Ratiometric Tuning of Luminescence: Interplay between the Locally

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C: Energy Conversion and Storage; Energy and Charge Transport

Ratiometric Tuning of Luminescence: Interplay Between the Locally Excited and Inter-Ligand ChargeTransfer States in Pyrazolate-Based Boron Compounds Deng-Gao Chen, Revathi Ranganathan, Jia-An Lin, ChunYing Huang, Mei-Lin Ho, Yun Chi, and Pi-Tai Chou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11100 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 28, 2019

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The Journal of Physical Chemistry

Ratiometric Tuning of Luminescence: Interplay Between the Locally Excited and Inter-ligand Charge-transfer States in Pyrazolate-Based Boron Compounds Deng-Gao Chen,‡,a Revathi Ranganathan,‡,b Jia-An Lin, ‡,a Chun-Ying Huang,a Mei-Lin Ho*,d Yun Chi*,b,c and Pi-Tai Chou,*,a a b

Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan Department of Chemistry and Frontier Research Center on Fundamental and Applied

Sciences of Matters, National Tsing Hua University, Hsinchu 30013, Taiwan c

Department of Chemistry and Department of Materials Science and Engineering, City

University of Hong Kong, Hong Kong SAR d

Department of Chemistry, Soochow University, Taipei 11102, Taiwan

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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

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Abstract In an aim to access the potential application toward organic light emitting diodes (OLEDs), a series of boron compounds NBN-1 to NBN-5 bearing both 1-isoquinolinyl pyrazolate, phenyl, substituted aryl or fused biphenyl appendage were designed and synthesized. Dual emissions specified as F1 and F2 bands were observed for NBN-1 and 2 in various solvents. The F1 emission features solvent independence and is assigned to the intra-ligand ππ* transition (i.e. LE state) over the isoquinolinyl pyrazolate moiety, while the F2 band shows significant solvatochromism, which originates from the inter-ligand charge transfer (i.e. CT state) from isoquinolinyl to aryl appendages. In comparison, NBN-3 bearing ortho-methyl on the phenyl appendages (cf. para-methyl for NBN-2) shows only F1 (LE) emission. In sharp contrast NBN-4 and NBN-5 with fused biphenyl-like appendage reveal solely the F2 (CT) emission. Comprehensive time-resolved fluorescence measurements, in combination with the computational approach, let us propose the occurrence of a through-space, photoinduced electron transfer (PET) from the LE to CT states. Depending on the characteristic of aryl appendages, the energetics between LE and CT states plays a key role for the locally excited LE versus inter-ligand CT emission. A pre-equilibrium type PET for NBN-1 and 2 and hence dual emissions are observed, whereas the energetically unfavourable PET for NBN-3 leads to the LE emission only. The highly exergonic PET for NBN-4 and NBN-5 renders solely the CT emission. This work thus demonstrates a strategy of facile appendage tuning of boron compounds that can afford both the LE and inter-ligand CT emissions spanning over the entire visible spectral region.

1. Introduction Owing to their potential application toward organic light emitting devices (OLEDs), boron compounds with judiciously designed π-system have received considerable attention, and have been actively utilized as electron conducting materials and/or luminescence emitters in OLEDs.1-9 In comparison to the well-known AlQ3 and related luminescent main group complexes, boron compounds exhibit efficient luminescence while having comparable or even better chemical and photochemical stability.10 Accordingly, it has been ‒2‒ ACS Paragon Plus Environment

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a core interest to modify these boron derivatives with either electron donating or withdrawing groups for adjusting energy gap between HOMO and LUMO, such that the associated photophysical properties can be fined-tuned to achieve the designated luminescence.11-13 To date, similar molecular designs have been successfully employed in preparation of highly efficient, thermally activated delay fluorescent (TADF) emitters, with emission spanning the entire visible spectral region.14-22 Along this line, we have been focusing on the design, preparation and characterization of tetravalent boron compounds bearing either β-diketonate23-24 or pyridyl pyazolate chelates. Furthermore, successful tuning of emission has been achieved by employment of a series of 2-pyridyl, 2-quinolinyl and 2-quinoxalinyl pyrrolide chelates.4 In yet another approach, the obtained boron compounds bearing distinctive 2-pyridyl pyrazolate chelates exhibit remarkable dual emission,11 among which the higher energy, normal emission is a mirror image with respect to the lowest lying absorption band originating from the locally excited (LE) pyridyl  pyrazolate (ππ*) transition, while the anomalous (visible) emission resulted from an inter-ligand pyridyl  phenyl group (ππ*) transition. The latter revealed solvatochromism which was highly sensitive to the selected solvent polarity. The results were plausibly rationalized by a mechanism incorporating a photo-induced electron transfer (PET) process from the phenyl group to pyrazolate chelate.11,

25-26

Unlike the ultrafast,

intra-ligand excited-state charge transfer (ESCT) process, the slow PET reaction dynamics (tens to hundreds of picoseconds) in the ordinary organic solvents render dual emissions from the locally excited (LE) and the charge transfer (CT) states. In the abovementioned works, the boron atom does not contribute to the frontier molecular orbitals for the first several low-lying electronic transitions (vide infra); hence, it may virtually be treated as an innocent core to bridge the ligating chromophores, forming a stable and highly rigidified entity. Due to the lack of through-bond boron-to-chelate interaction, the inter-ligand PET may take place via a through-space transition between pyrazolate and phenyl moieties in the case of 2-pyridyl pyrazolate substituted boron compounds in solution. These results are significant as they provide clues to a general observation that the associated boron compounds have shown a greater tendency in ‒3‒ ACS Paragon Plus Environment


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forming exciplex upon contacting with the hole transporting layer, giving a red-shifted, broadened emission band.27-29 In view of fundamental, such an inter-ligand charge transfer reaction should be influenced by the electronic orbital coupling constant (Hel, vide infra), the difference between donor and acceptor energy level, and environment viscosity and/or polarity.13, 30 Especially, the appendages may play a crucial role to influence both kinetics and thermodynamics of PET, which may alter the interplay between the LE and CT states, giving ratiometric tuning of luminescence. This issue is fundamentally important for the advance of boron-containing compounds in lighting applications. To shed light on the CT phenomenon for the boron compounds, in this study, we strategically designed a series of new boron compounds (NBN-1 to 5, see Scheme 1) bearing both 1-isoquinolinyl pyrazolate and various aryl functionalities. Our strategies are three folds. (i) We attempt to gain more insights into the influence of structural restrain to PET by introducing methyl substituents at different positions of aryl groups. In this approach, the energetics of PET can be altered as well. (ii) Next, the phenyl substituents were linked with a direct

C-C

linkage

or

a

bridging

oxo

group,

forming

9-borafluorene

or

9-oxa-10-boraanthracene as in NBN-4 and 5, to illustrate the variation of frontier molecular orbitals. (iii) We employed 1-isoquinolinyl fragment to replace the 2-pyridyl moiety of chelate. The decrease of ππ* and hence the LE transition energy (vide infra) may shift the emission from UV into the visible (blue) region. The results show a correlation for the kinetics and thermodynamics of PET versus the structural modification of aryl appendages; hence, the ratiometric luminescence and the color hue are able to be widely tuned. Detail of results and discussion are elaborated below.

2. Experimental Methods Herein, a tetravalent boron compound (iqfpz)BPh2 (NBN-1) and their functional derivatives (NBN-2 to 5), incorporating various aryl appendages have been designed and synthesized. Their structures are summarized in Scheme 1, together with the synthetic protocols as described in the Supporting Information. It is notable that the use of 1-isoquinolinyl pyrazolate (iqfpz) is intended to extend the π-conjugation, lowering the ‒4‒ ACS Paragon Plus Environment

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energy gap of chelate, and hence the dual emission bands are further red-shifted to the visible region (c.f. 5-(2-pyridyl) pyrazolate), enabling color tuning and the possible white light generation. In addition, for NBN-2 and 3, the methyl substituents are strategically introduced at the different position of aryl appendages to hinder their free rotation in solution, while NBN-4 and 5 are designed by fusing phenyl appendages with either a direct C-C linkage or an oxo bridge, so that the aryl groups can be in a planar configuration. The syntheses of these boron compounds requires (iqfpz)H prochelate, which was synthesized with three consecutive steps, including acetylation of isoquinoline to afford 1-acetylisoquinoline, Claisen condensation with ethyl trifluoroacetate to give the 1,3-dione intermediate, followed by hydrazine cyclization to induce formation of pyrazole. After then, treatment of (iqfpz)H with BPh3 afforded (iqfpz)BPh2 (NBN-1) in high yield. Modification of the phenyl appendages of NBN-1 to other aromatic groups was best synthesized via a chloride substituted intermediate (iqfpz)BCl2, followed by treatment with in-situ generated Grignard reagents, namely: (p-tolyl)MgBr and (o-tolyl)MgBr in affording the respective derivatives (iqfpz)B(p-tolyl)2 (NBN-2) and (iqfpz)B(o-tolyl)2 (NBN-3). Notably, the yields for NBN-3 were relatively lower than that of NBN-2 owing to the steric hindrance imposed by the o-methyl substituent. Finally, the reaction of (iqfpz)H with 9-bromo-9-borafluorene31 or with 10-bromo-9-oxa-10-boraanthracene32 yielded the corresponding spiro-substituted (iqfpz)B(frene) (NBN-4) and (iqfpz)B(oxane) (NBN-5), in absence and presence of a base promoter, respectively.

3. Results and Discussion 3.1 Structural Determination. Single crystal X-ray analyses of NBN-3 and 4 were conducted. As shown in Figure 1, their N(1)-B(1)-N(2) bond angles are recorded to be 92.95(15)° and 94.4(2)°. The o-tolyl groups of the BPh2 moiety in NBN-3 are slightly rotated along their C-C linkage, giving an orthogonal arrangement to the iqfpz entity due to the steric hindrance imposed by the o-methyl groups. Moreover, the B(1)-N(2) distance (1.578(3) Å and 1.560(4) Å) is shorter than the corresponding B(1)-N(1) distance (1.619(3) and 1.587(4) Å) in NBN-3 and 4, ‒5‒ ACS Paragon Plus Environment


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respectively. This is due to the stronger chemical bonding between boron and N(2) atom of pyrazolate than that of the N(1)→B dative bond of isoquinolinyl fragment. This result is consistent with the structural characteristics of relevant pyrazolate and pyrrolide compounds documented in the literature.11

3.2 Photophysical Properties. Figure 2 depicts the normalized absorption and emission spectra of NBN-1 to 5 in various solvents, and selected photophysical properties are summarized in Tables 1, S1 and S2. First, the lowest lying absorption bands for these boron compounds are observed at around 340  380 nm, which originate from the intra-ligand ππ* transition mainly associated with the iqfpz moiety according to the theoretical analyses (vide infra). Among these derivatives, NBN-1 and 2 exhibit remarkable dual emission, for which the shorter and longer wavelength emissions are denoted as the F1 and F2 bands, respectively. Further, the excitation spectra monitored at F1 and F2 bands are identical, which are also identical to that of the absorption spectra (Figure S1), indicating that F1 or F2 bands are originated from the same ground state. It is notable that the F1 band in NBN-1 and F2 band in NBN-2 are much more intense compared to their F2 and F1 counterparts, respectively. While the F1 band is nearly independent of the solvent polarity, the F2 band evidently shows pronounced solvent dependence. For example, the F2 band of NBN-2 is bathochromically shifted from 558 nm in cyclohexane to 607 nm in CH2Cl2 solution. In our preliminary approach, the F1 band is ascribed to the ππ* transition localized on the iqfpz moiety, i.e. the locally excited state (LE), while the solvent-polarity dependent F2 band is attributed to the charge transfer (CT) emission from iqfpz moiety to the aryl groups. Interestingly, by switching the methyl group from para- (versus boron atom, NBN-2) to the ortho- position in the aryl groups, the resulting NBN-3 shows only the LE relevant F1 band. Because methyl group is expected to cause similar inductive effect for both NBN-2 and NBN-3, the results manifest the influence of additional steric effect to the PET properties. Lastly, NBN-4 and NBN-5, for which two phenyl appendages are linked either directly or by


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an oxo bridge, merged to a planar chelate, and giving the CT emission band (F2) only. These results confirm the importance of the aryl pendants in influencing the PET properties. The coexistence of both LE and CT emissions indicates relatively slow PET that the emission rate of the optically excited LE state can compete with. This can be rationalized by the weak coupling between LE and CT states, i.e. a category of the nonadiabatic type PET (Figure 3(a)), due to the through-space inter-ligand charge transfer occurred for these boron compounds (vide infra). To gain more insight into the dual emissions, we then examined the corresponding relaxation dynamics of all boron compounds NBN-1 to 5. The transient photoluminescence for NBN-1 and NBN-2 in cyclohexane is shown in Figure 3(b) to (e). Also, pertinent data for both the F1 and F2 bands are tabulated in Table 1. The relaxation dynamics of NBN-1 monitored at the F1 band in cyclohexane can be fitted by two decay components of τ = 85 ps and τ = 6.69 ns. In comparison, the F2 band shows an 87 ps rise component, as indicated by a negative pre-exponential value (Table 1), followed by a 6.76 ns decay component. Similar photophysical behaviour was observed in both toluene and CH2Cl2, i.e., a short and a long decay components of the F1 emission, for which the time constants correlate well with the time constants of rise and decay of the F2 band (see Figure S3, S4 and Table S1, S2). Note that the uncertainty of the time-resolved fluorescence measurement is about ± 3 %. Therefore, within the experimental error, the fast decay component of the F1 band is essentially identical to the rise of the F2 band, and both F1 and F2 bands possess the same population decay time constant. These results clearly indicate a precursor (LE)-successor (CT) type of relationship as well as the establishment of a pre-equilibrium between LE and CT for NBN-1 in various solvents. As shown in Table 1, similar precursor-successor and pre-equilibrium types of LE and CT relationship, were observed for NBN-2 (Figure 3(d), (e)) except for the different fitted values of time constants (see Table 1, S1 and S2). For NBN-3 only the LE emission is observed, the relaxation of which follows a single exponential decay, and the lifetime was fitted to be 0.52 ns in cyclohexane. NBN-4 and NBN-5 exhibit solely the CT emission and the population decay time was fitted to be 1.22 and 1.79 ns, respectively. Note that the rise component of CT band of NBN-4 and



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NBN-5 is beyond the instrument response limit of 12 ps, indicating an ultrafast PET process. The above results show the importance of methyl substitution and ring fusion of aryl groups in harnessing the PET properties and hence the ratiometric emission. As for the pre-equilibrium type of PET, the equilibrium constant Keq between LE and CT states can be deduced by Keq = k+et/k-et where k+et and k-et denote forward and reverse electron rate constants, respectively (see Figure 3(a)). Kinetically, k+et / k-et is equivalent to the ratio of the pre-exponential value for the short decay component versus long decay component (see SI for detailed derivation). In cyclohexane, according the fitted pre-exponential values shown in Table 1, Keq of the PET reaction is calculated to be 1.83 and 166 for NBN-1 and NBN-2, which corresponds to a G value of -0.36 kcal/mol and -3.03 kcal/mol, respectively. Evidently, the electron donating ability of methyl substituents in NBN-2 is sufficient to modulate the G value, i.e. to increase the exergonicity of PET (cf. NBN-1), resulting in more favourable PET and hence the CT emission. In yet another approach, the subtle difference in para- and ortho- methyl substitution leads to a reversal of reaction pattern, i.e., dual emission versus solely LE emission for NBN-2 and NBN-3, respectively, which is of great fundamental interest. Despite the electronic effect exerted by the methyl group, other factor should be responsible for the different PET pattern between NBN-2 and NBN-3. Of equal importance is the predominant CT emission in NBN-4 and NBN-5 upon fusing the aryl appendages. The rationalization of these issues is pending according to current spectroscopic and dynamics data. Alternatively, we then carried out the theoretical approach elaborated below for resolution.

3.3 Theoretical approaches Computations were then performed to gain insights of the transition properties. The results reveal several key aspects responsible for the lower-lying electron transfer process. Based on time-dependent density functional theory (TD-DFT) using m062x function associated with the 6-311+G(d,p) basis sets33 (see experimental section in SI for details), all calculated pertinent energy gaps and assignments of lower-lying transitions and the ‒8‒ ACS Paragon Plus Environment

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associated frontier orbitals are depicted in Figure 4 and Tables S3 to S6. After geometric optimization, all results, except for NBN-3, show that HOMOs and LUMOs are mainly located at the aryl appendages and isoquinolinyl pyrazolate moieties, respectively, which can be virtually treated as a donor-acceptor (D-A) dyad separated by boron atom. Therefore, the lowest lying transition is expected to possess substantial degree of inter-ligand charge transfer character. For NBN-1, it appears that both phenyl groups are located at the equivalent positions; as a result, the electron densities of HOMO are localized at both phenyl groups in equal weighting. Interestingly, upon geometry optimization the two para-tolyl groups in NBN-2 are in slightly inequivalent position accordingly to the analysis; nevertheless, such a difference is negligible and the increase of HOMO energy is mainly from the electron donating effect of the methyl group (see Table S3 and S6). Distinct difference in molecular structure was then obtained for ortho-tolyl substituted NBN-3. Despite the same tetrahedral arrangement on boron, upon geometry optimization, the two ortho-tolyl groups of NBN-3 are arranged in an opposite orientation with respect to the methyl groups in order to reduce the steric encumbrance, which is also consistent with the X-ray analysis (see Figure 1(a)). This results in two distinctive o-tolyl groups with respect to the isoquinolinyl pyrazolate, so that HOMO resides at one of the o-tolyl groups with a greater proportion. In other words, the two o-tolyl groups are not in equal electron density distribution. In theory, the mixing of two equivalent o-tolyl groups lead to the resonance among the molecular orbitals and hence increases HOMO energy. Therefore, the decrease of the degree of resonance for o-tolyl groups in NBN-3 stabilizes the HOMO energy, which is evidenced by its increased oxidation potentials obtained in CV measurement (Table 2) as well as by theoretical approach (Table S6). For HOMO of NBN-3, the frontier orbital is mainly residing at the pyrazolate group rather than the aryl appendages (see Figure 4); therefore the lowest lying electronic transition is ascribed to the intra-ligand pyrazolate-to-isoquinoline transition, which is the locally excited (LE) state in nature, consistent with the experimental result. As for NBN-4 and NBN-5, their fused aryl group fixed their relative position and hence afforded equal contribution in HOMO, in which the



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extended π-conjugation further destabilize the HOMO energy, showing the dominant inter-ligand charge transfer characteristics. Next, we moved one further step to reveal the energetics between LE and CT states by estimate the change of free energy between LE and CT states. The slow PET process can be described by a weak electronic coupling between these two states, so that the Marcus-Weller equation is valid in estimating the associated free energy of the PET process, which can be expressed as:34-35 ∆G = ox(D) − red(A) − 00 − (e2∕ 4ϵsϵ0rc) − (e2∕ 8ϵ0) (1∕rD++1∕rA-)(1∕ϵDCM − 1∕ϵs)

(1)

where ox(D) and red(A) are the oxidation and reduction potentials of the titled molecules based on CV data tabulated in Table 2. In the CV experiment, the oxidation and reduction experiments were conducted in CH3CN and THF solutions, respectively, and the glassy carbon and gold electrode were served as the working electrode for oxidation and reduction processes (see Figure S6 and Table 2 for details). 00 is the energy of the 0-0 transition of the titled molecules where PET takes place. ϵDCM and ϵs denote the dielectric constant of CH2Cl2 and the designated solvent applied, respectively. Based on the photoluminescence results shown in Figure 2, 00 in eq. (1) should be identical to the 0-0 transition energy for the titled molecules, which is based on the observed 1LE transition. It is notable that rD+ and rA- are effective ionic radii and are estimated to be 3.54 and 3.78 Å on the basis of the geometry optimized structure, respectively; while rc is the center-to-center distance between the donor and acceptor moieties, and is estimated to be 4.89 Å. Plugging all of the values into eq. (1), the associated free energy for PET in cyclohexane were then calculated and shown in Table 2. As a result, G of PET for the studied boron compounds follows a trend of NBN-5  NBN-4 < NBN-2 < NBN-1 < NBN-3. Therefore, the exergonicity of PET is expected to be in the order of NBN-5  NBN-4 > NBN-2 > NBN-1 > NBN-3. It is notable that NBN-3 shows a positive G of 3.92 kcal/mol, which is unfavourable for the electron transfer from 1LE to 1CT, consistent with the observed 1LE emission.36 In contrast, NBN-4 and 5 with the solely CT emission showed the largest negative G (see Figure 2). It is also believed that the uncertainty of calculation should be appreciable due to the level, function used for the approach and the uncertainty ‒ 10 ‒ ACS Paragon Plus Environment

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in assessing the energy gap in solution. For example, experimentally deduced G of -0.36 kcal/mol and -3.03 kcal/mol were overestimated to be as -4.38 kcal/mol and -7.38 kcal/mol for NBN-1 and NBN-2, respectively. Experimentally, equilibrium type PET takes place for NBN-1 and NBN-2, resulting in both LE and CT emission bands (vide supra). It is notable that, in this study, none of the titled compounds show TADF (thermally activated delayed fluorescence) properties. We have performed photoluminescence measurements for NBN-1 to 5 in the frozen CH2Cl2 matrix at 77 K, revealing dominant LE bands as shown in Figure S5. This result is reasonable because both the structural relaxation and solvation are hampered in the rigid matrix, which destabilize the 1CT states and give the dominant 1LE emission. Importantly, the phosphorescence for NBN-3 to 5 in frozen CH2Cl2 matrix appears at around 570 nm, which was next assigned to the 3LE transition (see Figure S5) due to the vibronic progression observed. The onset of the3LE state is estimated to be 520 nm, which is lower in energy than the onset of the 1CT band (observed at RT) for NBN-1, NBN-2, NBN-4 and NBN-5 by > 4.8 kcal/mol. In fact, the energy of 3LE state is lower than that of 1CT in all studied compounds. It is reasonable to assume the proximity in energy between 1CT and 3CT due to spatial separation by boron atom and hence the lack of electron exchange energy. As a result, 3CT should be higher in energy than 3LE, so that fast 3CT → 3LE

take places. Due to the large separation of energy, the reverse 3CT →

3LE

→

1CT

is

energetically unfavourable, eliminating the TADF pathway.

4. Conclusion In summary, a series of boron compounds bearing both isoquinolinyl pyrazolate and various aryl ancillaries or even fused biphenyl group were designed and synthesized. Complexes NBN-1 and 2 exhibit notable dual emission, in which the peak wavelength of the F2 band, i.e., the CT emission, is evidently dependent on solvent polarity. In sharp contrast, only LE emission was observed in NBN-3, while sole CT emission is present in NBN-4 and 5. The LE emission is attributed to the intra-ligand ππ* transition within the iqfpz moiety, while the CT emission originates from through-space, inter-ligand electron transfer from isoquinolinyl to aryl moieties. Further dynamics studies, together with computational ‒ 11 ‒ ACS Paragon Plus Environment


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approach, prove the kinetic and thermodynamic relationship of LE and CT transitions as a function of methyl substitution and structure of the aryl appendages, which can be rationalized by the calculated free energies between the 1LE and 1CT states according to Marcus-Weller equation, giving positive G value (3.92 kcal/mol) for NBN-3 in cyclohexane and becomes more negative in the order of NBN-1 < NBN-2 < NBN-4  NBN-5. This is consistent with the experimental observation of sole LE emission in NBN-3, equilibrium between LE and CT in NBN-1 and NBN-2 and fully conversion to CT emission in NBN-4 and NBN-5. The present studies thus demonstrate that both the spatial and electronic properties of substituents could lead to a remarkable tuning of the photophysical properties in the corresponding boron compounds, rendering a combination of locally excited and charge transfer emissions spanning the entire visible spectral region.

Associated content Supporting Information. Additional computational and spectroscopic data are provided in Supporting Information. Notes The authors declare no competing financial interests. Corresponding Author Pi-Tai Chou, E-mail: [email protected] Yun Chi, E-mail: [email protected] Mei-Lin Ho, E-mail: [email protected] Author Contributions ‡These

authors contributed equally.

Acknowledgment This work was supported by the funding from Ministry of Science and Technology (MOST), featured areas research program within the framework of the Higher Education Sprout Project administrated by Ministry of Education (MOE) of Taiwan, and City University of Hong Kong, Hong Kong SAR. We are grateful to the National Center for the High-performance Computing (NCHC) of Taiwan for the valuable computer time and ‒ 12 ‒ ACS Paragon Plus Environment

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facilities. Single crystal X-ray diffraction studies were conducted by Dr. GH Lee of the Instrumentation

Center

of

National

Taiwan

University.

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CF3

CF3

N N

N

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N N

N

B

CF3

NBN-1

B

NBN-2

NBN-3

CF3 N N

N B

N N

N

B

CF3

N N

N B O

NBN-4

NBN-5

Scheme 1. Structural drawings of all titled 2-isoquinolinyl pyrazolate boron complexes.


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Figure 1. Molecular structure of (a) NBN-3 and (b) NBN-4 with thermal ellipsoid drawn at the 30% probability level. The recorded B(1)-N(1) and B(1)-N(2) distances and N(1)-B(1)-N(2) bond angles of NBN-3 and 4 are 1.619(3) Å, 1.578(3) Å and 92.95(15)°, and 1.587(4) Å, 1.560(4) Å and 94.4(2)°, respectively.



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Figure 2. Normalized absorption and emission spectra of NBN-1 to 5 recorded in cyclohexane (blue -o-), toluene (green -o-) and CH2Cl2 (red -o-) at RT and with excitation at λex = 310 nm. Insert: The zoom-in picture of the emission spectra.


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Figure 3. (a) Relationship between LE and CT states, in which k+et, k-et, kL* and kC* denote the rate constant of the electron transfer, reversed electron transfer and radiative decay processes, respectively. HEL involves an electron-coupling matrix element between LE and ET states. (b) to (e) Transient photoluminescence characteristics of the titled boron compounds NBN-1 and 2 in cyclohexane. IRF (red line) denotes instrument response function. Insert: the measurement conducted using shorter time window. (ex = 380 nm).



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Figure 4. Optimized molecular structures in the excited states and the associated frontier orbitals involved in the lower-lying transition.


Page 19 of 25 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


Table 1. Photophysical properties of boron compounds NBN-1 − 5 in cyclohexane. λabs / nma

λem / nmb

Q. Y.

τ / (pre-exp. factor, A1) F1c

τ / ns (pre-exp. factor, A2) F2c

1

379

410 / 500

99.85%

430 nm / 85 ps (0.6488) 6.69 ns (0.3512)

510 nm / 87 ps (-0.4770) 6.76 ns (0.5230)

2

377

407 / 558

18.42%

430 nm / 23 ps (0.9940) 10.55 ns (0.0060)

550 nm / 28 ps (-0.2606) 10.79 ns (0.7334)

3

374

414

2.85%

430 nm / 525 ps (1.0000)

-

4

373

490

0.32%

-

500 nm / 1.22 ns (1.0000)

5

375

473

0.26%

-

470 nm / 1.79 ns (1.0000)

a

Taken from the first vibronic peak of the absorption spectra.

maximum wavelength of the emission spectra.

c

b

Recorded from the

Denotes that the spectra were recorded

under 380 nm excitation wavelength and the monitored wavelength were recorded in the same column, respectively. The transient photoluminescence figures were fitted by the following equation: (t) = A1 exp(−t ∕1) + A2 exp(−t ∕2)



Page 20 of 25

Table 2. Electrochemical properties and the calculated free energy of the boron compounds under investigation. E1/2ox (V)a

E1/2red (V)a HOMO (eV) b

LUMO (eV) b

G (kcal/mol)c

NBN-1

0.94 [irr]

-1.81

-5.74

-2.99

-4.38

NBN-2

0.80 [irr]

-1.82

-5.60

-2.98

-7.38

NBN-3

1.30 [irr]

-1.81

-6.10

-2.99

3.92

NBN-4

0.77

-1.78

-5.57

-3.02

-8.99

NBN-5

0.77 [irr]

-1.77

-5.57

-3.02

-8.99

a

E1/2ox and E1/2red are the anodic and cathodic peak potentials referenced to the Fc+/Fc

couple. b HOMO = | -4.8 – E1/2ox|, LUMO = | -4.8 – E1/2red|. c Notes that the free energies are calculated from eq. (1) by Marcus-Weller equation.


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