Excitation energy transfer from a fluorophore to single-walled carbon nanotubes

We study the process of electronic excitation energy transfer from a fluorophore to the electronic energy levels of a single-walled carbon nanotube. The matrix element for the energy transfer involves the Coulombic interaction between the transition densities on the donor and the acceptor. In the Foumlrster approach, this is approximated as the interaction between the corresponding transition dipoles. For energy transfer from a dye to a nanotube, one can use the dipole approximation for the dye, but not for the nanotube. We have therefore calculated the rate using an approach that avoids the dipole approximation for the nanotube. We find that for the metallic nanotubes, the rate has an exponential dependence if the energy that is to be transferred, h is less than a threshold and a d(-5) dependence otherwise. The threshold is the minimum energy required for a transition other than the k(i,perpendicular to)=0 and l=0 transition. Our numerical evaluation of the rate of energy transfer from the dye pyrene to a (5,5) carbon nanotube, which is metallic leads to a distance of similar to 165 A degrees up to which energy transfer is appreciable. For the case of transfer to semiconducting carbon nanotubes, apart from the process of transfer to the electronic energy levels within the one electron picture, we also consider the possibility of energy transfer to the lowest possible excitonic state. Transfer to semiconducting carbon nanotubes is possible only if>=epsilon(g)-epsilon(b). The long range behavior of the rate of transfer has been found to have a d(-5) dependence if h >=epsilon(g). But, when the emission energy of the fluorophore is in the range epsilon(g)>h >=epsilon(g)-epsilon(b), the rate has an exponential dependence on the distance. For the case of transfer from pyrene to the semiconducting (6,4) carbon nanotube, energy transfer is found to be appreciable up to a distance of similar to 175 A degrees.

Fluorescent resonant excitation energy transfer in linear polyenes

We have studied the dynamics of excitation transfer between two conjugated polyene molecules whose intermolecular separation is comparable to the molecular dimensions. We have employed a correlated electron model that includes both the charge-charge, charge-bond, and bond-bond intermolecular electron repulsion integrals. We have shown that the excitation transfer rate varies as inverse square of donor-acceptor separation R-2 rather than as R-6, suggested by the Foumlrster type of dipolar approximation. Our time-evolution study alsom shows that the orientational dependence on excitation transfer at a fixed short donor-acceptor separation cannot be explained by Foumlrster type of dipolar approximation beyond a certain orientational angle of rotation of an acceptor polyene with respect to the donor polyene. The actual excitation transfer rate beyond a certain orientational angle is faster than the Foumlrster type of dipolar approximation rate. We have also studied the excitation transfer process in a pair of push-pull polyenes for different push-pull strengths. We have seen that, depending on the push-pull strength, excitation transfer could occur to other dipole coupled states. Our study also allows for the excitation energy transfer to optically dark states which are excluded by Foumlrster theory since the one-photon transition intensity to these states (from the ground state) is zero.

Excitation energy transfer in covalently linked pheophorbied—porphyrin systems

Steady-state fluorescence, lifetime measurements and time-resolved absorption spectra of the covalently linked hetero dimers consisting of pheophorbide and porphyrin revealed rapid (1011–1012s−1) and efficient singlet—singlet excitation energy transfer from porphyrin unit to pheophorbide.

Excitation energy transfer from dye molecules to doped graphene

Recently, we have reported theoretical studies on the rate of energy transfer from an electronically excited molecule to graphene. It was found that graphene is a very efficient quencher of the electronically excited states and that the rate infinity z(-4). The process was found to be effective up to 30 nm which is well beyond the traditional FRET limit. In this report, we study the transfer of an amount of energy (h) over bar Omega from a dye molecule to doped graphene. We find a crossover of the distance dependence of the rate from z(-4) to exponential as the Fermi level is increasingly shifted into the conduction band, with the crossover occurring at a shift of the Fermi level by an amount (h) over bar Omega/2.

Excitation energy dependent raman signatures of ABA- and ABC-stacked few-layer graphene

The dependence of the Raman spectrum on the excitation energy has been investigated for ABA-and ABC- stacked few-layer graphene in order to establish the fingerprint of the stacking order and the number of layers, which affect the transport and optical properties of few-layer graphene. Five different excitation sources with energies of 1.96, 2.33, 2.41, 2.54 and 2.81â €...eV were used. The position and the line shape of the Raman 2D, G*, N, M, and other combination modes show dependence on the excitation energy as well as the stacking order and the thickness. One can unambiguously determine the stacking order and the thickness by comparing the 2D band spectra measured with 2 different excitation energies or by carefully comparing weaker combination Raman modes such as N, M, or LOLA modes. The criteria for unambiguous determination of the stacking order and the number of layers up to 5 layers are established.

Excitation energy distribution between two photosystems in Porphyra yezoensis and its significance in photosynthesis evolution

Comparative investigation on energy distribution between two photosystems were carried out in the sporophytes and gametophytes of Porphyra yezoensis. By performing 77 K fluorescence spectra, we suggested that there probably existed a pathway for energy transfer from PS II to PS I to redistribute the absorbed energy in gametophytes, while no such a way or at minor level in sporophytes. Electron transfer inhibitor DCMU blocked the energy transfer from PS II to PS I in gametophytes, but no obvious effects on sporophytes. These indicated that excitation energy distribution between two photosystems in gametophytes was more cooperative than that in sporophytes. These data in ontogenesis reflected the evolution process of photosynthetic organisms and supported the hypothesis of independent evolution of each photosystem.

Synechococcus sp. PCC 7002 CpcB lyase null mutations alter phycocyanin chromophore function and, consequently, affect the redistribution of excitation energy via the light state transition /

Phycobilisomes are the major light harvesting complexes for cyanobacteria and phycocyanin is the primary phycobiliprotein of the phycobilisome rod. The phycocyanobilin lyases responsible for chromophorylating the phycocyanin p subunit (CpcB) have been recently identified in the cyanobacterium Synechococcus sp. PCC 7002. Surprisingly, mutants missing the CpcB lyases were nevertheless capable of producing pigmented phycocyanin. 10K absorbance measurements revealed that the energy states of the p phycocyanin chromophores were only subtly shifted; however, 77K steady state fluorescence emission spectroscopy showed excitation energy transfer involving the targeted chromophores to be highly disrupted. Such evidence suggests that phycobilin orientation within the binding domain is specifically modified. We hypothesized that alternate, less specific lyases are able to act on the p binding sites. A phycocyanin linker-polypeptide deficient mutant was similarly characterized. The light state transition, a short term adaptation of the photosynthetic light harvesting apparatus resulting in the redistribution of excitation energy among the photo systems, was shown to be dominated by the reallocation of phycocyanin-absorbed excitation energy. Treatment with a high M phosphate buffer effectively prevented the redistribution of both chlorophyll a- and phycobilisome- absorbed excitation energy, suggesting that the two effects are not strictly independent. The mutant strains required a larger redistribution of excitation energy between light states, perhaps to compensate for their loss in phycobilisome antenna function.

Distribution of excitation energy in photosynthesis: a study of heat loss, photo chemical activity and fluorescence emission in intact calls of cyanobacterium.

Photosynthetic state transitions were investigated in the cyanobacterium Synechococcus sp. PCC 6301 by studying fluorescence emission, heat loss, and PS I activity in intact cells brought to state 1 and state 2. 77K fluorescence emission spectra were modelled with a sum of 6 components corresponding to PBS, PS II, and PS I emissions. The modelled data showed a large decrease in PS II fluorescence accompanied with a small increase in the PS I fluorescence upon transition to state 2 for excitation wavelengths absorbed by both PBS and ChI ll.. The fluorescence changes seen with ChI .a. excitations do not support the predictions of the mobile PBS model of state transition in PBS-containing organisms. Measurements of heat loss from intact cells in the two states were similar for both ChI it. and PBS excitations over three orders of magnitude of laser flash intensity. This suggests that the PBS does not become decoupled from PS II in state 2 as proposed by the PBS detachment model of state transition in PBS-containing organisms. PS I activity measurements done on intact cells showed no difference in the two states, in contrast with the predictions of all of the existing models of state transitions. Based on these results a model for state transition In PBScontaining organisms is proposed, with a PS II photoprotectory function.