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Selective Complexation of Cyanide and Fluoride Ions with Ammonium Boranes: A Theoretical Study on Sensing Mechanism Involving Intramolecular Charge Transfer and Configurational Changes Haamid Rasool Bhat, and Prakash Chandra Jha J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b00502 • Publication Date (Web): 26 Apr 2017 Downloaded from http://pubs.acs.org on May 3, 2017

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The Journal of Physical Chemistry

1

Selective Complexation of Cyanide and Fluoride Ions with Ammonium Boranes: A Theoretical Study on Sensing Mechanism Involving Intramolecular Charge Transfer and Configurational Changes

Haamid R. Bhat‡ and Prakash C. Jha*† ‡

Computational Chemistry Laboratory, School of Chemical Sciences, Central University of Gujarat, Gandhinagar, India, 382030.

†

Centre for Applied Chemistry, Central University of Gujarat, Gandhinagar, India, 382030.

*Corresponding author. Tel.: +91 886 682 3510 E-mail address: [email protected]

Abstract The anion binding selectivity and the recognition mechanism of two isomeric boranes namely 4[bis(2,4,6-trimethylphenyl)boranyl]-N,N,N-trimethylaniline ( [p − (Mes 2 B)C 6 H 4 (NMe 3 )]+ , 1, where ‘Mes’

represents

mesitylene

and

‘Me’

trimethylphenyl)boranyl]-N,N,N-trimethylaniline

represents

methyl)

and

( [o − (Mes 2 B)C 6 H 4 (NMe 3 )]+ ,

2-[bis(2,4,6has

2)

been

investigated using density functional theory (DFT) and time dependent-density functional theory (TD-DFT) methods. Natural population analysis indicates that the central boron atoms in 1 and 2 are the most active centers for nucleophilic addition of anions. The negative magnitude of free energy changes ( ∆G ) reveals that out of binding of

CN −

CN − , F − , Cl

−

, Br − ,

NO

− 3

and

HSO

− 4

only the

and F − with 1 and 2 is thermodynamically feasible and spontaneous. In addition, CN −

the calculated binding energies reveal that the both with 1 and 2 while as other ions viz

NO

− 3

,

HSO

is showing lesser binding affinity than F − − 4

, Br − and

Cl

−

either don’t bind at all or

show very insignificant binding energy. The first excited states (S1) of 1 and 2 are shown to be the local excited states with π → σ* transition by frontier molecular orbital analysis, whereas fourth excited states (S4) of 4-[bis(2,4,6-trimethylphenyl)boranyl]-N,N,N-trimethylaniline cyanide ( [p−(Mes2B)C6H4 (NMe 3 )] CN, 1CN, the cyano form of 1) and 4-[bis(2,4,6-

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2

trimethylphenyl)boranyl]-N,N,N-trimethylaniline fluoride ( [p − (Mes 2 B)C 6 H 4 (NMe 3 )] F , 1F, the fluoro form of 1) and fifth excited state (S5) of 2-[bis(2,4,6-trimethylphenyl)boranyl]-N,N,Ntrimethylaniline fluoride ( [o − (Mes 2 B)C 6 H 4 (NMe 3 ) ] F , 2F, the fluoro form of 2) are charge separation states which are found to be responsible for the intramolecular charge transfer (ICT) process. The synergistic effect of ICT and partial configuration changes induce fluorescence quenching in 1CN, 1F and 2F after a significant internal conversion (IC) from S4 and S5 to S1. Introduction Cyanide anion ( CN − ) is known to be very deadly poison and hence toxic to human body.1 The paralysis of cellular respiration due to the strong binding between CN − and a heme unit of cytochrome c causes severe damages to the central nervous system.2,3 Since in industry, the versatility of cyanide reagents is obvious in synthesis and metallurgy, the accidental release of CN − into

the environment is thus inevitable4,5. Fluoride ( F − ) being an indispensable element of

human body has an important role to play in the treatment of osteoporosis and dental health protection.6,7 However, the fluoride ingestion in excess causes kidney disorders, urolithiasis and fluorosis in humans and even to their death if taken to a very high levels.8–10 In this connection, the World Health Organization (WHO) has therefore set the maximum permissible limit of cyanide and fluoride in drinking water at 1.9 µM and 1.5 mg/L respectively.11,12 Due to the continuous release of CN − and

F−

to the environment, their accurate quantification is therefore

necessary in the environmental samples. Though analytical methods like voltammetry, potentiometry and chromatography are exploited for successful detection of a very low levels of CN −

and

F−

(< 0.1 µM) but require tiresome sample pretreatment and expensive instrumentation.

Thus developing new methods and designing molecular probes for monitoring of cyanide and fluoride ions in particular and other anions in general has become a primary goal of research.13–15 Due to high selectivity, sensitivity and real-time detection, fluorescence sensing method has been exploited as a tool for detecting anions and metal ions.16–18 Fluorescent sensors usually consist of two parts: the acceptor part and the signal part. The acceptor part differentiates target analytes and the signal part converts the binding event into fluorescence signals which are either turned on or off depending upon the molecular structure. Out of several strategies utilized so far like hydrogen bonding,19–22 nucleophilic addition 23–26 and anion affinity to metal complexes,27,28 one based on chemical reaction of molecular fluorescent probe attracts attention because of its


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high selectivity and sensitivity. The complete descriptions for sensing mechanism of most of the investigations are still missing as their focus is on the new fluorescent chemosensors synthesis and surmising the sensing mechanism. However, understanding the sensing mechanism of fluorescent chemosensors is highly significant and crucial as far as their design and application in human health and environmental protection is concerned. For the designing of cyanide and other anion fluorescent chemosensors, various sensing mechanisms viz intramolecular charge transfer (ICT),29,30 excited state proton transfer (ESPT),21,30,31 photoinduced electron transfer (PET)32–34 etc. have been proposed. To explore the anion binding and its sensing process, different spectroscopic techniques, like proton nuclear magnetic resonance (1H NMR) spectra, time-resolved absorption spectra and time-resolved fluorescence spectra are being utilized, but only the indirect information regarding photophysical properties and geometries is provided. The density functional theory and time-dependent density functional theory (DFT/TDDFT) methods can be exploited to comprehend in detail the anion binding and the sensing process, as DFT/TDDFT methods are proved to be effective for the study of anion binding, ICT and other sensing mechanisms of anion fluorescent chemosensors.35– 38

The possibility of binding of cyanide to water stable triarylboranes has gone unnoticed for a long irrespective of the observed precipitation of the cesium ions by [Ph 3 BCN]− .39 Jakle et al presented that triarylboranes containing polymers can act as probes for cyanide in organic solvents.40 Hudnall et al. reported ammonium boranes 4-[bis(2,4,6-trimethylphenyl)boranyl]N,N,N-trimethylaniline,

(

[p − (Mes2 B)C6 H 4 (NMe3 )]+ ,

trimethylphenyl)boranyl]-N,N,N-trimethylaniline, ( scheme 1, as selective receptors for CN − and

F−

1)

and

2-[bis(2,4,6-

[o − (Mes2 B)C6 H 4 (NMe3 )]+ , 2), shown in

respectively in aqueous solution. Actually in

practice the ammonium borane triflate salts, 4-[bis(2,4,6-trimethylphenyl)boranyl]-N,N,Ntrimethylaniline

triflate

([1]OTf)

and

2-[bis(2,4,6-trimethylphenyl)boranyl]-N,N,N-

trimethylaniline triflate ([2]OTf) upon reaction with sodium cyanide (NaCN) in organic solvent methanol (MeOH) are converted to their corresponding cyanide complexes, 4-[bis(2,4,6trimethylphenyl)boranyl]-N,N,N-trimethylaniline cyanide ( [p − (Mes2 B)C6 H 4 (NMe3 )] CN , 1CN) and

2-[bis(2,4,6-trimethylphenyl)boranyl]-N,N,N-trimethylaniline

[o − (Mes2 B)C6 H 4 (NMe3 )] CN , 2CN) while as

cyanide

(

their fluoride complexes 4-[bis(2,4,6-



4

trimethylphenyl)boranyl]-N,N,N-trimethylaniline fluoride ( [p − (Mes2 B)C6 H 4 (NMe3 )] F , 1F) and 2[bis(2,4,6-trimethylphenyl)boranyl]-N,N,N-trimethylaniline fluoride ( [o − (Mes2 B)C6 H 4 (NMe3 )] F , 2F) are formed when allowed to react with tetrabutylammonium fluoride (TBAF) in organic solvent trichloromethane (CHCl3). These cyanide and fluoride complexes 1CN, 2CN, 1F and 2F are the zwitterions and have been well characterized by NMR spectroscopy and elemental analysis. As always aimed the anion sensing ability of these ammonium boranes is applicable in aqueous medium. The anion binding of 1 and 2 has been checked in aqueous solution ( H2O/DMSO 95:5 vol) where the UV-Vis absorption spectrum of 1 is not affected in the presence of fluoride ions while as the cyanide ion addition leads to absorption quenching. The behavior of 2 is has been observed to be quite opposite to that of 1. It is demonstrated that in the unusual selectivity of these cationic boranes 1 and 2, both steric and electronic effects are found to contribute.41 We carried out theoretical computations over the ground and excited states of the molecules 1, 2 and their various anion complexes to reinforce the experimental observations by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods and to provide some additional insights for molecular sensor designing. The energy changes viz free energy changes and binding energy have been calculated to demonstrate the feasibility of anion binding and their selectivity to 1 and 2. The frontier molecular orbitals, electronic excitation and de-excitation energies and respective oscillator strengths for 1, 2 and their various anion complexes were analyzed and are presented in this contribution.

Scheme 1: Structures of [p − (Mes 2 B)C 6 H 4 (NMe 3 ) ]+ (1) and [o − (Mes 2 B)C 6 H 4 (NMe 3 ) ]+ (2)


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Theory and Computational Details In the present contribution, all our calculations were performed with the Gaussian09 program.42 The simulation of spectroscopic properties was done as follows. The geometrical parameters of ground states (S0) of all the molecules were determined with density functional theory (DFT) and vibrational frequency analysis was carried out to confirm the nature of stationary points (minima) without imposing any constraint. Within the vertical approximation, the electronically excited-state energies on the S0 geometries have been calculated with time dependent-density functional theory (TD-DFT). The geometrical parameters of the first excited states (S1) were determined by utilizing the analytical TD-DFT gradients as implemented in Gaussian09.43,44 The excited state vibrational signatures were also determined by the help of numerical differentiation of the TD-DFT gradients to ascertain the absence of imaginary frequencies. In our calculations, CAM-B3LYP, coulomb-attenuating method based on Becke three-parameter Lee-Yang-Parr functional (65% exchange and 35% correlation weighting at long-range) was employed for DFT and TD-DFT studies. The selection of the functional was done because CAM-B3LYP, a rangeseparated hybrid, is most appropriate for charge transfer (CT) excited states and functional with 50% exact exchange part provide more consistent de-excitation energies than the functional with lesser amounts of exact exchange.45–48 The 6-31G(d) split-valence atomic basis set was used for all the DFT and TD-DFT calculations as it is well documented that 6-31G(d) basis set is well constructed for use in molecular orbital calculations over molecules containing the first and second row elements.49–52 Furthermore, to justify the use of CAM-B3LYP functional for ground and excited state DFT/TD-DFT calculations, a test with a series of functionals ( CAM-B3LYP, M06-2X, B3PW91, HCTH and LSDA) was performed as reported in literature.53,54 The excitation energies and the de-excitation energies attained at M06-2X, B3PW91, HCTH, LSDA/6-31G(d) levels don’t show good agreement with the experimental observations.41 While as these energies show good consistency with the experiment at CAM-B3LYP/6-31G(d) level of theory (see Table 1 and Table 2). In addition, the CAM-B3LYP/6-31G(d) obtained Stoke’s shift for S0 and S1 states are very close to that of the experimentally observed Stoke’s shift (See Table S1 of supporting information, e.g, in case of CAM-B3LYP, difference between the experimental and calculated stoke’s shifts are 57 nm for 1 and 29 nm for 2; while as in case of M06-2X it is 90 nm for 1 and 52 nm for 2, in B3PW91 it is 104 nm for 1 and 41 nm for 2, in HCTH it is 104



6

nm for 1 and 83 nm, in LSDA it is 101 nm for 1 and 45 nm for 2). Therefore, CAM-B3LYP functional was employed and considered appropriate for the DFT/TD-DFT calculations for triarylboranes and their derivatives. We carried out the potential energy surface scanning (PES) for S0 and S1 along the angles with maximum configurational changes in going from the ground to the excited states. This PES further authenticated that the optimized geometries obtained at CAM-B3LYP/6-31G(d) are same as that of the configurations obtained at S0 and S1 local minima on corresponding potential energy surfaces. In all the calculations, the effect of the solvent was accounted for by utilizing the Conductor-like Polarizable Continuum Model (CPCM) as implemented in Gaussian 09 with the dielectric constant of water (ε = 80.1). Natural bond orbital (NBO) method was employed for natural population analysis using NBO 3.1 version of Gaussian 09 package at CAM-B3LYP/6-31G(d) level of theory.55 Free energy changes ( ∆G ) and binding energies ( ∆E ) were calculated for complexes of 1 and 2 with various anions viz CN − , F − , Cl − , Br − , NO 3 − and HSO 4 − . In order to reduce basis set superposition error (BSSE) in these energy

calculations, the Boys-Bernardi scheme was applied to yield the counterpoise corrected energies.56

Results and Discussion Ground State Geometries The ground state optimized structures of 1, 1CN, 2 and 2F, are displayed in Figure 1. The minima of the S0 potential energy curves (Figure 2) show the configurations similar to that of the corresponding excited state optimized geometries for all the molecules (Figure 1). For the sake of convenience, the mesitylene rings attached to boron in all the molecules are assigned the names A and B and the benzene ring containing the trimethylammonium moiety is named as C (Figure 1) and this nomenclature will be followed throughout the discussion. The calculated geometrical parameters of all the optimized structures shown in Figure 1 are accommodated in Tables S2 and S3 of supporting information. The geometrical parameters for 1 and 2 are in close agreement with the reported X-ray data.41 This fine agreement validates the reliability of calculations performed and the use of optimized geometries for further property calculations. In order to check the binding sites for nucleophilic anions in 1 and 2, natural charges were calculated. The natural charges of some selected atoms in 1, 1CN, 2 and 2F are accommodated in Table S4. The highly positive charge on the central boron atoms in 1 and 2


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7

indicate that these are the most active centers for nucleophilic addition of anions. To understand the thermodynamic feasibility of the process of binding of various anions viz CN − , F − , Cl − , Br − , NO3− and HSO4 − with 1 and 2, we calculated the free energy change ∆G about the complexes with 1 and 2 and the fragments 1, 2, CN − , F − , Cl − , Br − , NO3− and HSO4 − following the equation:

∆G = (G complex ) − ( G receptor + G anion )

(1)

The calculated ∆G values are tabulated in Table S7/S5 of the supporting information. The calculated values clearly reveal that only the binding of CN − and F − with both 1 and 2 is thermodynamically feasible and spontaneous. In order to check the anion binding selectivity of 1 and 2, their binding energies have been calculated with the anions following the equation:

∆E = (E complex ) − ( E receptor + E anion )

(2)

The binding energies are accommodated in Table S6 of the supporting information. It is important to mention that the CN − is showing lesser binding affinity than F − both with 1 and 2 while as other ions NO3− , HSO4 − , Br − and Cl − either don’t bind at all or show very insignificant binding energy. The reason behind this is the governing principle of the effect of steric hindrance on the nucleophilicity; more bulky CN − than F − is weaker nucleophile thus shows lesser binding affinity in comparison to fluoride. The non-feasibility of the binding of NO3− , HSO4 − , Br − and Cl −

is also supported by the positive ∆G values shown in Table S5 which indicates that their

binding is not thermodynamically feasible. The electron deficient boron atoms in the S0 state of 1 and 2 get captured by added CN − and F − leading to change of geometry at boron centers. The calculated values of 104.2°, 99.67°, 113.0°, 118.2°, 106.6° and 114.5° for C4 ̶ B1 ̶ C26, C47 ̶ B1 ̶ C26, C27 ̶ B1 ̶ C26, C4 ̶ B1 ̶ C47, C4 ̶ B1 ̶ C27 and C47 ̶ B1 ̶ C27 respectively show the geometrical change at boron center in 1CN. Likewise the geometrical change at boron center in 2F is reflected by the



8

calculated values of 114.5°, 108.9°, 116.2°, 105.7°, 103.7° and 106.8° for C46 ̶ B1 ̶ C26, C46 ̶ B1 ̶ C4, C26 ̶ B1 ̶ C4, F2 ̶ B1 ̶ C46, F2 ̶ B1 ̶ C26 and F2 ̶ B1 ̶ C4. Furthermore, change of hybridization from sp2 to sp3 at the boron center on CN − and F − addition as shown in hybrids and AO% columns of Table S7 further indicates the change in geometry.

Figure 1: Ground state (S0) optimized structures of 1, 1CN, 2 and 2F calculated at CAMB3LYP/6-31G(d) level with the CPCM solvation model. Hydrogen atoms are omitted for clarity. In 1 and 2, the cyanide and the fluoride ions are added at boron atoms with numbering 1. The numbering of atoms of added cyanide is C (26), N (2) and fluoride is F (2). Geometry at boron


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centers in 1 and 2 is trigonal planar while as geometry at boron centers in 1CN and 2F is tetrahedral.



10

Figure 2: Potential energy curves of corresponding S0 states of (I) 1, (III) 1CN, (V) 2 and (VII) 2F; and corresponding S1 states of (II) 1, (IV) 1CN, (VI) 2 and (VIII) 2F calculated at the CAMB3LYP/6-31G(d) level with the CPCM solvation model as functions of the angles mentioned. UV-Vis Absorption Spectra and Molecular Orbital Analysis


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The optimized structures of 1, 2, 1CN and 1F were utilized to calculate their electronic transition energies and the corresponding oscillator strengths. Only the first six absorbing transitions were theoretically calculated for all of the molecules. The comparison of calculated absorption profiles and experimental results is made in Table 1 of the supporting information.41 Table 3 and Table S8 (for functionals other than CAM-B3LYP) accommodates the electronic transition energies and the corresponding oscillator strengths (ƒ) of all the six absorption transitions for all the molecules. Figure 3 displays the comparison of calculated and experimental maximum intensity absorption/excitation energy values in terms of wavelength for 1, 1CN, 2 and 2F. The calculated first singlet transition S0 → S1 for 1 is found at 3.94 eV (314 nm) with an oscillator strength of 0.1545 and S0 → S1 for 1CN is located at 5.27 eV (235 nm) with an oscillator strength of 0.0031. The oscillator strength for S0 → S1 transition in 1CN signifies that there is quenching of S0 → S1 absorption when CN − gets bind to 1 which is in good agreement with the experimental observations. Likewise, in case of 2, the S0 → S1 transition at 4.07 eV (304 nm) undergoes hypsochromic shift to 5.29 eV (234 nm) along with the change in oscillator strength from 0.2017 to 0.0019 after fluoride binding (2F) reflecting the quenching of low lying S0 → S1 absorption transitions as observed experimentally. The frontier molecular orbital (FMOs) and the corresponding orbital energies of 1, 1CN and 2, 2F are displayed in Figure 4 and Figure 5 respectively which shows the most probable and dominant transitions. For 1, the S0 → S1 transition, the lowest energy and thus the most probable transition is assigned to HOMO→LUMO (67.6%). The HOMO is located on A and B attached to the central boron and LUMO is shifted towards the C which reveals that the S0 → S1 transition of 1 is a π → σ* transition. For 1CN, the calculated maximum intensity absorption peak (ƒ = 0.0190) of course less significant is due to the lowest energy fourth singlet transition (S0 → S4) and is assigned to HOMO-1→LUMO (49.6%) and HOMO→LUMO (29.9%). The HOMO-1 is budged at A and B but the LUMO gets shifted completely to C. The FMOs depict that the S0 → S4 transition of 1CN is an intramolecular charge transfer (ICT) considered to be due to the increased electron density after cyanide addition. It is to be mentioned that ICT is often confused with photoinduced electron transfer (PET). PET is well accepted mechanism for “turn-on” chemosensors that fluoresce in presence of analytes only.57–61 These sensors are electron donor-acceptor systems. In case of PET, HOMO of the donor has higher energy than acceptor’s HOMO and can transfer its electron to HOMO of acceptor. This electron transfer process being feasible competes with the



12

radiative decay to the ground state, thus fluorescence quantum yield gets diminished.62 On analyte binding to a sensor, the HOMO of donor lowers in energy than that of acceptor HOMO, thus electron transfer gets stopped and resulting into fluorescence. While as in case of ICT, the case is quite opposite. Hence, it is ICT character in S0 → S4 which induces absorption quenching in 1CN with respect to 1 (see Figure 4). In the case of 2, the lowest energy dominant transition is S0 → S1 which is assigned to HOMO→LUMO (66.2%). The HOMO is localized on A and B while the LUMO is somewhat shifted to C indicating that the S0 → S1 transition of 2 is a π→ߪ ∗ transition. While as for 2F, the calculated maximum intensity absorption transition though less significant (ƒ = 0.0187) is because of the lowest energy fifth singlet transition S0 → S5 and is allocated to HOMO→LUMO (46.7%). The HOMO is confined to A and B and the LUMO is transferred towards C. As in the case of 1CN, the FMOs reveal that S0 → S5 transition in 2F possesses charge transfer character which is due to the increased electron density at boron after fluoride addition. The B atoms attain negative charge (after cyanide and fluoride binding) which gets delocalized via the π-orbital more specifically frontier orbital through charge transfer, reducing the energy of these states. The delocalization takes place from A and B to C. Therefore, the induced absorption quenching in 1CN and 2F is due to the ICT character of less significant S0 → S4 and S0 → S5 transitions respectively and the geometrical changes at boron canters on cyanide and fluoride binding to 1 and 2 respectively. The change in the geometry at boron center in 1 and 2 on CN − and F − binding respectively is all consistent with the changes in the relative energies upon excitation. As the calculated free energy changes (Table S5) and binding energy (Table S6) revealed that F − binds strongly with 1 in comparison to CN − , we performed TD-DFT calculations for 1F. The optimized geometry and some geometrical parameters of 1F are displayed in Figure S1 and Table S9 respectively. The electronic transition energies and the corresponding oscillator strengths (ƒ) of all the six absorption transitions for 1F are accommodated in Table 3. The FMOs related to absorption of 1F are displayed in Figure 4. The calculated first singlet transition S0 → S1 for 1F is found at 5.29 eV (234 nm) with an oscillator strength of 0.0013 reflecting the quenching of S0 → S1 absorption transition. The calculated maximum intensity absorption peak (ƒ = 0.0197) though less significant is due to the lowest energy fourth singlet transition (S0 → S4) and is assigned to HOMO→LUMO (53.2%). The HOMO is budged at the A and B but the LUMO gets shifted completely to C. The FMOs depict that the S0 → S4 transition of 1F


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undergoes intramolecular charge transfer (ICT) considered to be due to the increased electron density after fluoride addition. Hence, ICT character in S0 → S4 induces a hypsochromic-shift in 1F with respect to 1. The importance of ICT in 1CN, 2F and 1F upon excitation lies in dispersing the negative charge after cyanide and fluoride addition to 1 and 2. That is the anion species CN − and F − show a detrimental role in the process of ICT as they enhance the negative charge which need to be dissipated. The ICT process hence comes into force and disperses the attained negative charge. The ICT in 1CN and 2F and 1F leads to the blue-shift in the absorption spectrum in comparison to 1 and 2 and ultimately lead to absorption quenching. So, we can that in the process of ICT in general and in sensing mechanism in particular, anion species have a direct role to play.

Table 1: Calculated Electronic Excitation Energies and Corresponding Oscillator Strengths of the Low-Lying Singlet Excited States of 1, 1CN, 2 and 2F using Different Functionals. molecule

1

1CN

calculated excitation energy in eV (wavelength in nm)a

ƒb

CAM-B3LYP

3.94 (314)

0.154

M06-2X

3.91 (317)

0.144

3.34 (371)

0.110

HCTH

2.98 (416)

0.081

LSDA

2.62 (473)

0.073

CAM-B3LYP

5.27 (235)

0.003

M06-2X

5.29 (234)

0.006

4.54 (273)

0.008

HCTH

3.73 (332)

0.001

LSDA

3.57 (347)

0.002

functional

B3PW91

B3PW91

experimental energy in eV (wavelength in nm)41

3.87 (320)

QUENCHING



Page 14 of 29

14

2

2F

CAM-B3LYP

4.07 (304)

0.202

M06-2X

4.06 (305)

0.171

3.55 (349)

0.149

HCTH

3.09 (400)

0.041

LSDA

2.89 (428)

0.032

CAM-B3LYP

5.29 (234)

0.001

M06-2X

5.32 (233)

0.003

4.66 (266)

0.001

3.85 (322)

0.008

3.71 (334)

0.008

B3PW91

B3PW91

3.86 (321)

QUENCHING

HCTH LSDA a

b

Only the Selected Low-lying Excited States are Presented. Oscillator Strength.

Table 2: Calculated Electronic de-Excitation Energies and Corresponding Oscillator Strengths of the Low-Lying Singlet Excited States of 1, 1CN, 2 and 2F using Different Functionals. experimental energy in eV calculated de-excitation molecule functional ƒb (wavelength in nm) 41 energy (nm)a

1

1CN

CAM-B3LYP

3.13 (395)

0.109

M06-2X

3.39 (365)

0.105

3.06 (405)

0.002

HCTH

1.88 (658)

0.006

LSDA

2.43 (510)

0.008

CAM-B3LYP

3.99 (310)

0.0003

M06-2X

4.11 (301)

0.0017

3.13 (396)

0.0052

HCTH

2.53 (490)

0.0039

LSDA

2.02 (613)

0.0026

B3PW91

B3PW91

2.70 (458)

QUENCHING


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15

2

2F

a

CAM-B3LYP

3.20 (387)

0.159

M06-2X

3.39 (365)

0.134

2.47 (502)

0.002

HCTH

2.08 (595)

0.013

LSDA

2.12 (585)

0.013

CAM-B3LYP

3.86 (321)

0.0006

M06-2X

3.55 (349)

0.0004

3.01 (411)

0.0023

HCTH

3.71 (334)

0.0073

LSDA

2.06 (600)

0.0047

B3PW91

B3PW91

2.86 (433)

QUENCHING

Only the Selected Low-lying Excited States are Presented. b Oscillator Strength.

Table 3: Calculated Electronic Excitation Energies and Corresponding Oscillator Strengths of the Low-Lying Singlet Excited States of 1, 1CN, 1F, 2 and 2F at DFT/CAM-B3LYP/631G(d) Level of Theory. electronic energy in eV molecule ƒb contrib.c CId transitiona (wavelength in nm) ܵ଴ → ܵଵ HOMO→LUMO 0.676 3.94 (314) 0.1545

1

1CN

ܵ଴ → ܵଶ

4.18 (296)

0.1161

HOMO-1→LUMO

0.652

ܵ଴ → ܵଷ

4.44 (279)

0.0443

HOMO-2→LUMO

0.643

ܵ଴ → ܵସ

4.48 (276)

0.0296

HOMO-3→LUMO

0.659

ܵ଴ → ܵହ

5.05 (245)

0.0218

HOMO-4→LUMO

0.481

ܵ଴ → ܵ଺

5.25 (235)

0.0154

HOMO-5→LUMO

0.499

ܵ଴ → ܵଵ

5.27 (235)

0.0031

HOMO-2→LUMO+2

0.252

ܵ଴ → ܵଶ

5.28 (234)

0.0073

HOMO→LUMO+5

0.291

ܵ଴ → ܵଷ

5.39 (229)

0.0078

HOMO-4→LUMO+1

0.409

ܵ଴ → ܵସ

5.51 (224)

0.0190

HOMO-1→LUMO

0.496

HOMO → LUMO

0.299

ܵ଴ → ܵହ

5.66 (218)

0.0024

HOMO→LUMO+2

0.339

ܵ଴ → ܵ଺

5.83 (212)

0.0012

HOMO-1→LUMO+1

0.440



Page 16 of 29

16

1F

2

2F

ܵ଴ → ܵଵ

5.29 (234)

0.0013

HOMO→LUMO+4

0.332

ܵ଴ → ܵଶ

5.32 (233)

0.0027

HOMO-1→LUMO+5

0.375

ܵ଴ → ܵଷ

5.39 (229)

0.0022

HOMO-4→LUMO+1

0.392

ܵ଴ → ܵସ

5.44 (227)

0.0197

HOMO→LUMO

0.532

ܵ଴ → ܵହ

5.69 (218)

0.0014

HOMO→LUMO+1

0.429

ܵ଴ → ܵ଺

5.75 (215)

0.0019

HOMO-1→LUMO+1

0.313

ܵ଴ → ܵଵ

4.08 (304)

0.2017

HOMO→LUMO

0.662

ܵ଴ → ܵଶ

4.26 (290)

0.1465

HOMO-2→LUMO

0.513

ܵ଴ → ܵଷ

4.43 (279)

0.0532

HOMO-1→LUMO

0.531

ܵ଴ → ܵସ

4.51 (274)

0.0380

HOMO-3→LUMO

0.655

ܵ଴ → ܵହ

4.98 (248)

0.0316

HOMO-4→LUMO

0.608

ܵ଴ → ܵ଺

5.28 (234)

0.0164

HOMO-5→LUMO

0.545

ܵ଴ → ܵଵ

5.28 (234)

0.0019

HOMO→LUMO+4

0.397

ܵ଴ → ܵଶ

5.30 (233)

0.0030

HOMO-1→LUMO+5

0.371

ܵ଴ → ܵଷ

5.39 (229)

0.0050

HOMO-4→LUMO

0.488

ܵ଴ → ܵସ

5.73 (216)

0.0060

HOMO-1→LUMO

0.292

ܵ଴ → ܵହ

5.77 (214)

0.0187

HOMO→LUMO

0.467

ܵ଴ → ܵ଺

5.91 (209)

0.0079

HOMO-1→LUMO+3

0.409

a

Only the Selected Low-lying Excited States are Presented. b Oscillator Strength. c Only the Main Configurations are Presented. d The CI Coefficients are in Absolute Values.


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17

Figure 3: Comparison of Experimental41 and Calculated Absorption/Excitation Energies obtained at CAM-B3LYP/6-31G(d) Level using CPCM Solvation Model. (Blue Dash) Calculated Absorption/Excitation Maximum Intensity Transition Wavelengths for (I) 1 (II) 1CN (III) 2 and (IV) 2F; (Green Lines) Experimental UV-Vis Maximum Intensity Absorption/Excitation Peaks for 1 and 2; and (Red Lines) Indicate the Quenching of Experimental UV-Vis Maximum Intensity Absorption Peaks for 1CN and 2F.



18

Figure 4: Calculated FMO Energies for (I) 1 in Ground State and Excited State (1_EXC) (II) 1CN in Ground State and Excited State (1CN_EXC) and (III) 1F in Ground State at CAMB3LYP/6-31G(d) Level using CPCM Solvation Model.

Figure 5: Calculated FMO Energies for (I) 2 in Ground State and Excited State (2_EXC) and (II) 2F in Ground state and Excited State (2F_EXC) at CAM-B3LYP/6-31G(d) Level using CPCM Solvation Model.


Page 18 of 29

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19

Excited State Geometries As followed from analytical science, the light absorption of a dye is less sensitive than its emission.63 Hence, in this contribution we also studied the emission properties for molecules 1, 2, 1CN and 2F to understand the sensing mechanism and to identify the overall ICT process in detail. The first excited state (S1) geometries of 1, 2, 1CN and 2F are illustrated in Figure S2. The minima of the S1 potential energy curves (Figure 2) show the configurations similar to that of the corresponding excited state optimized geometries for all the molecules (Figure S2). Table S2 depicts that there are no obvious spatial configurational differences from ground to excited state for 1 and 2 and maximum changes are 3.3° and 5.8° for angles C45 ̶ C46 ̶ C53 and C45 ̶ B1 ̶ C3 in 1 and 2 respectively. However, a noticeable changes of 10.65° are found in 1CN at C4 ̶ B1 ̶ C27 angle and 10.5° in 2F at C46 ̶ B1 ̶ C26 angle as shown in Table S3. It is well known that twist in excited state configuration can lead to a significant internal conversion, loss of energy and thus results in fluorescence quenching.64,65 Furthermore, as noted that both 1 and 2 show least configurational changes and no ICT process which suggests that the dipole moment of these molecules is not much different in excited state than in their ground state.66–68 While as 1CN and 2F show large configurational changes, significant ICT indicating that their dipole of these molecules is larger in the excited state than in the ground state.66–68 The dipole moment of 1, 1CN, 2 and 2F are accommodated in Table S10 which clearly reflect that the dipole moments of 1CN and 2F undergoing ICT are larger in the excited state than the ground state.

Sensing Mechanism The electron deficient boron atoms of 1 and 2 get captured by added CN − and F − leading to change of geometry at boron centers in 1CN, 1F and 2F. In order to get further understanding of the sensing phenomenon, the emission properties of all the molecules were examined using TDDFT method. The relevant FMOs involved in emission are displayed in Figure 4 and Figure 5 and the de-excitation energies, oscillator strengths and the corresponding transition compositions are accommodated in Table 4 and Table S11 (for functionals other than CAM-B3LYP). Figure 6 displays the comparison of calculated and experimental maximum intensity emission/deexcitation energy values in terms of wavelength for 1, 1CN, 2 and 2F. Following the Kasha’s rule, the fluorescence emission occurs exclusively from the lowest singlet excited state (S1).69



Page 20 of 29

20

Any higher electronic state excitation results in the fast relaxation (10-14 – 10-11 seconds) to the S1 state before the possible emission to the ground state.69 As presented in Figure 7, 1 and 2 quite expectedly do not show similar excited state deactivation processes in comparison to 1CN and 1F which is well predicted by the TDDFT results. For 1 and 2, the excitation process involves the electron excitation to S1, the π → σ* transitions with oscillator strengths 0.1545 and 0.2017 respectively and simple direct dropping back of electron to S0 as shown in Figure 4 and Figure 5. The S1 states being the bright states (as oscillator strengths of 0.1090 and 0.1594 for S1 → S0 transitions in 1 and 2 respectively make the relaxation transitions allowed) undergo radiative decay (fluorescence emission) mirrored by the calculated red-shifted emission wavelength of 3.13 eV (395 nm) and 3.20 eV (387 nm) (Table 4) compared to their absorption wavelength of 3.94 eV (314 nm) and 4.08 eV (304 nm) respectively (Table 3). On the other hand, 1CN and 2F involve the excitation process to their respective S4 and S5 states, with oscillator strengths 0.0190 and 0.0187 respectively (the less significant charge transfer transitions from A and B to C moieties). Each electron excited to S4 and S5 state in 1CN and 2F respectively descend step by step to S1 through internal conversion (IC), a non-radiative process. The S1 → S0 transitions involving the charge transfer from C to A and B are forbidden with very small oscillator strengths 0.0003 and 0.0006 implying that S1 is a dark state in 1CN and 2F. Therefore, S1 decays to S0 via non-radiative process inducing the fluorescence quenching appreciated by intramolecular charge transfer quite in agreement with the less significant absorption transitions.29,70

Table 4: Calculated Electronic De-Excitation Energies and Corresponding Oscillator Strengths of the Low-Lying Singlet Excited States of 1, 1CN, 2 and 2F at CAM-B3LYP/6-31G(d) Level. molecule

electronic deexcitationa

energy in eV (wavelength in nm)

ƒb

contrib.c

CId

1

ܵ଴ ← ܵଵ

3.13 (395)

0.1090

HOMO←LUMO

0.677

1CN

ܵ଴ ← ܵଵ

3.99 (310)

0.0003

HOMO←LUMO

0.698

2

ܵ଴ ← ܵଵ

3.20 (387)

0.1594

HOMO←LUMO

0.682

2F

ܵ଴ ← ܵଵ

3.85 (321)

0.0006

HOMO←LUMO

0.502

a

Only the Selected Low-lying Excited States are Presented. b Oscillator Strength. c Only the Main Configurations are Presented. d The CI Coefficients are in Absolute Values.


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21

Figure 6: Comparison of Experimental41 and Calculated Emission/De-excitation Energies obtained at CAM-B3LYP/6-31G(d) Level using CPCM Solvation Model. (Blue Dash) Calculated Emission/De-excitation Maximum Intensity Transition Wavelengths for (I) 1 (II) 1CN (III) 2 and (IV) 2F; (Green Lines) Experimental Maximum Intensity Emission/De-



22

excitation Peaks for 1 and 2; and (Red Lines) Indicate the Quenching of Experimental Maximum Intensity Emission/De-excitation Peaks for 1CN and 2F.

Figure 7: Scheme of the Different Mechanisms of Fluorescence Emission for 1, 1CN, 2 and 2F.

Conclusions We explored the cyanide and fluoride sensing mechanism of 1 and 2 by investigating their ground and excited state photophysical properties with DFT and TDDFT methods. The natural charges indicate that the boron centers are the most active nucleophilic sites. It is demonstrated via theoretical results that all the optimized geometrical parameters in 1 and 2 agree well with the experiment and both show the geometrical changes on cyanide and fluoride binding at boron centers. The negative values of free energy changes ( ∆G ) reveal that among CN − , F − , Cl − , Br − , NO3 − and HSO 4 − , binding process of CN − and F − only is thermodynamically feasible with both

1 and 2. The calculated binding energies infer that the CN − is showing lesser binding affinity than F − both with 1 and 2 while as other ions NO3 − , HSO 4 − , Br − and Cl − either don’t bind or show very insignificant binding energy. The calculated red-shifted emission energies of 1 and 2 compared to the absorption energies show a fine agreement with the reported experimental


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23

results. Furthermore, the first excited states of 1 and 2 are designated as local excited states undergoing π → σ* transition. While as the excited states of 1CN and 2F involved in the most dominant transitions show an evident charge transfer character and some configurational change resulting in the deactivation of these states via non-radiative manner. The processes of ICT and configurational changes stimulate the fluorescence quenching 1 and 2 on cyanide and fluoride binding in a synergistic way. Hence, the different features of the excited states of 1 and 2 in comparison to their cyano and fluoro addition products play a significant role in the sensing mechanism of these chemosensors. In addition, it is concluded that the DFT and TD-DFT calculations accomplished at CAM-B3LYP/6-31G(d)//CAM-B3LYP/6-31G(d) and CAMB3LYP/6-31G(d)//CAM-B3LYP/6-31G(d) levels adequately reproduce the excitation and fluorescence energies respectively with a fine accuracy and it will help in the design of new potential triarylborane based molecules for anion sensing. Supporting Information The excitation and de-excitation energies calculated by employing different functionals; comparison between calculated and experimentally observed Stoke’s shifts; Calculated important geometrical parameters for the fully optimized structures of 1, 1CN, 1F, 2 and 2F in ground state and 1, 1CN, 2 and 2F in excited states and the corresponding X-ray data; NPA charge on some crucial atoms of 1, 1CN, 2 and 2F; calculated ∆G and binding energies of CN − , F − , Cl − , Br − , NO3 − and HSO 4 − with 1 and 2; hybrids of 1, 1CN, 2 and 2F; calculated dipole moments of 1, 2,

1CN and 2F in ground and excited states; ground state optimized structure of 1F; excited state (S1) optimized structures of 1, 1CN, 2 and 2F. Acknowledgements The authors are highly grateful to: (1) Central University Gujarat, Gandhinagar, India for providing basic computational facilities; (2) University Grants Commission (UGC), Govt. of India, for providing financial assistance in the form of Start-Up Grant to P C Jha and non-NET Fellowship to H R Bhat; and (3) UGC-NRC, School of Chemistry, University of Hyderabad, India in arranging a training visit for H R Bhat and providing some computational facilities.



24

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Selective Complexation of Cyanide and Fluoride ... - ACS Publications

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