Easting Research Articles

Spectroscopy, resolution, and ab initio calculation of vibronic fine structure in the optical absorption of Rh(III) doped in NaCl

A combined spectroscopic and theoretical investigation is devoted to the vibrational fine structure in the broad band spectrum at 4.2 K of the 1T1g and 1T2g states of Rh(III) d6 doped in NaCl. The vibrational structure is resolved with a noise reduction technique using quadrature mirror filters. A weak signal at 16805 cm−1 is identified as the zero phonon line of the 1T1g←1A1g transition, 194 cm−1 below a false origin of a progression in a single 264 cm−1 mode. The observed spectrum can be reproduced quantitatively by ab initio calculations involving identification of the Jahn-Teller active distortions, derivation of the shapes of excited state potential energy surfaces, and calculation of the Franck–Condon factors. The dominant mode of the progression is identified as a combination of the a1g and eg(a) modes with a calculated frequency of 279 cm−1. The calculations further reveal the existence of a conical intersection between the 1T1g and 1T2g states, situated in the optical window between the two absorption bands and expected to play an important role in the deactivation of the 1T2g state.

Interchain interaction in a prototypical conjugated oligomer from polarized absorption at 4.2 K: α-sexithienyl single crystal

The polarized absorption spectra in the bc face of an α-sexithienyl (T6) single crystal have been measured at 4.2 K. The origin of the lowest electronic transition is at 18 360 cm−1 and has been assigned to the lowest b-polarized au Davydov component of the 1 1Bu molecular level. The second optically allowed Davydov component (bu) is polarized in the ac crystal plane and is located at 20 945 cm−1. Therefore, the interchain interaction leads to a Davydov splitting of about 2600 cm−1. A quantum chemical model, which considers the total molecular wavefunctions for each transition, shows good agreement with the experimental findings for the energy and polarization of the optically allowed crystal levels. The vibronic manifold in the absorption spectra has been interpreted in terms of the Herzberg–Teller and Franck–Condon vibronic coupling. In particular, two false origins due to the coupling of the molecular electronic levels 1 1Bu and 2 1Bu have been identified at 18 486 and 18 657 cm−1. Consistently, the emission counterpart of the two false orgins has been identified in the polarized fluorescence spectra. The totally symmetric modes involved in the coupling are in excellent agreement with the Raman scattering data of the single crystal.

Vibronic Activity in the Phosphorescence Spectra of Disklike Aromatic Molecules: A Combined Experimental and Theoretical Investigation

The phosphorescence spectra of triphenylene and truxene, recorded in glassy solvents at 77 K, are presented. Their vibronic structure is interpreted on the basis of intensities computed with the help of quantum-chemical calculations. It is shown to be due to Herzberg−Teller-active nontotally symmetric modes which provide false origins for progressions of totally symmetric modes. Similarly to benzene, the dominant contribution to the T1 → S0 transition in the two disklike molecules is shown to derive from triplet−triplet transition dipole moments. Radiative triplet lifetimes are estimated on the basis of computed oscillator strengths and compared with the correspondingly observed values.

Polarized fluorescence in α-sexithienyl single crystal at 4.2 K

The polarized fluorescence spectra of the bc plane of α-sexithienyl (αT6) single crystal at T=4.2 K have been measured and analyzed. The spectra show a sharp component superimposed on a broad component. The origin of fluorescence at 18332 cm−1 with a rather narrow linewidth (FWHM=8 cm−1) is completely polarized along b. We assign it to a shallow X-trap 18 cm−1 below the bottom of the 1 1Bu exciton band. The 1 1Bu molecular electronic excited level splits in the crystal in four Davydov components (ag, bg, au, and bu) of which au and bu are dipole allowed. The calculated exciton band structure based on the Ewald sums, assuming the point dipole–dipole approximation, predicts two lowest degenerate Davydov components: an au b polarized component and a forbidden ag component. The fluorescence with the intrinsic sharp vibronic progressions is analyzed in terms of ground state totally symmetric modes which are in excellent agreement with Raman scattering data of the single crystal. Sharp c polarized false origins are identified at 18 167, 18 084, and 18 026 cm−1 and discussed either in terms of X-traps or Herzberg–Teller vibronic coupling with the second higher 2 1Bu molecular level polarized along the in plane short axis. A broad c polarized component is attributed to aggregates.

Optical properties of solid C 60

The optical properties of materials are strongly related to the nature of the photoexcitations in the solid. Fullerene C 60 in the solid state is a typical molecular material in which the molecules are weakly held together by van der Waals forces and the electronic excitations are very close in energy and nature to those of the free molecule. The lowest electronic states in solid C 60 are dipole forbidden and can be described as tight bound (Frenkel) excitons. Optical emission of C 60 single crystals can be explained in terms of photoluminescence sub-spectra built on vibronically allowed false origins. Further to these neutral excitations which determine the optical gap there are charged excitations corresponding to the promotion of electrons from ball to ball and from ball to the free electron continuum. We observe intermolecular charge-transfer excitons by measuring the electroabsorption of thin films at liquid helium temperature. The energy necessary to generate directly the electron—hole (e—h) pairs is set at least 0.6 eV higher than the optical gap.

On the analysis of the phosphorescence spectrum of

The phosphorescence spectrum of , recently obtained in an alcane matrix at low temperature, has been modelled by means of semi-empirical quantum-chemical calculations. The transition is symmetry as well as spin-multiplicity forbidden. The spectrum is successfully analysed in terms of the Herzberg - Teller mechanism combined with spin - orbit interactions. Its vibrational structure appears to be dominated by false origins due to modes of symmetry.

Observation of a magnetic dipole origin in the spectrum of the square planar CuCl 42− ion

The low temperature d-d absorption spectrum of bis(2-aminobenzothiazolium) copper tetrachloride is reported. The lowest energy B 2g( xy) ← B 1g( x 2 − y 2) transition has a relatively sharp electronic origin which has not previously been observed in a CuCl 4 2− ion. The polarisation properties of this electronic origin show it to be magnetic dipole allowed. The transition also has extensive vibrational structure and several false origins due to coupling with odd-parity vibrations. The direct observation of the electronic origin allows an accurate determination of the excited state geometry.

Vibronic activity in trans,trans-1,3,5,7 octatetraene: The S0→S1 spectrum

Various levels of configuration interaction are used to investigate the vibronic intensity pattern of the false origins of the 1 1Ag→2 1Ag transition of all-trans octatetraene. The vibronically induced mixing of the Ag with the Bu states is best simulated when polarization, i.e., d, functions are added to the basis set of atomic orbitals. Normal mode rotation upon electronic excitation plays an important role in the intensity distribution of the bu false origins. The progressions of the totally symmetric modes built on the bu bands is satisfactorily simulated at the CASSCF/6-31G* level. The calculations also show that the overtones of at least four out-of-plane modes should be present in the spectra whose assignment is tentatively proposed. Through comparison of the pure electronic intensities calculated for cis–trans octatetraene with the vibronically induced intensities of trans–trans octatetraene, it is found that the presence of a cis linkage induces a spectral perturbation similar to that of the most active bu mode.

Vibronic structure and resonance effects in the optical spectra of the fullerenes C60 and C70

Optical emission spectra of solid C{sub 60} and C{sub 70} were recorded at 1.9 K using different detector systems. In the region of low energy an improved resolution could be obtained using an IR sensitive photodiode. The energy of the emitting states of C{sub 60} was estimated by means of resonant excitation using a dye laser. The proposed assignment of the vibrational fine structure is based on vibrational frequencies obtained from normal coordinate analysis and CNDO/S-calculated intensities of false origins, both reported in the literature. It is shown that the emission arises from two electronic states. Low-energy excitation of C{sub 70} supplies evidence for two emitting states as well. Resonance excitation supplies specific changes of the emission spectrum attributed to different excited levels. 23 refs., 3 figs., 2 tabs.

Fluorescence fromXtraps inC60single crystals

We show that low-temperature fluorescence spectra of large high-quality ${\mathrm{C}}_{60}$ single crystals are mainly composed of several, independent pairs of ${\mathit{T}}_{1\mathit{g}}$ false origins. In analogy to the case of anthracene single crystals the series of emission bands can be interpreted in terms of exciton (Frenkel) emission from so-called X traps. Temperature dependent luminescence experiments support this interpretation.

High-resolution one- and two-photon spectra of matrix-isolated anthracene

Single-site fluorescence and one- and two-photon fluorescence excitation spectra of anthracene in an argon matrix at about 12 K are presented. The spectra exhibit a resolution of less than 4 cm −1 and show almost no site contamination. The excitation spectra are discussed with respect to the transitions to the first and second excited singlet state ( 1L a and 1L b). The spectra confirm the results of earlier investigations that the onset of the hidden 1L b transition to the second excited singlet state lies at about 28000 cm −1 in nonpolar solvents. By choosing different sites for the excitation spectra the solvent shifts for the two transitions were measured. Extrapolation of these shifts leads to an estimate of the separation between the origins of the two transitions in the free molecule of less than 500 cm −1. For one-photon absorption a two-dimensional fluorescence/fluorescence excitation spectrum is presented. This spectrum shows that in the one-photon case none of the bands observed up to an energy of about 1600 cm − above the origine of the 1L a band can be assigned to the 1L b transition with any confidence. All observed long axis polarized vibrational bands are false origins of the 1L a transition.

The S(1A g)–S1(1B2u) vibronic transition in benzene: An ab initio study

The four e2g false origin bands and the a1g progression of the S0(1Ag)–S1(1B2u) transition in benzene are simulated ab initio with the recently introduced configuration interaction singles (CIS) with 6-31G orbitals. The ground and excited state CC and CH bond lengths are optimized and compared with the experiment; the CC bond elongation upon excitation is found to be slightly underestimated. The vibrational force fields are calculated at the stationary points of S0 and S1. The 1Ag force field is calculated at the Hartree–Fock level while the 1B2u force field is calculated at the CIS level of theory. The two force fields are scaled to fit the experimental frequencies and the normal mode rotation upon excitation, i.e., the Duschinsky matrix, is obtained. In agreement with previous empirical fitting of the S1(1B2u) vibrational frequencies, the Duschinsky matrix is found to be nearly diagonal with the exception of the b2u modes submatrix which shows a large amount of mixing. The mixing of the b2u modes is larger before scaling but is subsequently reduced after scaling. The normal modes and the optimized geometries are used to calculate the amount of displacement, upon excitation, of the equilibrium position of the totally symmetric modes. This displacement causes the Franck–Condon progression and is slightly underestimated by the calculation. The intensity of the four e2g false origins in the absorption spectrum of S1 is calculated and the Herzberg–Teller intensities of the four bands are found to be very close to the experiment. In particular, the relative intensity of the CCC bend (ν6) and CC stretch (ν8) bands is nicely reproduced. This result is discussed in light of similar calculations at the semiempirical level of theory. We conclude that CIS can be of great value for the unravelling of vibronic spectra of conjugated systems.

Vibrational structure of the S 0→S 2 absorption band of coronene in polymethylmethacrylate, studied by transient spectral hole-burning

The S 0→S 2 absorption band of the aromatic hydrocarbon coronene in polymethylacrylate (PMMA) was investigated with the method of transient spectral hole-burning (TSHB), which uses the availability of a long-lived metastable triplet state. The same vibronic transitions are observed as in the site-selective fluorescence excitation spectrum of coronene in a heptane Shpol'skii matrix. The lowest e 2g mode not only gives rise to the lowest false origin but also can form progression in combination with false origins. The degree of polarization of transient zero-phonon holes is close to the theoretical value of 1 7 that is expected for inplane polarized transitions of a molecule with the symmetry D 6h. From the homogeneous widths of resonant transient holes lifetimes ≲ 2 ps are obtained for the S 2 vibronic states. With optically thin samples, the lineshape of resonant zero-phonon holes remains Lorentzian in the steady state of TSHB despite nonnegligible ground-state depletion. The correlation of nonresonant holes is approximately Lorentzian.

Franck–Condon analysis of the S→T1 absorption and phosphorescence spectra of biphenyl and bridged derivatives

The equilibrium geometry and the vibrational force field of the ground and the lowest triplet electronic states of biphenyl and three bridged derivatives−biphenylene, fluorene and phenanthrene−are computed by using an updated version of the QCFF/PI (Quantum Chemical Force Field/π electron) Hamiltonian. The displacement parameters between T1 and S0 are obtained and used to model the S0→T1 absorption and the phosphorescence spectra. The calculated Franck–Condon envelopes are found to be in excellent agreement with the vibrational structure of the observed spectra. The common features of the phosphorescence spectra of biphenyl and fluorene are related to the same orbital nature of the lowest triplet state. The observed asymmetry between the phosphorescence and singlet–triplet absorption spectra of biphenyl is reproduced when the twisted equilibrium geometry of S0 is considered. It is shown that evidence of the nonplanarity of the ground state of biphenyl is manifested by the lower intensity of the band observed in the phosphorescence at 747 cm−1 with respect to the intensity of the same band in fluorene. The increased vibrational activity calculated in the lower frequency region for biphenylene and phenanthrene agrees with the observed spectra and reflects the different orbital nature of the lowest triplet state of the two strongly perturbed bridged derivatives with respect to biphenyl and fluorene. From the analysis of the computed vibrational frequencies, it is suggested that the false origin of the symmetry forbidden phosphorescence of biphenylene is due to the lowest out-of-plane mode of au symmetry.

Interpretation of the vibrational structure of the emission and absorption spectra of C60

The vibronic intensity borrowing activity of the lowest electronically excited singlet states of C60 has been obtained through quantum chemical calculations. The vibrational structure of the UV–visible spectra is found to be dominated by false origins. The calculated intensities of the false origins of the T1g state agree with the vibrational structure observed in the fluorescence spectrum. The same false origins are recognized to be responsible for the vibrational structure of the red edge portion of the absorption spectrum. Only two bands in the spectra are assigned as combination bands involving an ag or a Jahn–Teller active mode. Absorption bands that may be associated with false origins of the states T2g and Gg which are quasidegenerate with S1 are tentatively assigned.

On the emission and excitation spectrum of the NIR laser center in Cr:YAG

We use a tunable narrowband laser operating in the region 1.1–1.3 μm to probe the excitation spectrum of the NIR laser center in Cr doped yttrium aluminum garnet (YAG) at low temperatures. Our results reveal that the sharp emission bands observed at low temperatures around 1.28 μm are true electronic origins. This is in contrast to previous interpretations of this luminophore in which these sharp bands have been interpreted as false origins built on a zero phonon line supposed, but not observed, to lie midway between the 1.28 μm bands in emission and a sharp “false origin” absorption band at 1.125 μm. We discuss the possible nature of this luminophore in light of these new results.

Luminescence and excitation spectroscopy of ruthenocene

The luminescence and excitation spectra of the perdeuterated and the protonated ruthenocene have been carefully investigated. The electronic origins are at 21 459 cm−1 and at 21 390 cm−1 for the perdeuterated and protonated crystals, respectively. The two observed luminescence progressions in the metal-ring stretch a′1 (D5h notation) in the perdeuterated crystal are separated by 155 cm−1 compared to the 165–170 cm−1 reported for the protonated compound. The metal-ring stretch frequency is reduced to 318 cm−1 in the perdeuterated crystal from the 333 cm−1 found for the protonated compound. In both crystals the observed doubling of the vibrational progression is due to a false origin based on a skeletal bending mode δ(ring-Ru-ring) which transforms as e1 in D5h. This mode is split by 7–8 cm−1 via the low site symmetry (Cs) in the crystal. The excitation spectrum of both compounds shows only a single progression in the metal-ring stretch. This frequency is reduced by 16% in the excited state for both the protonated and the perdeuterated crystal. Deuteration increases the lifetime from 127 μs to 990 μs at 1.5 K.

A two-photon resonant multiphoton ionization study of the 3p-Rydberg .rarw. ~X transitions of cyclopentadiene

The polarization-selected two-photon resonant multiphoton ionization spectra of cyclopentadiene and cyclopentadiene-d 6 have been measured in the 3p-Rydberg←X region of the spectrum. The spectra were analyzed to provide experimental assignments for all three 3p-Rydberg origins and three a 1 , three b 1 , and one a 2 fundamental vibrational frequencies in the 3p-Rydberg states. The allowed origins of the B 1 and A 1 3p-Rydberg←X transitions were not observed; Herzberg-Teller induced false origins were observed from the ground to these states in their stead

The excited-state geometry of [Re 2Cl 8] 2−

We report the luminescence spectrum of a single crystal of [Bu 4N] 2[Re 2Cl 8] at ≈20 K. The spectrum shows progressions in the Re-Re stretching vibration v 1 based on an electronic origin at 14161 cm −1 and on false origins. The luminescence spectrum exhibits a “mirror-image” relationship to the absorption spectrum and is interpreted as showing that the transition originates from the 1δδ ∗ excited state of the eclipsed ion.

Electronic absorption and resonance raman spectra of the [ReS 4] − ion: Specific resonance enhancement in non-totally-symmetric modes

Resonance Raman spectra have been measured at = 20 K for a variety of excitation wavelengths within the 1T 2 ← 1A 1 (2e ← 1 1) visible absorption band of the [ReS 4] − ion. Excitation into vibronic sidebands associated with non-totally-symmetric false origins results in selective enhancement of the appropriate combination band progressions in the RRS.