Transport Properties of Pentacene, Hexacene and Their BN Analogues (original) (raw)
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The Journal of Physical Chemistry A, 1998
The efficient fluorescence probe 6-propanoyl-2-(N,N-dimethylamino)naphthalene (PRODAN) shows a significant Stokes shift in polar solvents. Neither experimental nor theoretical studies performed up to now yield a definitive description of the character of the emissive state. Results of the recently developed DFT/ SCI method are compared to semiempirical calculations with the spectroscopic ZINDO/S parametrization and the AM1/CISD Hamiltonian which was not parametrized for computation of spectroscopic properties. The most reliable results are obtained with the DFT/SCI and AM1/CISD approaches. The absorption spectrum of PRODAN is excellently reproduced by the DFT/SCI calculations, and the AM1/CISD results are superior to ZINDO/S. These results allow the assignment of the red-shifted fluorescence band to both emission of a highly polar dimethylamino twisted intramolecular charge transfer state (N-TICT) and a geometrically unchanged planar intramolecular charge transfer (PICT) state. A comparison with 2-(N,N-dimethylamino)naphthalene (DAN) and 2-propanoylnaphthalene (PRON) shows that each of the donor or acceptor substituents alone in combination with rotation about the naphthalene-substituent single bond is sufficient to generate a low lying charge-transfer state with a large dipole moment.
Journal of Physical Chemistry A, 2008
The photophysics of N-(4-cyanophenyl)carbazole (NP4CN) was investigated by using absorption and fluorescence spectra, picosecond fluorescence decays, and femtosecond transient absorption. In the nonpolar n-hexane as well as in the polar solvent acetonitrile (MeCN), a locally excited (LE) state is detected, as a precursor for the intramolecular charge transfer (ICT) state. A LE f ICT reaction time τ 2 at 22°C of 0.95 ps in ethyl cyanide (EtCN) and 0.32 ps in MeCN is determined from the decay of the LE excited state absorption (ESA) maximum around 620 nm. In the ESA spectrum of NP4CN in n-hexane at a pump-probe delay time of 100 ps, an important contribution of the LE band remains alongside the ICT band, in contrast to what is observed in EtCN and MeCN. This shows that a LE a ICT equilibrium is established in this solvent and the ICT reaction time of 0.5 ps is equal to the reciprocal of the sum of the forward and backward ICT rate constants 1/(k a + k d). In the photostationary S 0 f S n absorption spectrum of NP4CN in n-hexane and MeCN, an additional CT absorption band appears, absent in the sum of the spectra of its electron donor (D) and acceptor (A) subgroups carbazole and benzonitrile. This CT band is located at an energy of ∼4000 cm-1 lower than for N-phenylcarbazole (NPC), due to the larger electron affinity of the benzonitrile moiety of NP4CN than the phenyl subunit of NPC. The fluorescence spectrum of NP4CN in n-hexane at 25°C mainly consists of a structured LE emission, with a small ICT admixture, indicating that a LE f ICT reaction just starts to occur under these conditions. In din -pentyl ether (DPeE) and din -butyl ether (DBE), a LE emission is found upon cooling at the high-energy edge of the ICT fluorescence band, caused by the onset of dielectric solvent relaxation. This is not the case in more polar solvents, such as diethyl ether (DEE) and MeCN, in which a structureless ICT emission band fully overlaps the strongly quenched LE fluorescence. For the series of D/A molecules NPC, N-(4-fluorophenyl)carbazole (NP4F), N-[4-(trifluoromethyl)phenyl]carbazole (NP4CF), and NP4CN, with increasing electron affinity of their phenyl subgroup, an ICT emission in n-hexane 25°C only is present for NP4CN, whereas in MeCN an ICT fluorescence is observed with NP4CF and NP4CN. The ICT fluorescence appears when for the energies E(ICT) of the ICT state and E(S 1) of the lowest excited singlet state the condition E(ICT) e E(S 1) holds. E(ICT) is calculated from the difference E(D/D +)-E(A-/A) of the redox potentials of the D and A subgroups of the N-phenylcarbazoles. From solvatochromic measurements with NP4CN an ICT dipole moment µ e (ICT)) 19 D is obtained, somewhat larger than the literature values of 10-16 D, because of a different Onsager radius F. The carbazole/phenyl twist angle θ) 45°of NP4CN in the S 0 ground state, determined from X-ray crystal analysis, has become smaller for its ICT state, in analogy with similar conclusions for related N-phenylcarbazoles and other D/A molecules in the literature.
Crystals
Luminophores featuring thermally activated delayed fluorescence (TADF) are the workhorses of the third- and fourth-generation OLEDs. While these compounds have usually been used as dopants embedded in the host, non-doped TADF OLEDs have recently shown significant progress as well and have attained performances comparable to those of the host-dopant systems. For efficient operation of non-doped OLEDs, the charge transport in neat films and single crystals of TADF luminophores is important; however, this issue was nearly unexplored theoretically. In the current study, we calculated the charge-carrier mobilities in four single crystals of TADF luminophores that have different molecular packing motifs. Specifically, in one of them both the donor and acceptor moieties form uniform π-stacks, while in the others the donors (acceptors) show alternating lateral shifts along the stacks; the difference in the molecular packing resulted in the difference in the transfer integrals between the mo...
Journal of Physical Chemistry C, 2011
We report detailed studies of optoelectronic and charge transport properties at the organic-organic semiconductor interfaces formed between polymer chains (interchain) and within a polymer chain (intrachain). These interfaces are fabricated using poly(9,9-din -octylfluorene-alt-N-(4-butylphenyl)diphenylamine) (TFB [f8-tfb]) (electron-donor) and poly(9,9-din -octylfluorene-alt-benzothiadiazole) (F8BT [f8bt]) (electron-acceptor) conjugated polymers, by blending them together or by covalently attaching them via a main polymer backbone (copolymer). For optoelectronic properties, when a bulky and twisted tfb molecule is incorporated into a rigid F8BT conjugated backbone, it disturbs the conjugation of F8BT polymer, leading to a blue-shift in the lowest absorption transition. However, by acting as an effective electron donor, it assists the formation of an intrachain singlet exciton that has a strong charge-transfer character, leading to a red-shifted and longer-lived emission than that of F8BT. An extremely efficient and fast energy transfer from tfb donor to bt acceptor is observed in the copolymer (<1 ps) compared to transfer from TFB to F8BT in the blend (tens of ps). This efficient energy transfer in the copolymer is found to be associated with its low fluorescence efficiency (40-45% vs 60-65% for blend) because of the migration of radiative singlet excitons to low-energy states such as triplet and exciplex states that are nonemissive or weakly emissive. The presence of molecular-scale tfb-f8-bt interfaces in the copolymer, however, does not hinder an efficient transport of charge carriers at high drive voltages. Instead, it provides a better balance of charge carriers inside the device, which leads to slower decay of the device efficiency and thus more stable light-emitting diodes with increasing voltage than the blend devices. These distinctive optoelectronic and charge transport properties observed at different organic-organic semiconductor interfaces will provide useful input for the design rules of conjugated polymers required for improved molecular electronics.
Journal of Molecular Modeling, 2021
In this paper, three organic semiconductors which are 9-[(5-nitropyridin-2aminoethyl)iminiomethyl]-anthracene(a)and Nꞌ-((pyren-4-yl)methylene)isonicotinohydrazide (b), and novel organic semiconductor N-(2-((pyren-4-yl)methyleneamino)ethyl)-5-nitropyridin-2-amine (c) have been prepared. Their structure have been assessed using NMR and elemental analysis techniques. While compound (a) and compound (c) have same wing unit ([(5nitropyridin-2-aminoethyl) iminiomethyl]), compounds (b) and (c) have same core unit (5nitropyridin-2-amine). Based upon TD-DFT and Marcus theories, we have explored the effect of molecular structure on the opto-electronic properties for OLED applications. Our results show that, wing units of molecules effects the opto-electronics properties a lot than core units. Such that, compounds (a) and (c) which have same wing unit, have been exhibited quite similar behaviours from points of both structural and opto-electronic parameters. Wheares, similar sitiation has not been observed for compounds (b) and (c) which have same core unit. More importantly accordingly our results, compounds (a) and (c) exhibit obvious advantages for organic electronic devices in terms of calculated opto-electronic and charge transport properties such as with better absorption and emission parameters, lower energy gaps and reorganization energies, higher charge mobility, etc.
Photoresponse of the conductivity in functionalized pentacene compounds
Journal of Applied Physics, 2002
We report the first investigation of the photo-response of the conductivity of a new class of organic semiconductors based on functionalized pentacene. These materials form high quality single crystals that exhibit a thermally activated resistivity. Unlike pure pentacene, the functionalized derivatives are readily soluble in acetone, and can be evaporated or spin-cast as thin films for potential device applications. The electrical conductivity of the single crystal materials is noticeably sensitive to ambient light changes. The purpose, therefore, of the present study, is to determine the nature of the photo-response in terms of carrier activation vs. heating effects, and also to measure the dependence of the photo-response on photon energy. We describe a new method, involving the temperature dependent photo-response, which allows an unambiguous identification of the signature of heating effects in materials with a thermally activated conductivity. We find strong evidence that the photo-response in the materials investigated is predominantly a highly localized heating mechanism. Wavelength dependent studies of the photo-response reveal resonant features and cut-offs that indicate the photon energy absorption is related to the electronic structure of the material.
Journal of the American Chemical Society, 2004
Recent advances in time-dependent density functional theory (TDDFT) have led to computational methods that can predict properties of photoexcited molecules with satisfactory accuracy at comparably moderate cost. We apply these methods to study the photophysics and photochemistry of 4-(dimethyl)aminobenzonitrile (DMABN). DMABN is considered the paradigm of photoinduced intramolecular charge transfer (ICT), leading to dual fluorescence in polar solvents. By comparison of calculated emission energies, dipole moments, and vibrational frequencies with recent results from transient spectroscopy measurements, a definitive assignment of the electronic and geometric structure of the two lowest singlet excited states of DMABN is possible for the first time. We investigate the mechanism of the ICT reaction by means of minimum energy path calculations. The results confirm existing state-crossing models of dual fluorescence. Our study suggests that analytical TDDFT derivative methods will be use...
The Journal of Physical Chemistry A, 2003
To enable the design of efficient organic electroluminescence (OLED) devices with desirable charge carrier transport properties, the mobilities of hole and electron in a series of compounds were studied computationally based on the Marcus electron transfer theory. MO calculations were performed, using the DFT B3LYP/6-31G* method in the Gaussian 98 program suite, on the following compounds: biphenyl (Bp), 4,4′biphenyldiamine (BA), triphenylamine (TPA), trip -tolylamine (TTA), 4-biphenylphenyl-m-tolylamine (BPTA), 4,4′-bis(phenyl-m-tolylamino)biphenyl (TPD), naphthalene (Np), 1-naphthyldiphenylamine (NDPA), 1-biphenylnaphthylphenylamine (BNPA), and 4,4′-bis(1-naphthylphenylamino)biphenyl (NPB). The geometries of these compounds in their neutral, cationic, and anionic states were optimized. The optimized geometries were then used to calculate the ionization potential, electron affinity, and reorganization energies. For compounds containing a biphenyl moiety (Bp, BA, BPTA, TPD, BNPA, and NPB), the interring distance and torsional angle followed the trend neutral g cationic g anionic, except NPB in which these two parameters in anionic state were larger than the corresponding parameters in the cationic state because of a small contribution from the biphenyl moiety to its LUMO. Also, the ionization potentials follow the order Bp > BPTA ≈ BNPA > BA > NPB ≈ TPD. The electron affinities were calculated to range from-1.54 to-0.05 eV for all compounds except NPB which has a positive electron affinity 0.24 eV due to the dominant contribution of two naphthyl groups to LUMO. For most compounds, the reorganization energy λ + for the hole transport is larger than λfor the electron transport except NPB and BA py (constrained nitrogen pyramidal geometry). These exceptions were rationalized by the special structures for their anionic states. According to the magnitudes of λ + , compounds can be divided into two groups: λ + g 0.28 eV (BA pl (constrained planar nitrogen geometry) ≈ Bp > TPD ≈ NPB) for compounds containing biphenyl group with or without two amino groups and λ + e 0.2 eV (TPA ≈ TTA <BPTA < BNPA ≈ NDPA) for compounds with single triarylamine group. According to the magnitudes of λ-, compounds can be divided into three groups: λg 0.50 eV (TPD > Bp > BPTA) for compounds with a dominating biphenyl group in their LUMO, λe 0.32 eV (NDPA > BNPA > Np > NPB) for compounds with a dominating naphthyl group in their LUMO, and the other compounds (TPA and TTA). From these results, λ + is determined mainly by the moiety which contributes predominantly to its HOMO, whereas λis determined mainly by the moiety which contributes predominantly to its LUMO. Therefore, by controlling the major contributors to the HOMO and LUMO, and by incorporating substituents to fine-tune the energy levels of these frontier orbitals (HOMO and LUMO), a systematic design of materials for OLED with desirable charge carrier transport properties should be feasible.
Journal of Physical Chemistry A, 1997
A photoinduced intramolecular charge transfer (ICT) of p-N,N-dimethylaminobenzoic acid (DMABA) has been investigated in TiO 2 and CdS colloidal solutions by steady-state and time-resolved fluorescence spectroscopy as well as ns time-resolved transient absorption spectroscopy. The intensity ratio of the ICT emission to the normal emission of DMABA was largely enhanced by the addition of CdS colloids with the decay times of the ICT emission increasing from 1.7 to 2.3 ns. However, the ICT emission of DMABA adsorbed on TiO 2 colloids was greatly quenched with constant fluorescence decay times. Concomitantly, the dual-emission bands of CdS (530 and 630 nm) and the single-emission band of TiO 2 (340 nm) were significantly changed upon adsorption of DMABA. These illustrate that electron transfer takes place efficiently between CdS or TiO 2 colloids and DMABA. For an efficient electron transfer, CdS interacts with DMABA in the ICT excited state, while TiO 2 interacts with DMABA in the LE state. In addition, we observed that the formation of the ICT state of DMABA became more favorable for the electron transfer from CdS to DMABA at the DMABA-CdS interface, while the electron transfer at the DMABA-TiO 2 interface inhibited the formation of the ICT state of DMABA. The ns time-resolved transient absorption experiments demonstrated further that the locally excited singlet state of DMABA was more affected by the interfacial electron transfer than the triplet state.