Intermolecular vibrations and dynamics of the anisole—benzene complex studied by multi-resonance spectroscopy

The intermolecular vibrations of the anisole—benzene complex in the ground and excited electronic states have been observed by the LIF (laser-induced fluorescence) and fluorescence-dip techniques. Short progressions due to the intermolecular vibrations suggest a small structure change of the complex upon electronic excitation. The LIF excitation spectrum shows predominant progressions of 27 cm⁻¹, which is tentatively assigned to one of the intermolecular bending modes in the excited electronic state. On the other hand, the fluorescence-dip spectrum shows only a series of bands with irregular intervals due to the intermolecular modes in the ground electronic state. The decay rates of the vibrationally excited complex in the ground electronic state have also been measured with the SEP-LIF (stimulated emission pumping-laser-induced fluorescence) technique, where the complex vibrationally excited by SEP is probed by the delayed LIF measurements. The complex excited to its purely intermolecular mode stays in the initially prepared state after a delay time of 1 μs. On the other hand, the complex excited to the intramolecular vibrational states above 500 cm⁻¹ does not seem to stay in the prepared states. Neither the relaxed complex nor the dissociated monomer was detected. A possible reason for this observation is discussed.     

Developing Bright Green Fluorescent Protein (GFP)-like Fluorogens for Live-Cell Imaging with Nonpolar Protein−Chromophore Interactions

Fluorescence-activating proteins (FAPs) that bind a chromophore and activate its fluorescence have gained popularity in bioimaging. The fluorescence-activating and absorption-shifting tag (FAST) is a light-weight FAP that enables fast reversible fluorogen binding, thus advancing multiplex and super-resolution imaging. However, the rational design of FAST-specific fluorogens with large fluorescence enhancement (FE) remains challenging. Herein, a new fluorogen directly engineered from green fluorescent protein (GFP) chromophore by a unique double-donor-one-acceptor strategy, which exhibits an over 550-fold FE upon FAST binding and a high extinction coefficient of approximately 100,000 M⁻¹ cm⁻¹, is reported. Correlation analysis of the excited state nonradiative decay rates and environmental factors reveal that the large FE is caused by nonpolar protein−fluorogen interactions. Our deep insights into structure-function relationships could guide the rational design of bright fluorogens for live-cell imaging with extended spectral properties such as redder emissions.     

LCGTO MP LSD calculations for the diatomics PdX (X = C,Si, Ge,Sn)

Results are presented of a theoretical study of RhC, PdC, PdSi, PdGe and PdSn metal-alloy diatomics performed with the LCGTO MP LSD method. For the RhC molecule the computed electronic structure and spectroscpic constants are in good agreement with experimental data. The ground state of RhC is found to be ²Σ⁺ withe the lowest-lying excited state, ²II, at 10654 cm⁻¹ above the ground state, in good agreement with experiment and with ab initio CI calculations. For PdC and the other Pd-group IV A dimers the ground state is predicted to be ¹Σ and the two lowest-lying excited state are ³II (at 2718 cm⁻¹ for PdC) and ³gS (at 12164 cm⁻¹ for PdC). Our results are in disagreement with ab initio CI studies for PdC, which found a ³Σ ground state with a ³II lowest-lying excited state, and for PdGe, which found a ³II ground state followed by ³Σ⁻ and ¹Σ⁺.     

Molecular spectroscopy of 2,5-distyrylpyrazine

The molecular absorption and fluorescence spectra of 2,5-distyrylpyrazine (DSP) in a Shpol'skii matrix at 4 K together with the Raman spectrum of crystalline samples at 298 K have been investigated. The transition to the lowest excited singlet state of B_u symmetry is of π—π^* character and two sites with origins at 23621 cm^?1 and 23713 cm^?1 are observed in n-tetradecane at 4 K. The DSP molecule has the same conformation in the two sites and, furthermore, the geometry of the molecule in the first excited singlet states is unchanged as compared to the ground state. The vibrational progression observed for DSP is compared with the literature data for distyrylbenzene (DSB) and slilbene.     

Line broadening mechanisms in the low pressure laser-induced plasma

The spectral profiles of Ca and Rb lines have been studied in a laser induced plasma as a function of pressure (1–10 torr) and delay time with respect to the plasma initiation (1–10 μs). Measurements were made in a plasma induced by the 1064-nm output of a Nd:YAG laser on a calcium carbonate matrix, doped with Rb. Spectral profiles were measured in absorption using a narrow-band cw Ti:Sapphire laser. It was shown that in the case of a trace element (Rb in a CaCO₃ matrix), the broadening mechanism was Doppler-dominant, whereas for a major matrix component (Ca), resonance broadening was the main contributor to the line shape. The plasma was shown to be non-equilibrium provided by the difference between the kinetic (3000 K) and the excitation (8000 K) temperatures. The electron number density at delay times of 5–10 μs and pressures of 1–10 torr was estimated not to exceed 10¹⁵ cm⁻³. The number densities of Ca atoms in the ground and the excited (23 652 cm⁻¹) states were evaluated by measuring line width and peak absorption at 732.6 nm. They were found to be in the range of (1.5–2.2)×10¹⁷ cm⁻³ for the ground state and (1.5–33)×10¹¹ cm⁻³ for the excited state.     

The ab initio calculation of the band origin and vibrational frequencies of the Ã-X̃ system of HNCl using the non-rigid bender hamiltonian

A three-dimensional fit of ab initio MRD CI potential data has been made for the lowest two electronic states of the HNC1 molecule (X̃ ²A″ and à ²A'), and the corresponding vibrational frequencies and rotational energies have been computed using the non-rigid bender Hamiltonian. For the ground state the vibrational frequencies obtained are ν₁ = 2942 cm⁻¹, ν₂ = 1232 cm⁻¹, and ν₃ = 549 cm⁻¹, while the corresponding values for the first excited state are 3524,947 and 836 cm⁻¹ respectively. We calculate T_c(à ²A') 16200 cm⁻¹, T_o(òA') = 16400 cm⁻¹, and the Franck-Condon maximum, Ã(0,3,1)-X̃(0,0.0), is calculate at 19200 cm⁻¹(5200 Å).     

Microwave spectrum of cyclohexanone

The rotational spectrum of cyclohexanone has been observed within the frequency region from 18.0 to 40.0 GHz. Transitions in the ground state and six excited states have been assigned. The ground state rotational constants are (in MHz) A = 4195.316 +- 0.059, B = 2502.627 ± 0.005 and C = 1754.443 ± 0.005.From information obtained from relative intensity measurements, these excited states are estimated to be ~ 100 cm^?1 above the ground state for the first excited state of the ring-bending mode and ~ 180 cm^?1 for the first excited state of the ring-twisting mode.     

Laser Spectroscopy and Lifetime Measurements of the S1 State of Tetracyanoquinodimethane (TCNQ) in a Cold Gas-Phase Free-Jet

The S₁ electronic state of 7,7,8,8-Tetracyanoquinodimethane (TCNQ) has been investigated by laser induced fluorescence (LIF), dispersed fluorescence (DF) spectroscopy, and lifetime measurements under jet-cooled conditions in the gas-phase. The LIF spectrum showed a weak origin band at 412.13 nm (24262 cm⁻¹) with prominent progression and combination bands involving vibrations of 327, 1098, and 2430 cm⁻¹. In addition, very strong bands appeared at ∼363.6 nm (3300 cm⁻¹ above the origin). Both the LIF and DF spectra indicate considerable geometric change in the S₁ state. The fluorescence lifetime of S₁ at zero-point level was obtained to be 220 ns. This lifetime is 40 times longer than the radiative lifetime estimated from the S₁−S₀ oscillator strength. Furthermore, the lifetimes of the vibronic bands exhibited drastic energy dependence, indicating a strong mixing with the triplet (T₁) or intramolecular charge-transfer (CT) state. This study is thought to disclose intrinsic nature of TCNQ, which has been well known as a component of organic semiconductors and a versatile p-type dopant.     

Polyene spectroscopy: Vibronic evidence for a forbidden transition in DECA-2,4,6,8-tetraene

The dimethylpolyene deca-2,4,6,8-tetraene was studied by absorption, fluorescence excitation and fluorescence spectroscopy in glasses at 77 K and in n-alkane crystals at 4.2 K. A strong transition to a ¹B_u excited state is observed with an origin at 32400 cm^?1 in isopentane at 77 K and at 31280 cm^?1 in n-undecane at 4.2 K. A weak transition to a ¹A_g excited state is observed with an origin at 28738 cm^?1 in the n-undecane matrix. The radiative fluorescence lifetime is 500 ns. In undecane the transition from the ground state to the ¹A_g excited state exhibits a classic Herzberg—Teller vibronic pattern indicating a symmetry forbidden transition.     

Spectroscopic Studies of the Magnetic Excitation and Spin-Phonon Couplings in a Single-Molecule Magnet

Large separations between ground and excited magnetic states in single-molecule magnets (SMMs) are desirable to reduce the likelihood of spin reversal in the molecules. Spin-phonon coupling is a process leading to magnetic relaxation. Both the reversal and coupling, making SMMs lose magnetic moments, are undesirable. However, direct determination of large magnetic states separations (>45 cm⁻¹) is challenging, and few detailed investigations of the spin-phonon coupling have been conducted. The magnetic separation in [Co(12-crown-4)₂](I₃)₂(12-crown-4) ( 1 ) is determined and its spin-phonon coupling is probed by inelastic neutron scattering (INS) and far-IR spectroscopy. INS, using oriented single crystals, shows a magnetic transition at 49.4(1.0) cm⁻¹. Far-IR reveals that the magnetic transition and nearby phonons are coupled, a rarely observed phenomenon, with spin-phonon coupling constants of 1.7–2.5 cm⁻¹. The current work spectroscopically determines the ground–excited magnetic states separation in an SMM and quantifies its spin-phonon coupling, shedding light on the process causing magnetic relaxation.     

Laser-induced fluorescence and single vibronic level emission spectroscopy of chiral (R)-1-aminoindan and some of its clusters in a supersonic jet

Two low energy conformers of the chiral (R)-1-aminoindan molecule are identified in supersonic jet and their ground and excited states vibrational spectroscopy has been investigated by laser-induced fluorescence (LIF) excitation and single vibronic level (SVL) emission spectroscopy. Ab initio calculations confirm the existence of two lowest-energy structures, where the amino group is in equatorial position with its lone pair directed opposite to the aromatic electron cloud. Harmonic frequencies have been calculated for these two conformers at the DFT level with B3LYP functional. A low-frequency progression of 118 cm(-1) and 114 cm(-1), respectively, appears in the fluorescence excitation spectrum of the two conformers, with its ground state counterpart at approximately 147 cm(-1). It has been assigned to the puckering motion coupled with the ring flapping mode. The other calculated low-frequency mode corresponds to the puckering motion coupled with the ring twisting mode and its ground state frequency has been observed at 119 cm(-1) and 111 cm(-1) from SVL spectra. Both conformers form similar 1 : 1 water clusters, whose 0-0 transitions are shifted to the blue by 41 cm(-1) and 44 cm(-1), respectively, and whose SVL spectra are similar. Interestingly, one of the conformers seems to preferentially make complexes with (S)-methyllactate, while the other one shows selective complexation to (R)-methyllactate.     

Laser-induced Fluorescence and Dispersed Fluorescence Spectroscopy of NiB: Identification of a New 2II State in 19000-22100 cm-1

The laser-induced fluorescence excitation spectra of jet-cooled NiB radicals have been recorded in the energy range of 19000-22100 cm^-1. Eleven bands have been assigned to the [20.77]²II-X²∑⁺ transition system for the first time. The dispersed fluorescence spectra related to most of these bands have been investigated. Vibrationally excited levels of the ground electronic state, with v" up to 6, have been observed. In addition, the lifetimes for almost all the observed bands have also been measured.     

Observation of photodissociation of S2Cl2 and resonance Raman and fluorescence spectra of S2Cl under 514.5 nm radiation

The laser Raman spectrum of S₂Cl₂ varies with the sample temperature and/or the laser power. The Raman signals of S₂Cl₂ decreases as the sample molecules within the laser beam are dissociated by absorbing 514.5 nm photons. Above 540 K and 2 W of laser power, new resonance Raman and fluorescence bands appear. These bands were all assigned to S₂Cl. The fluorescence bands could be classified into two transition systems. Only one of them had the ground electronic state X̃ as its lower state. For the other, the low lying first excited state à was suspected. The fundamental frequencies suggested for the three vibrational modes were 664, 196 and 450 cm⁻¹ for the X̃ state and 630, 249 and 554 cm⁻¹ for the à state respectively.     

Impact of extended pi conjugation on methyl rotor-induced IVR in aromatic molecules

Laser-induced fluorescence excitation and resolved fluorescence spectra following excitations of the single vibronic levels (SVL) of p-vinyltoluene (p-VT) and p-vinylfluorobenzene (p-VFB) have been measured in a seeded supersonic free-jet expansion. A complete vibronic assignment of the fluorescence spectrum measured following excitation of the 0(0)0-band of p-VT has been presented. Normal vibrational modes in the S0 and S1 states of the molecule have been calculated by CASSCF method, and the correlation between the two set of modes is made by expressing the excited-state normal modes in terms of those of the ground state. The calculations predict that in the excited state methyl and vinyl torsional motions of p-VT are extensively mixed with many of the out-of-plane modes of the aromatic ring. Our resolved fluorescence spectral data measured following SVL excitations essentially agree with such predictions. In the excited state, the molecule exhibits a dramatically low threshold for the rotor-induced IVR in a supersonic jet expansion. Several mechanisms have been discussed to explain the phenomenon.     

The mechanism and kinetics of energy transfer processes in Xe–CCl4–M (M=CO,CO2) mixtures irradiated by xenon resonance light

The mechanism and kinetics of energy transfer from the Xe(6s[3/2]₁) resonance state to CO and CO₂ molecules have been investigated by XeCl(B–X) (λ_max=308 nm) fluorescence intensity measurements at stationary conditions in Xe–CCl₄–M systems. Steady-state analysis of the fluorescence intensity dependence on the xenon and M pressure at constant CCl₄ concentration shows that these processes occur in two- and three-body reactions: Xe(6s[3/2]₁⁰)+M→products; Xe(6s[3/2]₁⁰)+M+Xe→products. The two-body rate constants for above reactions have been found to be (0.7±0.2)×10⁻¹⁰ and (4.9±0.4)×10⁻¹⁰ cm³ s⁻¹ for CO and CO₂, respectively. The three-body rate constants have been found to be (3±1)×10⁻²⁹ and (2.4±0.3)×10⁻²⁸ cm⁶ s⁻¹ for CO and CO₂, respectively. It has been shown that the third order reaction is a very effective channel of xenon excited atoms decay at high xenon pressures (P(Xe)>50 Torr).     

Far infrared spectra of methyl nitrate and methyl-d3 nitrate

The far infrared spectra from 300 to 50 cm⁻¹ of methyl nitrate, CH₃ONO₂, and methyl-d₃ nitrate, CD₃NO₂, have been recorded at a resolution of 0.12 cm⁻¹. The fundamental methyl torsional mode has been observed at 204.5 cm⁻¹ (154.2 cm⁻¹ for CD₃ONO₂) with two excited states falling to lower frequencies which gives a V₃ barrier of 980 ± 40 cm⁻¹ (2.80 ± 0.11 kcal/mol). The NO₂ torsion (methoxy) has been observed with the 1 ← 0 transition being at 133.7 cm⁻¹ (119.5 cm⁻¹ for CD₃ONO₂) and eight successive excited states falling to lower frequencies. From these data the twofold barrier to internal rotation has been calculated to be 2650 ± 75 cm⁻¹ (7.69 ± 0.21 kcal/mol).     

Molecular orbital studies of the spectral and structural properties of chlorosilylidyne,SiCl

Si-Cl bond distances for the X̃² Π ground and low lying b̃⁴ Σ, ò Σ and B̃'² Δ excited states of SiCl have been optimized at the SCF and CI level with 6–31G basis set. Optimized bond distances are in good agreement with experimental values. Computed electronic excitation energies for the X̃²Π-Ã²Σ and X̃²Π-B̃'² Δ transitions compare well with the observed spectrum. The calculated harmonic vibrational frequency for the ground state, 525.2 cm⁻¹, also agrees with the experimental value 535.6 cm⁻¹.     

An Electron-Accepting aza-BODIPY-Based Donor–Acceptor–Donor Architecture for Bright NIR Emission

A bright near-infrared (NIR) fluorescent molecule was developed based on the donor–acceptor–donor (D–A–D) approach using an aza-BODIPY analog called pyrrolopyrrole aza-BODIPY (PPAB) as an electron-accepting chromophore. Directly introducing electron-donating triphenylamine (TPA) to develop a D–A–D structure caused redshifts of absorption and emission of PPAB into the NIR region with an enhanced fluorescence brightness of up to 5.2×10⁴ m ⁻¹ cm⁻¹, whereas inserting a phenylene linker between the TPA donor and the PPAB acceptor induced solvatochromic behavior in emission. Transient absorption spectra and theoretical calculations revealed the presence of a highly emissive hybridized locally excited and charge-transfer state in the former case and the contribution of the dark charge-separated state to the excited state in the latter case. The bright D–A–D PPAB as a novel emitter resulted in a NIR electroluminescence with a high external quantum efficiency of 3.7 % and a low amplified spontaneous emission threshold of ca. 80 μJ cm⁻², indicating the high potential for NIR optoelectronic applications.     

LIF spectroscopy of jet-cooled 1:1 complex between 7-azaindole and 2-pyrrolidinone

Laser-induced fluorescence (LIF) spectra of a 1:1 complex between 7-azaindole (7AI) and five-member cyclic amide 2-pyrrolidinone (2-PDN) have been measured in a supersonic free jet expansion. The bands in the excitation spectrum appear doublet, which has been attributed to splitting of the zero-point level in the ground state due to puckering of 2-PDN moiety of the complex in a symmetric double minimum potential. This feature is consistent with low puckering barrier (～260 cm^?1) predicted by electronic structure calculation. The complex emits only UV fluorescence from locally excited state in the jet, but visible tautomer fluorescence is observed in hydrocarbon solution.     

On the “anomalous” flourescence of 3,4-benzypyrene in the vapour phase

An analysis of the fluorescence of 3,4-benzpyrene in the vapour phase shows two contributions to the “anomalous” fluorescence: (i) the emission from the second excited state to the ground state and (ii) a vibronically induced S₁ → S₀ emission originating from the +520 cm^?1 vibrational level. A comparison between the intensities of the emissions indicates that in the vapour phase the vibrational redistribution from the 520 cm^? vibrational level of S₁ to modes of lower frequencies is relatively slow.