Theoretical study on the photophysical properties of boron-fused double helicenes

College of Resource and Environmental Changchun 130118, China Department of Ophthalmology, Second H 132400, China Centre for Advanced Optoelectronic Function UV Light-Emitting Materials and Technolo Normal University, Changchun 130024, Chi † Electronic supplementary information LUMO of compounds 2–6. Fig. S2: molec main electron transitions of compounds isosurfaces involved in the main electro molecular orbitals involved into the m Fig. S5: the calculated second-order NL four functionals (i.e. B3LYP, BHandHLY calculated absorption wavelength (nm) method at the PBE0, B3LYP, Cam-B3LY together with the experimental data. Tab for compound 1 between experiment an excitation energies, oscillator strengths a 1 in the gas phase at the TD-B3LYP/6-31+G Cite this: RSC Adv., 2017, 7, 56543


Introduction
2][3][4][5] Up to now, some organic materials have exhibited broad applications, including organic light emitting diodes, 6 organic eld effect transistors, 7 organic photovoltaic cells, 8 and nonlinear optics. 9The continuous performance improvement strongly depends on the discovery of new materials with desirable properties and the full understanding of the structure and property relationships at the quantum mechanical level.
Compared with inorganic and semiconductor nonlinear optical (NLO) materials, organic NLO materials exhibit a variety of merits, for example, larger NLO response coefficients, higher laser damage thresholds, lower dielectric constants, and faster response times. 10,11Among the various NLO response processes, second-order NLO closely and immediately correlates with the practical application.Based on the extensive studies, it is widely recognized that the two following strategies are the effective ways to enhance the NLO response of organic materials.One is the introduction of asymmetry substitutions (i.e.donor and acceptor) into the p-conjugated skeleton.The other is to use the chiral compounds, which can satisfy not only the requirement of non-centrosymmetric electron structure but also keep the merits of organic materials.6][17][18][19] In particular, the involvement of heteroatoms into helicene and synthesis of helicene with multihelicity have been proved to be an effective way to further improve their performance and modulate the photophysical properties.The long-desirable target, the introduction of boron atom into LUMO of compounds 2-6.Fig. S2: molecular orbital isosurfaces involved in the main electron transitions of compounds 1, 3, 5 and 6.Fig. S3: molecular orbital isosurfaces involved in the main electron transitions of compound 4. Fig. S4: molecular orbitals involved into the main ECD transitions of compound 2. Fig. S5: the calculated second-order NLO response values obtained from the four functionals (i.e.B3LYP, BHandHLYP, CAM-B3LYP and PBE0).Table S1: calculated absorption wavelength (nm) for compound 2 with the TDDFT method at the PBE0, B3LYP, Cam-B3LYP and BHandHLYP level, respectively, together with the experimental data.Table S2: the main concerned bond length for compound 1 between experiment and calculation.Table S3: the calculated excitation energies, oscillator strengths and rotational strengths for compound 1 in the gas phase at the TD-B3LYP/6-31+G(d) level.See DOI: 10.1039/c7ra11476a multihelical p-conjugated skeleton, has been recently synthesized, which exhibits unique three-dimensional brickwork stacking. 20Intriguingly, these compounds possess the bipolar carrier transport characters, which can greatly simplify device manufacturing process.Notably, their photophysical properties and relationship behind the structural characters, and the charge transport property remain elusive especially at the microscopic level, however it is very important to further elevate the performance.2][23][24][25] In this paper, we mainly focus on determining the geometry structures, electron transition properties, chiroptical properties, and electron and hole transport of boron-fused double helicenes, named as compounds 1 and 2, and establishing their structure-property relationship.7][28] As a consequence, the other ve compounds 3-7, including different electron donor/ acceptor units (i.e.NH 2 , NO 2 , tetrathiafulvalene (TTF) and 7,7,8,8-tetracyanoquinodimethane (TCNQ)) and their combinations were designed to probe the charge transfer cooperativity and nd excellent second-order NLO materials.

Computational details
The ground state geometries of the studied compounds were fully optimized at the B3LYP/6-31G(d) level of theory as performed in the Gaussian 09 program package. 29During the geometry optimization, no any symmetry or internal coordination constraints was done.The B3LYP functional is a combination of Becke's three-parameter hybrid exchange functional 30 and the Lee-Yang-Parr 31 correlation functional.All the positive vibrational frequencies indicate that our optimization structures are the local minima.
To ensure the accuracy of our calculation method, four wellaccepted functionals (PBE0, 32 B3LYP, CAM-B3LYP 33 and BHandHLYP 34 ) combined with the polarizable continuum model (PCM) in CH 2 Cl 2 solvent were performed to study the UVvisible absorption spectra of compound 2. The results indicated that PBE0 is the most suitable in reproducing the experimental data (Table S1, ESI †).Therefore, TDDFT-PBE0/6-31+G(d) was selected in the calculation of the electron excitation energies, oscillator strengths, and rotational strengths of the studied compounds.Both length and velocity representations were used to obtain the rotational strengths.It is noted that the velocitygauge representation of the dipole operator is gauge origin independent.Gaussian bandshapes 35 with a bandwidth of 0.20 eV were used to compare the calculated UV-vis/CD spectra with experimental ones.To test the effect of solvent on the UVvis/CD spectra, the polarizable continuum model 36,37 was utilized as implemented in Gaussian 09.According to the experimental condition, dichloromethane (CH 2 Cl 2 ) was treated as the continuous dielectric environment.
Each tensor of second-order NLO response was calculated as implemented in the Gaussian 09 program package, which was used to calculate second-order NLO response coefficients.Here, hyper-Rayleigh scattering (HRS) was used to measure the NLO response.In the case of plane-polarized incident light and observations made perpendicular to the propagation plane without polarization analysis of the scattered beam, the secondorder NLO response that can be extracted from HRS data can be described as: 38,39 b HRS ð0; 0; 0Þ hb ZZZ 2 i and hb XZZ 2 i correspond to the orientational average of the b tensor without assuming Kleinman's conditions. 40Here, we only were concerned with the static rst hyperpolarizability.Therefore, the frequency value in eqn (1) was set to zero.
Here, electron band structure and Projected Density of States (PDOS) calculations were carried out as implemented in the Vienna Ab initio simulation package (VASP). 41Band model was used to describe charge transportation property. 42Perdew-Burke-Emzerhof functional and a plane-wave basis set with an energy cut-off of 400 eV were adopted.

Geometrical and electronic structures
Here, seven boron-fused double helicenes were investigated, as shown in Fig. 1.Compounds 1 and 2 were synthesized and characterized by X-ray crystallography. 20To keep both the nonsymmetric structure and nd the effective way to enhance the second-order NLO response, compounds 3-7, containing different electron-donors or acceptors and their combinations, were designed based on compound 1.The geometric structures of the studied compounds were fully relaxed without any symmetry constraints at the B3LYP/6-31G(d) and PBE0/6-31G(d) level of theory (Table S2 †).Here, compound 1 was taken as an example to probe the suitability of our adopted method.Obviously, the resultant structural parameters at B3LYP/6-31G(d) level are in good agreement with experimental ones, indicating that functional B3LYP can reliably determine the geometry structures of the studied compounds.This can be further conrmed by the same average deviations of the bond lengths and the dihedral angles, obtained by functionals B3LYP and PBE0.On the other hand, the calculated vibrational frequencies of the considered compounds are all real, implying that the optimized structures are local minimum.
It is well-known that the distribution of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), and the energy gap between HOMO and LUMO play an important role in determining the electronic properties.As a consequence, we rstly examined the character of HOMO and LUMO and the variations of energy gap of the studied compounds.Their energy level contour plots and energy gaps were shown in Fig. 2.
The studied compounds exhibit a wide range of energy gaps from 2.09 to 3.40 eV.This indicates that different substituents of electron-donor or electron-acceptor and the combination have great effects on modulating energy gap.To be specic, compared to compound 1, tert-butyl ( t Bu) and NH 2 groups, acting as the electron donors in compounds 2 and 3, raise the energy levels of both HOMO and LUMO.Notably, both the HOMO and LUMO energy levels of compound 2 are elevated with a similar extent, which results in slightly change of energy gap.However, for compound 3, the increased magnitude of the HOMO energy level is much larger than that of the LUMO.As a consequence, the band gap of compound 3 is signicantly smaller than that of compounds 1 and 2. Intriguingly, compound 4 with TTF substitution, usually acting as electrondonor, has completely different variable trends in comparison with compounds 2 and 3. Specically, its HOMO energy level is raised, while the LUMO energy level is greatly reduced, leading to the obvious decrease of energy gap.On the other hand, for compounds 5 and 6, in which H groups are substituted with electron-acceptors of NO 2 and TCNQ groups, both the HOMO and LUMO energy levels are decreased simultaneously.Moreover, the rate of decline is almost the same.As a consequence, the band gaps of compounds 5 and 6 are unchanged compared to that of compound 1.As illustrated in Fig. 2, the band gap of compound 7 is the smallest, in which H groups are substituted with two electron donors (TTF) and two electron acceptors (TCNQ), respectively.Based on the above analysis, the effect of the substitutions of electron-donor or electron-acceptor or their combinations on frontier molecular levels are obvious.Therefore, it is expected that the studied compounds might exhibit different electronic properties.
Subsequently, we studied their HOMO and LUMO distributions.For compound 1, both HOMO and LUMO delocalizes over p-system of the benzene rings, whereas the contribution of the boron and oxygen atoms are vanishing small (Fig. 3).For the compounds 2, 3, 5 and 6, the distributions of the HOMO and LUMO are almost unchanged compared to that of compound 1, which can be found in Fig. S1.† Interestingly, for the compound 7, the LUMO is mainly localized at the benzene rings, whereas the HOMO is obviously different, which mainly resides on the one of the TTF groups (Fig. 3).This character is favor of intramolecular charge transfer, as shown in the electron transition analysis.And the HOMO and LUMO distribution of the compound 4 (Fig. S1 †) are similar to those of the compound 7.

UV-vis/CD spectra
4][45][46][47] In general, different basis sets and DFT functionals might inuence the electronic excitation energies and transition properties.Based on the previous studied experiences, the 6-31+G(d) basis set has proved to be enough for organic compounds. 48,49However, the different functionals usually have great effect on electronic excitation energy and the electronic transition properties. 50herefore, the 6-31+G(d) basis set combined with four popular DFT functionals were adopted to calculate the electron absorption wavelength of compound 2, which has been fully characterized by experiment. 20As mentioned in the computational details, TDDFT/PBE0 exhibits the best performance in reproducing the experimental absorption wavelength.In other words, there is just 4 nm difference between calculated and measured values (Table S1 †).Thus, PBE0/6-31+G(d) was used in the following calculations.
According to the calculated data, we simulated the UV-vis/CD spectra of compound 2 by using a Gaussian band shape with a bandwidth of 0.20 eV along with the experimental spectra 20 (Fig. 4).As illustrated in Fig. 4, the experimental UV-vis/CD absorption spectra were also reproduced, which can be used to reliably assign the electron transition properties and chiroptical origin.To probe the solvent effect on the UV-vis/CD spectra, 51,52 we also simulated the spectra obtained in CH 2 Cl 2 solution.The resultant absorption spectra in the gas phase are rather similar to those of in solution, which indicates that the inuence of the solvent could be negligible for this kind of compounds.It is noted that there is also small difference between simulated spectra and experimental ones.For example, there is a double peak at roughly 300 nm in experimental spectra, while it is one peak in our simulation.For compounds 1 and 2, the electron transition properties of the low-energy absorption peaks can be best described as p/p* transition (Fig. 5 and S2 †).It should be noted that the t Bu groups have almost no contribution to these transition orbitals.The electron transition properties of compounds 3, 5 and 6 are similar to those of compounds 1 and 2 as represented in Fig. S3, † indicating that the introduction of NH 2 , NO 2 and TCNQ groups has little effect on the electronic transition properties, whereas has certain inuence on the electron absorption wavelength.For example, the calculated absorption band of compound 3 is obviously red-shied compared to those of compounds 1 and 2 (Table 1).Again, there are some differences of compound 4 with TTF groups.The absorption band of compound 4 arises from HOMOÀ4 / LUMO and HOMOÀ3 / LUMO+2 excitations, involving certain contribution from TTF groups.Finally, the combination of the donors and acceptors greatly alters the electron transition characters.As illustrated in Fig. 5, obvious intramolecular charge transfer has been observed in compound 7, which might lead to a large nonlinear optical response.
In the simulated CD spectra of compound 2, there are four main bands.Among them, two negative bands are located at about 260 and 320 nm.And two positive bands are observed around 280 and 400 nm.Comparison of the experimental 20 and simulated spectra shows that the experimental spectra were well reproduced by our calculated CD ones in the solution.4][55] Besides, the differences between the rotational strengths calculated using the length-and velocity-gauge representation of the electric dipole operator are very small (Table S2 †), which conrms again  Table 1 Computed absorption wavelengths (l in nm), oscillator strengths (f), and major contribution for the studied compounds 1-7, as compared to the experimental data (in parentheses) the suitability of the 6-31+G(d) basis set for CD calculations.To further understand the chiral origin of compound 2, the molecular orbitals involved in the main transitions were shown in Fig. S4.† The observed CD bands mainly result from excitoncoupling of aromatic benzene rings, while there is not any contribution from t Bu groups.This is also in consistent with previous frontier molecular orbital analysis.

Charge transportation property
The studied compound 1, in its crystal form, has a unique packing pattern 20 (i.e. an offset face to face stacking arrangement), which is a benet for the application in organic semiconducting transportation materials.More intriguingly, hole mobility of compound 1 is only slightly larger than electron mobility.Thus, it exhibits ambipolar carrier transportation character.Subsequently, we investigated its charge transportation property from the standpoint of band model.According to band model, the mobility increases with the size of the band width. 56,57The valence band width corresponds to the hole transportation, while the conduction band width is the electron transportation.The calculated electron energy band indicates that crystal 1 is an indirect gap semiconductor with its maximum of valence band at Z point and minimum of conduction band at G point (Fig. 6).The PDOS analysis shows that the valence band mainly comes from the contribution of Op, C-p, and B-p orbitals, whereas to conduction band, C-p orbital does.The calculated width of valence band is comparable to that of conduction band, clearly indicating the ambipolar transportation properties.This observation mainly results from the contribution of Y / A direction, corresponding to intermolecular interaction (i.e. between a pure C 6 ring and another C 6 ring containing the boron and oxygen atoms), as illustrated in Fig. 6.In general, pure intermolecular p-p interaction is mainly responsible for hole transport (e.g.pentacene and rubrene). 58In this case, the involvement of oxygen and born atoms into C 6 ring obviously alters charge transportation property.This might be attributed to strong accepting electronic capabilities of oxygen.Similar observation has been found in phenyl-peruorophenyl (p-p F ) interaction. 59

Second-order NLO property
From the standpoint of the inherent asymmetric structures and large intramolecular charge transfer character, the studied compounds are expected to exhibit excellent second-order NLO response under the external electronic eld.To reliably determine the second-order NLO response, four widely used functionals (i.e.B3LYP, BHandHLYP, CAM-B3LYP and PBE0) were considered.The resultant b HRS values were shown in Fig. S5.† Although there are some differences between second-order NLO response values, the overall trend is consistent.This means that our adopted method can reliably describe the effects on b HRS values of different substituents and design the potential NLO materials.It is noted that following discussion was based on the results obtained from B3LYP/6-31+G(d) level.As shown in Table 2, their b HRS values of the studied compounds ranges from 0.34 to 25.48 Â 10 À33 esu, indicating that appropriate structural modications can effectively enhance the secondorder NLO response.In detail, among the studied compounds 1-4 with various electron-donor substitutions, compound 4 with TTF groups show the largest b HRS values.This means that TTF is the better donor for our studied compounds.For the electron-donor substitutions, TCNQ groups show the better performance.General speaking, the involvement of both donors and acceptors into one compound, at the opposite positions, is an effective way to enhance the second-order NLO response. 60,61As we expected, compound 7, containing TTF and TCNQ groups, shows the largest b HRS values of 25.48 Â 10 À33 esu.Notably, its b HRS values is about 190 times larger than that of the organic urea molecule 62 and 5 times larger than that measured for the highly p-delocalized phenyliminomethylferrocene complex, 63 indicating that this kind of compounds are the potential second-order NLO materials.The large NLO response of compound 7 originates from large intramolecular charge transfer, as discussed in electron structure analysis part.

Conclusions
In summary, we have investigated the electronic structure, chiroptical and linear optical properties, carrier transport, and second-order NLO response of seven boron-fused double helicenes and elucidated structure-property relationships, for the rst time.The experimental electron absorption wavelengths are well reproduced by our calculations.The simulated CD Fig. 6 The band structures and projected density of states (PDOS) of crystal 1. High symmetry k-points in the first Brillouin zone are Z ¼ (0, 0, 1/2), G ¼ (0, 0, 0), Y ¼ (0, 1/2, 0), A ¼ (À1/2, 1/2, 0), B ¼ (À1/2, 0, 0), D ¼ (À1/2, 0, 1/2), E ¼ (À1/2, 1/2, 1/2), C ¼ (0, 1/2, 1/2).spectra are in good agreement with the experimental CD spectra, not only wavelength position but also the relative rotational strength.Its chiral origin mainly results from excitoncoupling between the ortho-fused aromatic rings.The rst hyperpolarizability value of compound 7 is greatly larger than that of phenyliminomethylferrocene complex.The charge transfer from TCNQ to aromatic rings is responsible for the NLO response.The ambipolar carrier transportation character mainly comes from intermolecular interaction (i.e. between a pure C 6 ring and another C 6 ring containing the boron and oxygen atoms).Our work is also important for further designing and optimizing heterohelicenes with better performance.

Fig. 2
Fig. 2 Schematic energy levels of the studied compounds.The lower short line represents the HOMO level, while the upper one is the LUMO level.Fig.3HOMO and LUMO of compounds 1 and 7.

Fig. 4
Fig. 4 Calculated UV-vis (left) and ECD (right) spectra in both gas and solution phases of compound 2 along with experimental ones.

Fig. 5
Fig. 5 Molecular orbital isosurfaces involved in the main electron transitions of compounds 2 and 7.

Table 2
The calculated first hyperpolarizability (b HRS ) values (10 À33 esu) of the studied compounds at the B3LYP/6-31+G(d) level of theory