Conformation and Electronic Population Transfer in Membrane-Supported Self-Assembled Porphyrin Dimers by 2D Fluorescence Spectroscopy

Abstract

Two-dimensional fluorescence spectroscopy (2D FS) is applied to determine the conformation and femtosecond electronic population transfer in a dimer of magnesium meso tetraphenylporphyrin. The dimers are prepared by self-assembly of the monomer within the amphiphilic regions of 1,2-distearoyl-sn-glycero-3-phosphocholine liposomes. A theoretical framework to describe 2D FS experiments is presented, and a direct comparison is made between the observables of this measurement and those of 2D electronic spectroscopy (2D ES). The sensitivity of the method to varying dimer conformation is explored. A global multivariable fitting analysis of linear and 2D FS data indicates that the dimer adopts a "bent T-shaped" conformation. Moreover, the manifold of singly excited excitons undergoes rapid electronic dephasing and downhill population transfer on the time scale of ∼95 fs. The open conformation of the dimer suggests that its self-assembly is favored by an increase in entropy of the local membrane environment.

Coherent two-dimensional photocurrent spectroscopy in a PbS quantum dot photocell

“…These four excitation pathways contribute to the cumulative photocurrent signal according to GSB þ SE þ ESA 1 À ESA 2 , where ESA 2 ¼ G Á ESA 1 and GZ0 represents the photocurrent quantum yield associated with excitons created on state-|2i relative to the quantum yield associated with excitons created on state-|1i. We note that in previous fluorescencedetected 2D experiments performed on condensed phase molecular systems [16][17][18][19] , we typically observed the value of Go1, indicating that the fluorescence quantum yield from high-lying excited states is small (due to radiationless deactivation processes) in comparison with the fluorescence yield from lowlying excited states. For photocurrent-detected 2D experiments, we anticipate that the value of G may be much larger than for the corresponding fluorescence case (that is, G41), since we expect the photocurrent quantum yield to be enhanced for high-lying excited states in comparison to low-lying states.…”

“…We implemented phase-modulation 2DPS using a train of four collinear ultrafast laser pulses. The experimental set-up and laser pulse characteristics are very similar to those previously described for 2DFS performed at visible [15][16][17]19 and at ultraviolet wavelengths 18 . The phase-modulation method provides a means to isolate the fourth-order population contribution to the photocurrent signal, which is used to construct the 2D spectra.…”

“…This is the basis of 2D fluorescence spectroscopy (2DFS) that was previously developed by one of us to measure the 2D spectra of fluorescent systems 15 . 2DFS has been used to study the conformations and excited-state energy transfer of exciton-coupled molecular dimers in biological membranes and in nucleic acids [16][17][18][19] .…”

“…1c) 16,17 . It follows from time-dependent perturbation theory that the sign corresponding to each excitation pathway depends on whether there is an even or an odd number of field-matter interactions appearing on the bra side of the double-sided Feynman diagrams 21 .…”

Probing polaron excitation spectra in organic semiconductors by photoinduced-absorption-detected two-dimensional coherent spectroscopy

“…Usual implementations of 2D ES rely upon optical wavevector matching to measure a thirdorder mesoscopic polarization response [2].In this work, we focus upon the two-dimensional photoexcitation spectroscopy (2D PES), as a variant of 2D ES techniques, in which the detected nonlinear optical signals rely on the fourth-order excited-state population. According to the final population of interest, one can probe the excitation spectral signals in forms of photoluminescence (PL) [6][7][8] or photocurrent (PC) [9][10][11], for example. Indeed, this implementation of 2D coherent spectroscopy has the formidable advantage that it can exploit photocurrent detection as an extraordinarily sensitive population probe [12].…”