Resolving Vibrational from Electronic Coherences in Two-Dimensional Electronic Spectroscopy: The Role of the Laser Spectrum

Abstract

The observation of coherent quantum effects in photosynthetic light-harvesting complexes prompted the question whether quantum coherence could be exploited to improve the efficiency in new energy materials. The detailed characterization of coherent effects relies on sensitive methods such as two-dimensional electronic spectroscopy (2D-ES). However, the interpretation of the results produced by 2D-ES is challenging due to the many possible couplings present in complex molecular structures. In this work, we demonstrate how the laser spectral profile can induce electronic coherencelike signals in monomeric chromophores, potentially leading to data misinterpretation. We argue that the laser spectrum acts as a filter for certain coherence pathways and thus propose a general method to differentiate vibrational from electronic coherences.

Spectral Filtering as a Tool for Two-Dimensional Spectroscopy: A Theoretical Model

“…Example negative rephasing coherence pathway drawn onto the displaced harmonic oscillator potential energy surface in (a) and as a double-sided Feynman diagram in (b). The lower, hot band, transition frequency (highlighted in cyan) is not covered by the blue-shifted laser spectrum, identifying this as one of the eliminated pathways.For the simulation with the centred laser spectrum, analysis of the coherence pathways predicts peaks in the rephasing and non-rephasing amplitude spectra as per diagrams(1a)and (1b) of figure 4, respectively 26. Six positive coherence pathways produce a square arrangement of four peaks below the diagonal for the positive rephasing amplitude spectrum, as shown by the calculated and experimental spectra, respectively(2c)and (3c) of figure 4.…”

“…On inspection of all the possible Liouville pathways, the locations of peaks in the rephasing and non-rephasing amplitude spectra are easily identified. 26,69 q Energy |g 0 |g 1…”

“…6 These coherences have been utilized for the investigation of energy transfer in photosynthetic light harvesting complexes, 7-17 as well as the determination of the excitonic structure of quantum dots. [18][19][20][21] Organic dye molecules and macrocycles such as cyanines, [22][23][24] and porphyrins, 25,26 as well as chromophores such as chlorophyll, [27][28][29] have been extensively studied to explore the origins of these coherences in biological complexes. Homodimers and larger aggregates are used as analogues of the multichromophoric structures.…”

“…Camargo et al have shown experimentally that selective removal of peaks from the amplitude spectra can be achieved upon blue-shifting the laser spectrum, thereby eliminating Liouville pathways associated with lower energy transitions and allowing clearer analysis of the remaining coherences. 26 This kind of analysis has been performed recently to highlight the role of ground state vibronic wavepackets in the Fenna-Matthews-Olson complex. 44 In this work, we describe the development of a theoretical model which reproduces the experimental 2DOS spectra of the 5,15-bisalkynyl zinc porphyrin monomer used by Camargo et al In order to achieve this, the model requires an explicit description of the finite laser pulses, beyond the impulsive limit, for which we adopt the equation of motion-phase matching approach (EOM-PMA).…”

“…The maximum of the fundamental transition occurs at 15 650 cm −1 , with a vibronic shoulder on the blue side, identifying coupling to a vibrational mode of 375 cm −1 . 26 The amplitude spectra of the coupled vibrational mode are calculated using two different laser spectra, one which is centred at the fundamental transition frequency, and another which has been blueshifted such that the lower energy transition, equivalent to the first hot band, is covered by significantly less intensity. We begin with an overview of key features of the model, greater detail of all sections is available in the SI.…”