Tomas Gillbro

Annihilation of singlet excitons in J aggregates of pseudoisocyanine (PIC) studied by pico‐ and subpicosecond spectroscopy

The excited‐state dynamics of J aggregates of PIC have been studied by means of picosecond and subpicosecond absorption spectroscopy as well as integrated fluorescence yield measurements. The results of these measurements show that both the lifetime and the fluorescence yield are strongly dependent on excitation pulse intensity. At relatively high light intensity (1014 photons cm−2 pulse−1) the S1 lifetime is essentially pulse limited (<1 ps) and the fluorescence yield is very low. Upon decreasing the light intensity a gradual increase of the excited‐state lifetime and fluorescence yield is observed. At very low excitation intensity (1010 photons cm−2 pulse−1) a single exponential lifetime of 400 ps is observed. At intermediate intensities the excited‐state decay is strongly nonexponential. The observed intensity dependence of the excited‐state dynamics is attributed to efficient exciton–exciton annihilation between the highly mobile singlet excitons. By applying an expression for bimolecular exciton anni...

Singlet Excited States and the Light-Harvesting Function of Carotenoids in Bacterial Photosynthesis

Abstract— The recent results of stationary‐state and time‐resolved absorption, fluorescence and Raman spectroscopies of some typical carotenoids are summarized. Theoretical analyses of carotenoid singlet states and of carotenoid‐to‐bacteriochlorophyll singlet‐energy transfer are also included. On the bases of the energies, the lifetimes and other properties of singlet excited states of the carotenoids in solution and bound to the light‐harvesting complexes, the energetics and the dynamics of the light‐harvesting function in purple photosynthetic bacteria are discussed with emphasis on the 2Ag and Bu+ states.

Excitation-energy transport in the bacteriochlorophyll antenna systems of Rhodospirillum rubrum and Rhodobacter sphaeroides, studied by low-intensity picosecond absorption spectroscopy

Abstract We have studied the energy-transfer dynamics in chromatophores of Rhodospirillum rubrum and Rhodobacter sphaeroides (formerly called Rhodopseudomonas sphaeroides ) under conditions of closed and open reaction centers, using low-intensity picosecond infrared absorption recovery measurements. The possibilities of selective infrared excitation and probing as well as picosecond anisotropy decay measurements offered by this technique have allowed us to obtain a detailed picture of the transfer dynamics. Upon selective excitation of a bacteriochlorophyll (BChl)-protein complex (B800–850, B875 or B880) a very low initial value of the absorption anisotropy ( r (0) ≈ 0.1) is observed for the excited-state decays of BChl 850, BChl 875 and BChl 880. This is consistent with a very fast ( k ≈ 3 · 10 12 s −1 ) initial transfer of energy between similar bacteriochlorophyll molecules. We suggest that this fast transfer occurs within a minimum unit of 6–8 chromophores. Only direct excitation of BChl 800 shows highly polarized, very short-lived (1–2 ps) absorbance changes due to the excited state of BChl 800. On a slower time-scale, 5–10 ps, the energy then migrates between the units of 6–8 bacteriochlorophyll molecules. On a similar time-scale, 35–50 ps, an equilibration of the excitation density occurs between different pigment pools. Thus in Rb. sphaeroides there is an equilibration between B800–850 and B875 ( τ = 37 ± 4 ps) and in R. rubrum there is a similar process between B880 and B896 ( τ = 47 ± 4 ps). These slower processes are associated with a further decay of the anisotropy from the initial value of ≈ 0.1 to very low values (less than 0.01). Our results suggest that the protein-pigment complexes designated B800–850 and B880 are not spectrally homogeneous, but may consist of spectrally slightly different bacteriochlorophyll molecules.

ABSORPTION SPECTRAL SHIFTS OF CAROTENOIDS RELATED TO MEDIUM POLARIZABILITY

Abstract–Solvent induced absorption spectral shifts of the electronic transition from ground 1 Ag state to the excited 1Bu state in carotenoids have been studied. It is shown that the shift depends only on dispersion interactions in non‐polar solvents. In polar media there is just a small extra contribution to the red‐shift, due to other forms of interactions. The spectral shifts are well described by the theory, which expresses the shift relative to the gas phase value, as a function of solvent polarizability. The main conclusion is that the dominating mechanism behind the large red‐shifted absorbance of carotenoids in the proteinacous environment, in vivo, is the mutual polarizability interactions between the carotenoids and the surrounding medium. The solution‐phase values of the dipole moments of the lAg to 1Bu transitions and the differences of isotropic polarizability between 1Bu and lAg states of carotenoids in non‐polar solvents are calculated and found to be around 13 D and 360 Å3 respectively. From the great overlap of absorption spectra between carotenoids in quinoline and carotenoids in vivo in purple bacterial antenna complexes, it can be expected that the carotenoids are surrounded by several aromatic amino acids in vivo. Comparisons have been done between the exicted states in carotenoids and in linear conjugated polyenes.

Picosecond kinetics of radiationless relaxations of triphenyl methane dyes. Evidence for a rapid excited-state equilibrium between states of differing geometry

Abstract The photophysics of several triphenylmethane (TPM) dyes has been studied in n -alcohols and glycerol/water solutions. We have been able to measure the rate and characterize the viscosity dependence of several radiationless decay channels. We propose a kinetic scheme to account for our observations. Important points in this model are, the existence of more than one ground-state species (in some dyes), a rapid excited-state equilibrium between states of differing geometry and solvent-induced spectral shifts.

Effects of solvent on TMP photophysics. Transition from no barrier to barrier case, induced by solvent properties

The dynamics of the radiationless relaxation of triphenylmethane (TPM) molecules in the n‐alcohols methanol to octadecanol have been studied with picosecond absorption recovery techniques as a function of viscosity, temperature, and wavelength of the exciting and analyzing light. It is shown that the solvent dependence of the relaxation rate in TPM molecules cannot, as is usually done, be described by a viscosity dependence only. In going through the n‐alcohol series the excited state potential surface changes from a practically flat one (E0=0) in methanol to a surface having a definitive potential barrier (E0≊15 kJ mol−1) in the higher alcohols. This variation of the potential surface implies that the relaxation is largely controlled by rotational diffusion in methanol and ethanol whereas it is mainly controlled by the potential barrier in the higher alcohols. When the viscosity dependence of the relaxation rate is obtained, by compensating for the contribution from the potential barrier, a turnover beha...

Dynamics of the isomerization of trans-stilbene in n-alcohols studied by ultraviolet picosecond absorption recovery

Abstract We have used a simple At + synchronously pumped and cavity-dumped dye-laser system to generate UV picosecond pulses with energies sufficiently high for absorption recovery experiments. With these pulses, we have studied the dynamics of the isomerization of trans-stilbene in n -alcohols as a function of viscosity and temperature. It is concluded that the excited-state barrier of trans-stilbene in n -alcohols is less than 1 kcal mol −1 and that the trans → cis isomerization yield is 0-54 ± 0.05.

Photophysics and dynamics of the lowest excited singlet state in long substituted polyenes with implications to the very long‐chain limit

In this paper we explore the intramolecular relaxation processes within two long carotenoids, namely decapreno‐β‐carotene (M15) and dodecapreno‐β‐carotene (M19) with 15 and 19 conjugated double bonds (N), respectively. Amplified 200 fs pulses at 590 nm were used to excite the optically allowed S0→S2 (1 1Ag→1 1Bu) transition of the two carotenoids. The excited state dynamics were probed by continuum light between 400–890 nm in solvents with different polarizabilities. The transient absorption spectra consist of a bleaching region, due to loss of ground state absorption, and of an excited state absorption region at longer wavelengths, due to the S1→Sn transition. The Sn state was assigned to an n 1Bu state. The overall wavelength dependence of the measured kinetics could be well described by introducing three decay time constants. One reflects the S1 lifetime (τ1) and was determined to 1.1 and 0.5 ps for M15 and M19, respectively. A second lifetime, between 5 and 15 ps, was attributed to vibrational cooling...

Energy transfer within the bacteriochlorophyll antenna of purple bacteria at 77 K, studied by picosecond absorption recovery

Abstract The dynamics of energy transfer within the bacteriochlorophyll antenna of Rhodobacter sphaeroides and Rhodospirillum rubrum, with closed reaction centers, has been studied at 77 K using low-intensity infrared picosecond absorption recovery. Measurements of isotropic decay as well as decay of induced anisotropy yielded a detailed picture of the energy transfer pathways in the two antenna systems. The BChl antenna of Rb. sphaeroides seems to consist of at least four different BChl a species: BChl 800, BChl 850, BChl 875, and BChl 896. Upon excitation of the highest-energy pigment, a transfer sequence towards lower energy is initiated. The transfer steps between the different pigment pools are characterized by the following time constants: BChl 800 → BChl 850, т = 2 ± 1 ps ; BChl 850 → BChl 875, т = 40 ± 5 ps ; BChl 875 → BChl 896, т = 20 ± 5 ps . Once the excitations are localized on B896 a slower quenching, т = 190 ± 10 ps , by closed reaction centers (P+) occurs. From measurements of decay of induced anisotropy it is further concluded that efficient transfer between BChl 800 molecules takes place on a time-scale comparable to the BChl 800 → BChl 850 transfer. A marked increase in anisotropy in the red wing of the absorption spectrum offers a clear evidence of the presence of the long-wavelength antenna component B896. The BChl antenna of R. rubrum is composed of two BChl a species, BChl 880, and BChl 896, and the energy transfer kinetics are observed to be very similar to the corresponding part of Rb. sphaeroides. Some evidence of further spectral inhomogeneity (apart from B896) or spectral shifts induced by excitation of the B880 antenna pigment was also obtained. Several possible models are discussed for the origin and organization of the B896 pigment.

Viscosity dependent radiationless relaxation rate of cyanine dyes. A picosecond laser spectroscopy study

The viscosity dependent radiationless relaxation of several cyanine dyes has been studied by picosecond laser spectroscopy. It was found that the relaxation rate is proportional to η−α. The value of α, however, is not constant for a certain dye molecule, but is strongly dependent on the kind of solvent used. In n-alcohols for instance α is typically about 1. In glycerol/methanol or glycerol/water mixtures on the other hand α ≈ 0.5. A comparison is made with literature data on orientational relaxation lifetimes of some dyes in similar solvents. It is shown that the radiationless relaxation of cyanine dyes and the orientational relaxation of for instance xanthene dyes changes in roughly the same way as the solvent is changed. This is taken as proof of the proposal that a torsional motion of the heterocyclic quinolyl rings is the main course of the viscosity dependent relaxation of the cyanine dyes studied.