Excited State Relaxation Processes Research Articles

Excited State Dynamics of Zinc–Phthalocyanine Nanoaggregates in Strong Hydrogen Bonding Solvents

{Zinc 2,9,16,23-tetra-tert-butyl-29H,31H-phthalocyanine} (ZnPC) exists as monomeric species in DMSO and is reasonably strong fluorescent. But ZnPC forms H-aggregates in water and hexafluoroisopropanol, which are strong hydrogen bonding solvents. Nanoaggregates are nearly nonemissive. Transient absorption spectroscopic technique has been used to investigate the excited state relaxation processes in both monomeric and aggregated forms of ZnPC. The lifetime of the S1 state of the monomoric form in DMSO is long (τ = 3.4 ns) but the excited states of ZnPC nanoaggregates show much faster ground state recovery (within 100 ps). The longest lifetime component, τ3, which is independent of excitation density, has been assigned to the unimolecular decay of the S1-exciton in the absence of annihilation reaction, while τ1 and τ2 are the lifetimes obtained by the two-component fit of the nonexponential decay arising due to the time-dependent decay rates of the S1-excitons because of diffusive migration controlled exciton - exciton annihilation reaction. Rates of the annihilation reaction (2.0 × 10–6 cm3s–1) and exciton migration (4.3 × 10–5 m2/s) as well as diffusion length (about 85 nm) of the S1-exciton created in the ZnPC nanoaggregates in HFIP have been determined.

Origin of 2.7 μm luminescence and energy transfer process of Er3+: 4I11/2→4I13/2 transition in Er3+/Yb3+ doped germanate glasses

An investigation of spectroscopic property and energy transfer process of Er3+ doped and Er3+/Yb3+ co-doped germanate glasses is presented. The emission cross section of the 4I11/2 → 4I13/2 transition of Er3+ doped germanate glass is calculated to be 1.2 × 10−20 cm2. The microparameter of energy transfer from Yb3+:2 F5/2 to Er3+:4I11/2 is calculated to be 2.74 × 10−39 cm6/s, and that is not phonon dependent in the quasiresonant process. The intensity of the emission around 2.7 μm is enhanced obviously when Er3+ co-doped with Yb3+. The excited-state relaxation process of Er3+ is adequately described by a combination of the Judd-Ofelt model and the energy-gap law. With the exception of 4I13/2 and 4I11/2 levels, multiphonon relaxation is dominant for all excited states, making it possible to efficiently pump the 1.55 μm 4I13/2 → 4I15/2 and 2.7 μm 4I13/2 → 4I11/2 emission by excitation of Yb3+:2F5/2 and Er3+:4I11/2 at 980 nm.

Fluorescence characteristics of 5-aminoquinoline in acetonitrile: Water

Photophysical and spectral properties of 5-aminoquinoline (5-AQ) in acetonitrile: water binary mixture have been studied to understand the excited state relaxation processes occurring in the excited state. Fluorescence of 5-AQ has been found to be quite sensitive to the hydrogen bond donating ability of the solvent mixture and is quenched in the presence of water. Hydrogen bonding in the excited state appears to be responsible for the quenching.

Ultrafast spectroscopic investigation of a fullerene poly(3-hexylthiophene) dyad

We present the femtosecond spectroscopic investigation of a covalently linked dyad, PCB-P3HT, formed by a segment of the conjugated polymer P3HT (regioregular poly(3-hexylthiophene)) that is end capped with the fullerene derivative PCB ([6,6]-phenyl-C-61-butyric acid ester), adapted from PCBM. The fluorescence of the P3HT segment in tetrahydrofuran (THF) solution is reduced by 64% in the dyad compared to a control compound without attached fullerene (P3HT-OH). Fluorescence upconversion measurements reveal that the partial fluorescence quenching of PCB-P3HT in THF is multiphasic and occurs on an average time scale of 100 ps, in parallel to excited-state relaxation processes. Judging from ultrafast transient absorption experiments, the origin of the quenching is excitation energy transfer from the P3HT donor to the PCB acceptor. Due to the much higher solubility of P3HT compared to PCB in THF, the PCB-P3HT dyad molecules self-assemble into micelles. When pure C-60 is added to the solution, it is incorporated into the fullerene-rich center of the micelles. This dramatically increases the solubility of C-60 but does not lead to significant additional quenching of the P3HT fluorescence by the C-60 contained in the micelles. In PCB-P3HT thin films drop-cast from THF, the micelle structure is conserved. In contrast to solution, quantitative and ultrafast (<150 fs) charge separation occurs in the solid-state films and leads to the formation of long-lived mobile charge carriers with characteristic transient absorption signatures similar to those that have been observed in P3HT: PCBM bulk heterojunction blends. While p-stacking interactions between neighboring P3HT chains are weak in the micelles, they are strong in thin films drop-cast from ortho-dichlorobenzene. Here, PCB-P3HT self-assembles into a network of long fibers, clearly seen in atomic force microscopy images. Ultrafast charge separation occurs also for the fibrous morphology, but the transient absorption experiments show fast loss of part of the charge carriers due to intensity-induced recombination and annihilation processes and monomolecular interfacial trap-mediated or geminate recombination. The yield of the long-lived charge carriers in the highly organized fibers is however comparable to that obtained with annealed P3HT: PCBM blends. PCB-P3HT can therefore be considered as an active material in organic photovoltaic devices.

Synthesis, optical properties and ultrafast dynamics of a 2,1,3-benzothiadiazole-based red emitter with intense fluorescence and large two-photon absorption cross-section

A new 2,1,3-benzothiadiazole -based red fluorescent compound with a D–A–D type structure was synthesized and characterized. The central 2,1,3-benzothiadiazole core was symmetrically connected via the 4,7-positions with two donor groups in which the 7-position of a fluorenyl ring was substituted with a carbazol-9-yl moiety and the 2-position was substituted by a 5-thienyl moiety. The carbazol moieties were further derivatized by two 2-naphthyl moieties at the 3,6-positions. Femtosecond laser spectroscopic techniques including excited state fluorescence and pump-probe technique investigations, together with steady state absorption and one-photon fluorescence spectra, were employed to systematically investigate the optical properties and ultrafast dynamics of the new compound in tetrahydrofuran solution. It shows a large two-photon absorption cross-section and high fluorescence quantum yield, indicating potential application in two-photon fluorescence imaging field. The ultrafast dynamics results reveal competition between a pure excited state relaxation process and stimulated radiation in the red wavelength region.

Excited-state relaxation processes of DPA-DSB: Investigation of the reason for high fluorescence quantum yield of symmetric D-π-D molecule

The temporal evolution of excited states of a symmetric D-π-D structure two-photon absorption material, 1,4-di(4′-N,N-diphenylaminostyryl) benzene (DPA-DSB), was investigated by femtosecond transient absorption spectrum and solvent polarity dependent fluorescence properties. The results suggested that the structure of DPA-DSB did not change much during the intramolecular charge-transfer (ICT) process, which could be the reason for good radiative ability of the ICT state. The major non-radiative deactive channel may be a large structure-changed process, which was formed more slowly than the radiative ICT state. Symmetric charge-transfer in this D-π-D structure molecule could make the non-radiative structure-changed process slow and ineffective, which should be the reason for the high fluorescence quantum yield.

Photophysics of Self-Assembled Zinc Porphyrin−Bidentate Diamine Ligand Complexes

The effects of complexation--by bidentate nitrogen-containing ligands such as pyrazine and 4,4'-bipyridine commonly used for porphyrin self-assembly--on the photophysics of the model metalloporphyrin, ZnTPP, are reported. Ligation to form the 5-coordinate species introduces an intramolecular charge transfer (ITC) state that, depending on the oxidation and reduction potentials of the electron donor and acceptor, can become involved in the excited state relaxation processes. For ZnTPP, ligation with pyridine has little effect on excited state relaxation following either Q-band or Soret band excitation. However, coordination of ZnTPP with pyrazine and bipyridine causes the S(2) (Soret) state of the ligated species to decay almost exclusively via an S(2)-ICT-S(1) pathway, while affecting the S(1) decay route only slightly. In these 5-coordinate species the S(2)-ICT-S(1) decay route is ultrafast and nearly quantitative. Literature redox data for other bidentate ligands such as DABCO and multidentate ligands commonly used for pophyrin assembly suggest that the ITC states introduced by them could also modify the excited state relaxation dynamics of a wide variety of multiporphyrin arrays.

ChemInform Abstract: Transient Absorption and Two-Step Laser-Induced Fluorescence Studies of the Intramolecular Excited-State Proton Transfer and Relaxation Process in 2-Naphthyl-3-hydroxychromones.

ChemInform Abstract: Transient Absorption and Two-Step Laser-Induced Fluorescence Studies of the Intramolecular Excited-State Proton Transfer and Relaxation Process in 2-Naphthyl-3-hydroxychromones.

Excited-state levels and relaxation processes of chromone and 6-fluorochromone vapors

Emission and excitation spectra of chromone and 6-fluorochromone have been measured in the vapor phase at different temperatures and pressures along with the absorption spectra. The emission is shown to consist of the 3(n, π*) phosphorescence accompanied by weak S 1(n, π*) delayed fluorescence. It is inferred from the temperature dependence of the phosphorescence intensity that the 3(n, π*) state is located at energies below the 3(π, π*) state for chromone and 6-fluorochromone vapors. The relaxation processes in the vapor phase are discussed based on the pressure and excitation energy dependence of the relative phosphorescence yields. The data include the vibrational analyses of the emission spectra based on the results of the DFT calculations at the B3LYP/6-311 + G(p, d) level.

Fluorescence properties of ion pair charge transfer complex with bithiophene group

The fluorescence properties of ion pair charge transfer (IPCT) complex with bithiophene group were studied by steady-state and time-resolved measurements in various solvents. The photophysical properties of 5,5′-di(4-pyridyl)-2,2′-bithiophene (DPBT) were identical to the unsubstituted bithiophene except for the peak wavelength. The absorption of polymeric 5,5′-di(4-pyridiniumyl)-2,2′-bithiophene (PBT 2+) salts showed both the locally excited state (LE) and intramolecular charge transfer (ICT) bands in all solvents studied. In less-polar solvents such as cyclohexane and toluene, absorption spectra showed an IPCT band in addition to the LE and ICT bands. The fluorescence spectra of polymeric PBT 2+ salts showed dual emission which originated from the LE and ICT states in polar solvents. The ICT fluorescence lifetime strongly depends on the solvent polarity. The fluorescence from the PBT 2+ salts was effectively quenched by their counter ion in less-polar solvent. These results indicate that the counter halide anion affects on the excited-state relaxation process.

The structure of adenine monohydrates studied by femtosecond multiphoton ionization detected IR spectroscopy and quantum chemical calculations

We present femtosecond multiphoton ionization detected infrared spectra of jet-cooled monohydrates of adenine and 9-methyladenine. By quantum chemical vibrational analysis and comparison with available literature data we identified two isomers of adenine hydrate with one water molecule hydrogen-bonded to either the amino or the N9-H group. These two monohydrates revealed different fragmentation patterns in the ion depletion spectra, indicating isomer specific intermolecular dynamics. This different behaviour is discussed in terms of competing electronically excited state relaxation and dissociation processes.

Interaction of LDS-751 with the drug-binding site of P-glycoprotein: A Trp fluorescence steady-state and lifetime study

P-glycoprotein (ABCB1) is an ATP-driven efflux pump which binds drugs within a large flexible binding pocket. Intrinsic Trp fluorescence was used to probe the interactions of LDS-751 (2-[4-(4-[dimethylamino]phenyl)-1,3-butadienyl]-3-ethylbenzo-thiazolium perchlorate) with purified P-glycoprotein, using steady-state/lifetime measurements and collisional quenching. The fast decay component of P-glycoprotein intrinsic fluorescence ( τ 1 = 0.97 ns) was unaffected by LDS-751 binding, while the slow decay component ( τ 2 = 4.02 ns) was quenched by dynamic and static mechanisms. Both the wavelength-dependence of the decay kinetics, and the time-resolved emission spectra, suggested the existence of excited-state relaxation processes within the protein matrix on the nanosecond time-scale, which were altered by LDS-751 binding. The fast decay component, which is more solvent-exposed, can be attributed to cytosolic/extracellular Trp residues, while the slow decay component likely arises from more buried transmembrane Trp residues. Interaction of a drug with the binding pocket of P-glycoprotein thus affects its molecular structure and fast dynamics.

Experimental and Theoretical Study of Excited-State Structure and Relaxation Processes of Betaine-30 and of Pyridinium Model Compounds

The long-wavelength absorption band of 2,6-diphenyl-4-(2,4,6-triphenylpyridinium-1-yl) phenolate (betaine-30, B30) in ethanol and 1-chlorobutane shifts to the blue by cooling in the temperature range of 294-128 K. In addition, B30 shows fluorescence in both solvents at 77 K, which is absent at room temperature. The study of the ground and excited state of B30 and its model compound 4-(pyridinium-1-yl)-phenolate PyPo by DFT/TDDFT calculations indicates that for the perpendicular conformation, the ground state possesses a charge-separated closed-shell hole pair (hp) electronic configuration, and the S(1) state corresponds to a biradicaloid electronic structure (dd) with a small dipole moment caused by an unpaired electron on each of the orthogonal fragments. Following the absorption process, there is a driving force for geometrical relaxation within the S(1) state toward an orthogonal arrangement of the phenolate and the pyridinium ring. In this final S(1) equilibrium geometry, the energy gap between the excited and the ground state is strongly reduced and causes very efficient radiationless deactivation of the S(1) state at room temperature. At 77 K, the viscous barrier hindering large-amplitude motion enables the appearance of the fluorescence from the partially or nonrelaxed excited Franck-Condon structure. By variation of the donor and acceptor strength of the two moieties, the energy gap for perpendicular systems can be tuned, allowing, in principle, the switching between the two cases hp < dd and hp > dd. This enables a new access to the efficient construction of mnemonic systems and NLO dyes.

Excited-State Relaxation Process of Free-Base and Oxovanadium Naphthalocyanine in Near-Infrared Region

Excited-state relaxation process of free-base and oxovanadium naphthalocyanine (H(2)Nc and VONc) in solutions and in polymer films was studied by transient absorption measurements. In polymer films only H-aggregate was observed with H(2)Nc with increasing its weight fraction, whereas VONc formed both H- and J-type aggregates. The transient absorption of singlet excited-state of H(2)Nc and VONc in toluene solution decayed with time constant of 250 +/- 30 and 12 +/- 2 ps, respectively. The relaxation from singlet excited state of H(2)Nc and VONc in toluene solution is governed by the IC and ISC, respectively. The central metal ion, VO(2+), acceralated the ISC by the spin-orbital coupling due to unpaired electron. The excited-state relaxation in polymer films differed from that in toluene solution, which originates from the exciton-exciton annihilation process. The excitation power dependence of the excited-state dynamics and weight ratio dependence of absorption spectrum suggests that the aggregation of VONcs contributes to faster decay from the singlet excited-state. The excition-exciton annihilation occurs more efficiently in VONc system compared with H(2)Nc system. The dipole-dipole interaction depending on the aggregated structure controls excited-state relaxation processes in polymer films. This is mainly due to the differences in the transition dipole moment between H- and J-type aggregates. The ultrafast deactivation of VONc in polymer films can be applied to all-optical ultrafast modulation in the optical telecommunication wavelength region.

Thymine relaxation after UV irradiation: the role of tautomerization and πσ* states

Ab initio calculations and femtosecond pump-probe ionization experiments were carried out to identify excited state relaxation processes in isolated thymine monomer and small thymine-water clusters. Three transient species with life times of < or =100 fs, 7 ps and >1 ns were observed in the experiments on gas phase thymine. The longer-lived transients were weak or absent in thymine-water clusters. Available theoretical results on thymine agree with the assignment of low-lying pi-pi* and n-pi* excited states to the femtosecond and picosecond transients but the assignment of the third transient remains opaque. Our theoretical results seem to exclude the possibility of ground or excited state tautomerization as well as the involvement of states with pi-sigma* character. Remaining explanations for the observed transients are: very fast intersystem crossing to the triplet manifold or the observation of transient signals from local minima on the potential energy surfaces.

Excited State Relaxation Processes of Naphthalocyanatovanadium(IV) oxide in Polymer Films for Ultrafast All-optical Modulation in the Near-infrared Region

Abstract Excited state relaxation process of naphthalocyanatovanadium(IV) oxide (VONc) in solutions and in polymer films was studied by transient absorption measurements. The transient absorption decay of VONc in polymer films is faster than that in toluene solution. The excitation power dependence and weight fraction dependence of the excited state dynamics and absorption spectrum suggests that the aggregation of VONcs contributes to faster decay from singlet excited state. The aggregation of VONc is concluded to be useful to ultrafast all-optical light modulation in the near-infrared region.

Excitation wavelength dependent fluorescence behaviour of nano colloids of ZnO

In this paper, the fluorescence behaviour of nano colloids of ZnO has been studied as a function of the excitation wavelength. We have found that excitation at the tail of the absorption band gives rise to an emission that shifts with the change of the excitation wavelength. The excitation wavelength dependent shift of the fluorescence maximum is measured to be between 60 and 100 nm. This kind of excitation wavelength dependent fluorescence behaviour, which may appear to be in violation of Kasha's rule of excitation wavelength independence of the emission spectrum, has been observed for nano ZnO colloids prepared by two different chemical routes and different capping agents. It is shown that the existence of a distribution of energetically different molecules in the ground state coupled with a low rate of the excited state relaxation processes, namely, solvation and energy transfer, are responsible for the excitation wavelength dependent fluorescence behaviour of the systems.

Surface and bulk exciton recombination dynamics in GaN freestanding films via one- and two-photon excitations

We have measured the photoluminescence (PL) lifetime of a freestanding GaN film using one- and two-photon excitations to demonstrate the dramatic difference in exciton recombination dynamics at the surface and in the bulk. An ultra-long exciton PL lifetime of 17.2 ns at 295 K is observed from a GaN freestanding film using two-photon excitation, whereas less than 100 ps lifetime is observed for one-photon excitation, suggesting that nonradiative processes from surface defects account for the short PL lifetime measured. The room temperature exciton lifetime of 17.2 ns is the longest ever reported for GaN film. A monotonic increase in two-photon excited PL lifetime with increasing temperature and the linear dependence of the exciton lifetime with emission wavelength show good agreement with the theoretical predictions, indicating that radiative recombination dominates for bulk excited state relaxation processes.

Femtosecond Time- and Wavelength-Resolved Fluorescence and Absorption Spectroscopic Study of the Excited States of Adenosine and an Adenine Oligomer

By employing broadband femtosecond Kerr-gated time-resolved fluorescence (KTRF) and transient absorption (TA) techniques, we report the first (to our knowledge) femtosecond combined time- and wavelength-resolved study on an ultraviolet-excited nucleoside and a single-stranded oligonucleotide (namely adenosine (Ado) and single-stranded adenine oligomer (dA)(20)) in aqueous solution. With the advantages of the ultrafast time resolution, the broad spectral and temporal probe window, and a high sensitivity, our KTRF and TA results enable the real time monitoring and spectral characterization of the excited-state relaxation processes of the Ado nucleoside and (dA)(20) oligonucleotide investigated. The temporal evolution of the 267 nm excited Ado KTRF spectra indicates there are two emitting components with lifetimes of approximately 0.13 ps and approximately 0.45 ps associated with the L(a) and L(b) pipi* excited states, respectively. These Ado results reveal no obvious evidence for the involvement of the npi* state along the irradiative internal conversion pathway. A distinct mechanism involving only the two pipi* states has been proposed for the ultrafast Ado deactivation dynamics in aqueous solution. The time dependence of the 267 nm excited (dA)(20) KTRF and TA spectra reveals temporal evolution from an ultrafast "A-like" state (with a approximately 0.39 ps decay time) to a relatively long-lived E(1) "excimer" (approximately 4.3 ps decay time) and an E(2) "excimer-like" (approximately 182 ps decay time) state. The "A-like" state has a spectral character closely resembling the excited state of Ado. Comparison of the spectral evolution between the results for Ado and (dA)(20) provides unequivocal evidence for the local excitation character of the initially photoexcited (dA)(20). The rapid transformation of the locally excited (dA)(20) component into the delocalized E(1) "excimer" state which then further evolves into the E(2) "excimer-like" state indicates that base stacking has a high ability to modify the excited-state deactivation pathway. This modification appears to occur by suppressing the internal conversion pathway of an individually excited base component where the stacking interaction mediates efficient interbase energy transfer and promotes formation of the collective excited states. This feature of the local excitation that is subsequently followed by rapid energy delocalization into nearby bases may occur in many base multimer systems. Our results provide an important new contribution to better understanding DNA photophysics.

Radiative recombination and ultralong exciton photoluminescence lifetime in GaN freestanding film via two-photon excitation

We have measured the photoluminescence (PL) lifetime of a freestanding GaN film using one-photon and two-photon excitations to demonstrate the dramatic difference in exciton recombination dynamics at the surface and in the bulk. An ultralong exciton PL lifetime of 17.2ns at 295K is observed from a GaN freestanding film using two-photon excitation, whereas less than 100ps lifetime is observed for one-photon excitation, suggesting that nonradiative processes from surface defects account for the short PL lifetime measured. A monotonic increase in two-photon excited PL lifetime with increasing temperature and the linear dependence of the exciton lifetime with emission wavelength show good agreement with the theoretical predictions, indicating that radiative recombination dominates for bulk excited state relaxation processes.