INFLUENCE OF THE SUBSTITUTION ON THE ELECTRONIC PROPERTIES OF PERYLENE-3,4:9,10-BIS(DICARBOXIMIDES): DENSITY FUNCTIONAL THEORY STUDY

Geometries of the substituted perylene-3,4:9,10-bis(dicarboximides) (PDI) and their radical anions have been optimized at the B3LYP/6-31G** level of theory. The adiabatic and vertical electron affinities have been computed at the B3LYP/6-31+G*//B3LYP/6-31G** level. Substitution of the PDI with COOCF3 reduces the energies of both the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) with the only exception of PDI4 derivative. Calculations predict a decrease in the electron injection barrier in the materials based on the proposed compounds comparing to the parent PDI. Taking into account the calculated electron affinities (EA), the air stability and ambipolar behavior of the materials under study can be expected.


INTRODUCTION
In recent years, organic semiconductors have attracted considerable attention because of their potential for creating low-cost, portable electronic and optoelectronic devices such as organic light emitting diodes (OLEDs), organic field effect transistors (OFETs), flexible displays, and sensors [1][2][3].Some small molecule compounds with a large π-conjugated ring structure have been demonstrated as good semiconductors [4][5][6][7].All of these compounds show good crystallinity.The higher performance found in single crystal OFETs compared to thin-film OFETs is related to the high molecular ordering and the absence of grain boundaries in the crystals [8].Recently, a number of studies have been carried out to improve the performance and to design efficient OFETs materials [9][10][11].
Chang et al. calculated adiabatic EA to explain the air stability and the threshold value was predicted to be ca.2.797 eV [37] at the B3LYP/6-31+G*//B3LYP/6-31G** level.In the present study, our aim is to enhance the air stability.Thus we have substituted strong electron withdrawing group (-COOCF 3 ) at different positions of PDI derivatives (see Figure 1).We have shed light on the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs), energy gaps, and electron affinities (EA).On the basis of EA, we have explained the air stability of new designed PDI derivatives and electron transfer behavior.The influence of position and number of the substituents on the charge transfer and air stability has been also investigated.

RESULTS AND DISCUSSION
Electronic structure (PDI7).On the contrary, the substitution at R5, R6, R7 and R8 positions (PDI4) leads to slightly higher values of E HOMO and E LUMO (by 0.17 and 0.16 eV, respectively).
As can be expected, Table 1 shows that the effect of substitution on the HOMO and LUMO energies depends on the number of electron withdrawing substituents.With the only exception of PDI4, the substitution with electron withdrawing groups at different positions leads to lower HOMO and LUMO energies.From the results presented in Table 1, a general trend in the increase of E HOMO and E LUMO is seen: PDI7 < PDI3 < PDI2 < PDI5 < PDI6 < PDI1 < PDI4.On the other hand, the HOMO-LUMO energy gap (Egap) is almost independent of substitution.The Newman et al. pointed out that the materials with electron affinity (3.0 to 4.0 eV) would be good n-type materials for OFETs due to the efficient electron injection from common gold electrode, ca.5.1 eV and enough ambient stability [44].The work function of aluminum is −5.1 eV and the LUMO energy level of PDI1 is −3.58 eV (see Table 1), the injection energy is around 1.52 eV (= −3.58 − (−5.1)) from the PDI1 to Al electrode.Therefore, it is necessary to lower the LUMO level to enhance the electron injection ability.From Table 1 it can be found that by substitution with COOCF3 at different positions make the LUMO energy levels lower predicting the reduction of the injection barrier, i.e., 1.14, 0.78, 1.19, 1.31, 0.48 eV for PDI2, PDI3, PDI5, PDI6 and PDI7, respectively.

Electron affinities
PDI1 and its derivatives have promising potential for n-type organic semiconductors due to their appropriate EA a .The EA a is actually important when determining air stability of materials.Previous studies have shown that substitution of electron withdrawing groups for small band gap materials is an effective strategy for synthesizing air-stable ambipolar OFET materials [45,46].As we explained above that EA a more than 2.797 eV mostly afford air-stable OFET devices and that those with EA a smaller than 2.797 eV mostly afford air-unstable ones.The results presented in Table 1 and Figure 3 indicates that substituent positions can have different impacts on the EA values of materials.Chen and Bao et al. have reported electrical characterization of a series of N-phenylated PDIs with different number of fluorine atoms on the phenyl substituents.In their study, only the unfluorinated PDIsPh was deemed air-unstable when measured immediately after air exposure [47].
Here we have discussed the EA a at B3LYP/6-31+G*//B3LYP/6-31G** level of theory.By substituting the -COOCF 3 at R1 and R1' (PDI2) leads the EA to 3.46 eV which is 0.68 eV higher than the EA of PDI1.The substitution of -COOCF 3 at R3, R3', R4 and R4' positions (PDI3) further improve the EA a to 3.72 eV.The -COOCF 3 at R5, R6, R7 and R8 positions (PDI4) leads to higher the EA a to 2.89 eV.The -COOCF 3 at R9, R10, R11 and R12 positions (PDI5) escorts the EA a to 3.33 eV.It can be seen than substitution of -COOCF 3 at four positions within the core is more effective one while the substitution at peripheral phenyl rings is not so efficient especially when the substitution at peripheral phenyl is near to the core (PDI4).While the substitution of -COOCF 3 at outer peripheral phenyl hydrogens (PDI5) is proficient.The substitution of -COOCF 3 at R13 and R14 (PDI6) is leading EA a to 3.14 eV.The substitution of -COOCF 3 at R3, R3', R4, R4' R13 and R14 (PDI7) direct EA a 4.09 eV which is the highest one.It is might be due to the reason that four -COOCF 3 groups have been substituted within the core and two groups are at terminal of the peripheral phenyl rings.The EA a values of PDI2-PDI7 meet the needed energy levels for designing air-stable ambipolar OFET materials.The results presented in Table 1 showed that -COOCF 3 group substitution can effectively decrease the injection barrier for electron and therefore it would be potential ambipolar semiconductor materials.Thus before going to synthesize new n-channel materials, it would be better to compute the EA at B3LYP/6-31+G*//B3LYP/6-31G** level of theory.The new designed materials having EA a close or larger than 2.8 eV can be synthesized which would have more probability to be air stable.

CONCLUSIONS
The substitution of PDI with strong electron withdrawing group lowers the E LUMO except for PDI4.The HOMO and LUMO are delocalized on whole of the PDI core, terminal oxygens of the PDI also participate in the formation of HOMO.No participation has been observed in the formation of HOMOs and LUMOs for aryl and electron withdrawing group.The electron injection barrier for PDI2, PDI3, PDI5, PDI6 and PDI7 would be smaller than the parent molecule which revealed higher electron transport.The electron affinity of new designed materials is more than 2.797 eV afford air-stable OFET devices.The EA a values of PDI2-PDI7 meet the needed energy levels for designing air-stable ambipolar OFET materials.Finally, it would be better to compute the EA a before synthesizing new n-channel materials.The new designed materials having EA a close or larger than 2.8 eV can be synthesized which would have more probability to be air stable.

Figure 2 Figure 2 .
Figure 2 displays the HOMOs and LUMOs of compounds under study.The HOMO of the PDI1 is delocalized over the perylene fragment and the oxygens also participate in the formation of the HOMO.The LUMO is also delocalized over entire perylene fragment.The oxygens of the PDI1 also participate in the formation of LUMO.It is seen from Figure 2 that HOMOs and LUMOs of all compounds under study (PDI2-PDI7) are delocalized over the PDI unit and both aryl substituents do not participate in the formation of HOMOs and LUMOs.

Table 1
presents the energies of HOMO (E HOMO ) and LUMO (E LUMO ), HOMO-LUMO energy gap (E g ), and EAs of PDI derivatives.The substitution of PDI1 with COOCF 3 at R1 and R1' positions (PDI2) lowers the E HOMO and E LUMO by 0.34 and 0.38 eV, respectively.The substitution of DPI1 with COOCF3 at R3, R3', R4 and R4' positions (PDI3) lowers the E HOMO and E LUMO by 0.76 and 0.74 eV, respectively.Similar to the previous cases, the substitution of PDI1 at R9, R10, R11 and R12 positions (PDI5) lowers both the E HOMO and E LUMO by 0.33 eV, and by 0.21 and 0.19 eV, respectively, upon substitution at R13 and R14 positions (PDI6).The most pronounced reduction of the energies of the E HOMO and E LUMO (by 1.05 and 1.04 eV, respectively) was predicted upon the PDI1 substitution at positions R3, R3', R4, R4' R13