Fluorescence Spectroscopic and Time-Dependent DFT Studies for Intramolecular Excimer Formation of Di-9H-fluoren-9-yldimethylsilane: Dynamics and Energetics for Conformational Change

Boo B.H., Lee J.K., Lim E.C.

Fluorescence excitation and resonant two-photon ionization spectra were measured for 9-ethylfluorene (EFR) molecule cooled in pulsed supersonic expansion of He in the range of 286.5 ∼ 300 nm. The structures and energies of the global and local minima and the transition states separating them are evaluated with the B3LYP/6-31G(d) and MP2/cc-pVTZ methods. It is found that the vertical excitation energies of fluorene and the EFR conformers can be reliably predicted by the time-dependent DFT method within 8.72%. The vibrational bands above the electronic origin are assigned on the basis of the RCIS/6-31G(d) calculation. Ethyl (or ethene) elimination from the excited neutral and/or ionic molecule is presumed to occur as an activation process along the adiabatic potential energy surface.

Bendkowsky V., Heinecke E., Hese A.

We present rotationally resolved spectra of the S(1)

Kang H.K., Kang D.E., Boo B.H., Yoo S.J., Lee J.K., Lim E.C.

A concerted computational and experimental study has been undertaken to probe the conformational structure and excited-state dynamics of bis(9-fluorenyl)methane (BFM). We have observed that the relative intensity of the delayed excimer fluorescence of BFM is greatly enhanced in comparison with that of the normal fluorescence. This is presumably because the relative concentration of the triplet excimer is enhanced in comparison with the singlet excimer. B3LYP DFT/6-31G(d) calculations indicate that the sandwich conformer of BFM in the singlet ground state is unstable, whereas that in the triplet excited state has a bound state, being very slightly higher in internal and Gibbs free energies than that of the lowest state of the near-orthogonal conformer.

Boo B.H., Kang D.

Dynamics and energetics for intramolecular excimer formation of a diarylsilane, di-9H-fluoren-9-yldimethylsilane (DFYDMS) have been investigated by means of ps time-resolved fluorescence spectroscopy and ab initio calculation. Multiple fluorescence decay curves were globally deconvolved to generate time-resolved fluorescence spectra and decay-associated spectra (DAS), from which species-associated spectra (SAS) were obtained. It is shown in the global analysis that there are at least three excited states: Two states are the locally excited (LE) states (lambda(max) approximately 320 nm) having lifetimes of 0.70 +/- 0.04 and 1.75 +/- 0.02 ns, and another is the excimer state (lambda(max) approximately 400 nm) having a lifetime of 7.34 +/- 0.02 ns. The species which decays with 0.70 ns evolves into a species with a red-shifted spectrum, which in turn decays in 7.34 ns. The experimental and ab initio results indicate that the rise time of 0.70 ns corresponds to the conversion of the initial S(1) LE state having a near sandwich geometry to the S(1) excimer state adopting a true sandwich geometry.

Borisevich N.A., Dziarzhytski S.L., Povedailo V.A., Yakovlev D.L.

The spectra of the fluorescence excitation within the rotational contours of the bands of the pure electronic long-wavelength S 0-S 1 transitions of jet-cooled indole and carbazole molecules and their complexes with water are measured. For the carbazole-water complex, a contour with three maxima is registered, which is possibly related to the occurrence of two isomers, differing in a slight displacement of hydrogen between the nitrogen atom of the imine group of carbazole and the oxygen atom of the water molecule. The degrees of polarization of integral fluorescence upon excitation within the rotational contours of the S 0-S 1 electronic transition bands of the above molecules and their complexes with water are determined for the first time. The coincidence of the calculated (7.7%) and measured (7.3%) values of the degree of polarization upon excitation in the rotational Q branch of the b L 1-A electronic transition of indole confirms the accepted intramolecular orientation of the transition dipole moment at an angle of 38.3° with respect to the principal axis of inertia A. Upon excitation of indole, its complex with water, and carbazole into the P and R branches, the measured and calculated degrees of polarization are also close to each other and amount to 2–3%. This confirms the occurrence of contributions to the fluorescence polarization due to the rotations of the indole molecules around the principal axes of inertia A and C.

Stearns J.A., Zwier T.S.

The vibronic spectroscopy of ortho-, meta-, and para-diethynylbenzene (oDEB, mDEB, and pDEB) was studied by two-color resonant two photon ionization (R2PI). The symmetry allowed S0−S1 origins of oDEB, mDEB, and pDEB were located at 33 515, 33 806, and 34 255 cm-1, respectively, with the vibronic structure extending about 2000 cm-1 above the origin in oDEB and mDEB, but more than 3000 cm-1 in pDEB. Major peaks in each spectrum were attributed to vibronically induced bands, indicating strong coupling of the S1 state to the S2 state. Ground-state infrared spectra in the C−H stretch region (3000−3360 cm-1) were obtained using resonant ion-dip infrared spectroscopy (RIDIRS). In all three isomers, the acetylenic C−H stretch fundamental was split by Fermi resonance with a combination band composed of the C⋮C stretch and two quanta of the C⋮CH bend. This Fermi resonance was detuned in the overtone region of pDEB, which showed a single peak at 6556 cm-1. Infrared spectra were also recorded in the excited electroni...

van Walree C.A., Kaats-Richters V.E., Jenneskens L.W., Williams R.M., van Stokkum I.H.

Intramolecular excimer emission is observed for cis-1,4-di(1-pyrenyl)decamethylcyclohexasilane in nonpolar solvents. Time-resolved fluorescence spectroscopy and kinetic modelling indicate that the driving force of excimer formation is very small, and that the process is governed by the flexibility of the silicon ring. In the polar solvent acetonitrile, photoinduced electron transfer occurs, with the cyclohexasilane ring acting as electron donor and the pyrenyl group as electron acceptor. 2002 Elsevier Science B.V. All rights reserved.

Stratmann R.E., Scuseria G.E., Frisch M.J.

Time-dependent density-functional (TDDFT) methods are applied within the adiabatic approximation to a series of molecules including C70. Our implementation provides an efficient approach for treating frequency-dependent response properties and electronic excitation spectra of large molecules. We also present a new algorithm for the diagonalization of large non-Hermitian matrices which is needed for hybrid functionals and is also faster than the widely used Davidson algorithm when employed for the Hermitian case appearing in excited energy calculations. Results for a few selected molecules using local, gradient-corrected, and hybrid functionals are discussed. We find that for molecules with low lying excited states TDDFT constitutes a considerable improvement over Hartree–Fock based methods (like the random phase approximation) which require comparable computational effort.

Casida M.E., Jamorski C., Casida K.C., Salahub D.R.

This paper presents an evaluation of the performance of time-dependent density-functional response theory (TD-DFRT) for the calculation of high-lying bound electronic excitation energies of molecules. TD-DFRT excitation energies are reported for a large number of states for each of four molecules: N2, CO, CH2O, and C2H4. In contrast to the good results obtained for low-lying states within the time-dependent local density approximation (TDLDA), there is a marked deterioration of the results for high-lying bound states. This is manifested as a collapse of the states above the TDLDA ionization threshold, which is at −εHOMOLDA (the negative of the highest occupied molecular orbital energy in the LDA). The −εHOMOLDA is much lower than the true ionization potential because the LDA exchange-correlation potential has the wrong asymptotic behavior. For this reason, the excitation energies were also calculated using the asymptotically correct potential of van Leeuwen and Baerends (LB94) in the self-consistent field step. This was found to correct the collapse of the high-lying states that was observed with the LDA. Nevertheless, further improvement of the functional is desirable. For low-lying states the asymptotic behavior of the exchange-correlation potential is not critical and the LDA potential does remarkably well. We propose criteria delineating for which states the TDLDA can be expected to be used without serious impact from the incorrect asymptotic behavior of the LDA potential.

Wilson A.K., van Mourik T., Dunning T.H.

The family of correlation consistent polarized valence basis sets (cc-pVXZ) has been extended to include sextuple zeta sets (cc-pV6Z) for the atoms boron through neon. Potential energy functions have been calculated with these sets for the electronic ground states of N 2 and HF using a number of correlated wave functions: MP2, MP3, MP4, CCSD, CCSD(T) and CAS +1+2. Spectroscopic constants have been calculated for each level of theory and have been compared with experiment. Combining these results with those of prior studies, complete basis set limits have been estimated for E e , D e and r e . It is found that the cc-pV6Z basis sets yield dissociation energies that are within 0.6–0.8 kcal mol −1 (N 2 ) and 0.1 kcal mol −1 (HF) of the estimated CBS limits. Adding core-core and core-valence contributions to the CCSD(T) CBS limits yields D e 's that are within 0.1 kcal mol −1 of the experimental values.

Davidson E.R.

Abstract By using a general linear transformation of Dunning's contracted basis set, the contraction length can be reduced even more than noted in the Comment by Hashimoto et al. With GAUSSIAN 94, this results in a considerable savings in computer time. If one is willing to neglect small coefficients, even greater savings are possible.

Bauernschmitt R., Ahlrichs R.

Time dependent density functional methods are applied in the adiabatic approximation to compute low-lying electronic excitations of N2, ethylene, formaldehyde, pyridine and porphin. Out of various local, gradient-corrected and hybrid (including exact exchange) functionals, the best results are obtained for the three-parameter Lee-Yang-Parr (B3LYP) functional proposed by Becke. B3LYP yields excitation energies about 0.4 eV too low but typically gives the correct ordering of states and constitutes a considerable improvement over HF-based approaches requiring comparable numerical work.

Yip W.T., Levy D.H.

Van der Waals dimers between benzofuran, dibenzofuran, and fluorene were studied in a supersonic jet. These show red-shifted excimer/exciplex emission following excitation to their locally excited states. The dibenzofuran dimer forms an excimer from its zero-point level, while the dibenzofuran−benzofuran and fluorene−benzofuran dimers require a moderate amount of excess vibrational energy for excimer formation. Exciplex formation in dibenzofuran−fluorene was inferred from spectral broadening. Excimer/exciplex formation was attributed to strong mixing of a charge transfer state with the exciton state. The charge transfer state has a low enough energy at its equilibrium geometry to mix, but this requires a large change in geometry from the equilibrium geometry of the locally excited state. There is a barrier in the adiabatic potential surface between the minima of the locally excited state and the excimer/exciplex state. This barrier is larger for greater differences in equilibrium geometry.

Head-Gordon M., Maurice D., Oumi M.

A perturbative correction, termed RCIS(D), to restricted open shell single excitation configuration interaction (ROCIS) is formulated and implemented for excited states of radicals. RCIS(D) gives spin-pure energies (but not wavefunctions), is size-consistent, and has computational complexity scaling with the fifth power of molecular size. All these properties are analogous to second order restricted Moller-Plesset theory for the ground state. Comparison of calculated vertical excitation energies against experiment for a diverse series of small radicals, BeH, BeF, CH, CH 3 , CN, CO + , NO and OH, suggests that RCIS(D) constitutes a useful improvement over ROCIS.

van Dantzig N.A., Levy D.H., Vigo C., Piotrowiak P.

The fluorescence spectra of a series of bichromophoric molecules consisting of covalently linked fluorene units were investigated in a supersonic jet. In three of the systems (spirobifluorene, d8h8-spirobifluorene and 1-methyl spirobifluorene) no electronic coupling and no corresponding exciton splitting were detected in the zero-point level of the S1 state. Only 9,9′-bifluorene exhibited an exciton splitting in the v=0 state. The lack of coupling was attributed to symmetry; in the spirobifluorenes the planes of the fluorene moieties and the S1←S0 transition moments are perpendicular. When low vibrational levels were excited, state mixing, and energy transfer between the chromophores was observed. This behavior is characteristic of the ‘‘small molecule’’ regime of radiationless transition theory. When higher vibrational levels were excited, the systems exhibited typical ‘‘large molecule’’ behavior. In this limit, both electronic energy transfer, as well as intramolecular vibrational relaxation contribute to the decay of the initially excited state. Intramolecular dispersive interactions were also investigated by comparing the bifluorenes with a series of reference compounds.

Belova A.S., Kononevich Y.N., Ionov D.S., Sazhnikov V.A., Anisimov A.A., Safonov A.A., Surin N.M., Svidchenko E.A., Fedorov Y.V., Shchegolikhina O.I., Muzafarov A.M.

A series of new tetrachromophoric systems based on stereoregular tetracyclosiloxanes and dibenzoylmethanatoboron difluoride derivatives have been synthesized and characterized by a complex of physicochemical methods. The photophysical properties of the synthesized compounds are studied by electronic absorption, steady-state, and time-resolved fluorescence spectroscopy. In the synthesized compounds, four dibenzoylmethanatoboron difluoride (DBMBF2)-based fluorophores are in an all-cis arrangement with respect to a cyclotetrasiloxane scaffold. DFT calculations predict that they can form H-type dimers, trimers, or tetramers with an antiparallel orientation of their ground-state dipole moments. Under UV excitation, solutions of these compounds in polar and nonpolar solvents exhibit complex fluorescence consisting of monomer- and excimer-like emissions with different lifetimes. Global fitting analysis reveals the presence of at least four kinetically distinguishable species in the excited state. The studied compounds in solutions have CIE chromaticity coordinates very close to the white color point and are promising objects for the development of next-generation single-emission materials for white illumination.

Khan M.F., Wu J., Cheng C., Akbar M., Liu B., Liu C., Shen J., Xin Y.

Various single-ring aromatic compounds in water sources are of great concern due to its hazardous impact on the environment and human health. The fluorescence excitation-emission matrix (EEMs) spectrophotometry is a useful method to identify organic pollutants in water. This study provides a detailed insight into the fluorescence properties of the 14 selected toxic single-ring aromatic compounds by experimental and theoretical analysis. The theoretical analysis were done with Time-Dependent Density Functional Theory (TD-DFT) and B3LYP/6-31G (d,p) basis set, whereas, Polarizable Continuum Model (PCM) was used to consider water as solvent. The selected compounds displayed their own specific excitation-emission (Ex/Em) wavelengths region, at Ex < 280 nm and Em < 340 nm, respectively. Whereas the theoretical Ex/Em was observed as, Ex at 240 nm-260 nm and Em at 255 nm-300 nm. Aniline as a strong aromatic base has longer Em (340 nm) than alkyl, carbonyl, and halogens substituted benzenes. The lone pair of electrons at amide substituent serves as a π-electron contributor into the aromatic ring, hence increasing the stability and transition energy, which results in longer emission and low quantum yield for the aniline. The fluorescence of halogenated benzenes illustrates an increase in the HOMO-LUMO energy gap and a decrease in quantum yield associated with atomic size (F>Cl>Br>I). In this study the theoretical results are in line with experimental ones. The understanding of fluorescence and photophysical properties are of great importance in the identification of these compounds in the water.

Han M., Tian Y., Yuan Z., Zhu L., Ma B.

A butterfly-like phosphorescent platinum(II) binuclear complex can undergo a molecular structure change in which the Pt-Pt distance shortens upon photoexcitation, which leads to the formation of two distinct excited states and dual emission in the steady state, that is, greenish-blue emission from the high-energy excited state at the long Pt-Pt distance and red emission from the low-energy excited state at the short Pt-Pt distance. This photoinduced molecular structure change has a strong dependence on the molecule's surrounding environment, allowing its application as self-referenced luminescent sensor for solid-liquid phase change, viscosity, and temperature, with greenish-blue emission in solid matrix and rising red emission in molten liquid phase. With proper control of the surrounding media to manipulate the structural change and photophysical properties, a broad white emission can be achieved from this molecular butterfly.

Han M., Tian Y., Yuan Z., Zhu L., Ma B.

Boo B.H., Lee M., Jeon K., Kim S.

Intramolecular excimer formation of bis(9-fluorenyl)methane (BFM) and 9-(9'-ethylfluorenyl)-9-fluorenylmethane (EFFM), in which an ethyl group is substituted to a 9-H atom in BFM, was studied by means of steady-state and time-resolved fluorescence. Ab initio and DFT calculations enabled the prediction of three conformers as stable species of orthogonal, trans-gauche, and gauche-gauche. The theoretical and experimental results reveal that the substitution effect is also found to appreciably influence the energies, spectroscopy, and kinetics associated with the interconversion of various conformers of the diaryl compounds. We have not observed the rising components in the excimer fluorescence decay of BFM and EFFM in PMMA as observed in the liquid solutions probably because of the existence of the sandwich conformer responsible for the excimer fluorescence prior to the laser irradiation.

Boo B.H., Kim J.H.

E-mail: bhboo@cnu.ac.krReceived August 23, 2012, Accepted October 28, 2012Key Words : 5,8-Dihydroxy-1,4-naphthoquinone, Fluorescence, Fluorescence excitation, TD DFT, Orbitalpopulation analysisExcited-state intramolecular proton transfer (ESIPT) hasbeen a research topic of incessant and wide interest becauseof its importance in many photophysical,

Boo B.H., Kim H.S., Koh S.G., Lee M., No K.

Time-dependent DFT, fluorescence decay profile, fluorescence anisotropy. Fluorescence, fluorescence excitation, time-resolved fluorescence spectroscopic, time-dependent fluorescence anisotropy, and time-dependent (TD) DFT studies were carried out to elucidate the excitation energies, fluorescence decay kinetics, and excitation transfer dynamics of calix[4]arene. We found that TD DFT calculations help to analyze the absorption spectra by elucidating the variation of the oscillator strengths with the excitation energies of various conformers involved and to clarify their relative importance. The TD DFT calculations imply that a cone-shaped conformer plays the most important role in the absorption spectrum. Fluorescence in a methanol solvent has a lifetime of 3.11 ± 0.1 ns and the decay kinetics is not found to have particular propensity with respect to solvent polarity. When the molecules in a glass matrix at 77 K were excited by short laser pulses, fluorescence anisotropy was observed. From the time-dependent fluorescence anisotropy, we derived the energy transfer rate constant and fluorescence anisotropy decay time, respectively: k et = 1.7 × 10 9 s - 1 , τ h = 0.59 ns .

Boo B.H., Cho H., Kim Y.M., Lee S.J.

conjugation in terms of the density matrix evaluated with the TD DFT calculation; and finally (6) to predict the excitation energies with the aid of the TD DFT methods. We will compare the absorption and the transient absorption spectra with the simulated spectra evaluated with the TD DFT method, by which the electronic state relevant to the electronic absorption can be assigned. In this work, we present the various spectroscopic and quantum mechanical investigations concerning the static and transient absorptions. We also present the TD DFT calculation results of the geometrical change upon the change of the spin state. Our findings in this work could help to establish the excited-state spectroscopy and dynamics of the cross-conjugated molecules.

Boo B., Lee J.

We investigated fluorescence and fluorescence excitation of diphenylsilane (DPS) in a solution and molecular beams in combination with the aid of the DFT method. When the molecule was photoexcited at 250 nm in a cyclohexane solution, normal and excimer fluorescences were observed in the ranges of 260-320 and 330-450 nm, respectively. The fluorescence excitation spectrum indicates that the channel leading to the intramolecular excimer formation is not efficient in comparison with the normal fluorescence. Vibrationally resolved fluorescence excitation spectra were measured for the DPS molecules cooled in pulsed supersonic expansion of He in the range 262.2-271.7 nm, in which we can see several electronic excitation spectra exhibiting the electronic band origins. We found that the simulated absorption spectrum based on the time-dependent densityfunctional theory calculations accords well with the absorption spectrum.

Alamiry M.A., Benniston A.C., Copley G., Harriman A., Howgego D.

Photophysical properties have been recorded for a small series of covalently linked, symmetrical dimers formed around boron dipyrromethene (Bodipy) dyes. Within the series, a control dimer is unable to adopt a cofacial arrangement because of steric factors, while a second dimer possesses sufficient internal flexibility to form the cofacial geometry but with little overlap of the Bodipy units. The other three members of the series take up a cofacial arrangement with varying bite angles between the planes of the two Bodipy units. Fluorescence quantum yields and excited-state lifetimes indicate differing extents of electronic interaction between the two Bodipy head-groups, but only the compound with the smallest bite angle exhibits excimer emission in solution under ambient conditions. Time-resolved fluorescence studies show dual-exponential decay kinetics in each case, while temperature-dependent emission studies reveal reversible coupling between monomer and lower-energy excimer states. The latter is weakly fluorescent, at best, and is seen clearly only for dimers having small bite angles. The application of high pressure to dilute solutions of these dimers promotes excimer formation in certain cases and leads to loss of monomer-like fluorescence. Under high pressure, excimer emission is more evident, and the overall results can be discussed in terms of subtle structural rearrangements that favor excimer formation.