Exploring the optoelectronic properties of a chromene-appended pyrimidone derivative for photovoltaic applications

Abstract In the present study, a chromene-appended pyrimidone derivative (PBA) has been synthesized in order to account for the relationship between chemical structure and charge transport properties. The optical properties of PBA were studied in different solvents; it displays a weak emission profile in polar protic solvents but is highly emissive in polar aprotic solvents. Quantum chemical approaches on this molecule were performed in detail to highlight the importance of and to better understand the structural and electronic effects of introducing substituted pyrimidone rings in a polyaromatic molecule to support the development of new optoelectronic and photovoltaic devices. We shed light on the frontier molecular orbital, electron injection, electronic coupling constant, light harvesting efficiency, and photophysical properties of PBA by using density functional theory and time domain density functional theory. The absorption spectra (λa) and fluorescence emission spectra (λf) were computed in different solvents (Methanol, Ethanol, Butanol, Hexane, Chloroform and DMF) at the TD-B3LYP/6-31G** and TD-PBE/6-31G** levels of theory, and it was determined that the TD-B3LYP/6-31G** level is more accurate in the reproduction of experimental λa and λf in various solvents. Furthermore, no significant effect was observed on the λa and λf by changing the solvent polarity.


Introduction
Great effort has been made in the design and development of luminescent dyes, owing to their potential exploitation in materials sciences and biological applications [1,2]. Among them, the development of dyes with tunable emissions and large Stokes shifts has received significant attention in recent years [3,4]. Of the several fluorescent organic compounds that are known to date, molecules with a heterocyclic moiety with Sulphur or Nitrogen atoms appended to the main chromophore system have gained popularity as versatile fluorophores owing to easy chemical modifications and tunable optical properties, such as excellent fluorescence quantum yields and high absorption coefficients [5]. Hence, such compounds are extensively used to construct optoelectronic materials such as organic light emitting diodes (OLEDs) and dye sensitized solar cells (DSSCs) [6,7].
Molecular materials appended with pyrimidine or pyrimidine-like structures, including pyrimidines, have received great attention in the fabrication of photovoltaic and photorefractive materials.
Their utility in this application owes to specific photophysical properties that may arise due to the decrease in the HOMO−LUMO energy band gap on incorporation of pyrimidine-like structures in the backbone of the π-conjugated moiety [8]. Further, it has been proved that nitrogen-containing heterocyclic compounds are useful either as π-conjugated linkers between donor and acceptor groups or as electronaccepting TiO 2 anchoring groups into the scaffold of dyes for solar cells [9,10].
In the present study, a chromene-appended pyrimidone derivative (PBA) has been synthesized in order to account for the relationship between chemical structure and charge transport properties. The optical properties of PBA have been studied in different solvents, and the molecule showed a weak emission profile in polar protic solvents but was highly emissive in polar aprotic solvents. The computational investigations on this molecule have been studied in detail to highlight the importance and to better understand the structural and electronic effects of introducing substituted pyrimidone rings in small molecules and polymers in the development of new optoelectronic devices.

Experimental Methodology
PBA was synthesized according to a published method [11]. Briefly, an equimolar mixture of 2-Hydroxy-1naphthaldehyde and Barbituric acid was heated at reflux temperature for 1 h in 50 mL ethanol. After the completion of the reaction, the precipitate obtained was filtered off, and the resulting solid was recrystallized from hot ethanol to yield a bright yellow solid (Scheme 1). IR and NMR confirmed the structure of the compound. 1

Electronic properties
The energy levels of the frontier molecular orbitals (FMO), i.e. the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs), are noteworthy in the determination of various electrochemical, optical and photovoltaic properties. Ground and excited state energies of the FMOs (HOMO and LUMO) are displayed in Table 1. Figure 1 illustrates the charge density delocalization and localization pattern of these FMO. In the ground state, the HOMO is distributed on entire compound while LUMO is on the pyrimidone side. In the excited state, HOMO is distributed at the pyrimidone while LUMO is in the center of the compound. Clear intra-molecular charge transfer has been observed in the ground and excited states (see Figure 1). The dipole of the excited state is higher than that of the ground state, which demonstrates that PBA is more polar in the excited state than in the ground state.

Electron injection barrier
The hole and electron injection energies for PBA were calculated at the B3LYP/6-31G** level of theory. The electron injection energy (EIE) of was calculated by using the following equation (1).
where W is the work function of metal and ELUMO is the LUMO energy. Aluminum (Al) is conductive, easy to manufacture, and inexpensive, and it is easily accessible in sheets of many sizes; therefore, Al (W = -4.08 eV) has been used as an electrode in the present work. The EIE from PBA to the Al electrode was observed at 0.72 eV (=−3.36 − (−4.08)). The hole injection energy (HIE) of PBA is 2.72 eV (=−4.08 − (−6.80)) from PBA to the Al electrode, when the HOMO of the Compound and work function of Al are considered. Moreover, in previous studies, the electron affinity correlated with LUMO energy as EA=-LUMO energy. By considering this relation, it can be found that EA of PBA is 3.36, which is greater than 3, thus revealing that this compound would be more stable when it has electron transfer ability.

Optical properties
The electronic absorption and emission spectra of PBA (20 µM) have been recorded in solvents of varying different polarity and are displayed in Figures 2 and 3. PBA shows distinct broad absorption bands around 428 nm in all solvents studied except in chloroform, where the spectra are more structured. On excitation at the respective absorption maxima, PBA shows an emission band at 475 nm with emission intensity enhancement in polar aprotic solvents such as CHCl 3 and DMF. It was found that the change in absorption and emission maxima in solvents with different polarities was not significant, and only the intensity and spectral shape vary. Moreover, the observed fluorescence quantum yield was significantly lower in non-polar and polar protic solvents, suggesting charge transfer character in the excited state [5]. Moreover, calculated absorption spectra (λ a ) and fluorescence emission spectra (λ f ) and oscillator strengths (f) in different solvents (Methanol, Ethanol, Butanol, Hexane, Chloroform and DMF) at the TD-B3LYP/6-31G** and TD-PBE/6-31G** levels of theory are tabulated in Table 2. It has been found that the TD-B3LYP/6-31G** level is more accurate to reproduce the experimental absorption and emission spectra in various solvents. Additionally, no significant effect has been observed at the λ a and λ f by changing the solvent. Thus, this level of theory has been adopted to shed light on electronic coupling constant, electron injection and light harvesting efficiency.

Open-circuit voltage
The Open-circuit voltage (Voc) is an important photovoltaic parameter that significantly affects solar cell performance. Generally, Voc depends on the materials and some other factors, including light intensity, lightsource, energy levels, device temperature, work functions of the electrodes, external fluorescence effciency, and charge-carrier recombination [25,26]. The relationship between the electronic structure and V OC is still not clear. Previously, the relationship between Voc and the donor (D) HOMO and acceptor (A) LUMO energy gap was studied [27]. Scharber et. al. calculated the Voc by using the equation below (2). Previously, valance band (VB) and conduction band (CB) values for TiO 2 were found to be -7.40 and -4.20 eV, respectively [28]. According to Liu et al. VB and CB of Si are -5.43 and -3.92 eV [29]. (2) By considering the acceptor CB and HOMO of PBA, the Voc value was determined to be 2.3 eV from donor to TiO 2, respectively. When for considering Si as acceptor, the Voc value is 2.58 eV from HOMO of PBA to Si.

Electron coupling constants
The electron injection (ΔG inject. ), electron coupling constants (|V RP |) and light harvesting efficiency (LHE) of PBA were computed at the B3LYP/6-31G** and TD-B3LYP/6-31G** levels of theory and tabulated in Table 3. The ΔGinject of Dyenitro was estimated to be -0.39 and -0.59 by considering the CB of TiO 2 -4.0 and -4.20, respectively. The |V RP | of Dyenitro was estimated to be 0.195 and 0.295, respectively. From Table 3, it can be found that the ΔG inject and |V RP | of PBA are greater than the reference compound. It is expected that in PBA, the short-circuit current density (JSC) value would be higher due to high inject G ∆ and LHE.

Conclusions
In conclusion, a chromene-appended pyrimidone derivative (PBA) has been synthesized, and its optical properties have been studied in detail in different solvents. The observed shift in the absorption and emission spectra were found to be not significant, but the emission profile showed high emission intensity in polar aprotic solvents The computed values for absorption spectra (λ a ), fluorescence spectra (λ f ) and oscillator strengths (f) at TD-B3LYP/6-31G** and TD-PBE/6-31G** level of PBA in different solvents. and low fluorescence quantum yield in non polar and polar aprotic solvents. An intra-molecular charge transfer was observed from occupied to unoccupied molecular orbitals. No significant effect of solvent polarity on the absorption wavelengths was observed at the TD-B3LYP/6-31G** and TD-PBE/6-31G** levels of theory as well. The TD-B3LYP/6-31G** theory was found better to reproduce the experimental absorption and emission spectra. Moreover, LUMO energy, electron injection, LHE, ΔGi nject and |V RP | of PBA showed that it would be good electron transfer material to be used in photovoltaic and optoelectronic devices.