Ji-Kang Feng

Theoretical study on photophysical properties of ambipolar spirobifluorene derivatives as efficient blue-light-emitting materials.

The aim of this work is to provide an in-depth interpretation of the optical and electronic properties of a series of spirobifluorene derivatives. These materials show great potential for application in organic light-emitting diodes as efficient blue-light-emitting materials due to the tuning of the optical and electronic properties by the use of different electron donors (D) and electron acceptors (A). The geometric and electronic structures of the molecules in the ground state are studied with density functional theory (DFT) and ab initio HF, whereas the lowest singlet excited states are optimized by ab initio CIS. The energies of the lowest singlet excited states are calculated by employing time-dependent density functional theory (TD-DFT). The results show that the HOMOs, LUMOs, energy gaps, ionization potentials, electron affinities, reorganization energies, and exciton binding energies for these complexes are affected by different D and A moieties. Also, it has obtained that these blue-light-emitting materials have improved charge transport rate and charge transfer balance performance and can be used as efficient ambipolar-transporting materials in organic light-emitting diodes.

Structural, electronic, and optical properties of phosphole-containing π-conjugated oligomers for light-emitting diodes

The purpose of this work is to provide an in‐depth interpretation of the optical and electronic properties of a series of phosphole derivatives, including 2,5‐diphenylthiooxophosphole (2a), 2‐phenyl‐5‐biphenylthiooxophosphole (3a), 2‐phenyl‐5‐stilbenylthiooxophosphole (4a), 2,5‐dithienylthiooxophosphole (2b), 2‐thienyl‐5‐biphenylthiooxophosphole (3b), 2‐thienyl‐5‐stilbenylthiooxophosphole (4b), and dibenzophosphole 1. These thiooxophospholes show great potential for application in OLEDs as efficient red emitters due to the tuning of the optical and electronic properties by the use of various substituents at the 2,5‐positions of the phosphole ring. The geometric and electronic structures of the oligomers in the ground state were investigated using density functional theory (DFT) and the ab initio HF, whereas the lowest singlet excited states were optimized with ab initio CIS. To assign the absorption and emission peaks observed in the experiment, we computed the energies of the lowest singlet excited states with time‐dependent DFT (TD‐DFT). All DFT calculations were performed using the B3LYP functional and the 6‐31G (d) basis set. The results show that the HOMOs, LUMOs, energy gaps, ionization potentials, and electron affinities for the phosphole derivatives are significantly affected by varying the phosphole ring substituents at the 2,5‐positions, which favor the hole and electron injection into OLEDs. The absorption and emission spectra exhibit red shifts to some extent [the absorption spectra: 339.63 (1) < 358.65 (2a) < 373.77 (3a) < 443.89 nm (4a) and 403.03 (3b) < 449.11 (2b) < 460.19 nm (4b); the emission spectra: 418.42 (1) < 513.62 (2a) < 556.51 (3a) < 642.59 nm (4a) and 568.31 (2b) < 631.11 (3b) < 647.35 nm (4b)] and the Stokes shifts are unexpectedly large ranging from 78 to 228 nm resulting from a more planar conformation of the excited state for the phosphole derivatives.

Color-tuning mechanism in firefly luminescence: theoretical studies on fluorescence of oxyluciferin in aqueous solution using time dependent density functional theory.

The first singlet excited state geometries of various isomers and tautomers of firefly oxyluciferin (OxyLH2), as well as their fluorescence spectra in aqueous solution, were studied using time dependent density functional theory (TDDFT). With changing pH in aqueous solution, three fluorescence peaks, blue (450 nm), yellow-green(560 nm), and red (620 nm) correspond to neutral keto and enolic forms, the monoanionic enolic form,and the monocationic keto form respectively. A counterion, Na+, was predicted to cause a blue shift in the fluorescence of anionic OxyLH2. The contributions of a charge transfer (CT) state upon electronic excitation of the planar and twisted structures were predicted. CT was large for the twisted structures but small for the planar ones. The differences between pK and pK* of various oxyluciferin species were predicted using a Forster cycle. A new possible light emitter, namely, the monocation keto form (keto+1), was considered.

Theoretical investigation of one- and two-photon absorption properties of platinum acetylide chromophores.

In this paper, we have theoretically investigated bis((4-phenylethynyl)phenyl) ethynyl)bis(trimethylphosphine)platinum(II) (PE2) and its analogs three platinum acetylide complexes (1-3) that feature highly pi-conjugated ligands (alkynyl-dimethylfluorene substituted with electron-donating or -withdrawing moieties). The geometrical and electronic structures are calculated at the ECP60MWB//6-31G*(H, C, P, N, S) basis set level by the density functional theory (DFT) method; one-photon absorption properties have been calculated by using time-dependent DFT (TDDFT) and Zerners intermediate neglect of differential overlap (ZINDO) methods, and two-photon absorption (TPA) properties are obtained with the ZINDO/sum-over-states method. The values of beta(sp) and beta(d) for Pt are adjusted to -1 eV and -28.5 eV, respectively, to make one-photon absorption spectra calculated by ZINDO closest to the experimental data and TDDFT results. The calculated results indicate that all molecules in this work (involving cis isomers of molecules 1-3) take on two TPA peaks in the 600-800 nm region. The peak at 700-750 nm should not be simply attributed to the appearance of noncentrosymmetric cis isomers in solution, although trans and cis isomers adhere to a different selection rule. Every TPA peak results from its transition character. Molecules 1-3 show greater two-photon absorption strength compared with PE2 and retain good transparency.

Theoretical studies on the electronic and optical properties of two new alternating fluorene/carbazole copolymers.

Poly(fluorene)‐type materials are widely used in polymer‐based emitting devices. During operation there appears, however, an additional emission peak at around 2.3 eV, leading to both a color instability and reduced efficiency. The incorporation of the carbazole units has been proven to efficiently suppress the keto defect emission. In this contribution, we apply quantum‐chemical techniques to investigate two series of alternating fluorene/carbazole oligomers and copolymers poly[2,7‐(N‐(2‐methyl)‐carbazole)‐co‐alt‐2,7‐m(9,9‐dimethylfluorene)], namely, PFmCz (m = 1,2) and gain a detailed understanding of the influence of carbazole units on the electronic and optical properties of fluorene derivatives. The electronic properties of the neutral molecules, HOMO‐LUMO gaps (ΔH‐L), in addition to the positive and negative ions, are studied using B3LYP functional. The lowest excitation energies (Egs) and the maximal absorption wavelength λabs of PFmCz (m = 1,2) are studied, employing the time‐dependent density functional theory (TD‐DFT). The properties of the two copolymers, such as ΔH‐L, Eg, IPs, and EAs were obtained by extrapolating those of the oligomers to the inverse chain length equal to zero (1/n = 0). The outcomes showed that the carbazole unit is a good electron‐donating moiety for electronic materials, and the incorporation of carbazole into the polyfluorene (PF) backbone resulted in a broadened energy gap and a blue shift of both the absorption and photoluminescence emission peaks. Most importantly, the HOMO energies of PF1Cz and PF2Cz are both a higher average (0.4 eV) than polyfluorene (PF), which directly results in the decreasing of IPs of about 0.2 eV more than PF, indicating that the carbazole units have significantly improved the hole injection properties of the copolymers. In addition, the energy gap tends to broaden and the absorption and emission peaks are gradually blue‐shifted to shorter wavelengths with an increase in the carbazole content in the copolymers. This is due to the interruption of the longer conjugation length of the backbone in the (F1Cz)n series.

SEMI-EMPIRICAL CALCULATIONS ON THE BN SUBSTITUTED FULLERENES C60-2X(BN)X (X = 1-3) : ISOELECTRONIC EQUIVALENTS OF C60

Abstract The equilibrium structures and relative stabilities of BN substituted fullerenes C60−2x(BN)x (x=1–3) have been studied using AM1 and MNDO semi-empirical methods. The calculation results obtained by both methods indicated that the BN substituted fullerenes are less stable than C60. The homo–lumo splitting and heat of formation suggests that the isomer of C58BN in which two neighboring carbon atoms between the six- and six- membered ring are substituted by BN units is the most stable species. The stabilities of C58BN decrease with increasing the distance between the heteroatoms. For C56(BN)2, the calculation results show that N–N and B–B bonds should be avoided and the smallest number of the C–X bonds is preferred for the stable isomer of C56(BN)2 and the isomer in which the B–N–B–N bond is formed has the lowest heat of formation, thus the most stable species. The structure of the most stable isomer of C54(BN)3 has been proposed based on the calculations on various isomers of C58BN and C56(BN)2, i.e. the most energetic favorable C54(BN)3 should have three BN units located in the same hexagon to form B–N–B–N–B–N ring. The BN substituted fullerenes C60−2x(BN)x have somewhat smaller ionization potentials and bigger affinity potentials compared with C60, which suggests that it is easier to oxidize and reduce C60−2x(BN)x relative to C60, thus the redox characteristics of C60 can be enhanced by doping. Based on the optimized geometries, the electronic spectra for these BN doped fullerenes have been calculated using INDO/CIS method

Calculations on all possible isomers of the substituted fullerenes C58X2 (X=N,B) using semiempirical methods

A systematic investigation on the molecular structures of all the possible isomers of C58N2 and C58B2 has been performed using the semiempirical methods AM1, PM3 and MNDO. The equilibrium geometrical structures, heats of formation, HOMO–LUMO gap energies, heats of atomization, ionization potentials and affinity potentials of C58X2 (X=N,B) have been studied. The calculation results obtained by all these semiempirical methods show that the heterofullerenes are less stable than C60, and that C58N2 should be more stable than the boron analog C58B2. All the empirical methods in this work indicate that isomer-7, which corresponds to 1,4-substitution in the cyclohexatriene unit, is the most stable isomer for C58X2 (X=N,B). The stability decreases with the increasing distance between the heteroatoms. The heterofullerenes C58X2 have smaller ionization potentials and bigger affinity potentials compared with C60, thus the redox characteristics of C60 can be enhanced by doping. Both C58N2 and C58B2 are expected to have significantly different chemical and physical properties from those of the fullerenes. We propose that the change of hybridization from s to s may be the underlying reason why 1,4-substitution is favored in both C58N2 and C58B2. The electronic spectra for these doped fullerenes have been calculated using the INDO/CIS method based on the optimized geometries.

Theoretical studies on the substituted fullerene C60−x−yBxNy(x+y=2)

Abstract The equilibrium structures, relative stabilities of substituted fullerenes C 58 B 2 , C 58 N 2 and C 58 BN have been studied at the AM1, PM3, MNDO and INDO level. The calculation results obtained by all these semi-emperical methods show that the substituted fullerenes are less stable than C 60 , 6–6 isomers are the most stable species for C 58 B 2 , C 58 N 2 and C 58 BN, the stabilities decrease with the increase of the distance between the heteroatoms. The calculation results also show that the stability of C 58 BN is considerably higher than that of C 58 N 2 , and the stability of C 58 N 2 is considerably higher than that of C 58 B 2 . Calculations on the C 58 B 2 -(NH 3 ) 2 adducts have been employed in this paper to evaluate Smallys proposition on the substitute preference of C 58 B 2 . The elctronic spectra for these doped fullerenes have been calculated using INDO/CIS method

Computational Design of Two-Photon Fluorescent Probes for a Zinc Ion Based on a Salen Ligand

A two-photon fluorescent probe has become a critical tool in biology and medicine owing to its capability of imaging intact tissue for a long period of time, such as in two-photon fluorescence microscopy (TPM). In this context, a series of Salen-based zinc-ion bioimaging reagents that were designed based on an intramolecular charge-transfer mechanism were studied through the quantum-chemical method. The increase of one-photon absorption and fluorescence emission wavelength and the reduction of the oscillator strength upon coordination with a zinc ion reveal that they are fluorescent bioimaging reagents used for ratiometric detection. When the Salen ligand is incorporated with Zn(2+), the value of the two-photon absorption (TPA) cross-section (δmax) will decrease, and most of the ligands and complexes exhibit a TPA peak in the near-infrared spectral region. That is, a substituent at the end of the ligand can influence the luminescence property, besides increasing solubility. In addition, the effect of an end-substituted position on the TPA property was considered, such as ortho and meta substitution. The detailed investigations will provide a theoretical basis to synthesize zinc-ion-responsive two-photon fluorescent bioimaging reagents as powerful tools for TPM and biological detection in vivo.

Theoretical study of one-photon and two-photon absorption properties of perylene tetracarboxylic derivatives

The geometrical structure, electronic structure, one-photon absorption (OPA) and two-photon absorption (TPA) properties of the perylene tetracarboxylic derivatives (PTCDs) were studied theoretically by using density functional theory (DFT) and Zerners intermediate neglect of differential overlap (ZINDO) methods. The results revealed that increasing the number of naphthalene nucleus, extending the conjugated length on long axis, increasing the strength of donor group on lateral side, decreasing the DeltaE(H-L) (energy gap between the highest occupied orbital and the lowest unoccupied orbital) and keeping the conjugation effect and inductive effect along the same molecular axis are the efficient ways to enlarge TPA cross section of PTCDs compounds. The results that PTCDs compounds exhibited extremely large TPA cross section of around 800-1100 nm (near infrared region) shed light into the significance of the PTCDs compounds for applications in TPA labeling materials in vivo.