Vibronic Progression Research Articles

Vibronic states in single molecules: C60 and C70 on ultrathin Al2O3 films.

Vibronic states are observed in single C(60) and C(70) molecules by scanning tunneling microscopy. When single fullerene molecules are adsorbed on a thin layer of Al(2)O(3) grown on a NiAl(110) substrate, equally spaced features are observed in the differential conductance (dI/dV), which are clearly resolved in d(2)I/dV(2) spectra. These features are attributed to the vibronic states of the molecule. The vibronic progressions are sensitive to the molecular orientations and can have different spacings in different electronic bands of the same molecule. For C(60,) these vibronic states are associated with the intramolecular A(g) and H(g) vibrational modes. Vibronic states are not resolved in molecules adsorbed on the metal surface. However, inelastic electron tunneling spectroscopy exhibits a vibrational mode at 64 meV for C(60) and 61 meV for C(70) adsorbed on NiAl(110).

Temperature- and pressure-dependent luminescence spectroscopy on the trans-[ReO2(pyridine)4]+ complex — Analysis of vibronic structure, luminescence energies, and bonding characteristics

Resolved vibronic structure in electronic spectra provides a detailed view into how molecular structure changes after absorption or emission of a photon. We report temperature- and pressure-dependent luminescence spectra of trans-[ReO2(pyridine)4]I. Low-temperature spectra reveal long vibronic progressions in the totally symmetric O=Re=O (907 cm–1) and Re-pyridine (211 cm–1) stretching modes, indicating large structural displacements along these normal coordinates. The luminescence band maximum is at ca. 15 500 cm–1. Room-temperature spectra are somewhat less-resolved; however, intervals closely matching the O=Re=O frequency (~870 cm–1) persist at higher temperatures. The variable pressure spectra exhibit distinct changes in the vibronic patterns, and luminescence energies decrease by 16 ± 2 cm–1/kbar (1 bar = 100 kPa). Low-temperature spectra are modeled using two-dimensional potential energy surfaces to represent the initial and final electronic states, from which the quantitative normal coordinate offsets can be determined. We then adapt this model to the room-temperature, pressure-dependent data where it is possible to determine how the offsets and other important spectroscopic parameters vary with the pressure-induced changes of the molecular structure. Key words: trans-[ReO2(pyridine)4]I, low-temperature luminescence spectroscopy, high-pressure luminescence spectroscopy, vibronic structure, emitting state distortions.

Electronic Absorption Spectra of CnCl Radicals (n = 5, 6) and Their Cations in Neon Matrices

Electronic absorption spectra of C6Cl, C6Cl+, C5Cl, and C5Cl+ have been recorded in 6 K neon matrices. The bands observed are assigned to the B 2Π ← X 2Π electronic transition of C6Cl, 3Σ- ← X 3Σ- of C6Cl+, and 1Σ+ ← X 1Σ+ of C5Cl+ with band origins at 545.8, 527.4, and 226.1 nm, respectively. Two electronic band systems are apparent for C5Cl and these are tentatively assigned to a 2Π ← X 2Π transition with a band origin at 247.1 nm and to Σ+ ← X 2Π at 532.3 nm. For each of the four molecules, several transitions due to the excitation of vibrational modes in the excited electronic states are observed. The spectral assignments in each case are based upon the observation of clear, vibronic progressions with appropriate spacing for C−C and C−Cl stretching modes and by comparison with the absorption spectra of the isoelectronic sulfur-terminated carbon chains.

Phosphorescence of aluminum tris(quinoline-8-olate)

Phosphorescence from an aluminum tris(quinoline-8-olate) (Alq3)-based organic light-emitting diode was observed by measuring the delayed electroluminescence at low temperatures. The spectra as well as the transient intensities were investigated. Vibronic progressions of the phosphorescence (T1→S0 transition) were resolved and the triplet energy of Alq3 was measured to be 2.05±0.1 eV. The lifetimes of the triplet state were determined from the delayed fluorescence as well as from the phosphorescence decay to be 8.1±0.5, 7.0±0.5, and 5.6±0.5 ms at 30, 50, and 80 K, respectively.

Triplet excitons in acyl- and alkyl-substituted polycarbazolyldiacetylenes: A spectroscopical and photophysical study

Absorption, photoluminescence, and photoinduced absorption (PIA) spectra of films of red phase polycarbazolyldiacetylenes (polyCzDAs) carrying selected acyl (polyDPCHD) or alkyl (polyDCHD-HS) groups are reported. The absorption spectrum of polyDCHD-HS, which is almost independent of the temperature down to 20 K, shows an excitonic absorption followed by an inhomogeneously broadened vibronic progression. Very different is instead the case of polyDPCHD whose thin films show a very sharp excitonic transition and a vibronic progression with a lower electron-phonon (el-ph) coupling with respect to that of polyDCHD-HS. These peculiar characteristics allow the detection of additional features in the spectra, which are assigned on the basis of quantum chemical calculations to string modes of the backbone. As far as thick films of polyDPCHD are concerned, broad electronic spectra are observed. Raman spectroscopy studies suggest that this broadening is due to the presence of two forms of the polymer. Unlike other polydiacetylenes (PDA's) including polyDCHD-HS films, polyDPCHD films are also strongly luminescent, thus suggesting a different ordering of the dipole forbidden and allowed states. For these materials, the photoinduced absorption spectra are dominated by triplet excitons. Effects of different forms in acyl substituted polymers are observed also in the PIA spectra. The rise and decay of the triplet signal for polyDCHD-HS are accounted for through a dynamical model, which includes saturation effects in the monomolecular decay regime. A more complex kinetics is instead observed for polyDPCHD. From the study of the kinetics of the PIA spectra both the triplet generation efficiency and the density of traps are evaluated.

Influence of molecular anisotropy on the optical properties of nanocrystalline films of $pi;-conjugated systems

Absorption and electroabsorption spectra of nanocrystalline films of sexithiophene (T6), quaterthiophene (T4) and 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) are presented and analysed by dielectric response theory and accounting for the coherent polarization induced by the incident light. Due to the anisotropic crystal structure absorption peaks do not occur at exciton transition energies but depend on the respective orientation of molecules and the wave vector of light. This directional dispersion causes the broad and rather featureless absorption spectra of nanocrystalline films. Electroabsorption spectra, however, resolve the fundamental exciton and its vibronic progression clearly. The analysis reveals that excited states interact via their dipole moments while exchange interaction and overlap of molecular states is negligible.

Ab initio investigations of the unimolecular decay of CS+2 for internal energies lower than 10 eV

One-dimensional potential energy curves of the lowest doublet and quartet states of CS+2 and their mutual spin–orbit interactions have been calculated using the complete active space self-consistent field and internal contracted multi-reference configuration interaction methods. Between the A2Πu and the C2Σ+g states, four doublet satellite states (two 2Πu, one 2Πg and one 2Φu) in addition to the valence B2Σ+u state are found. The two 2Πu states have been already observed in the photoelectron spectra where they were associated with a long vibronic progression. Several quartet states have been also located in the 3–10 eV internal energy range. The avoided crossing regions and the conical intersections for the doublets and the quartets are located. The predissociation mechanisms of the doublet states of CS+2 leading to the first dissociation limit (S+(4Su) + CS(X1Σ+)) are also discussed. They are found to involve vibronic and/or spin–orbit interactions between the doublet and the quartet states.

Room temperature vibrational photoluminescence and field emission of nanoscaled tris-(8-hydroxyquinoline) aluminum crystalline film

The nanoscaled tris-(8-hydroxyquinoline) aluminum (AlQ3) crystalline film was synthesized by vapor condensation. It was stacked with nanometer-sized rods, approximately 100 nm wide and 1 μm long, and had a surface roughness of about 100 nm. The vibronic progression with several separated peaks was observed in the photoluminescence spectrum at room temperature. It is attributed to the crystallinity of AlQ3 and the coupling of vibrations of the individual ligands to the fluorescence transition. The emission current was also observed with a turn-on field of 12.0 V/μm, and a current density of about 0.8 mA/cm2 at 22 V/μm. Therefore, the AlQ3 crystalline film provides a choice for field emission.

The photophysics of triplet excitons in substituted polycarbazolyldiacetylenes

Absorption and photoinduced absorption (PIA) spectra of thin films of polycarbazolyldiacetylenes carrying alkyl (polyDCHD-HS) or acyl (polyDPCHD) substitutions are reported. The absorption spectra are dominated by an excitonic absorption followed by a vibronic progression, and are quite insensitive to the temperature. The PIA spectra show the typical spectral feature of a triplet excitation whose rise and decay times are different. The PIA signal shows a strong sublinear pumping intensity dependence, which also affects the rise time.

Influence of Dendronization on Spectral Diffusion and Aggregation in Conjugated Polymers

AbstractDendronization has previously been suggested as a means of controlling the level of intermolecular interactions in macromolecular and polymeric compounds. We therefore studied the spectral relaxation dynamics of dendronized and non‐dendronized polyfluorenes at different temperatures. Dendronization strongly affects inhomogeneous spectral broadening, resulting in a blue‐shift and broadening of the otherwise narrow vibronic progression of polyfluorene at low temperature. Using gated spectroscopy we are able to identify emissive keto defects on isolated chains of both dendronized and non‐dendronized polymers, along with a strong aggregation band in the solid state. Surprisingly, the emission from chemical and structural defects is found to be stronger in the case of the dendronized material. The observation of aggregate emission in dendronized polyfluorenes suggests that aggregation in these materials does not occur through linear π‐stacking, but rather through electronic interactions at point contacts between chains introduced by structural kinks along the backbone.

Solid-state absorption and luminescence spectroscopy of nitronyl nitroxide radicals

Low-temperature absorption and luminescence spectra of three crystalline nitronyl nitroxides are presented. For 2-(2-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide and 2-(2-benzimidazolyl)-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide, luminescence with resolved vibronic structure is observed between 700 nm and 1100 nm. The modes forming the resolved structure have frequencies of approximately 600 cm−1 and 1400 cm−1. They are characterized by Raman spectroscopy and electronic structure calculations. The intensity distribution within the vibronic progressions indicates small structural changes between the ground and emitting states. The lowest-energy absorption bands observed between 450 nm and 700 nm of the two luminescent compounds and of 2-cyano-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide also show resolved structure, but the overall width of these absorption band systems is larger by a factor of 2 than that of the luminescence spectra. This difference is rationalized in terms of overlapping electronic transitions to at least two excited states arising from the (SOMO − 1)1(SOMO)2 and (SOMO)0(SOMO + 1)1 electron configurations, supported by transition energies and oscillator strengths from density functional calculations.

Some spectroscopic aspects of electron transfer in ruthenium(II) polypyridyl complexes

The metal-to-ligand charge transfer (MLCT) absorption and emission properties of several ruthenium(II)-bipyridine am(m)ine complexes are compared. The Gaussian deconvolution of the spectra indicates that: (a) the emission MLCT bandwidths are smaller than the absorption bandwidths for the first components of the apparent vibronic progressions; (b) the emission bands decrease in energy and width when a polypyridyl is replaced by an am(m)ine. The observations can be interpreted in terms of a two state model and the perturbation theory-based treatment of the attenuation of the effective reorganizational energy, λr =~ λr o(1- 4α2 DA), where λr o is the reorganizational energy corresponding to no mixing between the two electron transfer states and αDA = (HDA/EDA) is the mixing coefficient. Both the solvent and molecular contributions to λr are attenuated. The MLCT excited state lifetimes also decrease with am(m)ine substitution, and the non-radiative decay rate constant at 77 K is roughly proportional to the number of am(m)ine moieties coordinated to the ruthenium center.

Electronic and infrared absorption spectra of NCCN+

The D2Πu←X2Πg electronic transition of the NCCN+ cation has been observed in a 5K neon matrix with origin band at 33,522±23cm−1. The origin of the B2Σu+←X2Πg system is assumed to be the band at 11,486±3cm−1. In the infrared, the σu+ vibrational mode is detected at 1799.5±1cm−1. Theoretical calculations demonstrate that D2Πu is a nonlinear shake-up state and possesses cis- and trans-bent/bent Renner–Teller minima, in contrast to the X2Πg and C2Πu states which are linear. The complex vibronic progression in the absorption spectrum in a neon matrix to the D2Πu state does not provide direct evidence for the two possible isomers. The stretching modes are assigned while the active bending modes coupled by the Renner–Teller effect will require additional studies. In the energy region of the D2Πu state lie several quartet states. The D2Πu state forms a conical intersection in Cs symmetry with B2Σu+ close to its barrier to linearity. The latter state crosses in linear structures also the C2Πu state and along the cis-bending coordinates the A2Σg+ state. Because the B2Σu+ state intersects conically with three electronic states, the vibronic and Renner–Teller couplings complicate the assignments of the vibronic progressions observed.

Effect of pressure on coupled electronic ground and excited states determined from luminescence spectra of trans-dioxorhenium(V) complexes.

Pressure-dependent luminescence spectra of trans-dioxo complexes of rhenium(V) with ancillary ethylenediamine ligands exhibit resolved vibronic structure in the O=Re=O symmetric stretching mode at room temperature. The intensity distribution within the vibronic progression changes with pressure, leading to band shapes that are also pressure-dependent. These spectroscopic features arise from coupled electronic states and depend on the energy differences between ground and excited states, which vary by 2500 cm(-1) for the three complexes with ethylenediamine, tetramethylethylenediamine, and tetraethylethylenediamine ancillary ligands. We describe the pressure-dependent vibronic structure and band shapes with anharmonic adiabatic potential energy surfaces for the ground states of all complexes. The calculated spectra reveal the pressure dependence of the energies of electronic origins, luminescence band maximums, offsets between ground- and emitting-state potential minimums, and vibrational frequencies. The largest pressure effects are observed where the coupled electronic states are close in energy.

Optical studies of molecular aggregates: The photophysics of a thienylene vinylene oligomer

The effect of aggregation on the photophysical properties of ${5}^{\ensuremath{'}}$${,5}^{\mathrm{\ensuremath{''}}}$-bis[${2}^{\ensuremath{'}\ensuremath{'}\ensuremath{'}}$${\ensuremath{-}(2}^{\ensuremath{'}\ensuremath{'}\ensuremath{'}\ensuremath{'}}$${\ensuremath{-}\mathrm{t}\mathrm{h}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{y}\mathrm{l})\ensuremath{-}1}^{\ensuremath{'}\ensuremath{'}\ensuremath{'}}$-ethenyl]${1,2\ensuremath{-}\mathrm{b}\mathrm{i}\mathrm{s}(2}^{\ensuremath{'}}$-thienyl)ethene (4TV) has been studied by absorption and photoluminescence-detected magnetic resonance (PLDMR) spectroscopy. Upon aggregation the molecules align themselves in a herringbone configuration with an estimated angle between the dipoles of 61\ifmmode^\circ\else\textdegree\fi{}. Intermolecular interactions in 4TV dimers cause Davydov splitting of the lowest-energy absorption band. The absorption spectrum of isolated 4TV molecules has a 0-0 transition at 2.52 eV with an associated vibronic progression and the absorption bands of 4TV dimers have transitions at 2.28 and 3.48 eV. The morphology of a blend of 4TV and PMMA can be reversibly converted between aggregated and isolated phases by simple thermal processing or adjustment of the 4TV concentration. The PLDMR spectra of 4TV films show a direct correlation between aggregation and charge photogeneration.

Vacuum ultraviolet pulsed field ionization-photoelectron study of OCS in the energy range of 15–19 eV

Vacuum ultraviolet pulsed field ionization-photoelectron (PFI-PE) spectra for OCS have been obtained in the energy range 15.0–19.0 eV, covering the vibronic bands of OCS+ (A 2Π, B 2Σ+, and C 2Σ+). The ionization energies for the formation of the ground vibrational levels of OCS+ (A 2Π3/2, A 2Π1/2, B 2Σ+, and C 2Σ+) from the ground OCS(X 1Σ+) state have been determined as 15.0759±0.0005 eV, 15.0901±0.0005 eV, 16.0403±0.0005 eV, and 17.9552±0.0005 eV, respectively. We have also generated the theoretical adiabatic three dimensional potential energy functions (PEFs) for OCS+(A 2Π) by employing the complete active space self-consistent field and internally contracted multireference configuration interaction methods. Using these PEFs, the spectroscopic constants and low-lying rovibronic energy levels for OCS+(A 2Π) are calculated variationally. These calculations have made possible the identification of many PFI-PE vibronic bands for OCS+(A 2Π), which are originated from vibronic and Fermi resonance interactions. Owing to the different equilibrium geometries between the OCS+(A 2Π) and OCS(X 1Σ+) states, the PFI-PE spectrum for OCS+(A 2Π) exhibits a long vibronic progression extending well above the OCS+(B 2Σ+) state. On the contrary, the PFI-PE spectra for OCS+ (B 2Σ+ and C 2Σ+) are overwhelmingly dominated by the ground (0,0,0) bands, exhibiting only weak vibrational progressions.

Absorption and emission in oligo-phenylene vinylene nanoaggregates: The role of disorder and structural defects

The impact of exciton-phonon coupling and defect states on the photophysical properties of p-distyrylbenzene nanoaggregates is studied numerically. Molecular packing within aggregates is based on the known crystal structures of poly-p-phenylene vinylene (Type I) and the five phenyl group oligomer (Type II). Calculations of absorption and emission are conducted using a reduced basis set consisting of all one- and two-particle vibronic states. The calculated spectra are very similar for both aggregate types, the only substantial difference being the polarization directions for the J-band and 0-0 emission line. Under the noninteracting domains approximation the calculated nanoaggregate absorption spectrum is in excellent agreement with experiment, assuming an exciton coherence length of approximately 20 Å. In the calculated emission spectrum the 0-0 emission is uniquely polarized compared with the rest of the vibronic progression, also in agreement with experiment. The 0-0 emission intensity in defect-free Type I and II aggregates is linearly proportional to the total number of molecules, becoming superradiant beyond a certain size threshold. The 0-0 emission is highly sensitive to stacking faults and dislocations. These defects account for the measured Stokes shift, but quench the 0-0 emission (and superradiance) while only slightly affecting the rest of the vibronic progression. Adding orientational point defects to an aggregate with stacking faults and/or dislocations enhances the 0-0 oscillator strength, bringing the 0-0 emission intensity into good agreement with experiment.

Oxalate-bridged complexes of dimolybdenum and ditungsten supported by pivalate ligands: ((t)BuCO(2))(3)M(2)(mu-O(2)CCO(2))M(2)(O(2)C(t)Bu)(3). Correlation of the solid-state, molecular, and electronic structures with Raman, resonance Raman, and electronic spectral data.

The compounds ((t)BuCO(2))(3)M(2)(mu-O(2)CCO(2))M(2)(O(2)C(t)Bu)(3) (M(4)OXA), where M = Mo or W, are shown by analysis of powder X-ray diffraction data to have extended lattice structures wherein oxygen atoms from the oxalate and pivalate ligands of one M(4)OXA molecule are linked to metal atoms of neighboring molecules. Raman, resonance Raman, electronic absorption (2-325 K in 2-MeTHF), and emission spectra are reported, together with corresponding spectra of the mu-O(2)(13)C(13)CO(2) isotopomers. To aid in the assignment, the Raman spectra of K(2)C(2)O(4).H(2)O and K(2)(13)C(2)O(4).H(2)O have also been recorded. The visible region of the electronic spectra is dominated by intense, fully allowed MLCT transitions, M(2) delta to oxalate pi*, which show pronounced thermochromism and extensive vibronic progressions associated with the oxalate ligand at low temperatures. With excitation into these charge-transfer bands, strong resonance enhancement is seen for Raman bands assigned to the oxalate nu(1)(a(g)) and, to a lesser extent, nu(2)(a(g)) modes. Electronic structure calculations for the model compounds (HCO(2))(3)M(2)(mu-O(2)CCO(2))M(2)(O(2)CH)(3), employing density functional theory (gradient corrected and time-dependent) with the Gaussian 98 and ADF 2000 packages, predict the planar oxalate D(2h) configuration to be favored, which maximizes M(2) delta to oxalate pi* back-bonding, and indicate low barriers (<8 kcal mol(-1)) to rotation about the oxalate C-C bonds.

Photoluminescence and Near-Edge Optical Absorption in the Low-Temperature Phase of Pristine C60Single Crystals

Photoluminescence and near-band-edge electronic structure of pristine C 60 single crystals have been investigated in the low temperature phase below 90 K. By applying two-photon excitation spectroscopy, a monomorphic spectrum is extracted from the several crystals which exhibit apparent diversity for ordinary one-photon excitation. Also, a localized absorption band with discrete vibronic sub-peaks is clearly confirmed at 1.685 eV below the fundamental absorption edge (1.815 eV). Overall features of the luminescence-line-narrowing spectrum obtained by site-selective excitation into the localized absorption band are assigned to the vibronic progressions coupled with the Raman-active gerade-symmetry modes of individual C 60 molecules (i.e., eight h g , two a g , and their combinations), accompanied by phonon-sidebands due to cooperative librations and intermolecular translations. The origin of photoluminescence and its relaxation kinetics are fully discussed in connection with the inhomogeneously broadened localized states which are presumably due to perturbed Frenkel excitons stabilized with symmetry-lowering by adjacent intrinsic defect or some lattice disorder.

On the assignment of the absorption bands in the optical spectrum of Alq 3

The optical absorption and/or excitation spectrum of tris(8-hydroxyquinolinato) aluminium(III) (Alq 3) in thin solid film is similar to the spectra in organic solvents, in vapour at ∼600 K and under jet-cooled conditions. The spectrum of Alq 3 shows two weak but prominent bands at ∼334 and ∼320 nm which have not been assigned before. These weak bands are now assigned to a vibronic progression due to the ring deformation modes at ∼1300 cm −1 associated with an electronic transition whose origin is at 346 nm (28 862 cm −1). This assignment is based on the one-to-one correspondence of the absorption bands of Alq 3 and the ligand, 8-hydroxyquinoline in its protonated form. In addition, the long wavelength absorption band of Alq 3 at ∼390 nm is concluded to be a superposition of two or more electronic transitions to account for the large discrepancy between the calculated radiative lifetime of Alq 3 and measured fluorescence lifetimes in condensed phase and jet-cooled beam.