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Insights into the Photobehavior of Fluorescent Oxazinone, Quinazoline, and Difluoroboron Derivatives: Molecular Design Based on the Structure–Property Relationships Jianyong Yuan, Yizhong Yuan, Xiaohui Tian, Yidan Liu, and Jinyu Sun J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b01360 • Publication Date (Web): 08 Mar 2017 Downloaded from http://pubs.acs.org on March 8, 2017

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The Journal of Physical Chemistry

Insights into the Photobehavior of Fluorescent Oxazinone, Quinazoline, and Difluoroboron Derivatives: Molecular Design Based on the Structure–Property Relationships

Jianyong Yuan, Yizhong Yuan,* Xiaohui Tian,* Yidan Liu, and Jinyu Sun

Key Laboratory for Ultrafine Materials of Ministry of Education and Shanghai Key Laboratory of Advanced Polymeric Materials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China

*(Y.-Z.Y.) E-mail: [email protected]. *(X.-H.T.) E-mail: [email protected].

ACS Paragon Plus Environment


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ABSTRACT Systematic comparisons among the Oxazinone, Quinazoline, and Difluoroboron series on optical absorption and fluorescence emission properties have been made. Weaker electron donor–acceptor (D–A) pairs in both the Oxazinone and Quinazoline series bring about the slight red-shifts of absorption spectra, whereas they significantly promote the fluorescence intensities of the Oxazinone series but bathochromically shift the maximum emission wavelengths of the Quinazoline series. Intrinsically, the CT modes govern both the electron excitation and de-excitation processes in above two series, but with respective different CT features, i.e., the inter-fragment CT mode for the Oxazinone series versus the intra-fragment CT mode for the Quinazoline series. In the Difluoroboron series, the Oxa-Cl-OCH3-BF2 undergoes a change of transition mode from the local excitation to the CT de-excitation, whereas a large variation of CT compositions can be observed in the Qui-Cl-OCH3-BF2. Experimentally, the Oxa-Cl-OCH3-BF2 exhibits higher fluorescence quantum yield, favorable thermo- and photostability, stronger fluorescence intensity, and appropriately large Stokes shift, while theoretically, the Qui-Cl-OCH3-BF2 benefiting from both the difluoroboron and quinazoline moieties is also promising to be a good fluorescent dye. The motif of combining oxazinone/quinazoline and difluoroboron moieties is believed to improve the prototypical architectures of oxazinone and quinazoline dyes.

2


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INTRODUCTION

Organic luminescent materials are nowadays in the limelight due to their various advantages over the traditional inorganic counterparts, including cheaper production, easier fabrication, and tunable emission wavelength.1-3 Among the diverse candidates of organic fluorescent dyes in the biological and medical fields, the oxazinone and quinazoline derivatives are more prominent because of their intriguing bioactivities: The oxazinone derivatives were reported to be the promising antimicrobial agents and HIV-1 reverse transcriptase inhibitors;4-9 however, as the fluorophores, their relatively lower fluorescence quantum yields restrict the practical uses of fluorescence labeling to report the biochemical reaction sites by themselves.10-11 Likewise, the quinazoline series have been extensively investigated on account of their various bioactivities such as the antibacterial, anticancer, and enzyme-inhibiting properties.12-16 Apart from the appealing bioactivities, their excellent fluorescence properties are also under the spotlight by virtue of the higher fluorescence quantum yields, good photostability, and larger Stokes shifts.17-19 The difluoroboron family, promising to enhance the molecular absorption/emission intensity or even alter the corresponding wavelength, is often employed to improve the optical properties of parent dyes,2,20-23 and thus various difluoroboron derivatives have been designed and reported, which promote the development and broaden the application of organic fluorescent dyes.23-29 Up to now, new derivatives of the Oxazinone, Quinazoline, and Difluoroboron series have been proposed and studied in succession, and their respective unique 3



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specialties including the bioactivities or fluorescence properties enable them to be dedicated to the relevant domains. However, the systematic comparisons on optical properties and the theoretical insights into these three series are still in shortage at present, which should be urgently required as guidelines for further molecular design and modification in these kinds of optical materials. In this article, we evaluated the effect of different substituents on the absorption/fluorescence properties of these three series on the basis of our previous work,30-31 and provided in-depth insights into their optical characteristics behind the experimental spectra by implementing theoretical calculations with the help of the density functional theory (DFT). The structure–property relationships were established to provide references for further theoretical/experimental investigations of these three families. Meanwhile, the motif of combining oxazinone/quinazoline and difluoroboron moieties reported here can offer many useful inspirations to both experimentalists and theorists on the way to improve the original oxazinone and quinazoline architectures. Furthermore, several theoretical methodologies and characterizations were employed and compared here, and their respective pros and cons were demonstrated in detail. Importantly,

the drawbacks of some

methods/descriptors were disclosed, which should be used with special caution.

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RESULTS AND DISCUSSIONS

1. Geometric and Electronic Structures in the Ground States

Figure 1. Molecular sketches of the Oxazinone, Quinazoline, and Difluoroboron series in this work. Relevant stereograms of isomers of the Oxazinone and Quinazoline series were gathered in Figure S1 and S2.

The Oxazinone series (panel A of Figure 1) including four benzoxazinone derivatives with different substituents (Oxa-Cl-OCH3, Oxa-H-OCH3, Oxa-Cl-CH3, and 5



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Oxa-H-CH3) were firstly presented in this paper. The prototype of this series was previously reported to have lower fluorescence quantum yield.11 Subsequently, the Quinazoline series, designed and prepared by further condensation of the Oxazinone series with the o-phenylenediamine (OPD), were gathered in the panel B of Figure 1 and marked as the Qui-Cl-OCH3, Qui-H-OCH3, Qui-Cl-CH3, and Qui-H-CH3. The difluoroboronated organic dyes own the merits of outstanding photochemical stability, large molar extinction coefficients, and higher fluorescence quantum yields; therefore, a protocol concerning the difluoroboronation of the Oxazinone and Quinazoline series was proposed in the panel C of Figure 1. The corresponding difluoroboronated analogues were denoted as the Oxa-Cl-OCH3-BF2 and Qui-Cl-OCH3-BF2, respectively.

1.1 Various Isomers of the Oxazinone and Quinazoline Series In view of the rotatable σ-bonds in our target dyes, we had carried out the conformation searching in advance, and the possible isomers of each system were summarized in Figure S1 and S2. There are two possible isomers in each case of the Oxazinone series, both of which exhibit L-type molecular skeletons from the side views (Figure S1). According to the relative Gibbs free energies (EGibbs, rel) of the Oxazinone series, the [B]-form isomers are ~13 kJ/mol more energetic than the [A]-form ones. In consequence, the [A]-form isomers of Oxazinone series with the face-to-face arrangement of two nitrogen atoms are more stable in vacuum. With regard to the Quinazoline series, generally three types of isomers were found in each 6


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case (except Qui-H-OCH3, where only two types were detected). Again, the [A]-form isomers with lying-U-type frameworks (Figure S2) are energetically more favored in vacuum, compared with the [B]- or [C]-form ones (~9 kJ/mol higher than [A]-forms). Overall, the [A]-form isomers of both the Oxazinone and Quinazoline series were selected for further theoretical and computational studies.

1.2 Geometric and Electronic Structures in Vacuum Considering the structure–property relationships in the molecular design, it is firstly worth looking into the geometric/electronic structures and their responses to the solvent environment. In this subsection, we employed bond lengths, bond orders, and two-plane (dihedral) angles to evaluate the structures of the Oxazinone, Quinazoline, and Difluoroboron series in the ground states. Moreover, the bond length alternation (BLA), nominally defined as the difference between the average bond length of double and single bonds, was utilized here to determine the extent of electron-delocalization. Note that, the BLA is not always a good reflector of the electron-delocalizability since it indirectly reflects the electronic structures via the straightforward measurement of corresponding geometric structures.32 Therefore, the bond order alternation (BOA) using Mayer bond orders instead of bond lengths in the BLA formula, was also considered here to directly and rigorously assess the electronic structures.

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Table 1. Bond Length Alternation (BLA) Values (Å), Mayer Bond Order Alternation (BOA) Values, and the Root-Mean-Square Deviation (RMSD, Å) of the Oxazinone, Quinazoline, and Difluoroboron Series in the Ground States in Vacuum/Ethanol at the B3LYP/6-311G(d) Level compound

geometric structures BLA(etha)

a

a

BLA(vacu)

electronic structures b

∆BLA

c

RMSD

BOA(etha)d

BOA(vacu)d

∆BOAe

Oxa-Cl-OCH3

0.0311

0.0277

0.0034

0.052

0.2379

0.2339

0.0040

Oxa-H-OCH3

0.0323

0.0286

0.0038

0.062

0.2685

0.2611

0.0073

Oxa-Cl-CH3

0.0316

0.0283

0.0033

0.080

0.2410

0.2370

0.0040

Oxa-H-CH3

0.0328

0.0291

0.0037

0.090

0.2711

0.2634

0.0076

Qui-Cl-OCH3

0.0371

0.0343

0.0028

0.177

0.2818

0.2841

-0.0022

Qui-H-OCH3

0.0378

0.0352

0.0026

0.164

0.3091

0.2975

0.0116

Qui-Cl-CH3

0.0368

0.0342

0.0026

0.070

0.2816

0.2836

-0.0020

Qui-H-CH3

0.0382

0.0357

0.0025

0.054

0.3108

0.3108

0.0000

Oxa-Cl-OCH3-BF2

0.0306

0.0241

0.0065

0.145

0.2223

0.2018

0.0205

Qui-Cl-OCH3-BF2

0.0300

0.0261

0.0039

0.104

0.2318

0.2263

0.0055

a

BLA = |(R1+R3+R5+R7)/4 − (R2+R4+R6+R8)/4|, where the relevant bonds were defined in Figure 1, and bond

lengths were collected in Table S1, respectively. b∆BLA = BLA(etha) − BLA(vacu). cRMSD values between the geometries in vacuum and in ethanol. dBOA = |(R1+R3+R5+R7)/4 − (R2+R4+R6+R8)/4|, where the Mayer bond orders were collected in Table S2. e∆BOA = BOA(etha) − BOA(vacu).

To begin with, the gas phase geometries of all the series were investigated irrespective of the solvent effect and visualized in Figure 1, Figure S1 and S2. In the Oxazinone series, the introduction of chlorine atoms can further delocalize the

π-electrons according to the decreasing BLA values in Table 1 (0.0277 Å for Oxa-Cl-OCH3 vs 0.0286 Å for Oxa-H-OCH3; 0.0283 Å for Oxa-Cl-CH3 vs 0.0291 Å for Oxa-H-CH3); similarly, the Oxa-Cl-OCH3 and Oxa-H-OCH3 where the methyl groups are replaced by the methoxy groups in the benzsulfamide moiety, are more favored in delocalized forms with smaller BLA values (0.0277 Å for Oxa-Cl-OCH3 vs 0.0283 Å for Oxa-Cl-CH3; 0.0286 Å for Oxa-H-OCH3 vs 0.0291 Å for Oxa-H-CH3). On the basis of these two facts, the Oxa-Cl-OCH3 with complete -Cl and -OCH3 substituents exhibits higher π-conjugation than other analogues. Notably, both BLA and BOA data consistently reveal the above results and 8


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the descending order of delocalization/conjugation in the Oxazinone series follows: Oxa-Cl-OCH3 > Oxa-Cl-CH3 > Oxa-H-OCH3 > Oxa-H-CH3. When the Oxazinone series were condensed with the OPD, the situation is slightly changed. The Quinazoline series, resultants of condensation (panel B of Figure 1), are prone to be more localized (less conjugated) compared with the corresponding oxazinone-forms (BLA/BOA values: ~0.035 Å / ~0.294 for Quinazoline series vs ~0.028 Å / ~0.249 for Oxazinone series). The BLA and BOA data, again, show the same conjugation order of Qui-Cl-CH3 > Qui-Cl-OCH3 > Qui-H-OCH3 > Qui-H-CH3, where interestingly, the relative conjugation degrees of the Qui-Cl-CH3 and Qui-Cl-OCH3 are interchanged in comparison to that of Oxazinone

series

(Oxa-Cl-OCH3

>

Oxa-Cl-CH3

versus

Qui-Cl-CH3

>

Qui-Cl-OCH3). Such an inversion in conjugation order is presumably attributed to the different molecular planarity (and will be illustrated later). As for the Difluoroboron series (Oxa-Cl-OCH3-BF2 and Qui-Cl-OCH3-BF2), the two smallest BLA/BOA values (0.0241 Å / 0.2018 for Oxa-Cl-OCH3-BF2, and 0.0261 Å / 0.2263 for Qui-Cl-OCH3-BF2) indicate obviously larger conjugation of the boronated system, promising to achieve quite different optical/electronic properties from the parent homologues (Oxa-Cl-OCH3 and Qui-Cl-OCH3). Furthermore, it seems that the BLA values can well reproduce the trend of delocalization/conjugation in each series derived from BOA results, and there’s no difference between the BLA and BOA results; if anything, BOA tends to magnify the difference of the electron-delocalization degrees among the analogues of the same 9



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series (Table 1).

1.3 Solvent Effect on Geometric and Electronic Structures In general, the solvent environment somehow affects the molecular geometries and electronic structures, which are intimately associated with relevant optical/electronic characteristics. Experimentally, various characterizations such as the absorption and fluorescence spectra are mostly measured in the solvents. Hence, it is necessary to evaluate the solvent effect on geometric/electronic structures. As our available experimental results were all obtained in ethanol, we herein drew a comparison between the geometric/electronic structures in ethanol and in vacuum. When the solvent effect is considered, the BLA and BOA values are in general larger than the isolated counterparts (in vacuum), suggesting a decrease of π-conjugation under the ethanol environment (Table 1). In addition, the conclusions of the delocalization orders of three different series drawn in the gas phase still remain unchanged in ethanol. To reflect the sensitivity (responses) of delocalization/conjugation to the solvent effect, we also offered the differences between the BLA/LOA values in ethanol and in vacuum, namely, the ∆BLA/∆LOA descriptors (Table 1). In the light of the ∆BLA descriptor, the Difluoroboron series exhibit largest variations of BLA values (0.0065 and 0.0039 Å), followed by the Oxazinone series with ~0.0035 Å, while the Quinazoline series seem to be insensitive to the ethanol with smaller changes of ~0.0026 Å. Nevertheless, the outcomes are quite different from the ∆BOA descriptor. 10


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An obvious contradiction can be found in the Qui-Cl-OCH3 and Qui-Cl-CH3 where the minus signs of ∆BOA values were observed (-0.0022 and -0.0020, respectively) on the contrary of the ∆BLA results (0.0028 and 0.0026 Å). This inconsistency can be rationalized by the fact that the BLA is not always the best reflector to predict delocalization/conjugation which is essentially related to the electronic structure, due to its limitation.32 Consequently, the somewhat irregular ∆BOA results are believed to be more reliable. After all, the delocalization/conjugation responses (variations) to the solvent effect are more subtle and complicated, and hence a direct descriptor (∆BOA) is necessary for capturing such changes.

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Figure 2. Solvent effect (ethanol) on (a) the bond lengths and (b) the Mayer bond orders of the Oxazinone, Quinazoline, and Difluoroboron series in the ground states. More details were collected in Table S1 and S2.

Some selected bonds in the conjugation systems were further checked upon the solvent effect. The relative deviation of bond length (RDBL), a metric of molecular geometry, was calculated for each given bond and presented in Figure 2a, while the relative deviation of bond order (RDBO) mirroring the electronic structure was provided in Figure 2b for comparison. From B1 to B7 in Figure 2a, we conclude that the Quinazoline series (marked in dashed lines) are the least sensitive to the ethanol, compared with the drastic changes of the Difluoroboron series (illustrated in orange and dark yellow lines). Interestingly, these two findings are in agreement with previous ∆BLA results, implying the capability of BLA as a measurement of geometric variations (though it is traditionally treated as the delocalization metric). 12


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Moreover, the slightly larger RDBL values of B8 can be seen (~0.6%), which means the docking bond B8 governing electron transportation from donor to acceptor moieties is more geometrically dependent on the solvent effect. From the perspective of electronic structures, the Difluoroboron series in ethanol are still more active than other series (Figure 2b). Noteworthily, the docking bonds (B8) in the Difluoroboron series underwent greatly changes of Mayer bond orders with the RDBO values up to ca. -5%. This phenomenon can be accounted for by the cooperative effect of the introduced difluoroboron moiety and the intrinsically sensitive response of B8 to the ethanol solvent (the RDBO values of B8 without -BF2 moiety are ca. -1.5%, still larger than other bonds). In addition to the bond lengths/orders of all series in vacuum/ethanol, the dihedral/two-plane angles were also considered. Here we defined the Ring A, B, and C, which corresponds to the oxazinone/quinazoline moiety, the phenyl ring, and the benzsulfamide moiety, respectively (Figure 3). Generally, the solvent effect can hardly alter the dihedral/two-plane angles; if anything, the Difluoroboron series are geometrically more susceptible to the solvent effect (Figure 3 and Table S3) which are in agreement with previous ∆BLA results. Furthermore, we found that the θAB values of the Oxazinone series are smaller than 5°, indicating the coplanar arrangement between Ring A and B which is caused by the formation of intramolecular hydrogen bond between the nitrogen atom of the Ring A and the H-substituted R4 group. Although this hydrogen bond remains in the Quinazoline series, the predominantly large steric hindrance of the bulky quinazoline moiety forces the drastic distortion 13



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between Ring A and B, along with the large θAB values of ~50°. The significantly decreased θAB values of the Difluoroboron series are attributed to the introduced -BF2 group which acts as a clamp gripping the Ring A and B, and thus restrains the distortion of geometry.

Figure 3. Solvent effect (ethanol) on dihedral angles (DA-B, θAB) and two-plane angles (TPB-C, φBC; TPA-C, φAC) of the Oxazinone, Quinazoline, and Difluoroboron series in the ground states. More details were supplied in Table S3.

We recall that the Oxazinone series are more delocalized than the Quinazoline series according to the smaller BLA/BOA values (Table 1). In fact, the larger conjugation of the Oxazinone series can be attributed to the preponderantly more planar geometries where the considerably reduced θAB values can be observed (Figure 3 and Table S3). Furthermore, the interchange of the conjugation orders, namely, 14


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Oxa-Cl-OCH3 > Oxa-Cl-CH3 versus Qui-Cl-CH3 > Qui-Cl-OCH3, can also be interpreted via the molecular planarity: The Oxa-Cl-OCH3 with smaller θAB value (3.7° in gas) in comparison to that of the Oxa-Cl-CH3 (4.1° in gas) is more preferred in the delocalized form, whereas the Qui-Cl-OCH3 owns the θAB value larger than that of the Qui-Cl-CH3 (48.1° vs 47.2° in gas), which means the latter is more delocalized. As a consequence, this contrast gives rise to the inverted conjugation order. We

finally

provided

the

geometric

deviation

parameter,

i.e.,

the

root-mean-square deviation (RMSD), to comprehensively investigate the solvent effect on the global molecular geometries (Table 1). The RMSD measures the overall geometric deviation between a target molecule in ethanol and in vacuum, and is defined as follows: "/

1 RMSD = − + − + − !

In most cases, the ethanol solvent does not greatly affect the molecular structures (RMSD values < 0.1 Å), but the RMSD values of Qui-Cl-OCH3, Qui-H-OCH3, and Oxa-Cl-OCH3-BF2 were found slightly larger (>0.14 Å). Therefore, the solvent effect upon the geometry variations should not be ignored. Eventually, our following calculations were all based on the optimized geometries in ethanol, and thereby more reasonable comparisons with the available experimental data could be made.

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2. Electron Excitation and Optical Absorption Properties In this section, electron excitation characteristics of the Oxazinone, Quinazoline, and Difluoroboron series in ethanol were explored. The experimentally maximum absorption wavelengths ($% ) were available to serve as the benchmark, and helped %&

to seek the best functional to reproduce the electron excitation energies. On the basis of the optimal calculation level, some basic excitation features such as the oscillator strengths and excitation assignments were supplied, followed by the plots of the electron density difference (EDD) and the correlative natural population analysis (NPA). These metrics shed light on the intrinsic characteristics of electron excitation processes.

2.1 Optimal Functional for Predicting Electron Excitation Energies In order to find the most suitable functional to reproduce the electron excitation energies extracted from the experimental absorption spectra, we herein tested various functionals including the traditional global hybrid functionals (B3LYP,33-34 PBE0,35 and M06-2X36), the range-separated exchange (RSE) functionals (CAM-B3LYP,37 ωB97XD,38 and LC-ωPBE39-41), and the burgeoning optimally tuned RSE functionals (denoted as ω*B97XD and LC-ω*PBE).42-46 The resulting collections of electron excitation energies were tabulated in Table 2. Overall, all the aforementioned functionals can give the correct order of electron excitation energies in the Oxazinone series (Oxa-H-CH3 > Oxa-H-OCH3 > Oxa-Cl-CH3 > Oxa-Cl-OCH3) despite the correspondingly various deviations to the experimental benchmark ('% ). As for the %&


16

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Quinazoline series, albeit most of the functionals fail to reproduce the experimental order (Qui-H-CH3 > Qui-H-OCH3 > Qui-Cl-CH3 > Qui-Cl-OCH3), they still reflect the similar '% values between the Qui-H-CH3 and Qui-H-OCH3 (the %&

Qui-Cl-CH3 and Qui-Cl-OCH3). Table 2. Calculated Electron Excitation Energies ((+,)*+ , eV) of the Oxazinone, Quinazoline, and Difluoroboron Series in Ethanol at Various Functionals with the 6-311+G(d) Basis Set, Compared with the Experimental Data (.)*+ , nm; ()*+ , eV) )*/

compound

exp.

Oxa-H-CH3

329

global hybrid

a

Oxa-Cl-CH3

Oxa-H-OCH3

Oxa-Cl-OCH3

Oxa-Cl-OCH3-BF2

Qui-H-CH3

Qui-Cl-CH3

Qui-H-OCH3

Qui-Cl-OCH3

Qui-Cl-OCH3-BF2 a

)*/

range-separated exchange (RSE)

optimally tuned RSE

B3LYP

PBE0

M06-2X

CAM-B3LYP

ωB97XD

LC-ωPBE

ω*B97XDc

LC-ω*PBEc

3.59

3.72

4.06

4.03

4.06

4.29

3.93

3.75

b

(3.77)

[-0.18]

[-0.05]

[0.29]

[0.26]

[0.29]

[0.52]

[0.16]

[-0.02]

344

3.40

3.53

3.90

3.88

3.91

4.17

3.76

3.56

(3.60)

[-0.20]

[-0.07]

[0.30]

[0.28]

[0.31]

[0.57]

[0.16]

[-0.04]

334

3.54

3.67

4.04

4.01

4.04

4.28

3.91

3.72

(3.71)

[-0.17]

[-0.04]

[0.33]

[0.30]

[0.33]

[0.57]

[0.20]

[0.01]

351

3.35

3.48

3.88

3.86

3.89

4.15

3.73

3.53

(3.53)

[-0.18]

[-0.05]

[0.35]

[0.33]

[0.36]

[0.62]

[0.20]

[0.00]

323

3.82

3.95

4.29

4.26

4.31

4.57

4.14

4.00

(3.84)

[-0.02]

[0.11]

[0.45]

[0.42]

[0.47]

[0.73]

[0.30]

[0.16]

325

3.46

3.60

4.04

4.04

4.09

4.40

3.88

3.71

(3.82)

[-0.36]

[-0.22]

[0.22]

[0.22]

[0.27]

[0.58]

[0.06]

[-0.11]

333

3.37

3.50

3.94

3.95

4.00

4.32

3.76

3.58

(3.72)

[-0.35]

[-0.22]

[0.22]

[0.23]

[0.28]

[0.60]

[0.04]

[-0.14]

326

3.46

3.59

4.03

4.03

4.08

4.39

3.87

3.69

(3.80)

[-0.34]

[-0.21]

[0.23]

[0.23]

[0.28]

[0.59]

[0.07]

[-0.11]

344

3.38

3.51

3.95

3.96

4.01

4.33

3.77

3.57

(3.60)

[-0.22]

[-0.09]

[0.35]

[0.36]

[0.41]

[0.73]

[0.17]

[-0.03]

/

2.95

3.09

3.56

3.57

3.61

3.98

3.33

3.10

The experimental electron excitation energies

%& ('% ,

b

eV) were given in parentheses. The differences between the

1 − ' calculated and the experimental values (Δ' = '% % ) were presented in the square brackets, where the %&

c 1 < ' negative values mean '% % . The best ω values of the ω*B97XD and LC-ω*PBE functionals were

%&

precalculated and summarized in Table 3.

Then we look further into the performance of various functionals relative to the experimental benchmark. The traditional global hybrid functionals (B3LYP, PBE0, and M06-2X), usually employed to optimize geometries and to calculate local

17



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Page 18 of 72

excitation (LE) energies, exhibit the strong dependence of the amount of exact Hartree–Fock (HF) exchange in predicting electron excitation energies; that is, more HF exchange mixed in the hybrid functional seems to bring forth higher calculated excitation energies: The B3LYP with 20%HF completely underestimates the '%

%&

values with the negative average ∆E values of ca. -0.22 eV, while the M06-2X greatly overestimates the '% values (∆E ≈ 0.3 eV) mainly due to the excessive HF %&

exchange (54%HF) involved in the hybrid functional.47 Therefore, the “satisfactory”

PBE0 (25%HF) results can be rationalized by the better compromise between the HF and the DFT exchange, giving rise to the mean absolute errors (MAEs) below 0.1 eV. Notwithstanding the fortuitously acceptable PBE0 results, we should point out that the conventional global hybrid functionals usually suffer from the charge-transfer (CT) issue,48-51 especially in the large conjugation systems,48-49,52 where the electron transition features are depicted in a physically improper manner. This failure is primarily attributed to the incorrect asymptotic behaviors of the global hybrid functionals without vast/full HF exchange in the long range region (>5 Å).53 Therefore, the plausible PBE0 results are still questionable in conjugated systems, and should be further checked and compared with the results of RSE functionals (CAM-B3LYP, ωB97XD, and LC-ωPBE). The RSE functionals with extremely large fractions of HF (or even full HF) exchange in the long-range limit, can provide correct asymptotic behaviors which are quite important for the CT excitations in the electronic spectroscopy,51,53 and hence are expected to outperform the global hybrid functionals. Unfortunately, three popular 18


Page 19 of 72

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RSE functionals are beyond our anticipation with the slightly large MAEs (~0.3 eV) of CAM-B3LYP and ωB97XD, and the outcome of LC-ωPBE is even deteriorated where the excitation energies are severely overshot by up to ~0.6 eV. It is supposed that these large deviations are correlated to the delocalization errors (DEs) of the original RSE functionals,43 where the default ω values (0.2~0.4 a.u.) seem to be still larger and should be optimally tuned. As a result, the untamed RSE functionals containing the superabundant HF components which stem from the larger default ω values eventually overestimate the electron excitation energies, thus losing their potential advantages on CT descriptions.54-55 From the above findings, we can deduce that the exact HF exchange plays a crucial role in depicting CT excitations,56-58 and the DEs should be necessarily eliminated to retrieve the superiority of RSE functionals on description of molecular optical properties.43,59 Table 3. Optimal ω Values (a.u.) for the LC-ω*PBE and ω*B97XD Functionals with the 6-311+G(d) Basis Set compound

LC-ω*PBE

ω*B97XD

Oxa-Cl-OCH3

0.1834

0.1434

Oxa-H-OCH3

0.1893

0.1487

Oxa-Cl-CH3

0.1845

0.1439

Oxa-H-CH3

0.1899

0.1489

Qui-Cl-OCH3

0.1691

0.1348

Qui-H-OCH3

0.1777

0.1407

Qui-Cl-CH3

0.1711

0.1352

Qui-H-CH3

0.1793

0.1406

Oxa-Cl-OCH3-BF2

0.1826

0.1403

Qui-Cl-OCH3-BF2

0.1707

0.1286

To corroborate this conjecture, we carried out the functional tuning procedures so as to reduce the DEs, and the optimal ω values for ω*B97XD and LC-ω*PBE, respectively, were collected in Table 3. More details of the tuning processes were 19



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Page 20 of 72

1 supplemented in Figure S3 and S4. Surprisingly, the '% values of both optimally

tuned RSE functionals perfectly repeat the '% values, with the MAEs of ~0.15 eV %&

(ω*B97XD) and ~0.07 eV (LC-ω*PBE). Such a success in predicting electron excitation energies is conspicuously ascribed to the substantial decreases of the DEs, in comparison to the corresponding original RSE functionals, so that the optimally 1 tuned RSE functionals display largely diminished MAEs of '% (~0.15 eV for

ω*B97XD vs ~0.3 eV for ωB97XD; ~0.07 eV for LC-ω*PBE vs ~0.6 eV for LC-ωPBE).

20


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Figure 4. (a) Percentage of exact-exchange (%HF) included as a function of the interelectronic distance 21



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Page 22 of 72

(R12) for various functionals. The Oxa-Cl-OCH3 is chosen as a representative. (b) Relationship between the total electronic energy of the Oxa-Cl-OCH3 and the fractional electron number, i.e., delocalization errors (DEs). The inset reflects the extent of the total electronic energy deviating from the corresponding exact electronic energy on the straight-line segments which are mathematically constructed by the cation [E(-1) − E(0)], neutral [0], and anion [E(1) − E(0)] states.

Note that our previous study on the spirooxazine systems, analogous to the Oxazinone series in this work, also revealed the preeminence of optimally tuned RSE functionals in estimating electron excitation energies,52 nevertheless, such an improvement was not further demonstrated. In order to elucidate how the optimally tuned RSE functionals overcome the original ones, we provided here the relationship of the exact-exchange percentage (%HF) versus the interelectronic distance (R12) in Figure 4a, where the four distinct curves revealed the different behaviors of tuned/original RSE functionals. Herein we take the tuned/default ω values of the Oxa-Cl-OCH3 as an example. Obviously, the optimally tuned RSE functionals own the decreased ω values (smaller curvatures) and their characteristic curves tend to be less bended, compared with the corresponding original ones. At R12 = 5.3 a.u. (which corresponds roughly to the width of a phenyl ring), the optimally tuned functionals provide 83.3% (LC-ω*PBE) and 77.7% (ω*B97XD) HF exchange, whereas the default ones give more than 99.5% (LC-ωPBE) and 89.7% (ωB97XD) HF exchange. Such a large contrast indicates that a lower fraction of HF exchange is really required to accurately predict the electron excitation energies. This conclusion is in line with the above-mentioned fact that the original RSE functionals generally possess too much HF exchange to precisely predict the electron excitation energies. On the other hand, the practical lower HF percentages (derived from the smaller tuned ω values) in 22


Page 23 of 72

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the optimally tuned RSE functionals, in turn, imply the truly more delocalized feature of the Oxa-Cl-OCH3 on account of the less needed localized character arising from the HF exchange. Furthermore, the diagram of total electronic energy versus fractional electron number for the LC-ω*PBE and LC-ωPBE functionals was also provided in Figure 4b to examine whether the DEs were cut down after optimal tuning process. Note that functionals

with

the

smaller

DEs

exhibit

linear

behaviors

(straight-line

segments),43,60-61 and hence the corresponding ∆[E(∆N) − E(0)] values should be close to zero. From Figure 4b, we can see that the tuned LC-ω*PBE with negligible ∆[E(∆N) − E(0)] values presents straighter behavior than the original LC-ωPBE does, indicating the greatly attenuated DEs of the LC-ω*PBE. Overall, the optimally tuned RSE functionals tailored to the corresponding systems are considerably free from the DEs, and thus are more suitable to predict the optical and electronic properties. The inverse relationship between the tuned ω value and the extent of global electron-delocalization was proposed in many previous studies,54,62 where the smaller tuned ω value is often associated with the more delocalized nature.63-64 In other words, the tuned ω value of a specific system can reflect the global delocalization degree. It is worth validating whether this relationship still holds in our systems. Here we take the tuned ω values of LC-ω*PBE as an example. The Oxa-Cl-OCH3 with the smaller tuned ω value (0.1834 a.u.) is prone to be more delocalized than the Oxa-H-OCH3 (0.1893 a.u.), which is in accord with the corresponding BLA/BOA results (0.0311 Å / 0.2379 for Oxa-Cl-OCH3 vs 0.0323 Å / 0.2685 for Oxa-H-OCH3). However, an 23



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Page 24 of 72

exception was found where the tuned ω values presented the completely inverted electron-delocalization order: According to the BLA/BOA values, the Oxa-Cl-OCH3 (0.0311 Å / 0.2379) is more favored in the delocalized form, compared with the Qui-Cl-OCH3 (0.0371 Å / 0.2818), but the former owns the distinctly larger tuned ω value (0.1834 a.u.) than the later does (0.1691 a.u.), which contradicts with the BLA/BOA results. This occurrence can be rationalized by the definition of BLA/BOA path in our work where a part of the conjugated quinazoline moiety is not included (Figure 1), leading to a certain underestimate of the global delocalization of the Qui-Cl-OCH3. Additionally, it is not strange that the tuned ω values of LC-ω*PBE are totally larger than those of ω*B97XD (Table 3). This phenomenon can be interpreted by comparison of the HF percentage in the short-range limit (R12 = 0) between the LC-ω*PBE and the ω*B97XD functionals (Figure 4a). The former with 0%HF in the short-range limit should achieve larger ω value so as to compensate for the lack of short-range HF exchange, compared with the latter (22.2%HF). To sum up, the optimally tuned RSE functionals (LC-ω*PBE and ω*B97XD) with greatly decreased DEs unveil their outstanding performance on prediction of electron excitation energies. Especially for the LC-ω*PBE functional where the MAEs were merely ~0.07 eV, the experimental order ('% ) of Quinazoline series %&

could be qualitatively reproduced, whereas most of other functionals failed. Considering the merits of optimally tuned RSE functionals, we ultimately chose the

24


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LC-ω*PBE/6-311+G(d) level, unless otherwise stated, to shed light on the electron excitation properties of all three series.

2.2 Origin of Electron Excitation Energies and Oscillator Strengths Both the Oxazinone and Quinazoline series display the same descending order of electron

excitation

energies

from

Table

4,

namely,

Oxa(Qui)-H-CH3

>

Oxa(Qui)-H-OCH3 > Oxa(Qui)-Cl-CH3 > Oxa(Qui)-Cl-OCH3, which are in accord with the available experimental spectra (Figure S5). Such relationships can be understood from the viewpoint of the electron-donating/withdrawing traits in the ground state, usually quantified by the NPA charges.

Table 4. Calculated Electron Excitation Energies ( (+,)*+ ), Oscillator Strengths (f), Excitation

Characteristics (∆r, S, D, and DCT), and Excitation Assignments of the Oxazinone, Quinazoline, and Difluoroboron Series at the TD-LC-ω*PBE/6-311+G(d) Level in Ethanol

compound

1 '%

(eV)

f

crucial a

state

∆r (Å)

S

D (Å)

S/D (Å-1)

DCT (Å)

excitation assignmentb

Oxa-H-CH3

3.75

0.380

S1

1.57

0.41

1.22

0.34

2.07

H → L (91%)

Oxa-Cl-CH3

3.56

0.364

S1

1.72

0.40

1.26

0.31

2.09

H → L (91%)

Oxa-H-OCH3

3.72

0.368

S1

2.47

0.40

1.29

0.31

2.15

H → L (81%)

Oxa-Cl-OCH3

3.53

0.350

S1

2.34

0.39

1.34

0.29

2.17

H → L (84%)

Oxa-Cl-OCH3-BF2

4.00

0.447

S2

2.27

0.48

0.64

0.75

1.23

H-2 → L (42%) H-1 → L (18%) H-2 → L+1 (16%)

Qui-H-CH3

3.71

0.105

S1

2.00

0.41

1.55

0.27

2.63

H → L (91%)

Qui-Cl-CH3

3.58

0.151

S1

1.88

0.41

1.52

0.27

2.58

H → L (91%)

Qui-H-OCH3

3.69

0.109

S1

1.96

0.41

1.53

0.27

2.60

H → L (91%)

Qui-Cl-OCH3

3.57

0.148

S1

1.81

0.41

1.52

0.27

2.57

H → L (91%)

Qui-Cl-OCH3-BF2

3.10

0.132

S1

2.65

0.31

1.14

0.27

1.67

H → L (76%)

a

The crucial state is defined as the lowest optically allowed excited state with large oscillator strength in this work.

b

H and L represent HOMO and LUMO, respectively; the percentage contributions of orbital pairs to the wave

functions of excited states were given in parentheses.

As is shown in Figure S6, the phenyl moiety (Frag. B) always obtains positive charges, indicating its role as the electron-donor (D), while the benzsulfamide moiety 25



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Page 26 of 72

(Frag. C) with significantly more negative charges than the oxazinone/quinazoline moiety (Frag. A), can be treated as the main electron-acceptor (A). Owing to the feebly negative charges of the Frag. A, we tentatively designate it as the auxiliary electron-acceptor (A’). Consequently, all the molecules in the Oxazinone and Quinazoline series can be described as the A’–D–A model. The different strengths of D–A pairs in the model lead to the evident changes of electron excitation energies: For instance, the Oxa-Cl-OCH3 with the weakest D–A 1 value of 3.53 eV, compared with the pair (+0.152/-0.118 a.u.) shows the lowest '%

Oxa-H-CH3 (3.75 eV) owning the strongest D–A pair (+0.179/-0.138 a.u.). Note that the descending order of D–A pair intensities in the Oxazinone series (Oxa-H-CH3 > Oxa-H-OCH3 > Oxa-Cl-CH3 > Oxa-Cl-OCH3) corresponds with that of the electron excitation energies. In consequence, the stronger D–A pairs in analogues can raise the electron excitation energies. The same conclusions can be drawn in the Quinazoline series. However, the difluoroboronation of Oxa(Qui)-Cl-OCH3 makes a great difference on the electron excitation energy and the relevant charge distribution. The 1 Oxa-Cl-OCH3-BF2 obtains the highest '% value up to 4.00 eV, while the lowest

one (3.10 eV) is found in the Qui-Cl-OCH3-BF2. It is not surprising that the former 1 displays the '% value noticeably larger than the Oxazinone series, because the

crucial excited state of Oxa-Cl-OCH3-BF2 lies in S2 rather than in S1 (Table 4). The introduction of the -BF2 moiety into the parent homologues greatly affects the charge distribution, and even alters the role of oxazinone/quinazoline moiety (Frag. A): The 26


Page 27 of 72

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Frag. A once acted as the auxiliary electron-acceptor (with negligible negative charges) in the Oxazinone and Quinazoline series, but now reveals the powerful electron-donating character (with large positive charges) in the Difluoroboron series (Figure S6). Such a great inversion will generate the completely different electron excitation characteristics (more details will be discussed later). In the end, the Difluoroboron series are viewed as the D–D–A model, which is quite different from the previous A’–D–A model. Table 5. Decomposition of Oscillator Strengths (f) into Electron Excitation Energies ((+,)*+ ) and 6 a Squares of the Transition Dipole Moments (45) ), Compared with the Overlap Degrees (I) of Key

Orbital Pairsb

1 '% (eV)

(a.u.) 78

Oxa-Cl-CH3

compound

Oxa-H-CH3

a

f

key orbital pairs

I (%)

4.13

0.380

H → L (91%)

67.6

3.56

4.17

0.364

H → L (91%)

65.1

3.75

Oxa-H-OCH3

3.72

4.04

0.368

H → L (81%)

60.2

Oxa-Cl-OCH3

3.53

4.05

0.350

H → L (84%)

58.1

Oxa-Cl-OCH3-BF2

4.00

4.56

0.447

H-2 → L (42%)

70.5

Qui-H-CH3

3.71

1.15

0.105

H → L (91%)

61.3

Qui-Cl-CH3

3.58

1.72

0.151

H → L (91%)

62.5

Qui-H-OCH3

3.69

1.20

0.109

H → L (91%)

61.7

Qui-Cl-OCH3

3.57

1.69

0.148

H → L (91%)

62.8

Qui-Cl-OCH3-BF2

3.10

1.74

0.132

H → L (76%)

44.9

b

At the TD-LC-ω*PBE/6-311+G(d) level in ethanol. The overlap degree of the orbital pair is defined as the

overlap integral between norm of two orbitals over the whole space, i.e., 9 |; (r>)||;@ (r>)| Ar>, which is calculated

numerically via Becke's grid-based integration approach at the B3LYP/6-311G(d) level in ethanol.

The oscillator strength (f), a dimensionless quantity reflecting the transition intensity between two different states, is often related to the overlap degree (I) of the orbital pair which depicts the transition composition. Larger oscillator strengths are expected to achieve higher overlaps of orbital pairs.52,65 In this context, we calculated the overlap integral between norm of these two orbitals over the whole space (Table 5) to quantify the overlap degree of the key orbital pair. 27



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Page 28 of 72

The Oxazinone series obtain the average oscillator strengths of ~0.37 that are totally larger than those of the Quinazoline series (~0.13), while all the excitation assignments involved in the transition description of both series are exclusively from the highest-occupied molecular orbital to the lowest-unoccupied molecular orbital (HOMO → LUMO). We take the Oxa(Qui)-H-CH3 as an example to make full illustration. Both the HOMO and LUMO of the Oxa-H-CH3 are uniformly located in Frag. A and B (Figure 5), which enable them to overlap each other more easily (I = 67.6%), thus with stronger oscillator strength (0.380). As for the Qui-H-CH3, almost the entire HOMO is concentrated in Frag. A (95%), while most of and part of the LUMO are assigned to Frag. A (68%) and Frag. B (28%), respectively. Such a large difference between the HOMO and LUMO distributions makes them slightly difficult to mutually interact (I = 61.3%). On the other hand, the Qui-H-CH3 exhibits greatly distorted geometry with the larger θAB value in ethanol (50.2°), which perceptibly impedes the electron transition, compared with the planar Oxa-H-CH3 (4.3°). Therefore, the Qui-H-CH3 conclusively shows the weaker oscillator strength (0.105 vs 0.380) in view of these two factors. The same reasons also hold for the Difluoroboron series where the oscillator strength of the Oxa-Cl-OCH3-BF2 prevails over that of the Qui-Cl-OCH3-BF2 (0.447 vs 0.132).

28


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Figure 5. Molecular orbitals (MOs, isovalue = 0.04 a.u.) and their compositions (based on the Hirshfeld method) of the Oxazinone, Quinazoline, and Difluoroboron series at the B3LYP/6-311G(d) level in 29



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Page 30 of 72

ethanol. All the MOs were decomposed into the oxazinone/quinazoline moiety (Frag. A), phenyl moiety (Frag. B), and benzsulfamide moiety (Frag. C).

However, we still found two special cases, that is, the Oxa(Qui)-H-OCH3 and Oxa(Qui)-Cl-OCH3, which cannot be plainly illuminated via the simple overlap-degree analysis. Fundamentally, the oscillator strength (f) originates from the 1 ) and the transition dipole moment (78 ), according to electron excitation energy ('%

the following equation:

B=

2 1 ' ∙7 3 % 8

Therefore, though the Qui-H-OCH3 and Qui-Cl-OCH3 present slightly larger overlap degrees (I = 61.7% vs 60.2%; I = 62.8% vs 58.1%), compared with their Oxazinone counterparts (Oxa-H-OCH3 and Oxa-Cl-OCH3), their weaker oscillator strengths (0.109 vs 0.368; 0.148 vs 0.350) are ultimately governed by the discernibly smaller transition dipole moments (78 = 1.20 a.u. vs 4.04 a.u.; 78 = 1.69 a.u. vs

4.05 a.u.). It is also worth noting that the Frag. C of Qui-Cl-OCH3-BF2 owns the majority (63%) of the HOMO, while its LUMO is primarily situated in the Frag. A (67%). Given the evidently little overlap degree (I = 44.9%) and the relatively large distance between these two fragments, a significant charge-transfer (CT) excitation can presumably

take

place

in

the

Qui-Cl-OCH3-BF2.

With

regard

to

the

Oxa-Cl-OCH3-BF2, the situation is somewhat complicated where three major orbital pairs are involved in excitation description, i.e., H-2 → L (42%), H-1 → L (18%), and H-2 → L+1 (16%). For convenience, we select the key orbital pair (HOMO-2 → LUMO) to examine the orbital composition. Unlike the Oxazinone series, the Frag. A 30


Page 31 of 72

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dominates both the HOMO-2 and the LUMO of Oxa-Cl-OCH3-BF2 (67% and 59%), while the Frag. B gains the minority (27% and 36%). Such a high similarity in orbital composition and a sufficient overlap (I = 70.5%) of the orbital pair indicate that this type of transition might belong to the local excitation (LE).

2.3 Electron Excitation Transitions: CT Mode versus LE Mode In order to judge whether an electron transition can be treated as the CT mode (or the LE mode), we employed several different descriptors in this paper. The ∆r index is a quantitative indicator of electron excitation mode originally proposed by CA Guido;66 the smaller the ∆r index is, the more likely the excitation is a LE mode. Special attention should be paid when one uses the ∆r index, since the larger ∆r value does not always guarantee the authentic CT mode (which will be exemplified later). The overlap integral of hole–electron distribution (S) and the distance between centroid of hole and electron (D), both derived from the hole–electron theory, were also considered here; the smaller S paired with the larger D often suggests the more evident CT character. For convenience, we further defined the S/D index to combine these two parameters, and self-evidently, the smaller S/D value means the more CT-like mode. From Table 4, the ∆r values of the Oxa-H(Cl)-OCH3 (2.47 and 2.34 Å) are found larger than those of the Oxa-H(Cl)-CH3 (1.57 and 1.72 Å), indicating more evident CT modes of the former, while the slight fluctuation of ∆r values in the Quinazoline series (1.81~2.00 Å) reflects their similar CT modes. These findings can 31



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Page 32 of 72

be qualitatively confirmed by the S/D results. Moreover, the smaller S/D values of the Quinazoline series (0.27 Å-1) show more remarkable CT modes, compared with the Oxazinone series (>0.29 Å-1). However, the ∆r values failed to reproduce this trend, and even worse, a pathological case was detected where the ∆r descriptor incorrectly predicted the “evident CT mode” of the Oxa-Cl-OCH3-BF2 (2.27 Å). In essence, its rather large S/D value (0.75 Å-1) suggests the LE mode, verified by the electron density difference (EDD) plot where no obvious transferred-electron (only 0.079 e) is found indeed (Figure 6). Fortunately, the large ∆r value of Qui-Cl-OCH3-BF2 (2.65 Å) still gives the correct CT mode which is in line with the S/D result (0.27 Å-1) and is substantiated by the corresponding EDD plot as well (0.115 e, transferred from Frag. C to Frag. A). Overall, the ∆r descriptor tends to exaggerate the CT character in some special cases (such as the Difluoroboron series), while the S/D index can give satisfactory results without the distinct deficiency. If anything, it seems that the S/D index can hardly distinguish the little difference of CT modes among the analogues in the Quinazoline series and hence shows the same value of 0.27 Å-1. In order to look into the subtle changes among the analogues in each series, we provided here the distance of charge-transfer (DCT) which is based on the electron density variation,67 and the amount of transferred-electron during electron transition in Table 4 and Figure 6, respectively. In the Oxazinone series, the evident CT features with large amounts of transferred-electron can be observed (~0.22 e), and these CT transitions mainly occur from Frag. B & C to Frag. A, showing a typical inter-fragment CT transition (Figure 6). Especially, the Oxa-Cl-OCH3 presents the 32


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larger DCT value (2.17 Å) and more amount of transferred-electron (0.239 e), compared with those of the Oxa-H-CH3 (2.07 Å and 0.218 e). This phenomenon can be rationalized by the molecular planarity (Table S3) where the smaller θAB value of the Oxa-Cl-OCH3 (3.4°) are more beneficial to the CT transition with respect to that of the Oxa-H-CH3 (4.3°).

Figure 6. Electron density difference (isovalue = 0.002 a.u.) of the Oxazinone, Quinazoline, and Difluoroboron series from the ground state to the crucial excited state at the TD-LC-ω*PBE/6-311+G(d) level in ethanol. Quantitative charge-transfer analysis is based on the NPA charges collected in Figure S6.

In comparison to the Oxazinone series, the smaller S/D and larger DCT values (~0.27 Å-1 and ~2.59 Å) of the Quinazoline series indicate even more significant CT features, promising to own large amounts of transferred-electron. Nonetheless, little 33



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Page 34 of 72

transferred-electron was found from Frag. A to Frag. B (merely ~0.065 e). We recall that the dihedral angles (θAB) of the Quinazoline series are substantially increased, which hinder the electron transitions, compared with those of the Oxazinone series (Figure 3 and Table S3); therefore, the strong CT transitions must happen within a certain fragment. As is shown in Figure 6, most of the atoms in the quinazoline moiety exhibit the prominently decreased electron densities (blue area), while only the N7=C8-N9 subfragment (Figure S6) displays greatly increased electron densities (green area). More surprisingly, the amounts of transferred-electron between these two areas are ~0.136 e, twice larger than those between Frag. A and B (~0.065 e). On the basis of these facts, the strong CT features of the Quinazoline series mostly originate from the intra-Frag. A CT transition (Figure 6). Furthermore, all the analogues

in

the

Quinazoline

series

provide

very

close

amounts

of

transferred-electron and similar DCT values, and hence it is not weird that the S/D index apparently shows the same value (Table 4). In summary, the analogues with weaker D–A pairs in the Oxazinone or Quinazoline series own the relatively lower electron excitation energies, leading to the slight red-shifts of absorption spectra (Figure S5). The larger oscillator strengths of the Oxazinone series and the Oxa-Cl-OCH3-BF2 are mostly associated with more sufficient overlaps of orbital pairs (due to the planar geometries), and essentially pertain to the bigger transition dipole moments. In all three series, only the Oxa-Cl-OCH3-BF2 exhibits the typical LE mode, mainly derived from the evenly distributed orbital pair (HOMO-2 → LUMO). The Qui-Cl-OCH3-BF2, another 34


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derivative in the Difluoroboron series, reveals a moderate CT transition from Frag. C to Frag. A due to the formation of larger conjugation with the help of the inserted -BF2 moiety. However, its excessive electron-delocalization without more powerful D–A pairs to drive, dampens the oriented charge-transfer and thus cuts down the net transferred-electron.52 Although both the Oxazinone and Quinazoline series present strong CT transitions, the essence of them is quite different: The former are classified as the inter-fragment CT transition, i.e., from Frag. B & C to Frag. A, while the latter are principally assigned to the intra-fragment CT transition (from other atoms in the quinazoline moiety to the N7=C8-N9 subfragment). Such a special intra-fragment CT case can be rationalized by their distorted geometries (on account of the bulky quinazoline moiety with large steric hindrance), which block the conventional inter-fragment CT transition and thus promote the electron transition within the same moiety (Frag. A). Additionally, the analogues with weaker D–A pairs in the Oxazinone series seem to display stronger CT behaviors, owing to the smaller dihedral angles (θAB); in contrast, no distinct changes are caught among the analogues in the Quinazoline series, which means the various substituents have negligible effect on the CT character of the Quinazoline series.

35



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Page 36 of 72

3. Electron De-excitation and Fluorescence Emission Properties The geometric/electronic differences between the relaxed excited states (ES-Minima) and the ground states (GS-Minima) were firstly discussed in this section. The experimental emission wavelengths ($FG ) were available to check the calculated %&

1 ). On the basis of the suitable methodology, a detailed emission energies ( 'FG

investigation on the electron de-excitation and fluorescence emission was performed and then compared with the corresponding electron excitation process.

3.1 Comparisons of Geometric/Electronic Structures between the Relaxed Excited States and Ground States To characterize the geometric/electronic structures of ES-Minima, we again used the RMSD and BLA/BOA descriptors here. The geometries of all series are more susceptible to the electron excitation (with larger RMSD values, seen in Table 6 and Figure 7) than to the solvent effect (with smaller RMSD values in Table 1), and therefore larger geometric variations can be observed in Figure S7 and Figure 8: The ES-Minima of both the Oxazinone and Quinazoline series show more stretched molecular skeletons (especially for the latter ones with even larger RMSD values, seen in Figure 7), whereas the ES-Minima of the Difluoroboron series tend to shrink, compared with the corresponding GS-Minima. Such large geometric changes bring about the significant variations of electronic structures: As is shown in Figure 7, the greatly attenuated BLA and BOA values of ES-Minima in all series indicate more delocalized/conjugated configurations in the 36


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relaxed excited states than in the ground states (GS-Minima). Interestingly, the ES-Minima of the Quinazoline series with smaller BLA/BOA values are more favored in delocalized configurations, compared with those of the Oxazinone series. This situation is contrary to that of the GS-Minima where the Oxazinone series exhibit more delocalized character (Figure 7). A possible reason is that the substantially decreased θAB values of ES-Minima of the Quinazoline series (~30° vs ~50° of GS-Minima, shown in Figure 8) vastly reduce the steric hindrance and thus promote the electron-delocalization, while those values of the Oxazinone series remain almost unchanged (still ~2°). Therefore, the quinazoline moiety with bulky aromatic-rings and smaller steric hindrance can sufficiently activate its nature of higher delocalization/conjugation, leading to the corresponding BLA/BOA values (of ES-Minima) even smaller than those of the Oxazinone series (Figure 7). Table 6. Bond Length Alternation (BLA) Values (Å), Mayer Bond Order Alternation (BOA) Values, and the Root-Mean-Square Deviation (RMSD, Å) of the Oxazinone, Quinazoline, and Difluoroboron Series in the Relaxed Excited States (S1, ES-Minima) and in the Ground States (GS) at the B3LYP/6-311G(d) Level in Ethanol compound

geometric structures a

a

electronic structures b

∆BLA

RMSD

BOA(GS)d

∆BOAe

0.0314

0.2379

-0.2065

0.269

0.0127

0.2685

-0.2557

-0.0036

0.320

0.0404

0.2410

-0.2006

-0.0026

0.305

0.0240

0.2711

-0.2471

0.0371

-0.0157

0.527

0.0235

0.2818

-0.2583

0.0199

0.0378

-0.0179

0.458

0.0052

0.3091

-0.3039

0.0209

0.0368

-0.0159

0.420

0.0209

0.2816

-0.2607

Qui-H-CH3

0.0201

0.0382

-0.0180

0.472

0.0048

0.3108

-0.3060

Oxa-Cl-OCH3-BF2

0.0010

0.0306

-0.0296

0.340

0.0855

0.2223

-0.1368

Qui-Cl-OCH3-BF2

0.0086

0.0300

-0.0214

0.324

0.0587

0.2318

-0.1731

BLA(ES)

BLA(GS)

Oxa-Cl-OCH3

0.0261

0.0311

-0.0050

0.247

Oxa-H-OCH3

0.0276

0.0323

-0.0047

Oxa-Cl-CH3

0.0281

0.0316

Oxa-H-CH3

0.0302

0.0328

Qui-Cl-OCH3

0.0214

Qui-H-OCH3 Qui-Cl-CH3

a

c

BOA(ES)

d

BLA = |(R1+R3+R5+R7)/4 − (R2+R4+R6+R8)/4|, where the relevant bonds were defined in Figure 1, and bond

lengths were collected in Table S4, respectively. b∆BLA = BLA(ES) − BLA(GS). cRMSD values between the geometries of relaxed excited states and of ground states. dBOA = |(R1+R3+R5+R7)/4 − (R2+R4+R6+R8)/4|, 37



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Page 38 of 72

where the Mayer bond orders were collected in Table S5. e∆BOA = BOA(ES) − BOA(GS).

Figure 7. Comparisons of BLA, BOA, and RMSD values of the Oxazinone, Quinazoline, and Difluoroboron series between the relaxed excited states (S1, ES-Minima) and the ground states (GS).

38


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Figure 8. Comparisons of θAB, φBC, and φAC values of the Oxazinone, Quinazoline, and Difluoroboron series between the relaxed excited states (S1, ES-Minima) and the ground states (GS). More details were supplied in Table S6.

As for the Difluoroboron series, their smallest BLA values of ES-Minima seem to predict the most conjugated configurations (0.0010 Å for Oxa-Cl-OCH3-BF2 and 0.0086 Å for Qui-Cl-OCH3-BF2, seen in Table 6). However, their largest BOA values reveal the least ones among all the ES-Minima (0.0855 and 0.0587) which contradicts with the BLA results (Figure 7). We recall that the electron-delocalization degree of homologues is intimately related to the molecular planarity, and consequently a check of corresponding θAB values should be required: From Figure 8 and Table S6, the Oxa-Cl-OCH3-BF2 owns evidently larger θAB value than the Oxazinone series do (18.2° vs ~2°), and the same situation is found between the 39



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Page 40 of 72

Qui-Cl-OCH3-BF2 and Quinazoline series (37.4° vs ~30°). On the other hand, though the BLA results can determine the electron-delocalization and reproduce the BOA results in most cases, its limitation should be noted (discussed before). As a result, the BOA results are more reliable where the Difluoroboron series are actually favored in less conjugated configurations among all the ES-Minima.

3.2 Electron De-excitation and Fluorescence Performance The experimental fluorescence spectra of all series (except Qui-Cl-OCH3-BF2) were collected in Figure S8. Interestingly, the descending order of fluorescence emission energies in the Quinazoline series is in agreement with that of electron excitation energies, that is, Qui-H-CH3 > Qui-H-OCH3 > Qui-Cl-CH3 > Qui-Cl-OCH3. This consistency further points to the fact that weaker D–A pairs such as the Qui-Cl-CH3 and Qui-Cl-OCH3 (Figure S6) can lead to a red-shift of absorption/fluorescence spectrum. Nevertheless, this rule is broken in the Oxazinone series where the maximum emission wavelengths range irregularly from 500 nm to 517 nm. Instead, their fluorescence intensities present the regularity of Oxa-Cl-OCH3 > Oxa-Cl-CH3 > Oxa-H-OCH3 > Oxa-H-CH3, which suggests that the relatively weaker D–A pairs can facilitate stronger fluorescence. Actually, stronger D–A pairs with very efficient CT transitions are often subject to a drastic fluorescence quenching;68 therefore, the Oxa-Cl-OCH3 with the weakest D–A pair (+0.152/-0.118 a.u.) exhibits the strongest fluorescence intensity (Figure S8). Moreover, the introduction of -BF2 group (Oxa-Cl-OCH3-BF2) can further enhance the fluorescence intensity, compared with 40


Page 41 of 72

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the Oxa-Cl-OCH3. However, the various D–A pairs can hardly affect the fluorescence intensities of the Quinazoline series. In short, we conclude that the D–A pairs play quite different roles between the Oxazinone series and Quinazoline series: They greatly alter the fluorescence intensities of the former whereas distinctly shift the maximum emission wavelengths of the latter. Table 7. Calculated Fluorescence Emission Energies ((+,)HI , eV) of the Oxazinone, Quinazoline, and Difluoroboron Series in Ethanol at Various Functionals with the 6-311+G(d) Basis Set, Compared )*/

with the Experimental Data (()HI , eV) compound

'FG

Oxa-H-CH3

2.40

%&

LC-ω*PBEa

1 'FG

Oxa-Cl-CH3

ω

3.18

[0.78] 2.48

PCM-tuned LC-ω*PBEa

0.1844 b

3.01

2.43

Oxa-Cl-OCH3

2.40

3.16

0.1796 0.1839

2.42

2.65

0.1865

2.62

Qui-Cl-CH3

2.51

2.66 [0.04] 2.58

2.61

2.67

0.1733

2.50

Qui-Cl-OCH3-BF2

/

2.57 [0.07]

a

2.21

2.53

0.0247 0.0245

2.64

9

2.63

4

2.53

7

[0.13]

1.60

0.0311

2.39

24

[-0.03]

2.05

0.0248

2.25

10

[-0.37]

2.01

0.0239

2.19

10

[-0.32]

2.06

0.0246

2.25

10

[-0.36]

2.01

0.0234

[-0.49] 0.1621

6

[0.20]

2.35

[-0.55] 0.1673

OHF(%)

2.77

[0.16]

[-0.50]

[0.06]

Qui-Cl-OCH3

0.0257

[-0.57] 0.1675

[0.07]

Qui-H-OCH3

2.44

[-0.82] 0.1733

1 'FG

[0.37]

[-0.05]

[0.23]

Qui-H-CH3

0.0259

[0.10] 0.1789

[0.59]

Oxa-Cl-OCH3-BF2

2.65

[-0.04]

[0.73] 2.99

ω

[0.25]

[0.53]

Oxa-H-OCH3

1 'FG

OHF-B3LYPc

2.19

10

[-0.31]

1.66

0.0235

1.85

11

The “PCM-tuned” means all the ω values were optimized in ethanol rather than in vacuum (the canonical tuning

scheme). More information about PCM-tuned RSE functionals was seen in ref 69 and 70. Both PCM-tuned LC-ω*PBE and canonical LC-ω*PBE functionals were implemented on the basis of the ES-Minima. bThe

1 differences between the calculated and the experimental values (Δ' = 'FG − 'FG ) were presented in the square %&

1 brackets, where the negative values mean 'FG < 'FG . cFor more details about the optimal HF (OHF) method, %&

one can refer to ref 47 and Table S7.

The optimally tuned RSE functionals were utilized to reproduce the fluorescence

41



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Page 42 of 72

emission energies. Here we assumed that all the fluorophores comply with the Kasha’s rule where the only S1 → S0 transition is considered during the emission process. From Table 7, we can find that the LC-ω*PBE functional is tailored to describe the Quinazoline and Difluoroboron series with merely MAEs of ~0.09 eV, while the PCM-tuned LC-ω*PBE functional gives agreeable prediction of the Oxazinone series (MAEs of ~0.11 eV). However, both functionals either greatly overshoot or underestimate the respective rest series. In other words, it is difficult to seek a unique functional to reliably describe all three series, as probably the intrinsic dissimilarity (of interactions between the solvent molecules and excited dyes) of ES-Minima lies between the Oxazinone series and the Quinazoline/Difluoroboron series. In fact, the combination of optimally tuned RSE functionals with continuum polarization models (e.g. the PCM-tuned LC-ω*PBE) has been recently investigated by De Queiroz and Kümmel in detail,69-70 and they argue that the PCM-tuned RSE functionals generally lead to an underestimate of excitation energies (due to too smaller ω values). Although this combinational method can be problematic for some cases (e.g., the Quinazoline/Difluoroboron series in our work), the Oxazinone series is not the case. Therefore, we believe the PCM-tuned LC-ω*PBE functional is still suitable to handle this special series, and such satisfactory outcomes can be ascribed to the adaptive error-cancellation where the solvent effect is taken into account during the ω tuning process. Moreover, we also employed the optimal HF (OHF) method47 where the MAEs were more moderate (uniform) relative to the optimal tuning methods (which seem to be more series-dependent), but it still shows slightly larger 42


Page 43 of 72

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MAEs of the Quinazoline series (~0.35 eV). As a consequence, we decided to apply the PCM-tuned LC-ω*PBE functional only to the Oxazinone series, whereas the Quinazoline and Difluoroboron series were still treated with the canonical LC-ω*PBE functional in the following calculations. Table 8. Fluorescence Emission Characteristics ((+,)HI , femi, τ, ΦF, S/D, and DCT), Stokes Shifts

(υStokes), and Reorganization Energies [∆E(abs) and ∆E(emi)] of the Oxazinone, Quinazoline, and Difluoroboron Series in Ethanol reorganization

emission characteristics compound

Oxa-H-CH3

1 '%

femi

(eV)

1 'FG

3.75

2.65

0.21

(eV)

τa

ΦFb

S /D -1

(ns) 15.7

energiesd

0.31

DCT

υStokes 3

3.56

2.44

0.20

19.9

0.28

(Å)

(10 cm )

(eV)

(eV)

3.84

8.9

0.20

0.79

(20%)

(80%)

0.08

4.29

3.72

2.53

0.15

24.0

0.35

0.06

4.41

Oxa-Cl-OCH3

3.53

2.35

0.16

26.7

0.27

0.07

4.46

4.00

2.65

0.08

42.8

0.69

0.10

2.70

Qui-H-CH3

3.71

2.66

0.15

22.5

0.49

0.17

2.97

Qui-Cl-CH3

3.58

2.58

0.16

21.3

0.42

0.17

3.04

Qui-H-OCH3

3.69

2.67

0.15

22.4

0.56

0.17

3.01

Qui-Cl-OCH3

3.57

2.57

0.17

20.8

0.45

0.17

3.02

Qui-Cl-OCH3-BF2

3.10

2.21

0.08

62.2

/

0.10

3.18

a

The fluorescence lifetime (τ) was calculated by the Einstein’s formula τ =

1 'FG

b

∆E(emi)

0.09

Oxa-H-OCH3

Oxa-Cl-OCH3-BF2

∆E(abs)

(Å )

(11.0)

Oxa-Cl-CH3

-1

c

9.1

0.28

0.84

(9.1)

(25%)

(75%)

9.6

0.30

0.87

(10.3)

(26%)

(74%)

9.5

0.38

0.92

(9.1)

(29%)

(71%)

10.9

0.48

1.22

(11.4)

(28%)

(72%)

8.5

0.01

1.07

(9.6)

(1%)

(99%)

8.1

0.10

1.12

(9.8)

(8%)

(92%)

8.2

0.11

1.15

(9.6)

(9%)

(91%)

8.1

0.18

1.22

(8.9)

(13%)

(87%)

7.2

0.39

1.34

(22%)

(78%)

1 2 46.7/[2('FG ) ·femi],

where the unit of

is eV, and the unit of τ is ns. The fluorescence quantum yields (ΦF) were experimentally measured by a

reference method where the quinine sulfate dehydrate worked as the reference compound (ΦF, ref = 0.55). cThe experimental Stokes shifts were given in parentheses. dThe total reorganization energies between S1 and S0 were calculated at the B3LYP/6-311G(d) level (in line with the geometry optimization level). The percentage contributions of respective part to the total reorganization energies [∆E(abs) + ∆E(emi)] were given in parentheses.

43



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Page 44 of 72

Figure 9. Electron density difference (isovalue = 0.002 a.u.) of the Oxazinone, Quinazoline, and Difluoroboron series from the relaxed excited state (S1, ES-Minima) to the ground state at the TD-(PCM-tuned)-LC-ω*PBE/6-311+G(d) level in ethanol. Quantitative charge-transfer analysis is based on the NPA charges at the ωB97XD/6-311G(d) level in ethanol.

The electron de-excitation is often regarded as the inverse process of the corresponding electron excitation, and thus the fluorescence behavior should be similar to the absorption characteristic but with the contrary transition direction. As is shown in Table 8, all series obtain smaller S/D and larger DCT values compared with those in the excitation processes, suggesting the even stronger CT transitions during de-excitation processes. This finding can be corroborated by comparison of EDD plots between the de-excitation and excitation processes (Figure 9 vs Figure 6) where 44


Page 45 of 72

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more amounts of transferred-electron can be observed during fluorescence emissions. Moreover, we can find from EDD plots that the transition directions of electron de-excitation in most cases are indeed contrary to those of electron excitation. In spite of many similarities between the electron excitation and de-excitation features, some differences should be noted. Firstly, a change of transition mode (LE → CT) is found in the Oxa-Cl-OCH3-BF2 which once showed typical LE mode (S/D: 0.75 Å-1; DCT: 1.23 Å) during the excitation process but now exhibits significant CT mode (S/D: 0.10 Å-1; DCT: 2.70 Å). Such a strong CT mode can be assigned to the enormous amount of electron (0.396 e) transferred from Frag. A to Frag. C (Figure 9). It is not surprising that the Oxa-Cl-OCH3-BF2 undergoes the mode change, since an internal conversion of S2 → S1 occurs between the electron excitation (S0 → S2, LE) and de-excitation process (S1 → S0, CT). As for the Qui-Cl-OCH3-BF2, though both excitation and de-excitation processes show the CT modes, their nature is quite different: During the excitation process, moderate amount of electron (0.115 e) is excited from Frag. C to Frag. A (Figure 6); however, in the de-excitation process, the majority of the electron (0.282 e) is transferred from the N7=C8-N9 subfragment to the other atoms in quinazoline moiety (i.e., the intra-Frag. A CT transition), and the minority (0.148 e) comes from Frag. B, a situation very similar to the de-excitation processes of the Quinazoline series seen below (Figure 9). In the case of the Quinazoline series, the de-excitation process can be decomposed into the major contribution of the intra-Frag. A CT transition (0.175~0.183 e) and the non-negligible contribution of the inter-fragment CT 45



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Page 46 of 72

transition from Frag. B to Frag. A (0.111~0.126 e). The latter contribution is slightly different from that of the excitation process where the contribution of Frag. A to Frag. B is only 0.060~0.073 e and thus can be neglected. Similarly to the excitation CT features, various D–A pairs can hardly alter the de-excitation CT characteristics (S/D: ~0.17 Å-1; DCT: 2.97~3.04 Å). In regard to the Oxazinone series, the Oxa-Cl-OCH3 shows the larger DCT value (4.46 Å) and more amount of transferred-electron (0.270 e), compared with those of the Oxa-H-CH3 (3.84 Å and 0.248 e). The conclusion that analogues with the weaker D–A pairs in the Oxazinone series exhibit stronger excitation CT behaviors, still holds in the de-excitation process. It is also worth noting that the -OCH3 substituent evidently improves the de-excitation CT character and greatly extends the CT distance: For instance, the Oxa-H-OCH3 displays smaller S/D and larger DCT values (0.06 Å-1; 4.41 Å), compared with the Oxa-H-CH3 (0.09 Å-1; 3.84 Å). More visualized comparison can be seen in Figure 9, where large amount of electron is accumulated in the -OCH3 moiety and its vicinity (green part), resulting in the larger DCT values. The fluorescence lifetime (τ) is a brief index to differentiate the fluorescence performance of various dyes. Here, we employed the Einstein’s formula71 to calculate τ values of all the series in Table 8. Large variations can be seen in the Oxazinone series (15.7~26.7 ns), compared with the Quinazoline series where tiny fluctuations are observed (20.8~22.5 ns). This phenomenon indicates that the Oxazinone series are more susceptible to the D–A pairs, which is in keeping with their sensitive responses of de-excitation CT characteristics to the various D–A pairs. As for the Difluoroboron 46


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series, their significantly larger τ values (42.8 ns for Oxa-Cl-OCH3-BF2 and 62.2 ns for Qui-Cl-OCH3-BF2) manifest that the introduction of -BF2 substituent into the Oxazinone/Quinazoline series gives rise to the pronounced change of fluorescence emission character. We note that the ES-Minima of the Oxazinone series possess CT states between the conjugated Frag. A & B (Figure 9). When the -BF2 group is introduced,

the

Oxa-Cl-OCH3-BF2

exhibits

CT

behavior

between

the

non-conjugated Frag. A & C, and consequently the corresponding orbital overlap between HOMO and LUMO sharply drops, leading to the smaller oscillator strength (femi = 0.08). As for the Qui-Cl-OCH3-BF2, the introduction of -BF2 group greatly enhances the CT transition (larger amount of transferred-electron can be seen in Figure 9), promotes the spatial separation of HOMO and LUMO distribution, and thus results in the decrease of oscillator strength (femi = 0.08). Eventually, these predominantly smaller oscillator strengths prolong the fluorescence lifetime of the Difluoroboron series. Experimentally, we provided the fluorescence quantum yields (ΦF) of all the series (except Qui-Cl-OCH3-BF2) in Table 8. The Oxazinone series own the relatively lower ΦF values (0.27~0.35), compared with the Quinazoline series (0.42~0.56). However, the introduction of -BF2 substituent into the Oxa-Cl-OCH3 strikingly improves the quantum yield (0.27 → 0.69). Such a surprising result can be rationalized by the restriction of intramolecular rotation (RIR) mechanism, which blocks the non-radiative relaxation channel and populates radiative excitons.72 Although the Oxa-Cl-OCH3-BF2 owns smaller oscillator strength, the introduced 47



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-BF2 group significantly improves the molecular rigidity, restrains the rotation between Frag. A & B, and hence cuts down the internal conversion through this channel, lifting the fluorescence quantum yield. The Stokes shift (υStokes) reflects the energy loss owing to the internal conversions and various vibrational or geometric relaxations of excited dyes. As is shown in Table 8, all three series display large υStokes values (7.2~10.9 × 103 cm-1), which can effectively restrain the reabsorption between the emission and absorption bands, promising to achieve better fluorescence performance. The reorganization energy is also provided in Table 8, which measures the change of electronic-state energies caused by the geometric relaxation. Here, we decomposed the total reorganization energy into the absorption part [∆E(abs)] and the emission part [∆E(emi)], seen in Figure S9. In the case of the Oxazinone and Difluoroboron series, the emission part owns the majority of the total reorganization energy (71~80%), while the absorption part contributes the minority (20~29%). With respect to the Quinazoline series, almost all the reorganization energy pertains to the emission part (87~99%). In short, the large Stokes shifts of all three series can be mainly attributed to the large emission reorganizations.

4. Thermo- and Photostability The thermo- and photostability, prerequisites for practical applications, were also measured in this work. In the case of the Quinazoline series, the Qui-Cl-OCH3 with the highest decomposition temperature (Td = 330 °C) suggests the weaker D–A pair 48


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can improve the molecular thermostability (Figure S10). Moreover, both the difluoroboron and quinazoline substituents can improve the thermostability of the prototypical Oxa-Cl-OCH3 in terms of their higher Td values (297 and 330 °C for Oxa-Cl-OCH3-BF2 and Qui-Cl-OCH3, vs 287 °C for Oxa-Cl-OCH3). The Oxa-Cl-OCH3-BF2 and Qui-Cl-OCH3 with two highest Td values were further compared with the Rhodamine 6G commodity (a widely-used fluorescent dye) on photostability. As is shown in Figure S11, both of them present the significantly smaller rate constants of photodecomposition (kd = 8.7 × 10-4 and 4.0 × 10-4 min-1 for Oxa-Cl-OCH3-BF2 and Qui-Cl-OCH3, vs kd = 58.0 × 10-4 min-1 for Rhodamine 6G), indicating their excellent photostability over the Rhodamine 6G. The Qui-Cl-OCH3-BF2, a theoretically designed fluorophore benefiting from both the difluoroboron and quinazoline moieties, is promising to inherit the merits of the Qui-Cl-OCH3 and Oxa-Cl-OCH3-BF2, and therefore should have outstanding thermo- and photostability. Further synthesis and experimental validation of the Qui-Cl-OCH3-BF2 and its homologues are ongoing.

49



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CONCLUSIONS

Weaker D–A pairs in both the Oxazinone and Quinazoline series lead to the slight red-shifts of absorption spectra, whereas they significantly promote the fluorescence intensities of the Oxazinone series but bathochromically shift the maximum emission wavelengths of the Quinazoline series. During the electron excitation, both the Oxazinone and Quinazoline series present strong CT transitions, but the nature is quite different: The former are classified as the inter-fragment CT transition from Frag. B & C to Frag. A, while the latter are assigned to the intra-Frag. A CT transition. Moreover, the weaker D–A pairs in the Oxazinone series show stronger CT modes, whereas the CT behaviors in the Quinazoline series are almost independent of D–A pairs. In the Difluoroboron series, the Oxa-Cl-OCH3-BF2 exhibits the typical LE mode, while the Qui-Cl-OCH3-BF2 reveals a moderate CT transition from Frag. C to Frag. A. During the fluorescence emission, more significant CT transitions were found in all series. Interestingly, a change of transition mode (LE → CT) is detected in the Oxa-Cl-OCH3-BF2 from the excitation process to the de-excitation process. Furthermore, both the intra-Frag. A CT transition and the inter-fragment CT transition from Frag. B to Frag. A contribute to the de-excitation processes of the Qui-Cl-OCH3-BF2

and

the

Quinazoline

series.

Experimentally,

the

Oxa-Cl-OCH3-BF2 shows higher fluorescence quantum yield, favorable thermo- and photostability, stronger fluorescence intensity, and appropriately large Stokes shift, 50


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and therefore can work as an excellent fluorescent dye. Theoretically, the Qui-Cl-OCH3-BF2 benefiting from both the difluoroboron and quinazoline moieties is also promising to achieve better fluorescence performance. We hope the motif of combining oxazinone/quinazoline and difluoroboron moieties can be regarded as guidelines and references for further molecular design and modification in these kinds of optical materials.

51



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EXPERIMENTAL AND COMPUTATIONAL DETAILS

Experimental Details. The Oxazinone, Quinazoline, and Oxa-Cl-OCH3-BF2 were prepared according to the synthetic routes presented in Scheme S1, and more synthesis details were available in our previous work.30-31 UV–visible absorption spectra (Figure S5) were recorded on the 756MC spectrometer, while fluorescence spectra (Figure S8) were achieved by the Fluorolog-3-P spectrometer (λexcitation = 365 nm); moreover, the fluorescence quantum yields (ΦF) were measured by a reference method73 where the quinine sulfate dehydrate worked as the reference compound (ΦF, ref

= 0.55). 1H NMR spectra (Figure S12) of the Oxa-Cl-OCH3, Qui-Cl-OCH3, and

Oxa-Cl-OCH3-BF2 were obtained from the AVANCE-AV500 instrument with the tetramethylsilane (TMS) serving as the internal reference and the CDCl3 as the solvent. Mass spectra (Figure S13) of the Quinazoline series were recorded on the G2577A spectrometer. Thermogravimetric analyses (TGA) were performed on the NETZSCH-STA409PC instrument where the heating rate was set to 10 °C/min starting from 30 °C to 600 °C under nitrogen atmosphere. Computational Details. The initial structures for geometry optimizations were prescreened with the aid of the conformation search at the semi-empirical PM6 level carried out by the Spartan 14 software.74 All the geometries were optimized at the B3LYP/6-311G(d) level in vacuum and in ethanol, and the geometric minima were confirmed by the absence of imaginary frequencies at the same level. In order to investigate the solvent effect on all three series, we employed the polarizable 52


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continuum model (PCM) to simulate the ethanol environment.75 Time-dependent DFT (TD-DFT), a useful method with desired compromise between computational efficiency and accuracy,42,76 is utilized here with the 6-311+G(d) basis set to compute the molecular optical properties on the basis of the optimized geometries. Especially, the optimally tuned RSE functionals (LC-ω*PBE and ω*B97XD) were implemented here, where the tunable parameter ω was optimized according to the error function

J (K ) given below:

R R J (K) = LMNOPO(Q) + IP()U + LMNOPO(QV") + IP( + 1)U

R R and MNOPO(QV") denote the corresponding HOMO energy of where the MNOPO(Q)

the N-electron (neutral) system and its anionic counterpart (N+1), while the IP(N) and IP(N+1) represent their ionization potentials (IP), respectively. The optimal ω value is then obtained when the J (K) reaches the minimum (Figure S3 and S4). During the

functional tuning process, the optDFTw tool77 was utilized to efficiently locate the

optimal ω value in each case. Furthermore, the B3LYP functional with the optimal HF% (OHF-B3LYP) was also considered to predict emission energies, and more details were collected in Table S7.47 Eventually, contour surfaces of the electron density difference (EDD) were generated by the Multiwfn 3.3.9 program78 and visualized with the VMD 1.9.2 tool,79 while the quantitative charge-transfer illustration in EDD plots was based on the natural population analysis (NPA) derived from the natural bond orbital (NBO) method.80-81 All the calculations were performed by the Gaussian 09 program package,82 except the ones involving fractional electron number which were supported by the NWChem 6.5 tool.83 53



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ACKNOWLEDGEMENTS

This research was supported by the Fundamental Research Funds for the Central Universities of China (22A201514002), Shanghai Leading Academic Discipline Project (No. B502), and Shanghai Key Laboratory Project (No. 08DZ2230500).

SUPPORTING INFORMATION

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.xxxxxxx. Optimized geometries and Gibbs free energies of isomers, bond lengths and Mayer bond orders, dihedral and two-plane angles, details of optimal tuning processes and optimal HF exchange for the B3LYP functional, NPA charges of the ground and excited states, geometric variations between GS- and ES-Minima; synthetic routes of dyes, 1H NMR spectra, mass spectra, UV–vis absorption spectra, fluorescence spectra, thermogravimetric analyses, and photooxidation kinetic curves (PDF).

AUTHOR INFORMATION

Corresponding Authors *(Y.-Z.Y.) E-mail: [email protected]. *(X.-H.T.) E-mail: [email protected]. ORCID Jianyong Yuan: 0000-0002-2068-4169 Yidan Liu: 0000-0002-1087-7362 54


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