On the Electrochromic Properties of Borepins: A Computational Prediction

Abstract

The spectroelectrochemical features of some recently synthesized borepins have been predicted herein using the methods based on density functional theory. The computed electronic spectra of neutral, radical anion, and dianion species clearly suggest that these molecules can be used as new electrochromic materials. The excellent agreement with the available structural and absorption experimental data for the neutral systems made us confident for the results obtained for charged species and suggests their potential use as electrochromic materials.

1. Introduction

Electrochromism is a phenomenon related to reversible color change in a material caused by an electrochemically induced oxidation–reduction reaction.¹⁻⁴ It results from the generation of different electronic absorption bands on switching between redox states. Commonly, color changes occur between a transparent state, in which the electrochromic material absorbs only in the UV region, and a colored state as well as between two colored states. If there are more than two redox states corresponding to different absorption bands in the spectrum, it is said that the material possesses multicolor electrochromism or it is named polyelectrochromic. In practice, this color change is achieved by a small potential pulse ranging from fractions of volts to few volts. When the pulse is removed, the electrochromic material persists in its new colored state until a reverse electric pulse is applied or a competitive reaction takes place. Although the term “electrochromic” originates from a color variation, now its meaning has been extended and it is used to indicate a change of electronic absorption bands in any region of the electromagnetic spectrum (visible, near-infrared,^5,6 thermal infrared, and microwave^7,8). These features make electrochromic materials very interesting for commercial applications. Among electrochromic devices, the so-called intelligent windows^3,9−11 are in a prominent position. They allow us to control the flow of light and heat, and they can be used for a variety of applications where the transmission or reflection modulation effect can be used significantly. For example, they can be used for the regulation of incident solar energy and glare for improving the energy efficiency of buildings, vehicles, aircraft, spacecraft, and ships. The electrochromic materials utilized for this type of applications are those for which one of the redox states corresponds to the total transmittance of the solar radiation (bleached state). Instead, materials lacking a transmissive state are useful for applications^3,12−14 where different colors are desired in different redox states.

Polyaromatic hydrocarbons have been the subject of intensive investigations due to their photophysical properties that allow multiple applications as optoelectronic materials.¹⁵ These properties, controlled by the energy of frontier molecular orbitals, can be modulated by varying the size and shape or also incorporating noncarbon main group atoms. For example, the introduction of electron-deficient three-coordinate boron atoms causes a lowering of the lowest unoccupied molecular orbital (LUMO) energy, making the materials, thus obtained, suitable for n-type or ambipolar semiconductor applications.^16,17 Although many studies on the synthesis and characterization of polyaromatic hydrocarbons are present in the literature, those on the analogues containing boron (borepins) have been carried out only in recent years.¹⁷⁻²³ Borepin is an unsaturated seven-membered ring system with tricoordinate boron, that, unlike other heteropins, in its neutral state has an unoccupied p-orbital on boron that together with π and π* orbitals of the heptatriene portion entails an aromatic planar structure.

Very recently, Wagner and co-workers have reported a facile route to attain quadruply benzannulated nonplanar borepins with potential applications as Lewis acid catalysts or optoelectronic materials (compounds a–c in Scheme 1).²⁴ The nonplanarity of such systems, recently reviewed by Messersmith and Tovar²⁵ assessing the aromaticity of borepin rings, helps also their solubility in the absence of solubilizing side chains. Other new borepins containing polycyclic aromatic compounds have been synthesized and characterized with the aim to promote borepin-centered aromaticity.²⁶ In this work, dibenzo[b]thiophene-fused borepins (d and e) with boron–mesityl groups that, differently from the other borepins, entail planarity of the whole structure have been presented.

Scheme 1. Schematic Structures of the Investigated Borepins a–e.

Besides the synthetic procedure, that always remains a critical challenge for large-scale application of a promising product,²⁴⁻²⁸ the modulation of the photophysical properties of compounds supposed to be suitable for optoelectronic applications can be achieved a priori by studying the properties that can help in enhancing their efficiency as electrochromic materials, such as absorption spectra of the reduced species and redox potentials. In this work, we report a theoretical investigation performed at density functional (DFT) and its time-dependent extension (TDDFT) levels of theory on some borepins (see Scheme 1) recently synthesized and characterized^24,26 that show some interesting electrochemical properties. In an effort to help the understanding of their properties, we found it interesting to theoretically predict electronic spectra of the charged species of two different classes of borepins, characterized by planar (molecules d and e) and nonplanar structures (molecules a and c). Furthermore, the spectra of the neutral species have also been described to compare the obtained data with those came out from experiments²⁴ and then to prove the reliability of the protocol used for the reduced species not characterized before. The same level of theory has been then used to simulate the redox potentials to find the most suitable functional able to reproduce such a property.

2. Results and Discussion

As reported above, the two classes of borepins studied here differ in extension of π electrons that in the first case, molecules a–c, returns nonplanar systems, contrariwise to the second one, compounds d and e, for which the high aromatic character of the borepin ring makes the molecules almost completely planar. The optimized structural parameters obtained using three exchange and correlation functionals, which are B3LYP, PBE0, and M06, are listed in Table S1 together with the available X-ray data for system a.²⁴

First, as expected, systems a–c are characterized by nonplanar structures with the mesityl (2,4,6-trimethylphenyl) group arranged orthogonally to the boron-containing plane, regardless of the exchange and correlation functional used. The dihedral angle C₁–C₂–C₃–C₄ (see labels in Scheme 1) assumes values of about 39° for a, in very good agreement with the corresponding crystallographic counterpart (38.9°). In compounds b and c, the extension of the systems entails a slight reduction of such a dihedral angle, returning values of 33 and 31°, respectively. It decreases by about 2° in the radical anion species, labeled as a^–, b^–, and c^–, and by about 4° in the dianion one of compound c, labeled c^2–. Also, the inspection of bond lengths and valence angles in a confirms a good agreement with the X-ray values.²⁴ In all of the systems, the addition of an electron causes only a small variation in the bond distances that, as expected, become a little bit large in the case of bicharged c^2–. Similarly to compounds a–c, the mesityl group in d and e assumes an orthogonal orientation with respect to the borepin plane. However, due to the conjugation, dihedral angles φ and θ assume values very close to the planarity in both neutral and anionic forms (see Table S1).

The computed vertical excitation energies for neutral and anionic systems are reported in Table 1. The comparison between the calculated and the experimental absorption spectra, possible only for the neutral molecules,²⁴ supports the reliability of the used computational tool. The average errors for B3LYP, M06, and PBE0 functionals in overestimation of absorption bands are about 17, 7, and 3 nm, respectively, with PBE0 that returns values closest to the experimental ones for the first class of compounds. However, for compounds d and e, all of the exchange and correlation functionals essentially underestimate the main absorption bands,²⁶ with average errors of 16, 24, and 27 nm, respectively.

Table 1. Main Vertical Singlet Electronic Energies (ΔE, eV), Wavelengths (λ_theo, nm) and Oscillator Strengths (f) Transitions for Borepins in Cyclohexane and Chloroform Environments for a−c and d−e Compounds, Respectively, Computed at the B3LYP/6-31+G*, M06/6-31+G* (in Brackets) and PBE0/6- 31+G* (in Square Brackets) Levels of Theory.

species	state	λtheo	ΔE	f	transitions	λexp
a	S1	430	2.89	0.287	H → L, 98%	408a
		(416)	(2.98)	(0.312)
		[412]	[3.01]	[0.306]
	S2	317	3.91	0.083	H → L + 2, 87%	303a
		(313)	(3.96)	(0.060)
		[308]	[4.03]	[0.079]
a–	S1	937	1.32	0.150	Hα → L, 100%
		(971)	(1.28)	(0.153)
		[909]	[1.36]	[0.154]
	S2	501	2.47	0.209	Hβ → L, 100%
		(507)	(2.44)	(0.184)
		[497]	[2.49]	[0.185]
b	S1	460	2.69	0.344	H → L, 97%	444a
		(445)	(2.78)	(0.383)
		[443]	[2.80]	[0.367]
	S2	347	3.57	0.138	H → L + 1, 82%	321a
		(344)	(3.61)	(0.143)
		[337]	[3.67]	[0.174]
b–	S1	939	1.32	0.174	Hα → L + 1, 100%
		(979)	(1.27)	(0.179)
		[909]	[1.36]	[0.181]
	S2	520	2.38	0.222	Hβ → L, 100%
		(527)	(2.35)	(0.115)
		[515]	[2.40]	[0.187]
c	S1	462	2.68	0.428	H-2 → L, 91%	444a
		(450)	(2.75)	(0.446)
		[444]	[2.79]	[0.453]
	S2	360	3.44	0.115	H-6 → L, 70%	353a
		(348)	(3.56)	(0.204)
		[345]	[3.60]	[0.184]
c–	S1	955	1.30	0.163	Hα → L, 100%
		(992)	(1.25)	(0.183)
		[939]	[1.32]	[0.183]
	S2	590	2.10	0.339	H-1β → L, 100%
		(596)	(2.08)	(0.303)
		[583]	[2.13]	[0.328]
c2–	S1	691	1.79	0.683	H → L, 99%
		(702)	(1.77)	(0.588)
		[679]	[1.82]	[0.704]
	S2	461	2.69	0.076	H-1 → L, 64%
		(456)	(2.72)	(0.067)
		[446]	[2.78]	[0.085]
d	S1	381	3.25	0.641	H → L, 98%	417b
		(378)	(3.28)	(0.634)
		[369]	[3.36]	[0.653]
	S2	311	3.98	0.499	H-1 → L + 1, 94%	303b
		(301)	(4.11)	(0.534)
		[301]	[4.12]	[0.565]
d–	S1	626	1.98	0.162	Hα → L + 1, 100%
		(628)	(1.97)	(0.057)
		[598]	[2.07]	[0.169]
	S2	469	2.64	0.490	Hβ → L, 100%
		(481)	(2.57)	(0.276)
		[459]	[2.70]	[0.488]
e	S1	399	3.10	0.087	H → L, 93%	437b
		(387)	(3.20)	(0.098)
		[388]	[3.19]	[0.097]
	S2	350	3.51	0.1412	H → L + 1, 85%
		(351)	(3.53)	(0.130)
		[342]	[3.62]	[0.153]
	S3	328	3.77	0.249	H-3 → L, 66%
		(320)	(3.86)	(0.245)
		[315]	[3.92]	[3.64]
	S4	316	3.92	0.973	H-4 → L, 78%	312b
		(309)	(4.01)	(0.654)
		[305]	[4.06]	[0.913]
e–	S1	763	1.62	0.234	Hα → L + 1, 100%
		(745)	(1.66)	(0.238)
		[713]	[1.74]	[0.270]
	S2	638	1.94	0.149	Hα → L + 5, 100%
		(626)	(1.98)	(0.139)
		[600]	[2.07]	[0.120]

^aRef (24).

^bRef (26).

The analysis of the excitation energies shows, as for the neutral systems, that the S₁ state is always originated by a HOMO → LUMO transition (over 90%) except for compound c, for which the transition occurs between HOMO – 2 and LUMO. The orbitals’ composition and energy gaps are shown in Figure 1, from which emerges that the HOMO–LUMO energy gap decreases in going from a to c with a more consistent reduction in c, whereas it is found to be the same for compounds d and e (3.70 eV), whose value falls between those of compounds a and b. In compound c, the HOMO – 2 → LUMO gap represents the smallest energy gap originating the first excited state, consistent with the predicted excitation energies. Our computations confirm that the lowest empty orbital is that of compound c, as previously underlined.²⁴

Figure 1.

Graphical representation of selected molecular orbitals for the studied neutral compounds a–e.

The recorded cyclic voltammograms clearly reveal a reversible redox event at E_1/2 (with respect to the FcH/FcH⁺ value of 4.8 eV) equal to −2.20, −2.38, −1.49, −2.14, and −2.30 V for a–e, respectively.^24,26 In addition, for system c, a second reversible reduction occurs at E_1/2 = −1.84 V. We have computed these reduction potentials, and the results (see Table 2) for all of the employed functionals are quite satisfactory, with the M06 functional giving values closest to the experimental counterparts²⁴ with an average error of about 0.13 V in underestimating the potential.

Table 2. Reduction Potentials (V) for Studied Compounds a–e in Tetrahydrofuran Solvent Computed Employing B3LYP, M06, and PBE0 Exchange–Correlation Functionals.

		EredI/V		EredII/V
compd	functional	calc	exp	calc	exp
	B3LYP	–2.85
a	M06	–2.35	–2.20a
	PBE0	–2.74
	B3LYP	–3.00
b	M06	–2.53	–2.38a
	PBE0	–2.89
	B3LYP	–2.08		–2.52
c	M06	–1.60	–1.49a	–2.00	–1.84a
	PBE0	–1.94		–2.41
	B3LYP	–2.74
d	M06	–2.24	–2.14b
	PBE0	–2.62
	B3LYP	–2.93
e	M06	–2.46	–2.30b
	PBE0	–2.81

^aRef (24).

^bRef (26).

The first reduction process leads to the formation of radical anion species whose stability depends on the ability of the compound to delocalize the unpaired electron on the whole system. Looking at the spin density distributions reported in Figure 2, it can be noted that a part is localized on the boron atom (which is an electron-deficient center), whereas the rest is distributed on almost the whole molecular structure in all of the investigated borepins. This behavior can also be seen considering the maps of the molecular electrostatic potential, which allow us to easily visualize what happens after electron addition. Figure 3 evidences how the negative charge distribution (red region) involves almost the whole structure in a, b, and c for the first reduction. As already seen for the spin density distribution, the same occurs for the second reduction of c.

Figure 2.

Spin density contour plot for radical anions of the investigated borepins.

Figure 3.

Maps of the molecular electrostatic potential for neutral and charged forms of all of the investigated borepins a–e. Electrostatic potentials have been obtained with a color scale going from −0.14 (red) to 0.14 au (blue).

As it can be deduced from the data reported in Table 1, the monoreduced systems show very interesting absorption properties (see also Figure 4). All of the employed functionals give the same trends and indicate that the presence of an extra electron strongly red-shifts the two absorption bands of the neutral species. The highest perturbation concerns the S₁ excitation, which, at the PBE0 level of theory, is red-shifted by 497 (a), 466 (b), and 495 (c) nm, while 229 and 325 are the shifts of the first band in the anionic form of d and e, respectively. As previously mentioned, compound c exhibits a further reversible reduction at −1.84 V. Under this potential, the absorption spectra must also be affected and our theoretical results predict red shifts of 235 and 101 nm for the wavelengths associated with S₁ and S₂, respectively. With respect to the first reduction, the second bathochromic shift in c is less evident.

Figure 4.

Comparison between neutral and reduced species of studied compounds a–e for absorption spectra computed at the M06/6-31+G* level.

3. Conclusions

Density functional theory, employing different exchange–correlation functionals, has been used to predict the spectrochemical behavior of three recently synthesized borepins. Results show that the computations give results in good agreement with the experiments, regarding geometries, absorption spectra of the neutral species and reduction potentials. Computations on reduced forms predict interesting optical properties. The monocharged species show a consistent red shift of the two main absorption bands with S₁ excitation falling in the near-infrared region. Also, the doubly reduced species of system c undergoes significant shifts in the red region of the spectrum, supporting its possible use as electrochromic materials. We hope that our predictions can stimulate experimental investigations on the spectrochemical properties of other borepins to propose the optoelectronic materials of this interesting class to be incorporated in electrochromic devices.

4. Computational Details

The electrochromic properties of several molecular systems²⁹⁻³⁵ have been well computed using B3LYP,^36,37 M06,³⁸ and PBE0³⁹ exchange–correlation functionals coupled with the 6-31G(d) basis sets as implemented in the Gaussian 09 code.⁴⁰ Also, for these systems, we chose to use these functionals to evaluate whether the influence of the amount of exact exchange, that is 20% for B3LYP, 25% for PBE0, and 27% for M06, could influence the electrochromic properties. The molecular geometries have been optimized without symmetry constraints, and the frequency calculations at the same level of theory have been done to properly establish the nature of minima for the structures found. Radical anions have been treated at the unrestricted Kohn–Sham level, and no spin contamination has been found, the ⟨S²⟩ value being equal to 0.77. The electronic spectra of neutral and charged species have been obtained employing the time-dependent density functional linear response theory (TDDFT) considering the lowest twenty-five vertical excitation energies on the geometries previously optimized at the same level of theory and employing the 6-31+G(d) basis set. Solvent effects, cyclohexane (ε = 2.02) for compounds a–c and chloroform (ε = 4.71) for d and e, have been evaluated using the nonequilibrium implementation⁴¹ of the polarizable continuum model.⁴² The choice of the medium has been dictated by the presence in literature of the experimental spectra for the neutral species.^24,26

Redox potentials, computed with respect to ferrocene (E(Fc⁺/Fc) = 4.63, 5.08, and 5.07 V at B3LYP/6-31G*, M06/6-31G*, and PBE0/6-31G* levels of theory, respectively), have been obtained following the Nernst equation⁴³

where F is the Faraday constant, n is the number of electrons transferred in the reaction and Δ(ΔG_s)^redox is the free energy change calculated on the basis of the Born–Haber thermodynamic cycle as follows

where ΔG_g is the change of standard Gibbs free energy for the reaction in gas phase and ΔG_s(red) and ΔG_s(ox) are standard solvation energies of reduced and oxidized species, respectively, of the given redox reaction.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b01288.

• Main geometrical parameters of all of the species for the investigated systems (PDF)

The authors declare no competing financial interest.