Vibronic Fluorescence Research Articles

A bidentate-anionic-group strategy for enhancing electron–phonon coupling and vibronic fluorescence in rare-earth crystals

We found that the bidentate-BO3 group, better than the monodentate-BO3 unit, can greatly enhance the electron–phonon coupling and phonon-assisted fluorescence at longer wavelengths in rare-earth borates.

Evidence of Increased Hydrophobicity and Dynamics inside the Tail Region of Glycolipid Self-Assemblies Using 2-n-Alkyl-Pyrene Derivatives to Probe Different Locations.

New designer biofluorophores are being increasingly used in the investigation of complex cellular processes. In this study, we utilized new derivatives of pyrene (Py), i.e., 2-n-alkyl-pyrenes (Py-C4 and Py-C8), in order to probe different regions inside the hydrophobic tail of n-dodecyl β-d-maltoside (βMal-C12) in two different phases (cubic ↔ lamellar). Although the sensitivity to the local environment is reduced compared to that of Py, attaching C4 and C8 at the 2-position of Py can provide a possible means to probe the local hydrophobicity in different parts of the tail region. The absence of excimer fluorescence and the ratio of the vibronic fluorescence peak intensities (I1/I3) in a lipid environment indicate the existence of Py as monomers in the hydrophobic region, similar to hydrophobic solvation, yet close to the headgroup region. When Py is replaced by Py-C4 and Py-C8, there is a small increase in hydrophobicity (reduction in I1/I3) as the Py moiety is pulled deeper inside the tail region of both cubic and lamellar phases. The larger space of the tail region in the lamellar phase is reflected as more local hydrophobicity measured by the probes which can penetrate deep inside, whereas the curved structure of the cubic phase limits the available space for the probes. Three fluorescence lifetime components were measured in lipid, indicating the heterogeneous nature of the hydrophobic region. In the lamellar phase, a large reduction in the average lifetime value, led by the long decay component, was measured for Py-C4 (reduction by 25%) and Py-C8 (45%) compared to that of the parent Py. This observation suggests the presence of a mechanism of interaction more collisional than static between the Py moiety and the tail region of the bilayer unit due to the ample space provided by the lamellar phase as the probe is buried deeper inside the hydrophobic region. A much smaller effect was observed in the cubic phase and was correlated with the tight environment around the probes, which stems from the increased curvature of the cubic phase. The current results provide a deeper understanding of the hydrophobic region during phase transition of lipid self-assembly which is important for better control during the process of membrane-protein crystallization.

Calculation and interpretation of vibronic absorption and fluorescence spectra of the first electronic nπ* transitions of pyridine and pyrimidine

We have calculated vibronic spectra of the first electronic nπ* transitions of pyridine and pyrimidine in the isolated state using the DFT method in the Franck-Condon approximation. Vibrational spectra for the ground and excited states have been calculated in the anharmonic approximation, which allowed us to refine the assignment of normal vibrations of pyridine and pyrimidine. We have done a complete interpretation of the vibrational structure of the absorption and fluorescence spectra of pyridine and pyrimidine. It has been shown that Fermi resonances between fundamental and combination vibrations and overtones 12 and 16b + 4, 6a and 2 × 16b affect the formation of the vibrational structure of electronic spectra of pyrimidine. Good agreement between calculated and experimental spectra confirms the correctness of the models of the two molecules in their ground and excited states, which makes it possible to use the models in further investigations of various properties of these molecules in electronically excited states, e.g., tautomerism of pyrimidine bases of nucleic acids.

Development of a novel cryogenic microscope with numerical aperture of 0.9 and its application to photosynthesis research

A novel cryogenic optical-microscope system was developed in which the objective lens is set inside of the cryostat adiabatic vacuum space. Being isolated from the sample when it was cooled, the objective lens was maintained at room temperature during the cryogenic measurement. Therefore, the authors were able to use a color-aberration corrected objective lens with a numerical aperture of 0.9. The lens is equipped with an air vent for compatibility to the vacuum. The theoretically expected spatial resolutions of 0.39μm along the lateral direction and 1.3μm along the axial direction were achieved by the developed system. The system was applied to the observations of non-uniform distributions of the photosystems in the cells of a green alga, Chlamydomonas reinhardtii, at 94K. Gaussian decomposition analysis of the fluorescence spectra at all the pixels clearly demonstrated a non-uniform distribution of the two photosystems, as reflected in the variable ratios of the fluorescence intensities assigned to photosystem II and to those assigned to photosystem I. The system was also applied to the fluorescence spectroscopy of single isolated photosystem I complexes at 90K. The fluorescence, assigned to be emitted from a single photosystem I trimer, showed an intermittent fluctuation called blinking, which is typical for a fluorescence signal from a single molecule. The vibronic fluorescence bands at around 790nm were observed for single photosystem I trimers, suggesting that the color aberration is not serious up to the 800nm spectral region.

Calculation and interpretation of the absorption and fluorescence spectra of indole in the isolated state and aqueous solution

We have calculated the vibronic absorption and fluorescence spectra of the first (1 L b ) and second (1 L a ) electronic transitions of indole in the isolated state and aqueous solution. The vibrational structure of the absorption and fluorescence spectra has been interpreted. The influence of the aqueous solution on the vibronic spectra has been shown.

Vibration and Fluorescence Spectra of Porphyrin- CoredBis(methylol)-propionic Acid Dendrimers

Bis-MPA dendron-coated free-base tetraphenylporphyrin and zinc-tetraphenyl-porphyrin (TPPH2 and TPPZn) were studied in comparison with simple porphyrins (H2P, ZnP) by theoretical simulation of their infrared, Raman and electronic absorption spectra, as well as fluorescense emission. Infrared and fluorescence spectra of the dendrimers were measured and interpreted along with time-resolved measurements of the fluorescence. The 0–1 emission band of the dendron substituted TPPZn was found to experience a “heavy substitution”-effect. The 0–1 vibronic emission signal is associated with a longer decay time (approx. 7 - 8 ns) than the 0-0 emission (approx. 1 - 1.5 ns). The former contributed with more relative emission yield for larger dendron substituents, in agreement with the appearance of steady-state emission spectra showing increased contribution from the 0–1 vibronic fluorescence band at 650 nm. No such substitution effect was observed in the electronic or vibrational spectra of the substituted free-base variant, TPPH2. Vibration spectra of the parent porphyrins (H2P, ZnP, TPPH2 and TPPZn) were calculated by density functional theory (DFT) using the B3LYP/6-31G** approximation and a detailed analysis of the most active vibration modes was made based on both literature and our own experimental data. Based on the results of theoretical calculations the wide vibronic bands in the visible region were assigned. The vibronic structure also gave a qualitative interpretation of bands in the electronic absorption spectra as well as in fluorescence emission depending on the size of dendrimer substitution. From the results of time-dependent DFT calculations it is suggested that the TPPZn-cored dendrimers indicate strong vibronic interaction and increased Jahn-Teller distortion of the prophyrin core for larger dendrimer generations. Specifically, this leads to the entirely different behaviour of the emission spectra upon substitution of the TPPH2 and TPPZn variants, which was also experimentally observed. Since TPPH2 is originally of lower symmetry the specific distortion upon dendron substitution is not expected to the same extent, which also was in agreement with the experimental findings.

Non-planar distortion of single molecules revealed by vibronic fluorescence excitation: Terrylene in p-terphenyl and naphthalene crystals

Fluorescence excitation spectra of spatially resolved single terrylene molecules in p-terphenyl and naphthalene crystals were studied at 5 K. The observation of a single vibronic line at 251 ± 4 cm −1 above the purely electronic line indicates that the terrylene molecules in p-terphenyl crystals are planar in the S 1 excited state. The observation for terrylene in naphthalene crystals of two vibronic lines located 239 ± 2 and 252 ± 2 cm −1 above the purely electronic origin in naphthalene crystals indicates that there is non-planar distortion of the terrylene skeleton. The fully saturated fluorescence detection rate for the purely electronic (0, 0) transition was found to be about two times smaller than that for the vibronic components.

Specific features in the vibronic fluorescence spectra of analogues of 1,4-bis(5-phenyl-2-oxazolyl)benzene with oxadiazole and furan rings

The fine-structure fluorescence and fluorescence excitation spectra of conjugated chain compounds, namely, 2,5-bis(5-phenyl-1,3,4-oxadiazol-2-yl)furan (PDFDP) and 2,5-bis[5-(2,4-dimethylphenyl)-1,3,4-oxadiazol-2-yl]furan (XDFDX), were obtained by the Shpolskii method in an n-octane matrix at a temperature of 4.2 K. These spectra were simulated by representing the band of each of the vibronic transitions as the sum of a zero-phonon line and a phonon wing with the corresponding parameters, such as the half-widths of the spectral lines and the Debye-Waller factors. The results obtained made it possible to estimate the relative intensities of the vibronic transitions between the S 0 and S*1 states. The anharmonicity revealed in the conjugate spectra of fluorescence and fluorescence excitation of the PDFDP and XDFDX compounds was explained in terms of the interference of the Franck-Condon and Herzberg-Teller interactions occurring in the molecules under investigation. The influence of the substitution of the furan heterocycle (F) for the central benzene ring (P) in 1,4-bis-(5-phenyl-2-oxadiazolyl)benzene (PDPDP) on the parameters of the intramolecular interactions responsible for the formation of the vibronic spectra was considered.

1,3,6,8-Tetraethynylpyrene and 1,3,6,8-tetrakis (trimethylsilylethynyl) pyrene: Photophysical properties in homogeneous media

The photophysical properties of two new tetra substituted derivatives of pyrene: 1,3,6,8-tetraethynylpyrene (TEP) and 1,3,6,8-tetrakis(trimethylsilylethynyl)pyrene (TEP-TMS) have been studied. Studies were done with respect to mirror image symmetry in the absorption and emission spectra and permissive or forbidden nature of S 0–S 1 transition, solvent sensitivity of the first and third vibronic bands and fluorescence anisotropy. Both the derivatives exhibited a strongly allowed S 0–S 1 transition, high fluorescence quantum yield, shorter fluorescence lifetime compared to pyrene and invariance of the vibronic band intensity ratio to solvent polarity. The behavior of the two pyrene derivatives validates the hypothesis “solvent polarity mediates vibronic coupling and therefore the emission band intensities, for forbidden S 0–S 1 transitions”. The trimethylsilyl derivative (TEP-TMS) was characterized by a strong fluorescence in solid state. The tetraethynyl derivative (TEP) showed high fluorescence anisotropy comparable to the well-known anisotropy probe DPH in glycerol at 0 °C. The fluorescence intensities of TEP and TEP-TMS did not show any significant change in the temperature ranger 0–40 °C for a low viscous solvent like ethanol and in the range 0–60 °C in glycerol. Unlike pyrene, no excimer emission was observed even up to 10 −3 M for TEP and TEP-TMS.

Vibronic fluorescence and fluorescence excitation spectra of diphenyl oxazolyl benzene and its oxadiazol analogue

The fine-structure fluorescence and fluorescence excitation spectra of conjugated chain compounds—1,4-di(5-phenyl-2-oxazolyl) benzene and its oxadiazol analogue—are obtained at 4.2 K in an n-octane matrix using the Shpolskii method. The spectra are modeled by representing the band of each of the vibronic transitions as the sum of a zero-phonon line and a phonon wing with certain parameters (half-widths, Debye-Waller factors). The spectra calculated in this fashion coincide with the experimental spectra. This makes it possible to determine the relative intensities of vibronic transitions and refine the frequencies of the normal vibrations in the S0 and S*1 states. The parameters of the Franck-Condon and Herzberg-Teller interactions in the molecules considered are determined. The influence of the replacement of one of the CH groups in the heterocycles with a N atom on the parameters determining the formation of vibronic spectra is considered.

Electronic spectra of cationic forms of meso-tetrapropylporphin in a nanoporous silicate gel matrix

We have studied the fluorescence and fluorescence excitation spectra at 300 K, 77 K, and 4.2 K for silicate gel matrices colored with meso-tetrapropylporphin by impregnation of the matrix with a solution of the pigment. Comparison of the data obtained with the absorption spectra in acidified solutions and analysis of the low-temperature fine-structure vibronic spectra, and also taking into account data obtained earlier for octaethylporphin in a xerogel showed formation of two cationic forms of meso-tetrapropylporphin in the gel matrix: the short-wavelength form has a dicationic structure, while the long-wavelength form has a monocationic structure. We have traced out the correlations of the vibrational structure in the spectra of the dicationic form with data for the porphin dication, and we have drawn a number of conclusions concerning the normal vibrational modes that are active in the vibronic fluorescence and absorption spectra of the studied cationic forms. Using the AM1 semiempirical quantum chemical method, we optimized the geometry of the mesotetrapropylporphin dication: the most stable of the possible conformers is the dication structure with saddleshaped macrocycle nonplanarity.

Vibronic absorption, fluorescence, and phosphorescence spectra of psoralen: a quantum chemical investigation

Excited state potential energy hypersurfaces of 7H-furo[3,2-g][1]benzopyran-7-one (psoralen) have been explored employing (time-dependent) Kohn-Sham density functional theory. At selected points, we have determined electronic excitation energies and electric dipole (transition) moments utilizing a combined density functional/multireference configuration interaction method. Spin-orbit coupling has been taken into account employing an efficient, non-empirical spin-orbit mean-field Hamiltonian. Franck-Condon factors have been computed for vibrational modes with large displacements in the respective Dushinsky transformations. The simulated band spectra closely resemble experimental band shapes and thus validate the theoretically determined nuclear structures at the S(0), S(1), and T(1) minima. In the S(1) (pi(HOMO)-->pi*(LUMO)) state, the lactone bond of the pyrone ring is significantly elongated. From excited vibrational levels of the S(1) state a conical intersection between a (pi-->sigma*) excited state and the electronic ground state may be energetically accessible. Fast non-radiative decay via this relaxation pathway could explain the low fluorescence quantum yield of psoralen. The T(1) (pi(HOMO-1)-->pi*(LUMO)) exhibits a diradicaloid electronic structure with a broken C(5)-C(6) double bond in the pyrone ring. A variational multireference spin-orbit configuration interaction procedure yields a phosphorescence lifetime of 3 s, in excellent agreement with experimental estimates.

Probing phase relaxation by measuring fluorescence interference: polarization beating and electron–phonon coupling in conjugated polymers

We report on the measurement of coherent optical-excitation transients in the low-energy states of the strongly disordered multi-chromophore poly(para-phenylene-vinylene) at low phonon temperatures. Interferometric femtosecond (fs) excitation/probing measurements, employing freely propagating 70 fs pulses have been used to generate spatially overlapping electronic and nuclear wavepackets. High-resolution detection in combination with the unique spectral features of the sample was used to project-out two vibronic fluorescence transitions with discrete arrival states α and β. By monitoring the correlations of the α- and β-photons on the detector, the coherent superposition can be probed as a typical polarization beating. The recorded fluorescence interferograms show strong damping on a timescale of 200 fs . The relaxation of the coherent signal is indicative of, mostly, homogeneous site-dephasing and hence, gives rise to the observation of the structural relaxation of the initial Franck–Condon excitation, i.e. the formation of the dressed state.

Luminescence and absorption spectra of polyatomic molecules subjected to conformational transitions

A general approach and results of specific calculations of conjugated vibronic absorption and fluorescence spectra of “soft” molecules subjected to conformational transitions are presented. The vibronic spectra of these molecules exhibit significant anomalies such as a large energy gap between the fluorescence and absorption spectra, a strong deviation from the mirror symmetry, etc., which cannot be explained within the framework of the theory of vibronic spectra based on single-well adiabatic potentials.

Design and synthesis of a two compartment micellar system based on the self-association behavior of poly(N-acylethyleneimine) end-capped with a fluorocarbon and a hydrocarbon chain

The synthesis, characterization, and association behavior of a new class of amphiphilic polymer surfactants are described. Living cationic polymerization of 2-methyl-2oxazoline was utilized to create a series of water-soluble polymer surfacants FP x L n , where F denotes a well defined flourinated end group C 8 F 17 CH 2 CH 2 , P x denotes the degree of polymerization and l n a hydrocarbon end group of variable length C n H 2+1 , n = 6,8, ..., 18. The molecular weights and polydispersities of the polymers were determined by gel permeation chromatography (GPC) in chloroform. 1 H NMR spectroscopy was used to calculate the number average degree of polymerization and the degree of end group modification with the hydrocarbon chain containing termination agent. The molecular weights and degree of polymerization corresponded closely to the values expected from the monomer/initiator ratio. The polydispersities were low, ranging from 1.15 to 1.20. The extent of end group modification was calculated to be a minimum of 85% for all polymers. Fluorescence experiments employing pyrene as a probe showed that these polymers form aggregates in water through self-association in the concentration range from 0.018 to 0.061 wt. -%. In addition, the micropolarity experienced by the peak ratio I 1 II 3 in the vibronic fluorescence spectrum of pyrene above the cmc up to 1.0 wt. -% changed from hydrocarbon-like for FPL 12 , FPL 14 , FPL 16 , and FPL 18 to fluorocarbon-like for FPL 6 and FPL 8 , and intermediate behavior for FPL 10 . Dynamic light scattering of FP 25 L 16 showed single micelles at a concentration of 0.50 wt. -% and 2.0 wt.-% with a hydrodynamic radius of R h =5.5 and 6.7 nm, respectively. 19 F NMR experiments were applied for FP 25 L 16 to study the aggregation behavior of the fluorocarbon end group. T 2 relaxation experiments indicated that the degree of aggregation increased linearly with polymer concentration. Moreover, the results of the T 2 relaxation experiments suggested the presence of pure fluorocarbon phases consistent with those found by fluorescence spectroscopy for FP 25 L 16 .

Intramolecular relaxation processes in a singlet excited naphthalene–acridine bichromophoric molecule in solution and in a supersonic jet expansion

Intramolecular electronic relaxation processes in a flexible naphthalene–acridine bichromophoric molecule I were studied in solution and in a supersonic jet expansion. Intramolecular electronic energy transfer (intraEET) from the electronically excited naphthalene unit to the acridine moiety and an additional non-radiative decay process, probably intramolecular electron transfer (intraELT) to the acridine unit, of the excited naphthalene group was observed in solution. In a supersonic jet expansion, no indication for intraEET or other interchromophoric interactions was found. The relaxation of the excited naphthalene chromophore in I, under the conditions of supersonic jet expansion, is dominated by intrachromophoric radiationless transitions. The dependence of single vibronic fluorescence lifetimes of jet-cooled I on the excitation frequency is discussed.

Ab initio calculations of vibronic spectra for indole

Detailed vibronic fluorescence spectra from the 1L b and 1L a state origins of indole are computed from the geometry differences and ground state normal modes determined by GAUSSIAN 92 and a program to calculate Franck-Condon factors. The combination of using CIS/3-21G and HF/3-21G basis sets for excited and ground state geometries and MP2/6-31G ∗ for the ground state vibrational modes captures many subtle details seen in the experimental 1L b fluorescence.

Excitation wavelength-dependent superradiant decay in PIC J aggregates

Excitation wavelength-dependent fluorescence decay is observed in PIC J aggregate at 77 K. In the case of zero-phonon excitation, exciton decays with cooperative radiation (superradiance). On the other hand if the J aggregate is excited at the vibronic band fluorescence decay is affected by exciton-exciton annihilation.

Near-infrared Yb 3+ vibronic sideband spectroscopy: application to Ca 2+-binding proteins

We have used near-infrared (NIR) vibronic fluorescence spectroscopy to study the vibrational structure of ligands associated with model complexes of the lanthanide Yb 3+. This technique exploits the similar binding properties of the lanthanide Yb 3+ to probe Ca 2+-binding sites in proteins. The (NIR) fluorescence of complexed Yb 3+ exhibits, in addition to main 0-0 ( 2F 5 2 → 2F 7 2 ) electronic transition of Yb 3+, weak vibronic sidebands which provide infrared-like, local vibrational spectra of the chelates (inner sphere ligands) of Yb 3+. A similar approach has been used for the lanthanide Gd 3+ (MacGregor, R.B. Jr. (1989) Arch. Biochem. Biophys. 274, 312–316) which fluoresces in the UV and which is usually complicated by amino-acid residues fluorescing in the same spectral region. In this same spectral region, other complications in studying photosynthetic membranes occur in the form of the excitation wavelength being actinic, promoting photodegradation of the membranes, as well as the reabsorption of Gd 3+ fluorescence. NIR excitation and fluorescence detection of Yb 3+ avoid these problems when studying photosynthetic membranes. A preliminary study has been conducted here on rat muscle parvalbumin.

Electronic levels and vibrational structure of the cis form of a cyano derivative of methylated isobacteriochlorin

Isobacteriochlorins, tetrahydroporphin derivatives with adjacent hydrogenated pyrrole rings (iBC), are of great importance in biochemistry as prosthetic groups of redox enzymes (siroheme) and intermediates in vitamin B12 biosynthesis. The quasi-line vibronic fluorescence and absorption spectra of unsubstituted iBC in the trans form were first obtained at 77 K [i] and then at 4.2 K [2] (see also [3], pp. 331-342); a parallel investigation was made of bacteriochlorin (BC), a tetrahydroporphin with an opposite arrangement of the hydrogenated pyrrole rings. In [2] a method of determining the polarization of vibronic transitions and the symmetry of the normal vibrations of molecules, based on a study of the anisotropy of the emission of a doped n-aikane single crystal, proposed in [4], was used. The spectroscopic data confirmed the presence of a center of symmetry in the BC molecule and its absence in the iBC molecule. The quasi-line spectra (QLS) of BC were also investigated in [5].