Excited Singlet State Absorption Research Articles

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.

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.

Synthesis and Ultrafast Broadband Optical Limiting Properties of a Two-Branched Twistacene

A novel two-branched twistacene (PyDN) has been designed and synthesized for application on ultrafast optical limiting. This twistacene exhibits excellent two photon absorption and two photon absorption-induced excited singlet state absorption, which was systematically investigated with a femtosecond Z-scan experiment, transient absorption spectrum, and two-photon excited fluorescence experiments. The admirable two photon absorption is attributed to the high degree of π electron delocalization in twistacene which is caused by introduction of two strong donors. The excited singlet state absorption cooperates with two-photon absorption to provide an excellent ultrafast optical limiting behavior with high linear transmittance, where the thresholds are 2.3–5.3 mJ/cm2 in the spectral region of 532–800 nm of femtosecond laser and 133 mJ/cm2 for picosecond pulse at 532 nm. These thresholds are lower than that of most of the optical limiters reported previously, which indicates PyDN is a promising candidate for ultrafast optical limiting.

Influence of structural and morphological features of zinc (II)-tetraphenylporphyrin thin film on its third order optical nonlinearity at pico and nano second regimes

Implications of laser pulse width on the third-order nonlinear optical properties of Zinc(II) 5, 10, 15, 20-Tetraphenyl-21H, 23H-Porphine (ZnTPP) thin films is investigated. Thin films of ZnTPP are developed on glass substrate by means of thermal vapor deposition technique. X-ray diffraction analysis illustrates polycrystalline nature of these films. Atomic force microscopy and field emission scanning electron microscope images confirm the homogeneous film surface with mean surface roughness value of 8.45 nm. UV–Visible absorption spectrum depicts signature peaks attributable to π-π* orbital transitions. Photoluminescence spectrum illustrates fluorescence emission in the wavelength range of 590 nm–650 nm from S1 state. Nonlinear optical investigations are carried out using single-beam Z-scan technique. ZnTPP thin-film at pico-second regime depicts saturable nonlinear absorption nature, attributable to the collective effect of singlet excited state absorption and filling effect of localized defect states. Whereas, at nanosecond timescale the film exhibits reverse saturable nonlinear absorption, sensitive to triplet excited state absorption. Closed aperture Z-scan measurements at pico and nano second regime highlight self-defocusing optical nonlinearity. Enhanced nonlinear optical coefficient (βeff and n2) values are strongly influenced by the heavy zinc metal ion and modified excited state dynamics due to the strong intermolecular interactions through J-aggregation.

Three-photon-induced singlet excited-state absorption for tunable ultrafast optical-limiting in distyrylbenzene: a first-principles study.

The ground and first singlet excited state absorption in distyrylbenzene (DSB) is simulated based on linear-response time dependent density functional theory (LR-TDDFT). It is found that distyrylbenzene shows a strong reverse saturable absorption effect around the near-infrared range. Combining the calculations of cubic response functions to simulate the three-photon absorption in distyrylbenzene, we are able to show that distyrylbenzene is a promising ultrafast optical limiter for the light with wavelengths around 775 nm. The primary mechanism for the optical limiting behavior can be well understood by the three-photon induced excited state absorption (3PA-ESA). This result in that DSB has high transmittance for low-intensity ambient light levels and the ultrafast response of optical-limiting. In addition, the limited optical window can be tuned by changing the length of the π-electron conjugated structure. It was also discovered that the molecular aggregation has an inhibitory effect on the optical limiting efficiency of distyrylbenzene. The present results may serve as a theoretical guideline for the design of distyrylbenzene-based optical limiting materials.

The Leap from Organic Light-Emitting Diodes to Organic Semiconductor Laser Diodes

In recent years, organic light-emitting device technology has expanded from organic light-emitting diodes (OLEDs) to organic semiconductor laser diodes (OSLDs) with the progress of sophisticated mo...

Investigation of excited singlet state absorption and intersystem crossing mechanism of isomeric meso-tetra(pyridyl)porphyrins containing peripheral polypyridyl platinum(II) complexes

In the present work, the excited states (singlet and triplet) for three different porphyrins, with and without coordinated Pt(II) complex derivatives, were investigated by three different temporal laser regimes were employed in the Z-Scan technique. Intersystem crossing time (singlet to triplet conversion mechanism) was obtained by a combination of Z-Scan with single or with a train of picosecond laser pulses. These measurements have shown that the excited state absorption spectrum shows distinct absorption behaviors. Also, the absorption cross-section is equivalent to the ground state, describing no changes in the optical transmittance.Fluorescence lifetimes and quantum yields were determined, respectively, by time resolved fluorescence analysis. To determine the intersystem crossing time constant, two additional levels were incorporated to the rate equation model. Moreover, fluorescence decays, internal conversion and ISC times have shown a strong dependence in Pt(II) derivatives and their position to the porphyrin ring (meta or para position).

Picosecond dynamic of aqueous sodium-copper chlorophyllin solution: An excited state absorption study

This work reports on the singlet excited-state absorption spectrum of sodium-copper chlorophyllin using three distinct methods of Z-Scan measurement: white-light continuum Z-Scan technique ranging from 470 to 780 nm, tunable Z-Scan technique with femtosecond pulses (490–700 nm) and 532 nm Z-Scan technique with 100 ps pulse width. A transition from reverse saturation absorption (RSA) to saturable absorption (SA) was observed at 570 nm, and at 532 nm RSA was tested showing potential application as optical limiting device. Pump-probe technique was used to measure lifetime of the first singlet excited state (22 ps) and no evidence of triplet conversion was observed.

Impact of electronegative character on ultrafast nonlinear optical absorption of azine derivatives

Two azine derivatives (NO(Cl) and NS(Cl)) with different electronegative central core were prepared and spectroscopically characterized. Their ultrafast third-order nonlinear optical (NLO) responses were investigated using open aperture Z-scan and optical limiting method with 190 fs laser pulses at 515 nm. The frontier molecular orbitals and energy gaps of samples were obtained through quantum chemical calculation. Transient absorption spectra demonstrated that the origin of strong NLO absorption of two molecules was attributed to excited-state absorption. A simplified three-energy-level model was used to determine their photo-physical parameters. Our results show that the stronger the electronegative cationic groups is, the stronger the NLO absorption performs. The azine compounds have large excited-state lifetime and large ratio of singlet excited-state absorption cross-section to that of ground state cross-section (∼20.2), indicating it is a candidate material for future ultrafast optical limiters.

Solvent dependent trans → cis photoisomerization of N, N′-diacetylindigo studied by femtosecond time-resolved transient absorption spectroscopy

Solvent dependence of trans→cis photoisomerization of N,N′-diacetylindigo (DAI) was investigated by femtosecond transient absorption (TA) spectroscopy. A strong and sharp singlet excited state absorption band centered at 595nm was observed in the TA spectrum and the differential absorbance (ΔAbs) and the wavelength at the maximum of the band were periodically modulated by coherent vibrational wavepacket motions. However, even in the “reactive solvent”, ethyl acetate (EtAc), where the isomerization takes place, the formation of cis-DAI was unobservable in our experimental time window of 2ns which means that isomerization is occurring in a longer time scale. Preceding studies suggest that intermolecular hydrogen-bond (H-bond) with the solvent quenches the excited state and blocks the isomerization. Interestingly, we have observed that dimethylformamide (DMF) blocked the isomerization more efficiently than methanol. The excited state lifetime was shorter in DMF compared to EtAc solution suggesting that weak H-bonding with the formyl group of DMF is sufficient to prevent the slow trans→cis photoisomerization of DAI.

Investigating the intersystem crossing rate and triplet quantum yield of Protoporphyrin IX by means of pulse train fluorescence technique

Photophysical investigations of PPIX were described in order to determine the triplet conversion efficiency. Time resolved fluorescence and pulse train fluorescence were employed to characterize the main mechanism responsible for deactivation of the first singlet excited state (excited singlet and triplet states). Single pulse and Z-Scan analysis were employed to measure the singlet excited state absorption cross-sections. Theoretical calculations were performed in order to get some properties of PPIX in ground state, first singlet and triplet excited state. A TD-DFT result shows a great possibility of ISC associated to out-of-plane distortions in porphyrinic ring. Furthermore, the B and Q bands in the calculated spectrum are assigned to the four frontier molecular orbitals as proposed by Gouterman for free-based porphyrins.

Ultrafast electronic and vibrational dynamics in brominated aluminum corroles: Energy relaxation and triplet formation.

We combined femtosecond (fs) VIS pump–IR probe spectroscopy with fs VIS pump–supercontinuum probe spectroscopy to characterize the photoreaction of the hexacoordinated Al(tpfc-Br8)(py)2 in a comprehensive way. Upon fs excitation at ∼400 nm in the Soret band, the excitation energy relaxes with a time constant of (250 ± 80) fs to the S2 and S1 electronic excited states. This is evident from the rise time of the stimulated emission signal in the visible spectral range. On the same time scale, narrowing of broad infrared signals in the C=C stretching region around 1500 cm−1 is observed. Energy redistribution processes are visible in the vibrational and electronic dynamics with time constants between ∼2 ps and ∼20 ps. Triplet formation is detected with a time constant of (95 ± 3) ps. This is tracked by the complete loss of stimulated emission. Electronic transition of the emerging triplet absorption band overlaps considerably with the singlet excited state absorption. In contrast, two well separated vibrational marker bands for triplet formation were identified at 1477 cm−1 and at 1508 cm−1. These marker bands allow a precise identification of triplet dynamics in corrole systems.

Signal Enhancement in Fluorescence Microscopy by Microsecond Pulsed Excitation

Although pulsed illumination is routinely used in multi-photon fluorescence microscopy, it is not quite common for its one-photon counterpart, i.e. (one-photon) confocal fluorescence microscopy. However, prolonged illumination with continuous wave (CW) light source leads to excited (singlet) state absorption (as well as absorption from triplet state) leading to photo-damage. Here, by photo-damage we mean any detrimental (irreversible) effect that leads to reduced fluorescence yield. Stroboscopic or pulsed light illumination has been recently introduced to reduce photo-bleaching as well as photo-toxicity.

Ultrafast optical limiting properties and transient dynamics of symmetrical phenoxazinium bromine salt

A symmetrical phenoxazinium bromine salt with resonant D–π–D structure was prepared and spectroscopically characterized. The ultrafast nonlinear optical (NLO) response of the compound dissolved in DMF was investigated using open aperture Z-scan method with 190fs laser pulses at 515nm. And the photo-physical dynamics of the compound was studied using transient absorption spectra at femtosecond time regime. The titled sample showed strong reverse saturable absorption (RSA) at 515nm. Transient absorption results demonstrated that the compound exhibited RSA signals at 478–580nm and SA signals at 600–780nm. A simplified three-energy-level model was used to determine the photo-physical parameters. And the excited-state lifetime was evaluated to be around one nanosecond. Our results show that the phenoxazinium bromine salt has a large ratio of singlet excited-state absorption cross-section to that of ground state cross-section (18.7), indicating it is a candidate material for future ultrafast optical limiters.

Excited-state absorption of meso-tetrasulfonatophenyl porphyrin: Effects of pH and micelles

The influence of the environment on the excited state transitions of meso-tetrakis(p-sulfonatophenyl) porphyrin (TPPS) is reported. TPPS was investigated in protonated and non-protonated forms, and in the presence of the cationic cetyltrimethylammonium bromide (CTAB) micelles. The singlet excited-state absorption spectra were measured by using the white-light continuum Z-scan technique and the triplet–triplet absorption spectra were acquired employing an association of laser flash photolysis and Z-scan techniques. Our results show that the perseveration of the molecular symmetry, upon excitation, depends on the state of multiplicity of the molecules, as well as on the environment and structural characteristics of the porphyrin. Additionally, it was observed that for excited molecules, the ring distortion caused by the protonation of porphyrin ring has great influence on the changes observed for the symmetry and vibronic structure. The results clearly show that the porphyrin investigated is a promising candidate for optical limiting applications for all investigated environments.

Direct Observation of Ultrafast Excimer Formation in Covalent Perylenediimide Dimers Using Near-Infrared Transient Absorption Spectroscopy.

Energy transfer in perylene-3,4:9,10-bis(dicarboximide) (PDI) aggregates is often limited by formation of a low-energy excimer state. Formation dynamics of excimer states are often characterized by line shape changes and peak shift dynamics in femtosecond visible transient absorption spectra. Femtosecond near-infrared transient absorption experiments reveal a unique low-energy transition that can be used to identify and characterize this state without overlapping excited singlet-state absorption. Three covalently bound PDI dimers with differing PDI-PDI distances were studied to probe the influence of interchromophore electronic coupling on the PDI excimer transient spectra and dynamics.

Investigation of ground and excited state photophysical properties of gadolinium phthalocyanine

In this present work, we investigated the singlet excited state absorption (ESA) spectrum and intersystem crossing dynamics of gadolinium phthalocyanine (GdPc) in toluene. For that, we employed the femtosecond wavelength-tunable Z-scan and picosecond Pulse Train Fluorescence (PTF) techniques to determine, respectively, the singlet ESA cross-section on a wide spectral range (from 460 up to 740 nm) and the intersystem crossing time (singlet–triplet transition). We observed that the ESA spectrum presents distinct absorption behaviors, i.e., saturable (SA) and reverse saturable absorption (RSA). We have also been able to identify regions in which the excited state absorption cross-section has the same magnitude as the ground state one (at 600 and 720 nm). The RSA effect was observed for wavelengths shorter than 600 nm and longer than 720 nm, while SA occurs in the Q-band region, located between 600 and 720 nm. To describe the main singlet–singlet transitions, we measured the fluorescence decay time and employed a rate equation model considering a three-energy-level-diagram. Ratios between excited and ground state absorption cross-section from 0.4 to 4 were observed along the ESA spectrum. From these results, we calculated the transition dipole moment between the excited states for the main electronic transitions of GdPc. To obtain the intersystem crossing time, we incorporated an additional level in our model and used the PTF technique to probe the population transfer to the triplet state. Our results pointed out that the intersystem crossing time is higher than 300 ns, which corresponds to a maximum triplet quantum yield of 1.5%.

Study of singlet excited state absorption spectrum of lutetium bisphthalocyanine using the femtosecond Z-scan technique

In this study we investigate the singlet excited state absorption of lutetium bisphthalocyanine (LuPc2) over a wide spectral range. It was observed distinct nonlinear absorption behaviors; saturable (SA) and reverse saturable absorption (RSA). The RSA effect was observed below 640 and above 680nm, while SA occurs around the Q-band region, located around 660nm. To describe the main singlet–singlet transitions, we employed the rate equation model considering the simplified three-energy level diagram. Our results reveal a ratio between excited and ground state absorption smaller than 0.05 at the Q-band region, and of approximately 4 for the other regions.

Excited-state absorption of mono-, di- and tri-nuclear cyclometalated platinum 4,6-diphenyl-2,2′-bipyridyl complexes

The ground-state absorption cross-sections ( σ g), triplet excited-state absorption cross-section ( σ T) at 532 nm, singlet excited-state absorption cross-sections ( σ s) at various visible wavelengths, singlet and triplet excited-state lifetimes, and triplet quantum yields of three cyclometalated platinum(II) 4,6-diphenyl-2,2′-bipyridyl complexes, are reported. The presence of metal–metal and π–π interactions in the dinuclear and trinuclear complexes results in a significant increase in their respective σ g’s in the visible spectral region. As a result, the ratio of σ s/ σ g and σ T/ σ g at each wavelength is significantly greater for the mononuclear complex than for the dinuclear and the trinuclear complexes.

Effect of protonation on the singlet–singlet excited-state absorption of meso-tetrakis( p-sulphonatophenyl) porphyrin

This work reports on the excited singlet-state absorption cross-section spectra of both biprotonated and nonprotonated states of meso-tetrakis( p-sulphonatophenyl) porphyrin obtained by means of the Z-scan technique with white-light continuum pulses. Although our main purpose was to determine these photo-physical parameters in a wide spectral range, we also analyzed the role of accumulative effects arising as consequence of the pulse chirp caused by the group velocity dispersion. This effect leads to apparent changes in the measured excited-state absorption spectra, which can be corrected during the fitting procedure to yield meaningful absorption cross-section values.