Excited State Absorption Bands Research Articles

Investigation into the carrier recombination in Sb2Se3: Photo thermal effect, trapped carrier absorption and hot carrier cooling

Understanding the carrier recombination processes in Sb2Se3 is essential for its optoelectronic applications. In this work, carrier recombination dynamics in Sb2Se3 were studied by broad band transient absorption spectroscopy. Firstly, the contribution of photothermal effect to the transient absorption spectrum was thoroughly discussed. It is confirmed that the excited state absorption (ESA) band with lifetime of several nanoseconds results from co-contribution of photo thermal effect and deep trapped carrier absorption. Secondly, the features of transient absorption spectrum on picosecond time scale were interpreted. The short-lived ESA band around 1000 nm was assigned to shallow trapped carrier absorption, while not band gap renormalization (BGR) or free carrier absorption. By globally fitting the transient absorption spectrum, the hot carrier cooling time and time constant for free carrier relax into deep trap state were determined to be 0.25∼0.45 ps and 3.1∼8.7 ps, respectively. Finally, we built up the carrier recombination model of Sb2Se3. The experimental results in this work will improve the understanding on the carrier recombination in Sb2Se3.

Nonadiabatic Molecular Dynamics Simulations Provide Evidence for Coexistence of Planar and Nonplanar Intramolecular Charge Transfer Structures in Fluorazene.

Fluorazene is a model compound for photoinduced intramolecular charge transfer (ICT) between aromatic moieties. Despite intensive studies, both spectroscopic and theoretical, a complete model of its photophysics is still lacking. Especially controversial is the geometry of its ICT structure, or structures. In order to fill in the gaps in the state of knowledge on this important model system, in the present study I report the results of nonadiabatic molecular dynamics (NAMD) simulations of its photorelaxation process in acetonitrile solution. To afford a direct comparison to spectroscopic data, I use the simulation results as the basis for the calculation of the transient absorption (TA) spectrum. The NAMD simulations provide detailed information on the sequence of events during the excited-state relaxation of the title compound. Following initial photoexcitation into the bright S2 state, the molecule undergoes rapid internal conversion into the S1 state, leading to the locally excited (LE) structure. The LE structure, in turn, undergoes isomerization into a population of ICT structures, with geometries ranging from near-planar to markedly nonplanar. The LE → ICT isomerization reaction is accompanied by the decay of the characteristic excited-state absorption band of the LE structure near 2 eV. The anomalous fluorescence emission band of fluorazene is found to originate mainly from the near-planar ICT structures, in part because they dominate the overall population of ICT structures. Thus, the planar ICT (PICT) model appears to be the most appropriate description of the geometry of the ICT structure of fluorazene.

Transient “cis-Peak” Is Present in Excited-State Absorption of Central-cis Isomers of Carotenoids

Application of femtosecond time-resolved transient absorption spectroscopy on all-trans and central-cis isomers of two exemplary carotenoids, lutein and spirilloxanthin, performed at room and cryogenic temperatures in the spectral range expanded toward UV revealed new spectroscopic transient features for the cis isomers. Notably, particularly for the central-cis spirilloxanthin, a very distinct additional transient absorption band is observed on the short wavelength side of the main excited-state absorption band of the S1 state, having the same temporal characteristics as the latter one. This band is absent in transient absorption spectra of all-trans isomers, suggesting it could be assigned to "transient cis-peak." Overall, the results show that the "transient" counterpart of the spectral marker of cis-isomers of carotenoids, or the so-called cis-peak, is detectable in the excited-state absorption spectrum and could be attributed to electronic transition from the S1 state. This new transient spectral band could be very useful for spectroscopic experiments targeting the dynamics of carotenoid isomers and their severely distorted forms.

Ultrafast optical limiting ability of trans-stilbene enhanced and broadened by a donor-π-acceptor structure.

It has been shown that trans-stilbene (TSB) has great potential as an ultrafast optical limiting material through the process of three-photon absorption (3PA)-induced excited state absorption (ESA). The present paper shows that the main transitions in the absorption bands of TSB are mostly local excitation. In order to improve the optical limiting performance of TSB, a series of TSB derivatives with an electron donor-π-acceptor structure are designed. The analysis of π electron localized orbital locators (LOL-π) reveals that the distribution of π electrons in the derivatives of TSB is much more continuous compared to that in the original TSB. This results in the main transitions in the ground state absorption (GSA) and ESA of the TSB derivatives showing obvious charge transfer characteristics, and the GSA, ESA and 3PA bands are largely enhanced and broadened compared to those of the original TSB molecule. These observations are well supported by the enlarged transition dipole moments of the main transitions in GSA and ESA. With these results, it is clearly shown that the TSB derivatives are promising optical limiting materials. Our observations provide clues for the development of optical limiting materials based on TSB.

Effects of aggregation on the structures and excited-state absorption for zinc phthalocyanine.

In the present paper, the aggregated structures of zinc phthalocyanine (ZnPc) have been investigated by considering its dimers and trimers. Based on the density functional theory calculations, two stable conformations are obtained for the ZnPc dimer and trimer, respectively. The IGMH (independent gradient model based on the Hirshfeld partition of molecular density) analysis reveals that the π-π interaction between the ZnPc molecules causes the aggregation. Normally, stacked structures with a slight displacement are favorable for aggregation. In addition, the planar structure of the ZnPc monomer is largely maintained in the aggregated conformations. For the presently obtained structures, the first singlet excited state absorption (ESA) spectra of these aggregated conformations of ZnPc were calculated based on the linear-response time-dependent density functional theory (LR-TDDFT), which has been well applied by our group. The results of the excited state absorption spectra reveal that the aggregation causes the ESA band to blue shift compared to the ZnPc monomer. By using the conventional description of the interaction between monomer transition dipoles, this blue shift is elucidated by the side-by-side transition dipole moments in the constituted monomers. The present results for the ESA combined with the previously reported results for ground state absorption (GSA) will provide guidelines to tune the window of the optical-limiting effect for the ZnPc based materials.

Excitation-Dependence of Excited-State Dynamics and Vibrational Relaxation of Lutein Explored by Multiplex Transient Grating.

Multiplex transient grating (MTG) spectroscopy was applied to lutein in ethanol to investigate the excitation-energy dependence of the excited-state dynamics and vibrational relaxation. The transient spectra obtained upon low (480 nm) and high-energy (380 nm) excitation both recorded a strong excited-state absorption (ESA) of S1 → S n as well as a broad band in the blue wavelength that was previously proposed as the S* state. By means of Gaussian decomposition and global fitting of the ESA band, a long-time component assigned to the triplet state was derived from the kinetic trace of 480 nm excitation. Moreover, the MTG signal with a resolution of 110 fs displayed the short-time quantum beat signal. In order to unveil the vibrational coherence in the excited-state decay, the linear and non-linear simulations of the steady spectrum and dynamic signals were presented in which at least three fundamental modes standing for C-C stretching (ν1), C=C stretching (ν2), and O-H valence vibrations (ν3) were considered to analyze the experimental signals. It was identified that the vibrational coherence between ν1 and ν3 or ν2 and ν3 was responsible for quantum beat that may be associated with the triplet state. We concluded that upon low- or high-energy excitation into the S2 state, the photo-isomerization of the molecule and structural recovery on the time-scale of vibrational cooling are the key factors to form a mixed conformation in the hot-S1 state that is the precursor of a long life-time triplet.

Excited-state resonance Raman spectroscopy probes the sequential two-photon excitation mechanism of a photochromic molecular switch.

Some diarylethene molecular switches have a low quantum yield for cycloreversion when excited by a single photon, but react more efficiently following sequential two-photon excitation. The increase in reaction efficiency depends on both the relative time delay and the wavelength of the second photon. This paper examines the wavelength-dependent mechanism for sequential excitation using excited-state resonance Raman spectroscopy to probe the ultrafast (sub-30fs) dynamics on the upper electronic state following secondary excitation. The approach uses femtosecond stimulated Raman scattering (FSRS) to measure the time-gated, excited-state resonance Raman spectrum in resonance with two different excited-state absorption bands. The relative intensities of the Raman bands reveal the initial dynamics in the higher-lying states, Sn, by providing information on the relative gradients of the potential energy surfaces that are accessed via secondary excitation. The excited-state resonance Raman spectra reveal specific modes that become enhanced depending on the Raman excitation wavelength, 750 or 400nm. Many of the modes that become enhanced in the 750nm FSRS spectrum are assigned as vibrational motions localized on the central cyclohexadiene ring. Many of the modes that become enhanced in the 400nm FSRS spectrum are assigned as motions along the conjugated backbone and peripheral phenyl rings. These observations are consistent with earlier measurements that showed higher efficiency following secondary excitation into the lower excited-state absorption band and illustrate a powerful new way to probe the ultrafast dynamics of higher-lying excited states immediately following sequential two-photon excitation.

Low threshold amplified spontaneous emission and lasing from single crystals and polymer microspheres based on a blue aggregation-enhanced emission molecule

Organic semiconductors as a gain medium have attracted much attention in organic lasers due to their unique properties, i.e., large stimulated emission cross section, widely tunable spectrum region, and mechanical flexibility. Aggregation-caused quenching broadly exists in amorphous solid films and single crystals, which result in high threshold. Here, we studied amplified spontaneous emission (ASE) and lasing properties of an aggregation-enhanced emission (AEE) molecule, namely, 2PB-AC, who owns high photoluminescence quantum efficiency in a solid film and a single crystal. The grown long rod-like single crystals exhibited very good ASE characteristics with a threshold of 48 μJ cm−2 and a full width at half-maximum (FWHM) of 8.4 nm. Furthermore, when forming polymer microspheres by a surface tension-induced self-assembly method, a low lasing threshold of 0.32 mJ cm−2 and a FWHM of 0.41 nm were also obtained well, and their lasing modes could be modulated by changing the diameter of the microspheres. The investigations on transient photoluminescence dynamics and femtosecond transient absorption analysis demonstrated that the high radiative decay rate and the well separation between a stimulated emission band and an excited state absorption band are the main reasons of good lasing performance, indicating that AEE molecules are promising lasing materials.

Excellent ultrafast broadband optical limiting of functionalized twistacenes based on two-photon absorption-induced excited state absorption

Twistacene is an outstanding building block due to the large conjugated system. Herein, a series of twistacenes have been designed and synthesized with the assistant from a theoretical analysis based on the morphology of the natural transition orbitals of first excited state. These twistacenes exhibit two photon absorption and unique broadband excited singlet state absorption features, which is systematically confirmed by transient absorption spectra, two-photon excited fluorescence and z-scan experiments. The overlap of two-photon absorption band with excited state absorption band can improve the OL performance. The experimental results are consistent with theoretical prediction, which confirm the rationalization of molecular design strategy. The screened twistacene 5 exhibits excellent optical limiting behavior with extremely high transmittance and low threshold, where the threshold is 1.3–3.1 mJ/cm2 in a wide optical window from 532 nm to 900 nm in femtosecond regime and 69 mJ/cm2 in picosecond regime. The experimental results suggest twistacene is a promising candidate for nonlinear optical applications, especially for protection against ultrashort laser pulses.

Visualization of Macrophase Separation and Transformation in Immiscible Polymer Blends

Visualization of Macrophase Separation and Transformation in Immiscible Polymer Blends

Er<sup>3+</sup>,Yb<sup>3+</sup>:YGdSiO<sub>5</sub> Crystal as Gain Media for Lasers Emitting in the Spectral Range of 1.5–1.6 µm

Solid-state erbium lasers, emitting in the spectral range of 1.5–1.6 µm, are of great interest for several industrial applications. Nowadays the Er:glass is the most widespread laser material for obtaining laser radiation at the wavelength near 1.5 µm. However, the maximal output powers of such lasers are restricted by hundreds of milliwatts because low thermal characteristics of the glass host. By this reason the search for new crystalline hosts doped with erbium ions is the actual task.In this article the investigation results of spectroscopic properties of Er3+,Yb3+:YGdSiO5 (YGSO) crystals are reported. Polarized absorption and luminescence spectra were measured. The lifetimes of energy levels were determined. The excited state absorption spectra were measured. It was shown that excited state absorption band does not overlap with gain band in the range 1.5–1.6 µm. The energy transfer efficiency from ytterbium to erbium ions was estimated. The stimulated emission and gain cross-section spectra for Er3+ ions in YGSO were calculated.

Investigation of Thermally Activated Delayed Fluorescence in Donor–Acceptor Organic Emitters with Time-Resolved Absorption Spectroscopy

In this contribution, we utilize multiple time-resolved and nonlinear optical measurements and quantum chemical simulations to investigate the excited-state dynamics of organic chromophores with thermally activated delayed fluorescent (TADF) characteristics. We are most interested in probing the influence of a phenylene linker on the photophysical properties of emitters composed of carbazole-based donors linked to either a phthalonitrile (PN) or diphenyltriazine (Trz) acceptor. The PN-acceptor compounds display a near double-fluorescence quantum yield (ΦF) enhancement in oxygen-free conditions. The fluorescent lifetime measurements indicate that the Trz-acceptor compounds are more efficient fluorescence emitters and quickly go from a delocalized to localized state. They also reveal that only the PN-acceptor compounds display a long-lived emissive lifetime component associated with TADF activity. Analysis of the nanosecond transient absorption spectra and kinetics reveals long-lived excited-state absorption (ESA) bands associated with triplet states for the PN-acceptor compounds. No ESA bands were observed for the Trz-acceptor compounds, despite observing a quantum yield enhancement for the Trz-acceptor compounds after oxygen purging. From the transient absorption measurements, it was determined that the PN-acceptor compounds have reverse intersystem crossing rates (krISC) that are able to compete with other triplet decay pathways. From quantum chemical calculations, it is proposed that inclusion of the phenylene linker prevents sufficient highest occupied molecular orbital/lowest unoccupied molecular orbital separation and suppresses TADF activity and that directly linking multiple donors to the acceptor will aid in achieving TADF activity.

Simulation and Analysis of the Transient Absorption Spectrum of 4-(N,N-Dimethylamino)benzonitrile (DMABN) in Acetonitrile.

4-(N,N-Dimethylamino)benzonitrile (DMABN) is a well-known model compound for dual fluorescence—in sufficiently polar solvents, it exhibits two distinct fluorescence emission bands. The interpretation of its transient absorption (TA) spectrum in the visible range is the subject of a long-standing controversy. In the present study, we resolve this issue by calculating the TA spectrum on the basis of nonadiabatic molecular dynamics simulations. An unambiguous assignment of spectral signals to specific excited-state structures is achieved by breaking down the calculated spectrum into contributions from twisted and nontwisted molecular geometries. In particular, the much-discussed excited-state absorption band near 1.7 eV (ca. 700 nm) is attributed to the near-planar locally excited (LE) minimum on the S1 state. On the technical side, our study demonstrates that the second-order approximate coupled cluster singles and doubles (CC2) method can be used successfully to calculate the TA spectra of moderately large organic molecules, provided that the system in question does not approach a crossing between the lowest excited state and the singlet ground state within the time frame of the simulation.

Symmetry Breaking in an Excited Quadrupolar Acridine-Dione Derivative Driven by Hydrogen Bonding.

An acridine-dione derivative (3,3,11,11-tetramethyl-8,16-diphenyl-3,4,8,10,11,12,13,16-octahydroacridino[4,3-c]acridine-1.9(2H,5H)dion) with quadrupolar motif has been synthesized and its stationary and transient spectra have been measured. Stationary absorption and fluorescence spectra as well as nonstationary spectra show no signs of symmetry breaking (SB) in aprotic solvents, even of high polarity. The specific features of SB are revealed in alcohol solvents through a considerable red shift of stationary fluorescence spectra and the appearance of a new excited state absorption band in transient absorption spectra. SB is due to the formation of asymmetric strong hydrogen bonds, mainly on one side of the molecule. An unexpected regularity of symmetry breaking is found in mixtures of aprotic dimethylformamide and protic methanol, where methanol acts as a fluorescence quencher. It is revealed that there is no quenching as long as the methanol concentration is less than the critical value of 9 M. This leads to the conclusion that SB in such mixtures is possible only if the concentration of the protic solvent exceeds a certain threshold value.

Discovering of the L ligand impact on luminescence enhancement of Eu(Dibenzoylmethane)3.Lx complexes employing transient absorption spectroscopy

The effect of luminescent enhancement under exchange of the auxiliary ligand in Europium(III) tris(1,3-diphenyl-1,3-propanedionato) monohydrate was investigated by steady-state and time-resolved transient absorption spectroscopy. The excited state relaxation dynamics of this complex was analysed through a comparison of the experimental data obtained for several model compounds, namely Eu(DBM)3·NH3, Eu(DBM)3.EDA, Eu(DBM)3.Phen, Al(DBM)3 and dibenzoylmethane (DBM) in various solutions and polymer matrices. The results show there is no linear relationship between enhancement of the emission quantum yield and the luminescent lifetime, which suggests that the auxiliary ligand reduces the rate of nonradiative relaxation of the lanthanide ion, but also affects the excited state energy transfer from ligand to metal ion. Transient absorption data shows a clear correlation between the efficiency of the energy transfer and the degree of triplet state population expressed by an amplification of the signal for its excited state absorption band on going from Eu(DBM)3·H2O to the Eu(DBM) = .L complex. The results show that this auxiliary ligand exchange acts as a “switch” turning the intersystem crossing on or off as a competitive pathway for excited state relaxation of the europium(III) complexes.

Ultrafast Dynamics and Photoresponse of a Fungi-Derived Pigment Xylindein from Solution to Thin Films.

Organic semiconductor materials have recently gained momentum due to their non-toxicity, low cost, and sustainability. Xylindein is a remarkably photostable pigment secreted by fungi that grow on decaying wood, and its relatively strong electronic performance is enabled by π-π stacking and hydrogen-bonding network that promote charge transport. Herein, femtosecond transient absorption spectroscopy with a near-IR probe was used to unveil a rapid excited-state intramolecular proton transfer reaction. Conformational motions potentially lead to a conical intersection that quenches fluorescence in the monomeric state. In concentrated solutions, nascent aggregates exhibit a faster excited state lifetime due to excimer formation, confirmed by the excimer→charge-transfer excited-state absorption band of the xylindein thin film, thus limiting its optoelectronic performance. Therefore, extending the xylindein sidechains with branched alkyl groups may hinder the excimer formation and improve optoelectronic properties of naturally derived materials.

Influence of the Solvent Environment on the Ultrafast Relaxation Pathways of a Sunscreen Molecule Diethylamino Hydroxybenzoyl Hexyl Benzoate.

The excited-state dynamics of photoexcited diethylamino hydroxybenzoyl hexyl benzoate (DHHB), a UVA absorber widely used in sunscreen formulations, are studied with transient electronic and vibrational absorption spectroscopy methods in four different solvents. In the polar solvents methanol, dimethyl sulfoxide (DMSO), and acetonitrile, strong stimulated emission (SE) is observed at early time delays after photoexcitation at a near-UV wavelength of λex = 360 nm, and decays with time constants of 420 fs in methanol and 770 fs in DMSO. The majority (∼95%) of photoexcited DHHB returns to the ground state with time constants of 15 ps in methanol and 25 ps in DMSO. In the nonpolar solvent cyclohexane, ∼ 98% of DHHB photoexcited at λex = 345 nm relaxes to the ground state with a ∼ 10 ps time constant, and the SE is weak. DHHB preferentially adopts an enol form in its ground S0 state, but excited state absorption (ESA) bands seen in TEAS are assigned to both the S1-keto and S1-enol forms, indicating a role for ultrafast intramolecular excited state hydrogen transfer (ESHT). This ESHT is inhibited by polar solvents. The two S1 tautomers decay with similar time scales to the observed recovery of ground state population. For molecules that avoid ESHT, torsion around a central C-C bond minimizes the S1-enol energy, quenches the SE, and is proposed to lead to a conical intersection with the S0 state that mediates the ground state recovery. A competing trans-enol isomeric photoproduct is observed as a minor competitor to parent recovery in polar solvents. Evidence is presented for triplet (T1) enol production in polar solvents, and for T1 quenching by octocrylene, a common UVB absorber sunscreen additive. The T1 keto form is observed in cyclohexane solution.

Switching resonance character within merocyanine stacks and its impact on excited-state dynamics

Summary In this study, the optical properties and excited-state dynamics of the unique self-assembled donor-acceptor (DA) merocyanine dye stacks from dimer up to octamer, prepared via dipole-dipole interactions, are reported in terms of coherent exciton dynamics and formation of an excimer-like state. Our findings are based on the steady-state absorption/emission, time-resolved fluorescence, and transient absorption (anisotropy) measurements, including wavepacket analysis and quantum mechanical calculations. Coherent exciton of torsional motions-restricted dye stacks rapidly localizes into the weakly emissive excimer-like state, by shortening the inter-moiety distance and changing the bond-length alternation pattern. The inner merocyanine moiety, having two neighboring units, has a reversed resonance character (non-polar (N) Z) in the ground state. This difference has led to two conclusions: (1) tetramers and octamers exhibit different features of excimer-like state than the dimer, and (2) octamers exhibit slower localization dynamics due to the enhanced homogeneity (six inner-moieties) compared with tetramers (two inner moieties).

Icelike Vibrational Properties of Strong Hydrogen Bonds in Hydrated Lithium Nitrate.

The hydrogen bond network accounts for many of the extraordinary physical properties of liquid water and ice. Its vibrational dynamics are quite complex in their entirety but can be accessed in detail by investigating small groups of only a few water molecules. Here, aqueous salt hydrates turned out to be an exceptional model system for water molecules arranged in well-defined geometrical structures that can be accessed by means of femtosecond spectroscopy of the OH stretching vibration. In this study, we find striking resemblance between the vibrational properties of three water molecules connected via strong hydrogen bonds in the trihydrate of LiNO3 and those of ordinary ice Ih. As in ice, the vibrations of the hydrate water molecules show ultrafast excited state dynamics that are strongly accelerated when proceeding from deuterated to neat H2O samples. The latter is analyzed by means of an additional relaxation channel that is due to Fermi resonance between the OH stretching vibration and the bend overtone accompanied by delocalization of the vibration over neighboring water molecules in the H2O species. Moreover, in the hydrate and ice samples severe spectral broadening is examined when comparing fundamental and excited state absorption bands. Here, proton delocalization along the strong hydrogen bonds is given as a possible underlying mechanism.

Photoisomerization dynamics of trans-trans, cis-trans, and cis-cis diphenylbutadiene from broadband transient absorption spectroscopy and calculations.

The photoisomerization path and dynamics of trans-trans (ttD), cis-trans (ctD), and cis-cis (ccD) 1,4-diphenyl-1,3-butadiene (DPB) in solution are studied with broadband transient absorption (TA) spectroscopy and quantum chemical calculations. For ttD in n-hexane, 2-photon-excited TA spectra indicate that the 2Ag state is located above 1-photon allowed 1Bu (S1) by ∼1000 cm-1. Following S0 → S1 optical excitation, the isomerization occurs via torsion about a butadiene double bond to perpendicular molecular configuration P. The P-state is detected in ccD with an excited-state absorption band at 390 nm. This P-band develops during S1 → P half-torsion with time of 0.15 ps, followed further by P → S0 half-torsion and simultaneous decay with 1.6 ps in acetonitrile and 5 ps in n-hexane. In addition, two oscillation cycles between P and S1 population are observed before equilibration in n-hexane. For ctD, an indication of rising and decaying P is found in acetonitrile. The vast majority of ctD species photoisomerizes to ttD and not to ccD, in agreement a with calculated low torsional barrier about the cis double bond and high barrier about the trans double bond. Photoisomerization yield Y and time τi depend drastically on the solvent polarity. Thus, in n-hexane, the isomerization ttD → ctD has yield Ytt,ct = 0.1 and time τi = 829 ps, while in acetonitrile, Ytt,ct = 0.4 and τi = 27 ps. The 30-fold acceleration of the isomerization in acetonitrile clearly reflects a highly polar character of P, consistent with a dipole moment μP > 9.6 D. The results for DPB are discussed in comparison to stilbene.