Transition Density Cube Research Articles

Intermolecular Coulomb Couplings from Ab Initio Electrostatic Potentials: Application to Optical Transitions of Strongly Coupled Pigments in Photosynthetic Antennae and Reaction Centers

An accurate and numerically efficient method for the calculation of intermolecular Coulomb couplings between charge densities of electronic states and between transition densities of electronic excitations is presented. The coupling of transition densities yields the Förster type excitation energy transfer coupling, and from the charge density coupling, a shift in molecular excitation energies results. Starting from an ab initio calculation of the charge and transition densities, atomic partial charges are determined such as to fit the resulting electrostatic potentials of the different states and the transition. The different intermolecular couplings are then obtained from the Coulomb couplings between the respective atomic partial charges. The excitation energy transfer couplings obtained in the present TrEsp (transition charge from electrostatic potential) method are compared with couplings obtained from the simple point-dipole and extended dipole approximations and with those from the ab initio transition density cube method of Krüger, Scholes, and Fleming. The present method is of the same accuracy as the latter but computationally more efficient. The method is applied to study strongly coupled pigments in the light-harvesting complexes of green sulfur bacteria (FMO), purple bacteria (LH2), and higher plants (LHC-II) and the "special pairs" of bacterial reaction centers and reaction centers of photosystems I and II. For the pigment dimers in the antennae, it is found that the mutual orientation of the pigments is optimized for maximum excitonic coupling. A driving force for this orientation is the Coulomb coupling between ground-state charge densities. In the case of excitonic couplings in the "special pairs", a breakdown of the point-dipole approximation is found for all three reaction centers, but the extended dipole approximation works surprisingly well, if the extent of the transition dipole is chosen larger than assumed previously. For the "special pairs", a large shift in local transition energies is found due to charge density coupling.

Energy Transfer in the Nanostar: The Role of Coulombic Coupling and Dynamics

We present a theoretical investigation of energy transfer in the phenylene ethynelene dendrimer known as the nanostar. Data from extensive molecular dynamics simulations are used to model the dynamical effects caused by torsional motion of the phenyl groups. We compare rate constants for energy transfer between the two-ring chromophore and the three-ring chromophore obtained via the Förster model, the ideal dipole approximation (IDA), and the transition density cube (TDC) method, which has as its limit an exact representation of the Coulombic coupling. We find that the rate constants obtained with the TDC are extremely sensitive to the phenyl group rotation, whereas the constants computed with the Förster model and the IDA are not. The implications of these results for the interpretation of recent pump-probe experiments on the nanostar are discussed in detail. Finally, we predict the temperature dependence of the rate constant for energy transfer.

The Light Harvesting Process in Purple Bacteria

We present and review the results of fluorescence upconversion and photon echo experiments, and ab initio calculations performed in our group within the last few years with respect to the light harvesting process in purple bacteria. Carotenoids transfer energy to bacteriochlorophyll (BChl) mainly via the carotenoid S2 ->BChl Q pathway on a ~ 100 fs timescale. This transfer is reasonably reproduced by considering the Coulombic coupling calculated using the transition density cube method which is valid at all molecular separations. Carotenoids may also serve a role in mediating B800 —>B850 energy transfer in LH2 by perturbing the transition density of the B850 as shown by ab initio calculations on a supermolecule of two B850 BChls, one carotenoid and one B800 BCh1. Further calculations on dimers of B850 BChl estimate the intraand interpolypeptide coupling to be 315 and 245 cm, respectively. These interactions are dominated by Coulombic coupling, while the orbital overlap dependent coupling is 20% of the total. Photon echo peak shift experiments (3PEPS) on LH1 and the B820 subunit are quantitatively simulated with identical parameters aside from an energy transfer time of 90 fs in LH1 and oo in B820, suggesting that excitation is delocalized over roughly two pigments in LH1. 3PEPS data taken at room and low temperature (34 K) on the B800-B820 suggest that static disorder is the dominant mechanism localizing excitation in LH1 and LH2. We suggest that the competition between the delocalizing effects of strong electronic coupling and the localizing effects of disorder and nuclear motion results in excitation in the B850 and B875 rings being localized on 2-4 pigments within approximately 60 fs.

Carotenoid mediated B800–B850 coupling in LH2

Ab initio calculations of the excited state properties of a portion of the light harvesting 2 complex consisting of a supermolecule of two B850 bacteriochlorophyll (BChl), one B800 BChl, and one rhodopin glucoside carotenoid show significant mixing between the BChl and carotenoid states. Calculation of the B800–B850 coulombic coupling, through the transition density cube method, both with and without the carotenoid reveal that the carotenoid affects an increase in the B800–B850 coupling via an indirect (superexchange) mechanism. This carotenoid-mediated coupling explains a portion of the difference between the B800–B850 energy transfer rate determined experimentally and through traditional (Forster) calculations.

Calculation of Couplings and Energy-Transfer Pathways between the Pigments of LH2 by the ab Initio Transition Density Cube Method

ADVERTISEMENT RETURN TO ISSUEPREVAddition/CorrectionNEXTCalculation of Couplings and Energy-Transfer Pathways between the Pigments of LH2 by the ab Initio Transition Density Cube MethodBrent P. Krueger, Gregory D. Scholes, and Graham R. FlemingCite this: J. Phys. Chem. B 1998, 102, 47, 9603–9604Publication Date (Web):October 28, 1998Publication History Published online28 October 1998Published inissue 1 November 1998https://doi.org/10.1021/jp983589lCopyright © 1998 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views469Altmetric-Citations39LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (47 KB) Get e-Alerts Get e-Alerts

Calculation of Couplings and Energy-Transfer Pathways between the Pigments of LH2 by the ab Initio Transition Density Cube Method

A practical method for accurate evaluation of the Coulombic contribution to the electronic coupling for energy transfer at any donor−acceptor separation is reported. The method involves the exact interaction between transition densities of each chromophore which are calculated ab initio and may include electron correlation. The method is used to calculate coupling strengths between the pigments of the bacterial light-harvesting complex, LH2, and to compare with results using the ideal dipole approximation (IDA). The results suggest that the relatively symmetric transitions of bacteriochlorophyll a (Bchla) pigments are reasonably well described by the IDA for separations >15 Å, although deviations are significant at smaller separations. The less symmetric transition of the twisted carotenoid pigment is rather poorly described by the IDA and shows significant deviation even at separations of well over 20 Å. The calculated coupling strengths are combined with estimates of the spectral overlap integral to estimate energy-transfer rates and time scales. The total depopulation time scale of the carotenoid S2 state is estimated to be 85 fs, in reasonable agreement with experiment. The B800−B850 transfer time is estimated to be 1.3 ps (a factor of 2 slower than experiment). Rapid (<400 fs) B800−B800 energy transfer is also predicted. Moreover, the calculations suggest that energy flows both from the carotenoid and the B800 Bchla into pigments of several different protomer units, indicating that interaction between protomer units is important in the LH2 function.