**1. Introduction**

Dye-sensitized solar cell (DSSC) is a promising photovoltaic technology and part of several green technologies used for environmental remediation based on taking advantage of sunlight as the energy source. The number of researchers dedicated to working on the development of this technology has increased exponentially in late years [1]. An important part of this technology relates to dyes which are used to sensitize the semiconductor in DSSCs. The development of new more efficient dyes is part of the research trends to improve this technology [1–3]. Natural pigments represent one of the more important choices to have improved dyes in DSSCs.

DSSCs discovery by Michael Grätzel and Brian O'Regan is dated in 1991 [4]. This device is a photoelectrochemical cell that imitates the photosynthesis process in plants. The cell consists of a semiconductor-based photoanode covered with a dye layer, a summarized functioning of the device is described as follows to understand the dye's role and importance in a DSSC [5, 6]. Dye photoexcitation provides an electron injection into the semiconductor conduction band from the dye LUMO which is caused by energy bands overlap. Next, the oxidized dye is regenerated when an electron is given up from the redox electrolyte. Electrolyte species reduction is completed with the addition of an electron at the platinum-coated transparent conducting oxide (TCO). The remainder of the semiconductor Fermi level and the electrolyte redox potential is equivalent to the open-circuit voltage [7, 8].

The idea of using the reactions of photosynthesis to convert sunlight into electrical power was published in 1974 by Melvin Calvin and became a common technique in solar technology [9–11]. Solar cells started with silicon devices, but technology has advanced, and new materials and devices were created, this progress includes DSSCs as part of an emerging third-generation photovoltaic concept in which stands out the use of synthetic or natural dyes as light-harvesting pigments [5, 6]. DSSCs components require more research and development to reach higher efficiencies [12–14]. Photosensitizers based on natural pigments are more desirable in DSSCs than dyes from metal complexes and may reach similar performances and stability [9]. Our interest within this work relates to natural pigments' electronic structure and will be focused on anthocyanidins.

Selected pigments are among the more commonly found anthocyanidins in nature, six different aglycones or anthocyanidins are included within this work and its common name with its distribution in fruits and vegetables is as follows: cyanidin 50%, pelargonidin 12%, delphinidin 12%, peonidin 12%, petunidin 7%, and malvidin 7% [15–18]. Hydroxyl and methoxy groups differentiate these molecules by the number and position of their B-ring [19–21]. Prior work related to anthocyanidins by our research group has been published elsewhere and includes cyanidin, malvidin, and peonidin [22]. The methodology from such work was reproduced with an upgraded version of the Gaussian program as part of the continuity work by our research group and we developed further work with additional pigments and calculations which were included in the present work. This work is considered a deeper study because all calculations were re-done using Gaussian 16 [23] (prior work was developed with G09), in addition, this work includes three additional pigments which enrich twice the options to make the best choice among the more used anthocyanidins for photocatalysis applications. The new calculations not included in the prior work were developed using Cis-TDDFT which enabled an analysis/discussion of emission data and the respective spectra. After increasing to six anthocyanidins in the study, the lower value for gap energy is malvidin in its gas phase and remains as a general behavior that with the addition of solvents gap energy increases in all cases except for malvidin with n-hexane because it had the narrower gap followed by petunidin also with n-hexane. For charge transfer, based on conceptual DFT results, cyanidin, malvidin, and pelargonidin present the best results. Water as solvent followed by ethanol and methanol applied in cyanidin displayed the lower values for electron reorganization energy (λe). Also, TDDFT calculations were carried out to calculate absorption properties for each pigment. After increasing the sample from three

molecules up to six, it was again cyanidin, malvidin, and petunidin the pigments with the best performance indices for dye sensitization.
