Structure and Properties of [8]BN-Circulenes: Inorganic Analogues of

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Structure and Properties of [8]BN-Circulenes - Inorganic Analogues of [8]Circulenes Tapas Kar, Steve Scheiner, and Ajit K Roy J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b03929 • Publication Date (Web): 15 Jun 2015 Downloaded from http://pubs.acs.org on June 17, 2015

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The Journal of Physical Chemistry

Structure and Properties of [8]BN-Circulenes - Inorganic Analogues of [8]Circulenes Tapas Kar1*, Steve Scheiner1 and Ajit K. Roy2 1

Department of Chemistry and Biochemistry, Utah State University, Logan, UT 84322, United States 2 Materials and Manufacturing Directorate, Air Force Research Laboratory, Dayton, Ohio 45433, United States

ABSTRACT: Structure and properties of [8]BN-circulenes are calculated using density functional theory (DFT) and time-dependent DFT (TD-DFT). Structurally they are similar to [8]circulenes, but exhibit different electronic and optical properties. For example, carbon circulenes emit in the visible region while BN-circulenes exhibit higher emission energies which fall in the UV-region. Tuning of molecular properties was examined by derivatization of [8]BN-circulenes, analogous to that in tetraoxa- and tetrathioderivatives. Such materials may be used in optical devices and serve as an alternative to organic light emitting devices (OLEDs). Absorption in the UV-region by [8]BNcirculenes suggests that such compounds may be an excellent candidate for UV-light protection.

-------------------*

Corresponding author: Email: [email protected], Fax 1-435-797-7230

ACS Paragon Plus Environment


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INTRODUCTION Since boron and nitrogen bracket carbon in the periodic table, in their numbers of valence electrons, in their electronegativities, and various other properties, the replacement of CC by BN has enriched inorganic chemistry, from simple molecules to more complex nanomaterials. For example, one such interesting molecule is borazine, B3N3H6 1. Owing to its structural resemblance to benzene and their very similar physical properties, borazine is sometimes referred to as inorganic benzene 2-3. Similar to simple hydrocarbons, sp3, sp2 and sp-hybridized pure HnBNHn and various derivatives are also well known 4. Pure BN is primarily found in hexagonal h-BN (α-BN) form that resembles graphite, sphalerite c-BN (βBN) related to cubic diamond, and wurtzite type γ-BN related to hexagonal diamond 5. Similar to carbon nanotubes 6-7, BN-nanotubes 8-10 are also in the forefront of recent research. Although there are strong structural similarities between BN and analogous carbon compounds, their physical, chemical, and electrical properties differ significantly.

For

example, graphite is an excellent host material and is semi-metallic whereas h-BN is an insulator with limited intercalation properties. Although BNNTs are structurally similar to their organic cousin carbon nanotubes (CNTs), they exhibit extraordinary mechanical properties, larger thermal conductivity, higher field emission property, higher resistance towards oxidation, and thermal stability than CNTs 11-12. Attention to the family of circulenes, an interesting class of polycyclic aromatic hydrocarbons (PAHs), has been renewed following the discoveries of fullerenes and graphenes, due to possible applications in nanoscale electronics. In [n]circulenes, a central nsided polygon is completely surrounded and fused by benzenoids. Such compounds are considered as a new class of organic semiconductor and promising materials for organic light emitted diodes (OLEDs). Derivatives of [8]circulene, such as tetraoxa[8]circulene, has been an active field of current research

13-19

due to the change of the saddle-shaped structure of

parent [8]circulene to planar structure (allowing better π-conjugation and less stain on the octagon) of oxa-derivatives. Planarity of the tetraoxa[8]circulene has been preserved by further derivatization where hexagonal rings are added to the peripheral hexagons. DFT studies

13-17

using

B3LYP/6-31G(d) were found reliable in predicting experimental vibrational (IR and Raman)


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and UV-spectra of such π-extended tetraoxa-[8]circulenes. Also, a wide range of hetero[8]circulenes (such as tetrathio and tetraseleno

20

18, 21

, octathio

, tetraseleno-tetrathio

21

,

azatrioxa 22 and diazadioxa[8]circulene 23 have recently been synthesized, and their structure and properties investigated using DFT. Such materials are potential candidates for organic light emitting diodes. For example, some azatrioxa[8]circulenes

22

exhibit blue emission

properties. Since the significance of BN analogues of a wide-range of carbon compounds (simple molecules to nanomaterials) has already been established and such BN-compounds are synthesized, it is expected that BN-circulenes, where all carbon pairs are replaced by BN pairs, would most likely be possible to synthesize and might show some fascinating properties. The present theoretical investigation considers [8]BN-circulene and its teraoxaand tetrathio-derivatives, and compares their structures (ground and excited states) and properties with corresponding carbon [8]circulenes. These molecular models of [8]BNcirculenes may be considered as the building blocks of crystalline nanoscale materials.

METHOD OF CALCULATIONS The B3LYP variant of density functional theory (DFT)

24-25

, was used to include

correlation effects. A double-ζ quality basis set augmented with polarized d-functions and diffuse sp-functions for all heavy atoms (6-31+G*) was used. To verify the effect of basis functions on the structures and properties, a larger triple-ζ quality basis set (6-311+G*) was also used. Geometries were fully optimized using both basis sets followed by vibrational analyses that insure the identification of true minima. Excited state calculations were carried out using the TD-DFT method

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and first singlet (S1) and

triplet (T1) states were optimized to estimate absorption and emission energies. Besides structural and electronic properties, IR characterization was included in this study and theoretical frequencies were scaled by a factor of 0.961

27

. To obtained vertical and

adiabatic ionization energy (IE) and electron affinity (EA), cation-radicals and anionradicals of the studied circulenes were also calculated using unrestricted B3LYP method. Spin-contamination was found to be negligible, indicating all cations and anions are relatively pure doublet states.



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All calculations were performed using the Gaussian-09

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28

code. Initial structures

of circulenes were obtained using Chemcraft 29 software, which was also used to generate figures for geometry and vibrational analyses.

RESULTS AND DISCUSSION [8]circulene ([8]C) 30, tetraoxa[8]circulene (4O[8]C) 31 and tetrathio[8]circulenes (4S[8]C)

20

have been synthesized and their X-ray structural data are available in

literature. Several theoretical research groups reported

14-18, 20, 22, 32

structures and

properties of these [8]circulenes using the B3LYP functional and similar basis sets considered herein and results are in accordance with experimental findings.

This

consistency leads us to believe our method is reliable here as well. For the sake of comparison with [8]BN-circulenes, B3LYP/6-311+G* optimized structures of organic counterparts are also depicted in Figure 1. Extension of the basis set from 6-31+G* (Figure S1) to 6-311+G* has practically no effect on the structures and bond lengths. In general, a change of ±0.01Å in some CC or BN bonds is found. It may be concluded that the 6-31+G* basis set is a good choice. Similar to [8]C, [8]BN-C adopts a saddle-shaped structure with BN distance varying between 1.42 and 1.47 Å, suggesting some double bond character. The central octagon exhibits alternate BN bond lengths (1.45 and 1.47 Å) and the shortest BN distance of 1.42 Å is found at the rim of the peripheral borazinoids. Such differences in BN bonds in different sections are similar to [8]C, where the CC bond at the rims is 1.36 Å. Replacement of alternate borazinoids of [8]BN-C by 5-membered BN-furans resulted in a planar tetraoxo[8]BN-circulene (4O[8]BN-C), as is the case for organic analogue 4O[8]C. Enhancement of BN π-conjugation in oxa-derivative is reflected in BN distances, where BN bonds shorten by 0.02 to 0.05 Å, except the BN bonds at the rims which are stretched by 0.03 Å. Replacement of oxygen by sulfur as in terathio[8]BNcirculene (4S[8]BN-C) changes from flat to puckered structure, different from the corresponding flat 4S[8]C structure. The planar structure exhibits one imaginary frequency but is energetically very close to the nonplanar geometry. Interestingly, besides BN bonds at the rims, all BN bond lengths in the octagon of 4S[8]BN-C are equal to 1.46


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Å.

These results suggest derivatization of [8]BN-circulene can reorganize BN π-

conjugation (on the basis of equal BN distances). In Figure 2 MO energies of [8]circulenes and [8]BN-circulenes are compared. It is clear that in all cases, the HOMO energy is lowered (stabilized) in inorganic circulene compared to their corresponding organic counterpart. This difference between HOMO energies of circulenes and BN-circulenes is narrowed as one goes from pristine circulene (1.65 eV) to oxa- (0.84 eV) and then to thio-derivative (0.29 eV). In contrast, the LUMO energy of BN-circulenes is higher than that of the carbon circulenes; this difference shrinks from 1.84 (pristine) to 1.44 (oxa-) and to 1.21 eV (thio-derivative). Collectively, the gap between HOMO and LUMO energies of circulene is substantially reduced. Thus, it appears the gap between HOMO and LUMO may be tunable by structural and chemical modifications of [8]BN-circulene. Energy barrier for the injection of hole and electron, properties related to OLED, can be estimated from ionization energy (IE) and electron affinity (EA), respectively. A smaller IE indicates easier injection of hole and a larger EA favors electron injection. We have calculated vertical and adiabatic IE and EA. (The former refers to the energy difference between ground state (GS) and the cation/anion at the same GS geometry; the latter allows relaxation of the geometry of the excited state.) These two quantities can be estimated using Koopman’s theorem

33

(commonly used in several theoretical studies on

a wide range of molecules), where negative of HOMO and LUMO energies are IE and EA, respectively. As expected vertical ionization energy IE(v) is slightly higher than adiabatic IE(a), due to relaxation of ionic geometry. This difference between IE(a) and IE(v) is less pronounced in BN-circulenes than circulenes (Table 1). However, Koopman’s theorem underestimates IE significantly. Similarly, the vertical EA is lower than the adiabatic EA(a) energy and Koopman’s EA strongly overestimates this quantity by more than 1 eV. Thus Koopman’s theorem leads to substantial errors for IE and EA calculations in circulene or similar PAH systems. Injection of a hole in the oxa- and thio-derivatives of [8]circulene requires more energy than in the parent as the IE(a) increases by more than 0.8 eV. In contrast, IE(a) of 4O[8]BN-C increases by only 0.13 eV from parent [8]BN-C, and substitution by sulfur lowers the value by 0.70 eV. All carbon circulenes show positive EA indicating electron



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injection stabilizes the anion by more than 1.0 eV. In contrast, anions of all BNcirculenes are less stable or close in energy to the corresponding parent molecule. Thus electron injection is more favorable in [8]circulenes than in BN analogues. In the context of chemical properties, it can be argued that BN circulenes are more difficult to oxidize (loss of electron) and reduce (gain of electron) than carbon circulenes. These findings are in agreement with other BN compounds, such as BN nanotubes. In the last two columns of Table 1 optical band gaps are estimated using difference between HOMO and LUMO energies (most commonly used for molecular model calculations) and from lowest excited state energy using the TD-DFT method. The latter method was found 34-35 to be more reliable for a wide range of molecules. Extension of basis set from double- ζ to triple-ζ quality yields a negligible effect on these quantities; only a deviation of ± 0.04 eV is noted in a few cases. The gap estimated from the TDDFT method is lower for all systems than the HOMO-LUMO energy difference. A value of 2.98 eV of tetraoxa[8]circulene was reported earlier which is slightly higher than the present value of 2.96 eV and that may be due the effect of diffuse functions in the basis set used in present study. The most striking difference between [8]C and [8]BN-C is the change in gap upon derivatization. In the case of carbon circulenes this gap widens by 0.74 and 0.91 eV in oxa- and thio-derivatives respectively, while the gap is contracted by 0.36 and 1.40 eV in BN-circulene. This distinction suggests derivatization of BNcirculene may be a useful option to control the optical gap. Simulated absorption energies of [8]C and [8]BN-C based on the TD-DFT calculations are compared in Figure 3. Experimental spectra of 4O[8]C

13

exhibit two

distinctive electronic transitions at approximately 375 (weak) and 264 nm (very intense). Two additional peaks at 415 and 358 nm were also observed

18

in CH2Cl2 solution. The

theoretical absorption spectra in gas-phase exhibit transitions at 358 (intense), 260 (very intense) and 230 (weak) nm, close to the experimental quantities. Some deviation from experimental results may be due to the effect of solution. The tetrathio[8]circulene shows a strong absorption at 266 nm (in CH2Cl2) and moderate band in the longer-wavelength region, ranging from 320 to 420 nm

20

. Theoretical spectra at 269 and 387 nm (almost

same intensity) of 4S[8]BN-C reiterates the reliability of the method used. Strong


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absorption peaks in the UV-region by [8]BN-circulenes indicate that such compounds may be useful as UV light protectors. Electronic transitions (emission spectra) of BN-circulenes also occur in the UVregion and energies are blue-shifted (also less intense) compared to corresponding carbon circulene. For example, strongest absorption of [8]C of 286 nm shifts to 210 nm in the BN-analogue. The strongest peak at 260 nm of 4O[8]C up-shifts to 229 nm in 4O[8]BNC and 269 nm peak of 4S[8]C moves to higher energy at 233 nm in 4S[8]BN-C. Besides pure BN-circulene, both oxa and thio-derivatives exhibit two peaks which are within 30 nm of each other, whereas peaks are well separated in carbon-circulenes. Transition occurs at higher energy (286 vs. ~260 nm) for oxa- and thio-derivatives of [8]C. However, a reverse trend is noted for [8]BN-C where the strongest peak shifts from 210 to 229 (in oxa-) and to 233 nm in (thio-) derivatives. These results support the idea of tuning absorption energies by derivatizing BN-circulenes. The excited state energies of [8]circulenes and [8]BN-circulenes are compared in Figure 4. In all cases singlet (S1) and triplet states (T1) were fully optimized and energy differences (without vibrational correction) between ground state (S0) with these states show fluorescence (S1 → S0) and phosphorescence (T1 → S0) properties. Available experimental fluorescence spectra show peaks around 470 nm (2.64 eV) and 510 nm (2.40 eV) for derivatives of 4O[8]C 13, and around 464 nm (2.67 eV) with two shoulders at 447 nm (2.77 eV) and 485 nm (2.56 eV) for 4S[8]C

20

. Estimated energies of 2.63

(oxa-) and 2.67 eV (thio-) of [8]C are in excellent agreement with these experimental results. While carbon circulenes emit in the visible region (1.83– 2.67 eV, 670-464 nm), BN-circulenes exhibit higher emission energies which fall in UV-region (4.11 – 5.15 eV, 302 - 241 nm). This suggests such material may be a good source of UV-light. Although oxa- derivatives exhibit slightly higher fluorescence energy than pure [8]BN-circulene, thio-substitution reduces this energy by about 1.0 eV. The same trend is observed for T1 to S0 transition energy (phosphorescence). While S1 to T1 relaxation energies (intersystem crossing) are close to 0.50 eV for [8]circulenes, this quantity increases from 0.10 eV to 0.35 eV in oxa- and to 0.72 eV in thio-[8]BN-circulene. Thus the emission spectra can also be modulated by derivatization of BN-circulenes.



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Infrared spectra of [8]BN-circulene (5a), tetraoxa[8]BN-circulene 5(b) and tetrathio[8]BN-circulene (5c), shown in Figure 5, indicate absorption in the 1500 – 1300 cm-1 region and weak bands in the 2550-2600 cm-1 region for [8]BN-circulenes. The N-H stretching modes around 3500 cm-1 are found to be very weak and may not be a reasonable source for characterization. None of the modes in the 1500 – 1300 cm-1 region of non-planar structures are pure and vibrational analyses indicate a mixture of BN stretching vibration with

Structure and Properties of [8]BN-Circulenes: Inorganic Analogues of

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