Hot S1 State Research Articles

Symmetrically Functionalized Copper and Silver Corrole-Bis-Tetracyanobutadiene Push-Pull Conjugates: Efficient Population of Triplet States via Charge Transfer.

Copper and silver tritolylcorroles (TTC) are symmetrically functionalized to carry two tetracyanobutadiene (TCBD) entities via [2+2] cycloaddition-retroeletrocyclization reaction involving ethynyl functionalized corroles with an electron acceptor, tetracyanoethylene (TCNE) in excellent yields, as the first examples of corrole-TCBD push-pull systems. The strong push-pull effect resulted in charge polarization in the ground state resulting in a considerable hypsochromic shift of the spectrum extending it into the near-IR region. Electrochemical studies coupled with computational studies revealed considerable interactions between the two TCBD entities via the corrole π-system and the degree of such interactions was found to depend on the metal ion present in the corrole cavity. Energy considerations suggested charge transfer (CT) from the S2 or vibrationally hot S1 state but not the relaxed S1 state in the case of CuTTC(TCBD)2 while CT to occur from all these states in the case of AgTTC(TCBD)2 . Additionally, the high-energy CT states populate the low-lying triplet states. Systematic femtosecond pump-probe studies provided the ultimate proof for the occurrence of excited CT as a function of excitation wavelength followed by the efficient population of the triplet states. The present study brings out the significance of charge transfer in efficiently populating the triplet states in rather unusual copper and silver corroles carrying two TCBD entities.

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.

New Insights about the Photostability of DNA/RNA Bases: Triplet nπ* State Leads to Effective Intersystem Crossing in Pyrimidinones

The high photostability of DNA/RNA nucleobases is attributed to the effective internal conversions of their bright 1ππ* states to the ground state through conical intersections. Intersystem crossing (ISC) from singlet to triplet excited states is a minor decay pathway in nucleobases and it is observed with ∼1-2% quantum yields (QYs) in pyrimidine bases. Presumably, ISC in pyrimidines takes place from the dark singlet 1nπ* state to the lowest triplet 3ππ* state. However, recent studies showed that ISC from the initial populated bright 1ππ* state to higher energy triplet 3nπ* states indeed occurs in the subpicosecond timescale. Such a mechanism is still poorly understood since direct observation of this pathway is challenging. Herein, excited state dynamics of three pyrimidinones, which share the same skeleton with pyrimidine bases, is investigated in different solvents. Compared to canonical pyrimidine bases, removing the oxygen atom at the C4 position revokes the low-lying dark 1nπ* state in pyrimidinones, resulting in direct ISC from the S1 (1ππ*) state to triplet T3 (3nπ*) state with much higher QYs. Meanwhile, hydrogen bonding between the carbonyl group in pyrimidinones and protic solvents can accelerate vibrational cooling of the hot S1 (1ππ*) state, leading to higher fluorescence QYs and smaller ISC rate constants. These results not only evidence the hypothesis of the direct 1ππ* → 3nπ* ISC mechanism, but also contribute to a better understanding of triplet formation in pyrimidines.

Ultrafast nonlinear optical properties and excited-state dynamics of Soret-band excited D-π-D porphyrins

Herein, we report results from our studies on the ultrafast nonlinear optical (NLO) properties and excited-state dynamics in three different porphyrin molecules designed in D-π-D fashion (phenothiazine-porphyrin-phenothiazine) and with two different central metal ions [Zn(II) and Cu(II)]. We have performed extensive target analysis of the femtosecond transient absorption data obtained conveying the true exited state/species spectra of each process with respective lifetimes along with microscopic rate constants of each excited state. The obtained life time values were in the range of 250–500 fs, 20–142 ps, 1.5–6.2 ns and 0.1–1.23 μs and have been associated from relaxation with the Sn, Hot S1 (to S1), S1, and T1 states. Furthermore, the third-order nonlinear (NLO) coefficients [two-photon absorption (TPA) and nonlinear refractive index (n2)] of CPPHT and ZPPHT exhibited superior values compared to HPPHT recorded with ~50 fs, 1 kHz repetition rate pulses at a wavelength of 800 nm. The magnitude of the TPA coefficients of these molecules was compared with some of the recently reported porphyrin moieties and were found to be superior. The time-resolved degenerate four-wave mixing (DFWM) measurements confirmed a large magnitude and an ultrafast response of the χ(3) in these molecules suggesting potential photonic and all-optical switching applications.

Global and target analysis of relaxation processes of the collapsed state of P3HT polymer nanoparticles.

Organic-inorganic heterostructure materials have received significant research interest for designing light harvesting devices because of their efficient charge separation. Here, we design organic and inorganic nano-heterostructures using conjugated polymer nanoparticles (PNPs) [poly(3-hexylthiophene-2,5-diyl), P3HT] and Au nanoparticles. We investigate the carrier relaxation processes of this heterostructure at different time scales by ultrafast transient absorption spectroscopy. The lifetime of the singlet state (S1) of the pristine polymer shortens from 480.7 ps to 2.8 ps due to the formation of nanoparticles, and the formation of a delocalized collective state (CLS) is obtained in polymer nanoparticles whose lifetime is found to be 384.6 ps. The hot and ultrafast electron transfers occur from P3HT polymer nanoparticles to Au nanoparticles and the time constants are 253 fs and 37.7 ps, respectively, which are responsible for the efficient charge separation in such heterostructures. Such a fundamental study of relaxation processes of organic-inorganic nano heterostructures is very significant for designing light harvesting systems.

Energy transfer dynamics in a red-shifted violaxanthin-chlorophyll a light-harvesting complex

Photosynthetic eukaryotes whose cells harbor plastids originating from secondary endosymbiosis of a red alga include species of major ecological and economic importance. Since utilization of solar energy relies on the efficient light-harvesting, one of the critical factors for the success of the red lineage in a range of environments is to be found in the adaptability of the light-harvesting machinery, formed by the proteins of the light-harvesting complex (LHC) family. A number of species are known to employ mainly a unique class of LHC containing red-shifted chlorophyll a (Chl a) forms absorbing above 690 nm. This appears to be an adaptation to shaded habitats. Here we present a detailed investigation of excitation energy flow in the red-shifted light-harvesting antenna of eustigmatophyte Trachydiscus minutus using time-resolved fluorescence and ultrafast transient absorption measurements. The main carotenoid in the complex is violaxanthin, hence this LHC is labeled the red-violaxanthin-Chl a protein, rVCP. Both the carotenoid-to-Chl a energy transfer and excitation dynamics within the Chl a manifold were studied and compared to the related antenna complex, VCP, that lacks the red-Chl a. Two spectrally defined carotenoid pools were identified in the red antenna, contributing to energy transfer to Chl a, mostly via S2 and hot S1 states. Also, Chl a triplet quenching by carotenoids is documented. Two separate pools of red-shifted Chl a were resolved, one is likely formed by excitonically coupled Chl a molecules. The structural implications of these observations are discussed.

Laser-induced- and dispersed-fluorescence studies of rhodamine 590 and 640 ions formed by electrospray ionization: observation of fluorescence from highly-excited vibrational levels of S1 states.

The gas-phase fluorescence excitation and emission properties of rhodamine 590 (R590) and rhodamine 640 (R640) cations were studied using a novel instrument that allows extension of optical excitation to the UV region. Ions were generated by an electrospray ion source, and desolvated ions were accumulated in a room-temperature quadrupole ion-trap. In addition to the well-known S1-S0 transition observed for both dyes, two ultraviolet (UV) bands (that have not been reported previously) were observed for R590. When compared with transitions observed for the dyes in methanol solutions, the S1-S0 transitions in the gas phase are shifted to significantly higher energies. These shifts suggest that the S1 states for both dyes are more stabilized by favorable solvent interactions than the S0 states. The UV bands observed for R590 were assigned to the transitions involving the higher-energy S3 and Sn states. The DF spectra obtained upon UV excitation are similar to those recorded for S1 excitation; however, the emission after UV excitation is shifted slightly to the red region. These observations are consistent with the emission from high-vibrational levels of the S1 state formed upon fast internal conversion from S3 or Sn. In vacuo, the absence of collisions extends the lifetimes of vibrationally-excited S1 states, allowing fluorescence from these states to be observed. A similar DF spectrum was also observed for the UV excitation of R640, suggesting that fluorescence from vibrationally hot S1 states is a general phenomenon for ions in the gas phase.

Two-Photon Spectra of Chlorophylls and Carotenoid-Tetrapyrrole Dyads.

We present a direct comparison of two-photon spectra of various carotenoid-tetrapyrrole dyads and phthalocyanines (Pc) as well as chlorophylls (Chl) in the spectral range between 950 and 1360 nm, corresponding to one-photon spectra between 475 and 680 nm. For carotenoids (Car) with 8, 9, or 10 conjugated double bonds, the two-photon absorption cross section of states below the optical allowed carotenoid S2 is at least about 3-10 times higher than that of Pc or chlorophyll a and b at 550/1100 nm. A quantitative comparison of spectra from Pc with and without carotenoids of eight and nine conjugated double bonds confirms energy transfer from optically forbidden carotenoid states to Pc in these dyads. When considering that less than 100% efficient energy transfer reduces the two-photon contribution of the carotenoids in the spectra, the actual Car two-photon cross sections relative to Chl/Pc are even higher than a factor of 3-10. In addition, strong spectroscopic two-photon signatures at energies below the optical allowed carotenoid S2 state support the presence of additional optical forbidden carotenoid states such as S*, Sx, or, alternatively, contributions from higher vibronic or hot S1 states dominating two-photon spectra or energy transfer from the carotenoids. The onset of these states is shifted about 1500-3500 cm-1 to lower energies in comparison to the S2 states. Our data provides evidence that two-photon excitation of the carotenoid S*, Sx, or hot S1 states results in energy transfer to tetrapyrroles or chlorophylls similar to that observed with the Car S1 two-photon excitation.

Spectroscopic properties of the S1 state of linear carotenoids after excess energy excitation

Properties of the S1 state of neurosporene, spheroidene and lycopene were studied after excess energy excitation in the S2 state. Excitation of carotenoids into higher vibronic levels of the S2 state generates excess vibrational energy in the S1 state. The vibrationally hot S1 state relaxes faster when carotenoid is excited into the S2 state with excess energy, but the S1 lifetime remains constant regardless of which vibronic level of the S2 state is excited. The S∗ signal depends on excitation energy only for spheroidene, which is likely due to asymmetry of the molecule, facilitating conformations responsible for the S∗ signal.

Femtosecond Heterodyne Transient Grating Studies of Nonradiative Deactivation of the S2 (1(1)Bu(+)) State of Peridinin: Detection and Spectroscopic Assignment of an Intermediate in the Decay Pathway.

Femtosecond heterodyne transient grating spectroscopy was employed to investigate the nonradiative decay pathway from the S2 (1(1)Bu(+)) state to the S1 (2(1)Ag(-)) state of peridinin in methanol solution. Just as previously observed by this laboratory for β-carotene in benzonitrile, the real (absorption) and imaginary (dispersion) components of the transient grating signal obtained with Fourier transform spectral interferometry from peridinin exhibit ultrafast responses indicating that S2 state decays in 12 fs to produce an intermediate state, Sx. The excited state absorption spectrum from the Sx state of peridinin, however, is found to be markedly blue-shifted from that of β-carotene because it makes a substantial contribution to the signal observed with 40 fs, 520 nm pulses. The results of a global target analysis and numerical simulations using nonlinear response functions and the multimode Brownian oscillator model support the assignment of Sx to a displaced conformation of the S2 state rather than to a vibrationally excited (or hot) S1 state. The Sx state in peridinin is assigned to a structure with a distorted conjugated polyene backbone moving past an activation-energy barrier between planar and twisted structures on the S2 potential surface. The lengthened lifetime of the Sx state of peridinin in methanol, 900 ± 100 fs, much longer than that typically observed for carotenoids lacking carbonyl substituents, ∼150 fs, can be attributed to the slowing of torsional motions by solvent friction. In peridinin, the system-bath coupling is significantly enhanced over that in β-carotene solution most likely due to the intrinsic intramolecular charge transfer character it derives from the electron withdrawing nature of the carbonyl substituent. An important additional implication is that the Sx state, and the distorted structures reached subsequently along the torsional gradient on the S2 potential surface, may serve as the principal excitation energy transfer donors to chlorophyll a in the peridinin-chlorophyll a protein from dinoflagellates.

Efficient light-harvesting using non-carbonyl carotenoids: Energy transfer dynamics in the VCP complex from Nannochloropsis oceanica

Violaxanthin–chlorophyll a protein (VCP) from Nannochloropsis oceanica is a Chl a-only member of the LHC family of light-harvesting proteins. VCP binds carotenoids violaxanthin (Vio), vaucheriaxanthin (Vau), and vaucheriaxanthin-ester (Vau-ester). Here we report on energy transfer pathways in the VCP complex. The overall carotenoid-to-Chla energy transfer has efficiency over 90%. Based on their energy transfer properties, the carotenoids in VCP can be divided into two groups; blue carotenoids with the lowest energy absorption band around 480nm and red carotenoids with absorption extended up to 530nm. Both carotenoid groups transfer energy efficiently from their S2 states, reaching efficiencies of ~70% (blue) and ~60% (red). The S1 pathway, however, is efficient only for the red carotenoid pool for which two S1 routes characterized by 0.33 and 2.4ps time constants were identified. For the blue carotenoids the S1-mediated pathway is represented only by a minor route likely involving a hot S1 state. The relaxed S1 state of blue carotenoids decays to the ground state within 21ps. Presence of a fraction of non-transferring red carotenoids with the S1 lifetime of 13ps indicates some specific carotenoid-protein interaction that must shorten the intrinsic S1 lifetime of Vio and/or Vau whose S1 lifetimes in methanol are 26 and 29ps, respectively. The VCP complex from N. oceanica is the first example of a light-harvesting complex binding only non-carbonyl carotenoids with carotenoid-to-chlorophyll energy transfer efficiency over 90%.

Excited‐State Deactivation of Branched Phthalocyanine Compounds

The excited-state relaxation dynamics and chromophore interactions in two phthalocyanine compounds (bis- and trisphthalocyanines) are studied by using steady-state and femtosecond transient absorption spectral measurements, where the excited-state energy-transfer mechanism is explored. By exciting phthalocyanine compounds to their second electronically excited states and probing the subsequent relaxation dynamics, a multitude of deactivation pathways are identified. The transient absorption spectra show the relaxation pathway from the exciton state to excimer state and then back to the ground state in bisphthalocyanine (bis-Pc). In trisphthalocyanine (tris-Pc), the monomeric and dimeric subunits are excited and the excitation energy transfers from the monomeric vibrationally hot S1 state to the exciton state of a pre-associated dimer, with subsequent relaxation to the ground state through the excimer state. The theoretical calculations and steady-state spectra also show a face-to-face conformation in bis-Pc, whereas in tris-Pc, two of the three phthalocyanine branches form a pre-associated face-to-face dimeric conformation with the third one acting as a monomeric unit; this is consistent with the results of the transient absorption experiments from the perspective of molecular structure. The detailed structure-property relationships in phthalocyanine compounds is useful for exploring the function of molecular aggregates in energy migration of natural photosynthesis systems.

Broadband transient absorption spectroscopy with 1- and 2-photon excitations: Relaxation paths and cross sections of a triphenylamine dye in solution.

1-photon (382 nm) and 2-photon (752 nm) excitations to the S1 state are applied to record and compare transient absorption spectra of a push-pull triphenylamine (TrP) dye in solution. After 1-photon excitation, ultrafast vibrational and structural molecular relaxations are detected on a 0.1 ps time scale in nonpolar hexane, while in polar acetonitrile, the spectral evolution is dominated by dipolar solvation. Upon 2-photon excitation, transient spectra in hexane reveal an unexpected growth of stimulated emission (SE) and excited-state absorption (ESA) bands. The behavior is explained by strong population transfer S1 → Sn due to resonant absorption of a third pump photon. Subsequent Sn → S1 internal conversion (with τ1 = 1 ps) prepares a very hot S1 state which cools down with τ2 = 13 ps. The pump pulse energy dependence proves the 2-photon origin of the bleach signal. At the same time, SE and ESA are strongly affected by higher-order pump absorptions that should be taken into account in nonlinear fluorescence applications. The 2-photon excitation cross sections σ(2) = 32 ⋅ 10(-50) cm(4) s at 752 nm are evaluated from the bleach signal.

Following the decay dynamics of photoexcited 1,2,4-trimethylbenzene using femtosecond time-resolved photoelectron imaging

The decay dynamics in excited states of 1,2,4-trimethylbenzene has been investigated by combining time-resolved photoelectron imaging with time-resolved mass spectroscopy. The ultrafast internal conversion from the initially optical excited S2 state to the vibration hot S1 state on time scale of 72fs is observed. The lifetime of secondary populated S1 state was determined to be about 16.4ps, which might be originated from internal conversion to ground state or intersystem crossing to triplet states.

Tracking ultrafast relaxation dynamics of furan by femtosecond photoelectron imaging

Ultrafast internal conversion dynamics of furan has been studied by femtosecond photoelectron imaging (PEI) coupled with photofragmentation (PF) spectroscopy. Photoelectron imaging of single-color multi-photon ionization and two-color pump–probe ionization are obtained and analyzed. Photoelectron bands are assigned to the related states. The time evolution of the photoelectron signal by pump–probe ionization can be well described by a biexponential decay: two rapid relaxation pathways with time constants of ∼15fs and 85 (±11)fs. The rapid relaxation is ascribed to the ultrafast internal conversion (IC) from the S2 state to the vibrationally hot S1 state. The second relaxation process is attributed to the redistributions and depopulation of secondarily populated high vibronic S1 state and the formation of α-carbene and β-carbene by H immigration. Additionally, the transient characteristics of the fragment ions are also measured and discussed as a complementary understanding.

Investigation of ultrafast excited state dynamics of 2,2′,4,4′,6,6′-hexanitrostilbene using femtosecond transient absorption spectroscopy

A study on the dynamics and structures of the excited states of 2,2′,4,4′,6,6′-hexanitrostilbene shows equilibrium between vibrationally hot S1 (S*1) and S1 states with lifetimes of 0.8 and 6 ps, respectively.

Ultrafast Excited State Dynamics of trans-4-Aminoazobenzene Studied by Femtosecond Transient Absorption Spectroscopy

The ultrafast excited state dynamics of trans-4-aminoazobenzene (trans-4-AAB) in ethanol was investigated by femtosecond transient absorption spectroscopy. After being excited to the S2 state by 400 nm, trans-4-AAB decays from the S2 state to the hot S1 state by internal conversion with time constant of ∼70 fs. The photoisomerization through inversion mechanism on the S1potential energy surface and the internal conversion from the S1 state to the hot S0 state are observed, respectively. The average timescale of these two decay pathways is ∼0.7 ps. And the vibrational cooling of the hot S0 state of cis- and trans-4-AAB occur with time constants of ∼4 and ∼13 ps, respectively. Furthermore, the ultrafast intersystem crossing process are also observed. The timescale of intersystem crossing from the S2 state to the T4 state is about 480 ps while from the S1 state to the T2 state is ∼180 ps.

Photophysics of diiodine-substituted fluorinated boron–dipyrromethene: A time resolved study

In this letter nanosecond laser flash photolysis and femtosecond pump-probe spectroscopy are applied to clarify the nature, spectral and kinetic properties of transient species formed upon the excitation of 4,4-difluoro-2,6-diiodo-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene in ethanol solution. Picosecond dynamics observed after the excitation of this compound can be adequately interpreted as the following sequence of processes: ultrafast (0.6ps) internal S2−S1 conversion, vibrational relaxation of the hot S1 state (7ps) and intersystem crossing (115ps) to the long-lived T1 state (φТ=0.56). The latter species decays in a microsecond timescale via oxygen quenching and T–T annihilation leading to the recovery of the ground state.

Laser-Induced Carbene–Carbene Rearrangement in Solution: The Diphenylcarbene–Fluorene Rearrangement

Diphenylcarbene (DPC) generated by high-intensity laser photolysis of diphenyldiazomethane rearranges to fluorene (FL) by two distinct mechanisms as revealed by methyl-group labeling. Thus, excimer laser irradiation of p,p'-dimethyldiphenyldiazomethane generates 3,6-dimethylfluorene (3,6-DMF) and 2,7-dimethylfluorene (2,7-DMF), which were identified by fluorescence measurements as well as GC-MS and comparison with authentic materials. 3,6-DMF corresponds to direct bond formation between ortho positions in DPC, referred to as ortho,ortho' coupling. 2,7-DMF corresponds to a carbene-carbene rearrangement, whereby DPC undergoes ring expansion to phenylcycloheptatetraene (PhCHT) followed by ring contraction to o-biphenylylcarbene (o-BPC), which then cyclizes to FL. The carbene-carbene rearrangement dominates over the ortho,ortho' coupling under all conditions employed. The ortho,ortho' coupling must take place in a higher excited state (most likely S2 or T1) of DPC, because it is not observed at all under thermolysis conditions, where only S1 and T0 are populated. The carbene-carbene rearrangement may take place either in a hot S1 state or more likely in a higher excited state (S2 or T1).

Real-time observation of ultrafast internal conversion in ethylbenzene by femtosecond time-resolved photoelectron imaging.

The ultrafast dynamics of the second singlet electronically excited state (S2) in ethylbenzene has been studied by femtosecond time-resolved photoelectron imaging. The time evolution of the photoelectron signal can be well described by a biexponential decay: a rapid relaxation pathway with a time constant of 60 ( ± 9) fs and a longer-lived channel on a timescale of 2.58 ( ± 0.22) ps. The rapid relaxation is ascribed to the ultrafast internal conversion from the S2 state to the vibrationally hot S1 state. This internal conversion process has been observed in real time. The slow photoelectron signal reflects the depopulation of secondarily populated high vibronic S1 state.