures of the monotropic phases are revealed, that is an uncommon 2D crossed π-stacked arrangement for the B-phase versus a 1D slipped π-stack for G. Both phases exhibit a layered structure in the (100) plane with high structural integrity, driven by the hydrophobic contacts of the terminal CF3 groups; as (100) coincides with the film surface, this allows for exfoliation by scotch…

Design principles of chemiluminescence (CL) chemodosimeter for self-signaling detection: Luminol protective approach

Chemiluminescence (CL) sensors can provide convenience and high sensitivity because they do not require an external excitation light source to produce a fluorescence signal. However, most CL based detection systems do not have a built-in self-signaling process, leading to inefficient and complex protocols due to the required multistep cascade reactions. Here, we develop a CL based sensory system with a built-in self-signaling feature by adapting the chemodosimeter concept. We found that a masking group incorporated to luminol efficiently suppresses the CL of luminol and that selective removal of the masking group by a target analyte can turn on the CL process, generating a sensitive fluores…

Tetrakis{[(p-dodecacarboranyl)methyl]stilbenyl}ethylene: A Luminescent Tetraphenylethylene (TPE) Core System

The synthesis and spectroscopic characterization of the first set of tetra(p-dodecacarborane-stilbeno)ethylenes (TDSE), substituted either with a methyl or phenyl group in the 2-position (Ccluster) of the ortho-carborane, are described. The complex absorption properties are elucidated by TD-DFT calculations, stressing the importance of through bond conjugation. Enhanced conjugation and restriction of the conformational space are identified as the main factors for boosted luminescence properties in solution compared to tetraphenylethylene (TPE) core, effectively reducing internal conversion (IC). IC is further reduced when aggregate suspensions of our compounds in water are formed, providing…

Photoluminescence in Carborane–Stilbene Triads : A Structural, Spectroscopic, and Computational Study

A set of triads in which o- and m-carborane clusters are bonded to two stilbene units through Ccluster -CH2 bonds was synthesized, and their structures were confirmed by X-ray diffraction. A study on the influence of the o- and m- isomers on the absorption and photoluminescence properties of the stilbene units in solution revealed no charge-transfer contributions in the lowest excited state, as confirmed by (TD)DFT calculations. The presence of one or two B-I groups in m-carborane derivatives does not affect the emission properties of the stilbenes in solution, probably due to the rather large distance between the iodo substituents and the fluorophore. Nevertheless, a significant redshift o…

Naphthalenediimide Polymers with Finely Tuned In-Chain π-Conjugation: Electronic Structure, Film Microstructure, and Charge Transport Properties

Naphthalenediimide-based random copolymers (PNDI-TVTx) with different π-conjugated dithienylvinylene (TVT) versus π-nonconjugated dithienylethane (TET) unit ratios (x = 100→0%) are investigated. The PNDI-TVTx-transistor electron/hole mobilities are affected differently, a result rationalized by molecular orbital topologies and energies, with hole mobility vanishing but electron mobility decreasing only by ≈2.5 times when going from x = 100% to 40%.

Regio(ir)regular naphthalenediimide- and perylenediimide-bithiophene copolymers: How MO localization controls the bandgap

Absorption spectra of regio(ir)regular naphthalenediimide (NDI)- and perylenediimide (PDI)-bithiophene (2T) donor/acceptor (D/A) copolymers are surprisingly similar despite cross-conjugation in the regioirregular structures. This result is traced back to largely localized frontier molecular orbitals (FMOs) as revealed by (time-dependent) DFT calculations. Interestingly, while the FMOs of the P(PDI-2T) copolymer are localized solely in the PDI units, they are predominantly localized in the respective D/A units of the P(NDI-2T) copolymer. The pronounced CT character of the lowest singlet state in P(NDI-2T) should give rise to a close lying CT triplet state, generating small singlet–triplet ga…

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