Understanding the effects of building block rings of π electron-rich organic photocatalysts in CO2 transformation to amino acids

The industrial revolution coincided with excessive fossil fuel consumption by humans which disrupted the CO2 equilibrium in the environment. Thus, CO2 utilization becomes an urgent necessity for human civilization [1]. Any ignorance about irregular carbon dioxide levels will be led to the irreparable destruction of future human life. Thus, researchers in the CO2 conversion field have engaged their facilities to solve the CO2 issue. One of the approaches to CO2 utilization is its transformation to value-added materials such as fuels and chemicals [1]. However, CO2 is thermodynamically a stable molecule, as well as kinetically inert, also its carbon atom has the highest oxidization state [2]. Hence, regarding decreasing the energy barrier of CO2 activation, designing and applying efficient catalyst systems are essential steps in its transformations.

Two overall approaches can be considered for designing catalytic systems of CO2 activation. The first approach involves the catalysts having nucleophile characters such as superbase, N-heterocyclic carbene (NHC) [3,4], N-heterocyclic olefin (NHO) [5], frustrated Lewis pair (FLP) [6,7], and hydroxyl group-containing compounds [8,9]. These catalysts involve the carbon atom of CO2. In the second approach, the oxygen atom of CO2 is affected by the catalysts having Lewis acid orbital such as a transition metal which yields metal–CO2 complex [10].

Inspired by the fundamental role of CO2 as a synthon in natural photosynthesis, nowadays, photocatalytic CO2 conversion has been considered an interesting challenge among the scientific community [[11], [12], [13]]. Saving the input energy in photocatalytic CO2 activations and acquiring the reduced form of CO2 (CO2•–) without any reducing chemical are the main advantages of CO2 transformation with the aid of light. For example, there are various reports about light-driven CO2 reductions into one-carbon resources, such as CO, methane formic acid, and methanol [14,15]. Photocarboxylation is another green and sustainable approach in CO2 conversion to value-added materials in which a typical light-mediated C–C bond formation with CO2 progresses through photocatalytic reactions by forming various reactive intermediates [16].

Metal-based photocatalysts are prevalent compounds in light-driven CO2 transformation, however, due to higher eco-friendly aspects and structural flexibility relative to metal-based catalysts, metal-free approaches in CO2 utilization have been attracted higher consideration among the scientific communities [17,18]. Like the hydrocarbons such as alkanes, alkenes, alkynes, the CO2 molecule does not show any absorption of visible and UV radiations (200–700 nm), thus photocatalytic CO2 transformations normally need a photosensitizer to transform an electron to CO2. Hence, the excitation of a photosensitizer (PS) is a key step in photocatalytic CO2 utilization [16]. Jamison and coworkers reported a photocatalytic synthesis to produce α-amino acids through CO2 coupling with amines. The reaction is progressed via a single-electron reduction of CO2 by p-terphenyl as the organocatalysis [19]. The applied p-terphenyl is an Oligo (p-phenylene) which Matsuoka and coworkers reported as a promising compound in photocatalytic CO2 reduction in 1992 [20]. Afterward, Ma and coworkers showed that the conjugation length of the Oligo (p-phenylenes) (OPP-n) is an effective factor in the band gap values, as well as the optical properties of these compounds [21]. Sharada and coworkers studied the role of substituted OPP-n by Hammett parameter, σp, on the free energy values of the electron transfer to CO2 [22]. They showed that decreasing the σp values causes an increase in the free energy of electron transfers. Herein, based on previous studies [[23], [24], [25], [26], [27], [28], [29]], we focus on the effects of π electrons density and aromatic characters of some Oligo (p-phenylenes) (OPPs) and Polycyclic Aromatic Hydrocarbons (PAHs) as photocatalysts in CO2 utilization. Indeed, we want to investigate the role of these factors on the kinetic behavior of the studied OPPs and PAHs as photosensitizers (PSs) in CO2 reduction. Fig. 1(A) depicts the proposed mechanism of CO2 transformation to amino acids by Jamison and coworkers which has been considered in our investigations, they applied only p-terphenyl molecule as photocatalyst in CO2 conversion to amino acids, but we considered the effects of other OPPs and some PAHs, theoretically, which are illustrated in Fig. 1(B). Among the different constituent rings of OPPs and PAHs, some rings have a more significant function in CO2 reduction. The Anisotropy of the Current-Induced Density (ACID), Nucleus-Independent Chemical Shift (NICS) indices, Electrostatic Surface Potential (ESP) concept, the multi-center bond order (MCBO) and AV1245 index are applied descriptors in justification of observed barrier energies. These tools provide a deep insight into the performance of each ring of OPPs and PAHs in electron transfer steps.

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