Influence of hydrogen bonding on excitonic coupling and hierarchal structure of a light-harvesting porphyrin aggregate

Helical porphyrin nanotubes of tetrakis(4-sulfonatophenyl)porphyrin (TSPP) were examined in DCl/D(2)O solution using resonance Raman and resonance light scattering spectroscopy to probe the influence of hydrogen bonding on the excitonic states. Atomic force microscopy reveals similar morphology for aggregates deposited from DCl/D(2)O and from HCl/H(2)O solution. Deuteration results in subtle changes to the aggregate absorption spectrum but large changes in the relative intensities of Raman modes in the J-band excited resonance Raman spectra, revealing relatively more reorganization along lower-frequency vibrational modes in the protiated aggregate. Depolarization ratio dispersion and changes in the relative Raman intensities for excitation wavelengths spanning the J-band demonstrate interference from overlapping excitonic transitions. Distinctly different Raman excitation profiles for the protiated and deuterated aggregates reveal that isotopic substitution influences the excitonic structure of the J-band. The deuterated aggregate exhibits a nearly two-fold increase in intensity of resonance light scattering as a result of an increase in the coherence number, attributed to decreased exciton-phonon scattering. We propose that strongly coupled cyclic N-mers, roughly independent of isotopic substitution, largely decide the optical absorption spectrum, while water-mediated hydrogen bonding influences the further coherent coupling among them when they are assembled into nanotubes. The results show that, similar to natural light-harvesting complexes such as chlorosomes, hydrogen bonding can have a critical influence on exciton dynamics.     

Absorption spectra of blue-light-emitting oligoquinolines from time-dependent density functional theory

Recently, it has been discovered that a series of four conjugated oligomers, oligoquinolines, exhibits many desirable properties of organic materials for developing high-performance light-emitting diodes: good blue color purity, high brightness, high efficiency, and high glass-transition temperatures. In this work, we investigate the optical absorption of oligoquinolines in the gas phase and chloroform (CHCl3) solution, respectively, using time-dependent density functional theory with the adiabatic approximation for the dynamical exchange-correlation potential. Our calculations show that the first peak of optical absorption corresponds to the lowest singlet excited state, whereas several quasi-degenerate excited states contribute to the experimentally observed higher-frequency peak. We find that, compared with the gas phase, there is a moderate red shift in excitation energy in solution due to the solute-solvent interaction simulated using the polarizable continuum model. Our results show that the lowest singlet excitation energies of oligoquinolines in chloroform solution calculated with the adiabatic hybrid functional PBE0 are in a good agreement with experiments. Our simulated optical absorption agrees well with the experimental data. Finally, analysis of the natural transition orbitals corresponding to the excited states in question underscores the underlying electronic delocalization properties.     

Direct measurement of near-threshold rate constants for unimolecular dissociation of CF3I after IR multiphoton excitation

Excited-state populations of CF₃I after IR multiphoton excitation were monitored by time-resolved hot-band UV absorption spectroscopy. Using a calibration of the spectrum by shock-wave experiments, the absorption changes during the laser pulse are analyzed with respect to excited-state populations and dissociation at higher excitation energies. Dissociation of molecules near threshold is detected under collision-free conditions by absorption changes after the laser pulse. At higher pressures, absorption signals after the pulse are markedly influenced by energy transfer between excited and cold molecules. The measured dissociation rate constants near threshold are consistent with statistical calculations of k(E,J), showing pronounced rotational dependence.     

Linear and nonlinear optical studies of CdS1−x Se x quantum dots

In this contribution we present and discuss our measurements on CdS_1?x Se _x quantum dots in a glass matrix. In linear absorption measurements we find the typical blue shift of the transitions with decreasing crystallite radius due to quantization. The luminescence shows a significant Stokes shift with respect to absorption, which is interpreted in terms of strong exciton-phonon coupling and allows to deduce the Huang-Rhys factor S. Under high excitation we find an additional emission band on the high energy side, which can be attributed to the recombination of an excited two electron-hole pair state to a one electron-hole pair state in agreement with theory. Pump and probe beam experiments give a bleaching but no hole burning. Finally we discuss some open questions especially concerning the high energy structures in the absorption spectrum.     

The calculated magnetic and optical properties of (Mn,N) Co-doped ZnS

Based on the density functional pseudo-potential method, the density of states, the magnetic and the optical properties of the Mn-doped and (Mn,N) co-doped ZnS are calculated. The calculation results indicate that (Mn,N) co-doping can realize more effective p-type doping than Mn-doped does. Both Mn-doping and (Mn,N) co-doping exhibit spin polarization states, which can realize 100% spin polarization of the carrier injection by transfer of effective masses. Compared with that of pure ZnS, the optical absorption edges of both Mn-doped and (Mn,N) co-doped ZnS make red shifts. The peak value of the reflection coefficient increases and the main reflection peaks move to the lower energy after doping. Meanwhile, the location of peaks move toward the direction of lower energy which corresponds to the sharp decline of reflection spectrum.     

On the Influence of Higher Excited States on the ISC Quantum Yield of Octa-aL-alkyloxy-substituted Zn-Phthalocyanine Molecules Studied by Nonlinear Absorption

Abstract— Octa-aL-alkyloxy-substituted Zn-phthalocyanines are an interesting class of far red-absorbing photosensitizers. The chemical structure, the calculated steric conformation, the observed linear optical properties and an anomalous luminescence from a higher than S, excited state are reported. To study the optical properties of higher excited states and their occupation dynamics up to delay times of 15 ns we have carried out measurements of transient absorption spectra after 14 ps pulsed, resonant B-band and Q-band excitation. From these measurements the excited state singlet-singlet and triplet-triplet spectra as well as the intersystem crossing (ISC) quantum yields are obtained. The main result is an excitation wavelength-dependent ISC quantum yield that can be explained by an additional ISC channel between higher excited singlet and triplet states. The large rate of this channel is justified by the resonance between higher triplet states, observed in the triplet-triplet spectrum and the B, absorption band. Using kinetic model calculations, a lifetime of the higher excited singlet state of some picoseconds is predicted and the influence of a two-step absorption process on the population density of this higher excited singlet state is discussed.     

On the applicability of the Kasha—Vavilov rule to C60

Via full correction for the instrumental response function, the fluorescence excitation spectrum of C₆₀ in methylcyclohexane between 240 and 580 nm is shown to match the absorption spectrum. This proves that, in contrast to earlier reports, also after UV excitation into higher S_n states, deactivation proceeds virtually completely via the S₁ state, in accordance with the Kasha—Vavilov rule.     

Interaction of low-energy electrons with linear diphenylethynyl derivatives in the gas phase

The gas phase electron impact spectroscopy has been used to study the relative efficiency of excitation into singlet states and energies of singlet-triplet transitions for two electroactive organic materials, anthracene and biphenyl-containing diphenylethynyl derivatives. The probability of the lowest singlet-triplet transition in anthracene-containing molecule was found to be much higher than that in anthracene which is connected with triple bonds. No noticeable contribution of the triple bonds into singlet spectra of the studied molecules was observed. There are a number of intense transitions in the range higher than 10 eV. The optical spectrum calculated using the density functional theory is in good agreement with experimental electron energy loss and optical absorption spectra.     

Dimers of polyene antibiotic amphotericin B detected by means of fluorescence spectroscopy: molecular organization in solution and in lipid membranes

Fluorescence emission from amphotericin B dissolved in 2-propanol-water was recorded in the spectral region 500-650 nm. The fluorescence excitation spectrum corresponds to the absorption spectrum of the monomeric drug. The large energy shift between the excitation and emission bands indicates that emission takes place from an energy level different than that responsible for absorption. These levels were attributed to the 2(1)A(g) and 1(1)B(u) states, respectively. Excitation of the same sample with short wavelength radiation (below 350 nm) yields light emission between 400 and 550 nm. The fluorescence excitation spectrum corresponding to this emission band displays distinct maxima at 350, 334 and 318 nm. This band was analyzed in terms of the exciton splitting theory and assigned to amphotericin B in a dimeric form, in which chromophores are spaced by 4.9 A. The binding energy of the dimers, determined to be 4.9 kJ/mol, indicates that the structures are stabilized by van der Waals interactions. The same type of molecular structures was also detected in the lipid membranes formed with dipalmitoylphosphatidylcholine. Linear dichroism of amphotericin B embedded in lipid multibilayers indicates that molecules are distributed between two fractions: parallel (38%) and perpendicular (62%) with respect to the membrane. The biological importance of such membrane organization is discussed.     

Pseudoisocyanine J-aggregate to optical waveguiding crystallite transition: microscopic and microspectroscopic exploration

Using fluorescent microscopy and microspectroscopy, optical properties and morphology transformations in individual pseudoisocyanine (PIC) J-aggregates in aqueous electrolyte solutions have been explored. A stringlike structure of J-aggregates with a string diameter much less than 1 microm has been observed. Photodestruction of the strings under short-wavelength excitation has been revealed. Rodlike PIC crystallites, about 1 microm in diameter, have been observed with time. The fluorescence spectrum of rodlike crystallites has been found to differ from that of stringlike J-aggregate and from PIC crystal powder spectra. The crystallites are very stable, and their photodestruction has not been observed under any excitation conditions. It has been found that rodlike crystallites in contrast to stringlike J-aggregates possess optical waveguide properties. The luminescence of crystallites can be observed only at the excitation spot and at butt ends located up to hundreds of micrometers from the excitation spot.     

Optical Gain in Semiconducting Polymer Nano and Mesoparticles

The presence of excited-states and charge-separated species was identified through UV and visible laser pump and visible/near-infrared probe femtosecond transient absorption spectroscopy in spin coated films of poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) nanoparticles and mesoparticles. Optical gain in the mesoparticle films is observed after excitation at both 400 and 610 nm. In the mesoparticle film, charge generation after UV excitation appears after around 50 ps, but little is observed after visible pump excitation. In the nanoparticle film, as for a uniform film of the pure polymer, charge formation was efficiently induced by UV excitation pump, while excitation of the low energetic absorption states (at 610 nm) induces in the nanoparticle film a large optical gain region reducing the charge formation efficiency. It is proposed that the different intermolecular interactions and molecular order within the nanoparticles and mesoparticles are responsible for their markedly different photophysical behavior. These results therefore demonstrate the possibility of a hitherto unexplored route to stimulated emission in a conjugated polymer that has relatively undemanding film preparation requirements.     

Edge excitation red shift and energy migration in quinine bisulphate dication

Photoinduced excited state dynamical processes in quinine sulphate dication (QSD) have been studied over a wide range of solute concentrations using steady state and nanosecond time-resolved fluorescence spectroscopic techniques. The edge excitation red shift (EERS) of emission maximum, emission wavelength dependence of fluorescence lifetimes and the time dependence of emission maximum are known to occur due to the solvent relaxation process. With increase in solute concentration, the emission spectrum shifts towards the lower frequencies accompanied with decrease in fluorescence intensity, however, absorption spectrum remains unchanged. A decrease in EERS, fluorescence lifetimes, time dependent fluorescence Stokes shift (TDFSS), fluorescence polarization and the solvent relaxation time (τ_r) is observed with the increase in solute concentration. The process of energy migration among the QSD ions along with solvent relaxation has been found responsible for the above experimental findings.     

High-energy-resolution electron energy-loss spectroscopy study of the electronic structure of Cu- and Mg-Si-doped β-rhombohedral boron crystals

Electronic structures of Cu and Mg-Si-doped β-rhombohedral boron (β-r-B) crystals were studied by using a high-energy-resolution electron energy-loss spectroscopy microscope. Boron 1s electron excitation spectra, which show the density of states of the conduction bands, of the crystals were obtained from single crystalline areas of 100 nm in diameter. The spectrum of Cu-doped β-r-B showed a chemical shift to a lower binding energy side. It means an electron transfers from the doped Cu atoms to B atoms. The intensity distributions of the spectrum was almost the same as that of the non-doped β-r-B, which suggests that all of the doped electrons occupy the intrinsic acceptor level just above the valence bands. The spectrum of Mg-Si-doped β-r-B showed not only a chemical shift to a lower binding energy side but also a sharp intensity increase at the onset with a width of an energy resolution of the experiment. The sharp onset may be assigned to a Fermi edge. It indicates that the doped electrons fill up the acceptor level and occupy the conduction bands forming the Fermi edge, a metallization of β-r-B by the Mg-Si-doping.     

Ligand-Induced Donor State Destabilisation – A New Route to Panchromatically Absorbing Cu(I) Complexes

The intense absorption of light to covering a large part of the visible spectrum is highly desirable for solar energy conversion schemes. To this end, we have developed novel anionic bis(4H-imidazolato)Cu(I) complexes (cuprates), which feature intense, panchromatic light absorption properties throughout the visible spectrum and into the NIR region with extinction coefficients up to 28,000 M⁻¹ cm⁻¹. Steady-state absorption, (spectro)electrochemical and theoretical investigations reveal low energy (Vis to NIR) metal-to-ligand charge-transfer absorption bands, which are a consequence of destabilized copper-based donor states. These high-lying copper-based states are induced by the σ-donation of the chelating anionic ligands, which also feature low energy acceptor states. The optical properties are reflected in very low, copper-based oxidation potentials and three ligand-based reduction events. These electronic features reveal a new route to panchromatically absorbing Cu(I) complexes.     

Vibrational analysis of the electronic spectrum of ethylene based onab initio SCF-CI calculations

Ab initio calculations for CH₂ twisting and CC stretching vibrational wavefunctions and energy levels are reported for various electronic states of ethylene C₂H₄. Electronic transition moments between these states are also obtained to allow a calculation of the oscillator strengths for vibrational transitions involved in various electronic band systems; from this study it is concluded that thevertical electronic energy differenceΔE _e may differ significantly from the energy of the absorption maximumΔE _max with which it is often equated. In particular it is found in the case of theπ→π ^* singlet-singlet excitation of ethylene that theΔE _e value overestimates the most probable vibrational transition energy (7.89 eV) by some 0.4 eV, thereby offering an explanation for the fact that previous attempts to predict the location of theV-N Franck-Condon absorption maximum have consistently obtained substantially higher results than the 7.66 eV value actually observed. Similar calculations for various Rydberg species and for theN-T transition are also found to obtain a quite consistent representation of the electronic spectrum of this system.     

Threshold electron excitation of azulene

Intense energy absorption at 2.44 eV is observed in the threshold electron excitation spectrum of azulene vapor. Direct excitation of triplet states or an electron-molecule resonance may account for this striking feature.     

Disorder-induced exciton localization and violation of optical selection rules in supramolecular nanotubes

Using numerical simulations, we study the effect of disorder on the optical properties of cylindrical aggregates of molecules with strong excitation transfer interactions. The exciton states and the energy transport properties of such molecular nanotubes attract considerable interest for application in artificial light-harvesting systems and energy transport wires. In the absence of disorder, such nanotubes exhibit two optical absorption peaks, resulting from three super-radiant exciton states, one polarized along the axis of the cylinder, the other two (degenerate) polarized perpendicular to this axis. These selection rules, imposed by the cylindrical symmetry, break down in the presence of disorder in the molecular transition energies, due to the fact that the exciton states localize and no longer wrap completely around the tube. We show that the important parameter is the ratio of the exciton localization length and the tube's circumference. When this ratio decreases, the distribution of polarization angles of the exciton states changes from a two-peak structure (at zero and ninety degrees) to a single peak determined by the orientation of individual molecules within the tube. This is also reflected in a qualitative change of the absorption spectrum. The latter agrees with recent experimental findings.     

Unveiling the Hidden,Dark, and Short Life of a Vibronic State in a Boron Difluoride Formazanate Dye

Boron difluoride (BF₂) formazanate dyes are contenders for molecular species that exhibit a large Stokes shift and bright red emission. Excitation of 3‐cyanoformazanate complexes with 10 μs wide pulses of specific wavelengths resulted in strong luminescence at 663 nm at both room temperature in solution and at 77 K in a frozen solution. Analysis of the short‐lived excitation spectrum from this luminescence shows that it arises from a vibronic manifold of a higher‐lying excited state. This dark state relaxes to the emitting state over 10 μs. TD‐DFT calculations of the two lowest‐energy excited states show that the relaxed geometries are planar for S₁ but highly distorted in S₂. The specific time‐ and wavelength‐dependence of the excitation profile provides a unique optical encryption capability through the comparison of emission intensities between adjacent vibronic bands only accessible in the 0–12 μs time domain.     

Spectra and excitation properties of (AlP)8: Effect of external electric fields

On the basis of density functional theory (DFT), the geometry and infrared spectrum of the (AlP)₈ cluster have been calculated under external electric fields (EEFs). In addition, on the basis of time-dependent DFT, the ultraviolet–visible absorption spectra, oscillator strengths, wavelengths, and hole–electron orbits of the first 20 excited states have been calculated. Under EEFs, the energy of (AlP)₈ gradually decreases, the dipole moment increases, and the molecular configuration significantly changes. In the infrared spectrum, the vibration frequency corresponding to the stretching vibration of the Al–P bond decreases, and a red shift occurs. With increasing EEF, the infrared spectrum splits and shows an obvious Stark effect; the ultraviolet absorption intensity is enhanced, and the molecular excitation energy decreases. Additionally, the excitation wavelength increases with increasing EEF. It is conclusively shown that the (AlP)₈ cluster is easily excited under an EEF. Separation of the holes and electrons of the (AlP)₈ cluster is obvious. Theoretical investigation of the spectra and excitation properties of (AlP)₈ is an important step toward a comprehensive understanding of the effects of EEFs on the molecular structure, stability, and dynamics.     

Unveiling the Hidden,Dark, and Short Life of a Vibronic State in a Boron Difluoride Formazanate Dye

Boron difluoride (BF₂) formazanate dyes are contenders for molecular species that exhibit a large Stokes shift and bright red emission. Excitation of 3‐cyanoformazanate complexes with 10 μs wide pulses of specific wavelengths resulted in strong luminescence at 663 nm at both room temperature in solution and at 77 K in a frozen solution. Analysis of the short‐lived excitation spectrum from this luminescence shows that it arises from a vibronic manifold of a higher‐lying excited state. This dark state relaxes to the emitting state over 10 μs. TD‐DFT calculations of the two lowest‐energy excited states show that the relaxed geometries are planar for S₁ but highly distorted in S₂. The specific time‐ and wavelength‐dependence of the excitation profile provides a unique optical encryption capability through the comparison of emission intensities between adjacent vibronic bands only accessible in the 0–12 μs time domain.