Mechanistic insight into internal conversion process within Q-bands of chlorophyll a

Abstract

The non-radiative relaxation of the excitation energy from higher energy states to the lowest energy state in chlorophylls is a crucial preliminary step for the process of photosynthesis. Despite the continuous theoretical and experimental efforts to clarify the ultrafast dynamics of this process, it still represents the object of an intense investigation because the ultrafast timescale and the congestion of the involved states makes its characterization particularly challenging. Here we exploit 2D electronic spectroscopy and recently developed data analysis tools to provide more detailed insights into the mechanism of internal conversion within the Q-bands of chlorophyll a. The measurements confirmed the timescale of the overall internal conversion rate (170 fs) and captured the presence of a previously unidentified ultrafast (40 fs) intermediate step, involving vibronic levels of the lowest excited state.

Raman and 2D electronic spectroscopies: A fruitful alliance for the investigation of ground and excited state vibrations in chlorophyll a

“…Because of that, 2DES has the recognized advantage of making accessible simultaneously frequency and time information, identifying not only the frequency of the modes more strongly coupled with the electronic transition but also their dynamics and dephasing time. This could be crucial information to understand how the coupling with vibrational degrees of freedom affects the first steps of ultrafast relaxation dynamics [33,52]. However, as it has already been pointed out [39], the possibility of retaining information both in time and frequency domain can only be achieved at the cost of losing some resolution along the frequency axis.…”

“…The main diagonal positive peak at around (15250, 15250) cm -1 arises from stimulated emission and ground state bleaching of the Qy(0,0) band. The cross-peak at about (16500, 15250) cm -1 is instead originated by the coupling of the Qy(0,0) state with vibrational modes within the range 1000-1300 cm -1 , responsible also for the vibronic progression in the linear absorption spectrum, and by ultrafast relaxation processes from higher states [33].…”

“…Given the importance of this dye, the electronic and vibrational properties of Chla has been intensively studied since the 60's through several time-and frequency-resolved techniques. [19][20][21][22][23][24][25][26][27][28][29] More recently, Chla has also been the object of an intense investigation through 2DES spectroscopy in different conditions [30][31][32][33]. In these works, a lot of attention has been paid in identifying how the coupling with the environment and with vibrational degrees of freedom can affect the electronic and optical properties of the molecule.…”

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

“…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.…”

Optimization and selection of time-frequency transforms for wave-packet analysis in ultrafast spectroscopy

“…Whatever the nature of the studied material or the particular technique and time resolution used, the analysis of the frequency and time behavior of the beatings is a key ingredient to extract information about the system dynamics. To this aim, methodologies based on linear and bilinear time-frequency transforms (TFT) have been already proposed and applied to ultrafast 1D [14][15][16][17][18][19] and 2D spectroscopy measurements [20][21][22][23][24][25][26]. The advantage of TFTs is that they analyze a non-stationary oscillating signal providing both time and frequency resolution, spreading out the information content along the two dimensions of the time-frequency plane [27].…”