2.1. Second-Order NLO Properties
Boron compounds that exhibit nonlinear optical properties can generally be found in three forms:
Where boron is in neutral form, it acts as electron acceptor due its Lewis acid character. This is exemplified in Figure 6
for 5-(dimesitylboryl)-5′-(pyrrolidin-1-yl)-2,2′-dithienyl B1
Where boron is in ionic form, it acts as an electron donor, as observed in B2
Where the boron forms an adduct with nitrogen, for instance in a boronated pyridine (Figure 8
), the creation of the B–N bond turns the pyridine from a weak withdrawing unit to a much stronger one, which leads to enhanced intramolecular charge transfer and hence, larger NLO response.
Various three-coordinated and tetracoordinated boron containing materials have been reported in relation to potential NLO properties.
The most characteristic electronic properties of trivalent boron are related to its vacant pz
] (Figure 9
Three-coordinated boron atom is an electron acceptor, it is a Lewis acid, and is isoelectronic to a carbocation; with a trigonal planar geometry, it is electron deficient, which allows an effective π-p conjugation with an adjacent organic π-system. The Lewis acid character of three-coordinated boron atom can be inhibited by steric hindrance.
Tetracoordinated boron acts as an electron-donating substituent and possesses a negative charge and an occupied pz-orbital. Tetracoordinated boron compounds are more stable than the three-coordinated ones, and the modification of the electronic and hindrance effects allows the tuning of their nonlinear optical properties.
The development of functional materials which use boron as the main element started with the work of Williams from the Kodak group [20
]. Kanis et al. [11
] made the first theoretical calculations of the quadratic hyperpolarizabilities of organoboranes in stilbene chromophores, in 1991 using the semiempirical ZINDO calculations.
Yuan, Marder et al. [22
] synthesized air-stable ”push-pull” E-alkenes of the form E-D–CH=CH–B(Mes)2
by hydroboration of π-donor substituted alkynes with dimesitylborane as a π-electron acceptor, obtaining β values of 11 × 10−30
esu for 4-Me2
, and 9.2 × 10−30
esu for 4-H2
In 1991, Lequan et al. [23
] evidenced large changes in dipole moment occurring upon charge transfer transitions through an investigation of the solvatochromic shift in the push-pull derivatives 4-(dimethylamino)biphenyl-4′-yl]dimesitylborane (B4
) and [4-(dimethylamino)-phenylazophenyl-4′-yl]dimesitylborane (B5
). Solvatochromism provides an approach towards β, based on a dominant charge transfer transition accounting for the entire NLO response of the molecule. Under this assumption, the so called resulting βCT
were found to be equal to 37 and 210 × 10−30
esu. (Figure 10
Later on, the β values of these two chromophores were estimated more accurately by use of the EFISH techniques [13
] which allows to get the projection of β along the molecular dipole moment (μ0
) of the chromophores; assuming β parallel to μ0
leads to the actual hyperpolarizability β value of 42 × 10−30
and 72 × 10−30
esu for B5
] were obtained. The sizeable NLO response indicates an excellent withdrawing capability of the dimethyl boron comparable to that of a nitro substituent.
In 1993, Yuan et al. [25
] synthesized “push-pull” organoboranes (E)-Fc–CH=CH–B(Mes)2
, powder SHG signal was observed for the latter compound which crystallized in a non-centrosymetric space group. Evaluation of β by the EFISH technique, resulted in a value of −24 × 10−30
esu, comparable to many classical organic donor-acceptor systems.
In 1996, Branger et al. [26
] synthesized molecules with dimesitylboron as the acceptor group, bithiophene as the unsaturated chain and pyrrolidine-1-yl, dithianylidene and 3-thienyl as donor groups. EFISH measurements revealed β values of 37 × 10−30
esu for B6
and 31 × 10−30
esu for B7
derivatives. The replacement of the biphenyl unsaturated chain by bithiophene improved the dipole moment and the quadratic hyperpolarizabilities of the boron derivatives, despite a lack of planarity suggested at the computational level (Figure 11
In 1997, Branger et al. [27
] synthesized new polyurethanes with high glass transition temperature (Tg
) containing azo-dyes with NLO properties in which dimesityl boron groups are used as electron acceptors (Figure 12
). Having high Tg
polymers leads to the expectation that once aligned at high temperature under strong electric fields, this will result in non-centrosymmetry, and therefore the NLO signal will be maintained at room temperature all over the life time of the device.
In 1998, Lesley et al. [28
] reported on the second-order NLO properties determined by the EFISH technique in a series of neutral pyridyl adducts containing the strong Lewis acids BF3
. This study indicates that there is a good communication in chromophores with coordinated Lewis-acidic boranes, BF3
, acting as efficient electron acceptors. The β values range between 48 × 10−30
esu for B11
to 155 × 10−30
esu for B14
. The β(0) values range between 30 × 10−30
esu to 72.5 × 10−30
esu for the pyridine/BF3
Lewis adducts. The β × μ product is doubled in most cases upon complexation with the strong Lewis acids, due to an increase of the molecular dipole moments (μ) and the second-order NLO coefficients (β), the largest values being realized for the (dimethylamino)stilbazole derivatives. Finally, the NLO response is increased by 50% as compared to the disperse red 1 (DR1) standard reference (Figure 13
Further computational investigations by Su et al. in 2001 [29
] using the quantum chemical AM1/Finite Field method on pyridine, styryl pyridine, and phenylethinyl pyridine/borane adducts were in good agreement with the experimental values determined by Lesley [28
In 2002, Farfán, Santillan, Lacroix et al. [30
] studied ”push-pull” boronates, in which the boron atom is attached in the π-conjugated bridge. They observed that β may be influenced by the molecular geometry in the vicinity of the boron atom. A computational investigation revealed that tuning the hyperpolarizability becomes possible by a controlled rotation of the phenylboronic fragment (Figure 14
In 2002, Entwistle and Marder [31
] reviewed the nonlinear optical properties of three and four-coordinate boron (which have been discussed before in this manuscript), and related compounds and polymers, highlighting the contributions of Williams, Glowgowski, Kaim, Lequan, and Marder groups.
In 2004, Entwistle and Marder [32
] studied β by the EFISH technique for 4,4′(E
-dimethylamino)stilbene, which was found to be 45 × 10−30
esu at 1.907 μm.
In 2006, Yamaguchi and Wakamiya [33
] discussed the characteristic features of the boron element that their group use as a basis of molecular design. This is based on three fundamental features of the boron element: (1) The existence of an empty p-orbital; (2) the Lewis acidity of boron; and (3) the geometric feature of boron.
In 2006, Lamère, Farfán, Lacroix et al. [34
] proposed a molecular switch induced by an electric field applied on a tetracoordinated boron compound, containing two “push-pull” units engineered with free rotation along the boron-carbon bond of a bisboronate structure (Figure 15
). The NLO response was compared with that obtained for the monomeric derivative, by the EFISH technique. While the ground state conformation (the off state) is centrosymmetric and SHG silent, the application of an external electric field gradually aligned the subunits and increases the NLO response of the molecule. The EFISH signal is 1.95 times larger for bisboronate than for its monoborated analogue.
In 2008, Santillan et al. [35
] found that boronates derived from Schiff bases present second (by EFISH technique) and third order NLO properties that could be tuned by modifying the conformations of the aryl fractions around the boron-carbon bonded to a naphthyl fragment (Figure 16
In 2009, Lacroix, Farfán, Santillan et al. [36
] reported on the synthesis of three boronates derived from bidentate imine ligands. The β × μ product recorded by EFISH shows a trend for a general increase of the NLO response after boron complexation (Figure 17
Another synthetic effort undergone during 2015, was the one by Zhang [37
], whose team modified the (Mes)2
) group through substitution of the methyl groups of the boron atom by electron-withdrawing perfluorophenyl (B25
) and 3,5-bis(trifluoromethyl)phenyl (B26
) substituents, to produce the acceptor groups (2,6-Me2
B) and (2,6-Me2
B), respectively (Figure 18
). Zhang found that tuning the donor strength by exchanging the triphenylamine donor for the stronger donor 1,1,7,7-tetramethyljulolidine (B28
) gave further fine control over the optoelectronic properties. Even if the NLO properties of these compounds have not been recorded, they could be considered as good candidates due to their architecture.
In 2017, Ji, Griesbeck and Marder [38
] highlighted recent contributions using BMes2
moieties along with the development of alternative strong B-based π–acceptors, focusing on systems which retain or enhance the air-stability of such species, a property which is most desirable for ease of preparation and handling, and thus for use in electronic or optical devices, as well as other applications.
The above examples indicate that tri-coordinated and tetracoordinated boron compounds are promising materials for different NLO applications. The lack of stability presented by the three-coordinated compounds was surpassed by the introduction of bulky groups such as B(Mes)2 groups.
In this revision we have not addressed the group of compounds based on boron clusters since a comprehensive analysis of their nonlinear properties are out of the scope of this review. The readers are referred to the work published by Gao and Hosmane [39
] and Nuñez and co-workers [40
] for excellent overviews on the topic. Instead we have selected two reports on metallacarboranes that describe the supramolecular structure and its relationship with NLO response.
In 2014, Ma and co-workers [41
] reported a DFT analysis of how the redox-driven, switchable, rotatory movement of a series of nickel metallacarboranes can be regulated through second-order nonlinear optical properties. Figure 19
depicts graphically that the computed properties (axial rotation energy, redox pair potential and second-order hyperpolarizability) are substituent dependent in such a way that when electron donation increases in the substituted boron vertexes, the energy required to rotate the metallacarborane portion decreases, the redox potential decreases also, and the second-order response increases (Table 1
). This result provides a strong rational design for tuning and enhancing NLO properties in these species.
The interesting work by Gassin and collaborators [42
] ponders on the close relationship between interfacial properties of dispersed systems and their potential NLO responsiveness. It is well known that a surfactant is an amphiphilic solute that at certain concentration stabilizes its content in solution by saturating the interphase between two media. Usually, when we refer to this kind of compound we talk about species with two clear portions differentiated by polarity (one portion being hydrophilic and polar, and the other portion of hydrophobic nature), although rare, there is the case of some cobalt metallacarboranes, which in neutral form possess no interfacial properties, but when deprotonated to the anionic form, they behave like surfactants. Knowing the surfactant-like behavior of the cobalt-coordinated dimetallacarborane (COSAN) anion (Figure 20
), Gassin tested different solutions at concentrations of 0 mM, 0.06 M and 5.0 mM. SHG was measured in these solutions, varying the geometric dispositions of the fundamental frequency finding different second-order responses that depend on the orientation. This is indicative of a preferential supramolecular array, in which the COSAN anion molecules form a layer of axially arranged dimetallacarboranes. Adsorption isotherms determined for the COSAN solutions provided further insight on the interfacial arrangement, evidencing that certain axial anionic repulsion was taking place. With all these facts, it was proposed that the driving force for this supramolecular arrangement was the presence of dihydrogen bonds, as depicted in Figure 20
. This study is an example of a somewhat overlooked application of NLO properties: The second-order response as means to elucidate structural problems of fundamental supramolecular chemistry.
2.2. Third-Order NLO Properties
In the first half of 1990, boron compounds with NLO properties were studied carefully and systematically in terms of second-order hyperpolarizability (β), to understand the correlation between electronic and NLO properties.
Concise examples of these discoveries were the electron-acceptor character of trivalent boron due to the presence of an empty p-orbital which, combined with a π-bridged structure having an electron-donating group, such as an amine, formed a dipolar “push-pull” architecture with important electronic communication along the structure [23
], as depicted in Figure 21
The other main design used for boron nonlinear optics resulted in what could be called an electronic umpolung
], since the character of boron was reversed to be used as an electron-donor, rendering it tetravalent by adding an electron-accepting group. The design maintained a π-bridge of varying length [45
], as shown in Figure 21
At the end of the 1990, TPA-responsive organic structures without boron were synthesized using two main designs: (1) A strong dipolar “push-pull” structure (as mentioned before) and (2) a quadrupolar structure to enhance TPA. Examples of these approaches can be found in the study reported by Reinhardt and co-workers [46
] where both architectures were explored; using benzothiazole and substituted diphenylamines as electron-donor moieties, bi-thiophene and fluorene derivatives as π-bridges and a pyridine unit as the electron-acceptor (Figure 22
). The only drawback is the modest σTPA
values which barely surpass 115 GM in contrast with the synthetic difficulty.
In 1996 Yuan et al. [47
], reported the synthesis of a series of symmetric bis(dimesitylboryl) compounds and studied their second order hyperpolarizabilities (γ) by third harmonic generation (THG) at 1.907 μm.
provided a larger γ value (229 × 10−36
esu) than C-18
, which has a very similar architecture (Figure 23
). This is attributed to enhancement of the π-conjugation pathway due to the empty p-orbital on boron.
The enhancement of γ was achieved by increasing the length of the π-conjugation for both symmetric and unsymmetrical molecules. The MeS group was much more efficient than MeO for enhancing γ. Unsymmetrical “push-pull” organoboranes gave rise to large γ values (Figure 24
The γ values in these boron compounds increase dramatically as the π-conjugation length increases. For unsymmetrical compounds, when comparing B47 γ = 24 × 10−36 esu, B48 γ = 32 × 10−36 esu, and B49 γ = 23 × 10−36 esu with B60 γ = 93 × 10−36 esu, B58 γ = 81 × 10−36 esu and B59 γ = 54 × 10−36 esu, the effect of the additional vinyl moiety is clearly greater for the strongest π-donor (NMe2) group.
By enhancing the charge transfer capabilities within the quadrupole architectures, Albota et al. [48
] targeted TPA-efficient compounds by synthesizing molecules having the two different D-π-A-π
-D and A-π-D-π
-A topologies. Following this strategy, they reached σTPA
values in the range of 600–3700 GM (Figure 25
Belfield et al. [49
] synthesized “push-pull” molecules with different donating substituents, that exhibit σTPA
of 650 and 1300 GM (Figure 26
At the beginning of the century, there was renewed interest in the electron-acceptor character of trivalent boron, with a special focus on the dimesitylboron group.
Several organoboron compounds were investigated with the aim of designing new materials with potential application in two-photon excited fluorescence (TPEF). Sensors and imaging are the most targeted application for these materials, although applications in materials science were also envisioned during the early 2000s.
Chujo et al. [50
], synthesized π-conjugated low molecular weight polymers, that exhibited strong photoluminescence. The compounds have a structure related to poly-p
-phenylene vinylene with boron in the polymer chain and were prepared by hydroboration of aromatic and heteroaromatic diynes with mesitylborane. Compound B67
based on diethynylbenzene displayed third-order nonlinear optical susceptibility χ(3)
= 6.87 × 10−6
esu, more than a thousand times larger than that of all-trans
-polyacetylene, as measure by four-wave mixing (Figure 27
Liu et al. reported some of the most remarkable examples of these approaches both at the structural and synthetic level [51
] between 2002 and 2004. These contributions illustrate the transition from the molecular architectures explored during the previous decade that worked to increase boron NLO second-order properties as well as from the general NLO design behind organic dipolar and quadrupolar topologies, all the way towards the fusion of both, to get to NLO, third-order, boron-including chemical structures with potential application due to their fluorescent properties.
Beginning with the dipolar structure, Liu examined different donors, and concluded that dimethylamino and diethylamino share, to a degree, the same donor character while carbazolyl and diphenylamino groups are above in the series. Two different π-bridges were explored to determine whether phenyl or thienyl are capable of delocalizing and connecting in a better way the two ends of the dipolar groups. Interestingly, when there is a dipolar architecture of this kind containing a phenyl, it was possible to reach up to 300 GM of σTPA
for TPA (Figure 28
When considering thiophene as an extension of the π-conjugation, Liu reported that the value of σTPA reached 239 GM, surpassing the properties of carbazole as a donor. This phenomenon proved the weak-donor character of thiophene along with its usual role as π-bridge. Another aspect is that changing the organoboron portion for a somewhat weak donor as benzothiazole to form a quadrupolar structure leads to a larger decrease in σTPA compared to a dipolar architecture.
Nowadays it is considered that multipolar structures will present better σTPA, provided they have an adequate design that allows interaction of the dipoles that leads to a good charge transfer.
The quadrupolar structures depicted in Figure 29
were synthesized and their photophysical properties were measured to compare molecular architectures of the type Donor-π-Donor with systems of the kind Acceptor-π-Acceptor, containing trivalent boron species. Another factor studied was the length of the π-system with one and two styryl units as π-bridge. This study demonstrated that π -conjugation is a key factor for attaining better TPA. In the particular case of the studied molecules, lengthening the π-bridge had a tenfold increase of the σTPA
. This reasoning paved the way to build the molecule depicted in Figure 29
which presented an enhancement of 30 times its σTPA
, compared to the species with one styryl unit as π-bridge.
This architecture provides two somewhat strong dipoles at each end of the molecule (knowing that thiophene works as a weak donor), but the same thienyl portions have a second function to elongate the π-conjugation, combining in a remarkable manner both properties of that heterocycle.
Theoretical studies on this kind of systems (D-π-D and A-π-A motifs) by Tao et al. [54
] provided information on the influence of changing the direction of electron density in quadrupolar architectures, as well as the effect of increasing the length of the π-conjugated system, establishing it as a function of the difference in dipole moments for the two-photon transition alongside charge redistribution within the donor or acceptor portions of the molecules. Having a large difference in dipole moment from the ground state to the excited state leads to a huge contrast in the atomic charges in the diphenylamino or diarylboron portions of the molecules.
In the early 2000s two contributions on the same kind of boron-based ligands but with two different applications were reported. On the one hand, Halik et al. [55
] made use of the 1,3-dioxo ligand as acceptor unit to absorb two photons and through this photophysical process reduce and carry out the deposition of silver. The species used for this purpose, depicted in Figure 30
, showed acceptable σTPA
values, reaching up to 547 GM.
The second series of molecules synthesized through this kind of ligands is that of Cogné-Laage et al. [56
]. Mimicking the structural motif of boron dipyrromethene (BODIPY), a series of diaroyl(methanato)boron complexes were synthesized and their TPA response was examined. According to the authors, the advantage of these complexes is the strength of the B–O bond which is less labile to hydrolysis than the B–N bond in BODIPYs.
The molecular architectures reported feature a quadrupolar design of the A-π-A type. Even though tetravalent boron is considered an electron-donor moiety, its coordination modes entail a strong electron density deficiency on the ligands, especially when connected through a π-bridge to a donor portion. This behavior distinguished organoboron from boron complexes from an electronic and structural standpoint. The molecular assortment proved having moderate NLO properties of 5–85 GM for σTPA
values. Interestingly, one of the synthesized species reached 200 GM, this compound is depicted in Figure 31
In 2006, Yuan et al., [57
] synthesized and studied the nonlinear properties of a series of air-stable, conjugated dimesitylboranes (p
-R-phenyl)dimesitylboranes substituted with various donor and acceptor groups: (E
)-[2-(2-thienyl)]dimesityl-borane and (E
-carboranyl)ethenyl]dimesitylborane. The first and second order molecular hyperpolarizabilities, determined by EFISH at 1.907 μm in CHCl3
and THG measurements, revealed that compounds having strong R-substituent donors have the largest β. Hyperpolarizability values for these compounds were obtained from AM1 calculations with the exception of (E
)-[2-(2-thienyl)]dimesityl-borane and (E
-carboranyl)ethenyl]dimesitylborane, and are in reasonable agreement with those determined experimentally, as well as with several hypothetical compounds containing multiple C=C bonds. The 2-(p
-R-phenyl)ethenyl]- and (p
-R-phenylethynyl)dimesitylboranes showed larger β values than the analogous (p
-R-phenyl)dimesitylboranes. The authors conclude that the increases in β for (E
-R-phenyl)ethenyl]dimesitylboranes compared to (p
-R-phenylethynyl)dimesitylboranes may be due to larger changes in dipole moment in going from the ground to the excited state for the ethenyl derivatives.
THG measurements of γexp
at 1.907 μm showed again that (E
-R-phenyl)ethenyl]-dimesitylboranes and (p
-R-phenylethynyl)dimesitylboranes have much larger γexp
values than the analogous (p
-R-phenyl)dimesitylboranes, as shown in Figure 32
During the second part of the 2000s, Hayek et al. [58
] and Nicoud et al. [60
] introduced a new mode of delocalization of the type π–σ–π by using two boron and nitrogen containing heterocycles, alongside the now well established quadrupolar architecture. The systems used consisted of the 4-membered cyclodiborazane and the 6-membered pyrazabole. In both fragments, B–N sigma bonds are present and the cycles linked to the π-system, render the ring a delocalizing portion as well (as seen in Figure 33
Analysis of the development of boron-based complexes to improve their photophysical properties, lead Hayek et al. to a pioneering, tandem theoretical-experimental approach where computational methods are used to explain atomistically the photophysical properties observed. By portraying the molecular orbitals involved in the main transitions that give origin to the absorption phenomenon, it is possible to visualize how the charge transfer increases in the NLO process for the 4-membered cyclodiborazane, in accordance with the σTPA value of 1350 GM shown by B108.
It is important to mention that the pyrazabole family of compounds has been used in bioimaging of HeLa cells using a concentration of 10−6 M in DMSO, showing clear fluorescent staining of vesicles in the cytoplasm using laser power values up to 15 mW.
A study by Ramos-Ortiz et al. of the NLO response [61
] of a boron-imine complex formed by condensation between aminophenol and dimethylamino cinnamaldehyde (Figure 34
), along with a crystal structure analysis of the packing and supramolecular interactions gave a moderate σTPA
value similar to other reported molecules, which are more structurally complex.
In the last years, the development of TPA responsive boron-based compounds became multidisciplinary, increasing the knowledge of photophysical properties of the synthesized compounds as well as their applications.
It should be pointed out that TPA literature on BODIPY species is not included in the present work since this family of compounds has been extensively studied through the last 20 years.
To focus on the contributions of these last years to the field of TPA-responsive boron-based complexes, we have selected only some advancements of organoboron species in the field of NLO. These compounds have recently been reviewed by Ji and co-workers [38
] providing an excellent overview of the knowledge within the field of trivalent organoboron species in the last 20 years and covering in detail the different applications of this chemical family, including an NLO section.
At the beginning of the decade, the tetravalent quadrupolar architectures were studied again by Li and co-workers [62
] who synthesize a ligand based on a diazo dye. Reduction of the diazo group gave rise to a compound with tautomeric equilibrium, which is favored by the H-bonding present between the pyridine and the di-aza portion of the molecule. Coordination with BF2
to form the boron complex, leads to the compounds depicted in Figure 35
. Although having a moderate σTPA
value of 100 GM, computational calculations revealed that charge transfer was suppressed by coordinating with BF2
due to the fact that the HOMO–LUMO transition for the ligand shows better charge transfer from the pyridine portion to the H-bonded complementary subsystem. This is the main reason why the TPA measurements provide only a modest value for the photophysical property.
Jadhav et al. reinvestigated the pyrazabole core coupling this heterocycle to a series of π-spacers with ferrocene at the end to produce a number of molecules with the D-π-A-π-D architecture [63
From the no-π-spacer species to a set of phenylene derivatives, the photophysical and electrochemical properties were assessed to evidence the potential applicability of the synthesized compounds (as depicted in Figure 36
). The results showed that the compounds with ethinyl-phenyl and the one with diethinyl-phenylene as substituents connecting the pyrazabole and ferrocene portions, presented the best σTPA
values when testing their TPA responsiveness, providing values of 831 and 1016 GM, respectively. This is in accordance with the delocalization pattern present in the molecules. As the π-conjugation increases, the TPA response increases too. In addition, changing the direction of the delocalization affects importantly the magnitude of the σTPA
Finally, it is important to address the contribution of D’Aléo’s group revisiting the hydroxyketone functionality coordinated to a BF2
moiety exploited during the early 2000s by Halik [55
], Cogné-Laage [56
], D’Aléo [64
], Lanoë [65
] and Kamada [66
] in three separate studies.
Using curcuminoid derivatives, D’Aleo was able to tune this substituents into pseudoquadropolar architectures and actual quadrupoles that exhibited a TPA response. Besides having these two electron-donating-substitution differences, the direction of electron density is changed by varying the angle of the two dipoles assembled within the molecules.
The molecules in Figure 37
evidence the effect of different electron-donating groups and how rigidity of the conjugation is the key to reach high σTPA
values. The best results are obtained with nitrogen donors while the efficiency is reduced when using sulfur and finally oxygen. Additionally, a change in symmetry from quadrupolar to dipolar importantly reduces the TPA response.
Another way to affect the TPA response is to change the angle at the quadrupole. The three V-shaped quadrupolar boron complexes depicted in Figure 38
, show decreased values of σTPA
. This is important evidence to consider because when TPA response is the goal, then the quadrupolar and linear molecular architecture with good rigid donors should be chosen. Additionally, having V-shaped compounds with moderate σTPA
values opened the possibility for different applications, for example, greater viability to obtain crystalline solids, which show fluorescent properties. This could lead to new developments in boron-based emitting solids.
These two studies approach the problem of how the electron density distributes, through quantum-level computations and allows to visualize the way charge transfer proceeds via HOMO–LUMO from the ligands to the tetravalent boron moiety of the molecules. They also highlight the importance of an atomistic and electronic point of view that provides a connection between the design, the synthesis, the way the molecules interact electronically and the applications they present.