Excitation-Dependent Multiple Fluorescence of a Substituted 2-(2

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Excitation-Dependent Multiple Fluorescence of a Substituted 2-(2'-Hydroxyphenyl)benzoxazole Quinton J. Meisner, Ali Haidar Younes, Zhao Yuan, Kesavapillai Sreenath, Joseph J. M. Hurley, and Lei Zhu J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b07988 • Publication Date (Web): 09 Nov 2018 Downloaded from http://pubs.acs.org on November 14, 2018

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The Journal of Physical Chemistry

Excitation-Dependent Multiple Fluorescence of a Substituted 2-(2'-Hydroxyphenyl)benzoxazole Quinton J. Meisner, Ali H. Younes, Zhao Yuan, Kesavapillai Sreenath, Joseph J. M. Hurley, and Lei Zhu*, Department of Chemistry and Biochemistry, Florida State University, 95 Chieftan Way, Tallahassee, FL 32306-4390, USA [email protected]

ABSTRACT. Excitation-dependent multiple fluorescence of a 2-(2'-hydroxyphenyl)benzoxazole (HBO) derivative (1) is described. Compound 1 contains the structure of a charge transfer (CT) 4hydroxyphenylvinylenebipy fluorophore and an excited state intramolecular proton transfer (ESIPT)-capable HBO component that intersect at the hydroxyphenyl moiety. Therefore, both CT and ESIPT pathways, while spatially mostly separated, are available to the excited state of 1. The ESIPT process offers two emissive isomeric structures (enol and keto) of 1 in the excited state, while the susceptibility of 1 to a base adds another option to tune the composite emission color. In addition to the ground state acid-base equilibrium that can be harnessed for the control of emission color by excitation energy, compound 1 exhibits excitation-dependent emission that is attributed to solvent-effected ground state structural changes. Therefore, depending on the medium and

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excitation wavelength, the emission from the enol, keto, and anion forms could occur simultaneously, which are in the color ranges of blue, green, and orange/red, respectively. A composite color of white with CIE coordinates of (0.33, 0.33) can be materialized through judicious choices of medium and excitation wavelength.

Introduction Luminescent materials exhibiting multicolor emission have attracted considerable attention due to their potential applications in many areas. Mixing emission colors in the visible region may generate white light1-11 that could be used in energy-efficient organic light-emitting diodes (OLEDs).12-15 Two or more emission bands from a single fluorophore could be exploited in the self-calibrating ratiometric sensing technologies,16-20 which offer the advantage of minimizing systematic errors originating from concentration variation, excitation power fluctuation, and photobleaching of the fluorophore.21 Properties of the microenvironment surrounding a multipleemission fluorophore can be extracted through monitoring the changes of various emission bands that have uneven responses to environmental factors such as polarity19,22 or viscosity.23,24 In this work, “multiple emission” is defined as a collection of radiative relaxation processes of the excited electronic states that a single fluorophore has access to. We restrict the definition of multiple emission as such to distinguish from emissions from the same excited electronic state either from (rarely) or to (often) more than one vibrational states. A highly overlapping term is “panchromatic emission”,5,25 which carries the connotation of white light emission found in white OLEDs. Organic molecular systems capable of dual26-29 and triple fluorescence30-35 have been reported. Among them, twisted internal charge transfer (TICT),36,37 excited-state intramolecular proton transfer (ESIPT),38-41 valence tautomerism,42 excimer formation,43 slow internal conversion


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(IC),27 ground state proton transfer,33,44,45 and any combination of these occurrences,46 lead to the formation of two or more emissive excited states. Producing multiple emission from a single fluorophore, especially one whose fluorescence color is tunable by changing excitation wavelength, presents not only a challenge in molecular design from a fundamental chemistry perspective,47,48 but also an attractive strategy in driving applications requiring multicolor luminescence. Herein, we report compound 1 (Figure 1a), an organic fluorophore that undergoes medium-dependent ground and excited state proton transfer, and is capable of excited state charge transfer, to afford three emissive excited states that collectively cover a broad sector of the visible region (from blue to orange/red). This paper focuses on the fundamental photophysical properties of this compound, while its electroluminescence performance and its potential use in OLEDs will be described in a future article.

Results (a) Molecular Design. Compound 1 (Figure 1a) is built upon the structure of 2-(2'hydroxyphenyl)benzoxazole (HBO, Figure 1c), a fluorophore that has the ESIPT capacity.49,50 An electron-withdrawing 2,2’-bipyridyl group is conjugated via a trans-vinylene functionality51 at the para position to the hydroxyl in HBO (Figure 1a). Based on our previous work on dual-emitting fluorophores that are capable of two charge transfer (CT) transitions,27 we postulate that the newly designed molecule might be excited in a frequency-dependent manner to either of the two CT states that are divergent in the physical spaces they occupy. CT1 (Figure 1b) is the precursor of ESIPT, which would result in the dual-emission of the enol and keto tautomers in a solventdependent manner.35,52,53 A second charge transfer state CT2 shall operate in the conformers without intramolecular hydrogen bonds. In such structures, the ESIPT is hindered, while the C-C


3


bond connecting the benzoxazolyl and hydroxyphenyl moieties may rotate to reduce the electronic conjugation

in

the

HBO

moiety,

thus

favoring

the

CT2

transition

on

the

4-

hydroxyphenylvinylenebipy side. Deprotonation of the hydroxy group would remove the ESIPT by eliminating the intramolecular hydrogen bond, while amplifying the CT2 transition by enhancing the electron-donor strength. Compound 2 (Figure 1d) models the CT2 component of 1. Factoring into the ground state acid/base equilibrium, compound 1 shall carry a larger spread of frequency distribution of emissive excited states than HBO. Consequently, the composite emission of 1 could sample across the visible spectrum depending on excitation wavelength and acid/base or hydrogen bonding property of the medium.

b r2

a

pt o

N

Ac ce

CT1

2

N

CT

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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Donor

Acceptor 1 O 1-enol

N

H

H O

X

ESIPT

d

c

N

O HBO-enol N

2 HO

N

HO

Figure 1. (a) The structure of compound 1 in an intramolecularly hydrogen-bonded enol form; (b) a molecular design containing two CT (CT1 and CT2) and one ESIPT components; (c) HBO: 2(2'-hydroxyphenyl)benzoxazole; (d) compound 2: 4-hydroxyphenylvinylenebipy.


4

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(b) Synthesis of compound 1. As shown in Scheme 1, compound 3 was synthesized through the formylation (the Duff reaction)54 of 4-hydroxybenzaldehyde. Combining 3 and 2-aminophenol in ethanol resulted in a selective hydrogen bond-assisted imine formation to afford 4. A 5-endo-trig oxidative cyclization mediated by PhI(OAc)2 converted 4 to benzoxazole 5.55 After introducing methoxymethyl ether (MOM) in compound 6, Horner-Wadsworth-Emmons reaction between 6 and phosphonate 756,57 afforded compound 1-MOM. Compound 1 was obtained after the removal of the MOM protecting group with trifluoroacetic acid.

Scheme 1. Synthesis of compound 1. OH

O

OH (a)

O O

(b)

O

(c)

OH N

O

3 (35%)

N O

(f)

1-MOM (79%)

5 (76%)

6 (quant.)

O

N

O N HO

N

O

HO

4 (81%)

O

(e)

N

N OH

O

O O

(d)

1 (55%)

O EtO

N

P

OEt

N 7

O N

N

Reagents and conditions: (a) i. hexamethylenetetramine, TFA, 120 °C, 2 h, ii. 10% (v/v) H2SO4, 1 h; (b) 2-aminophenol, ethanol, 0 °C to rt, 3 h; (c) PhI(OAc)2, CH3OH, rt, 3 h; (d) DIPEA, MOMBr, THF, 0 °C to rt, overnight; (e) KHMDS, 7, THF, -78 C to rt, 3 h; (f) TFA, DCM:ACN (1:1), reflux, overnight.

(c) Absorption spectra. The absorption spectra of compound 1 were recorded in six solvents (Figure 2a). The absorption band centering at 340-350 nm in all solvents was assigned to the enol form of 1 (1-enol). The appearance of a longer wavelength (~ 450 nm, data not shown) band with inconsistent relative absorptivity was observed in dimethylsulfoxide (DMSO). The nature of this


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band puzzled us in the early phase of this project. Ground state keto tautomer was initially postulated based on the reported interpretation of similarly long, weak absorption bands of HBO in DMSO.52,58 Yet we eventually concluded that the evidence to support that assignment was insufficient. It was later found that the absorption of the ESIPT component of 1, the model compound HBO, is susceptible to deprotonation by basic contaminants in the hydrogen bondaccepting solvent DMSO.53 The adventitious base contamination may be introduced in sample preparation.53 The absorption spectrum of 1 in DMSO free of the deprotonated band was thereafter obtained (Figure 2, red) via careful sample preparation to eliminate the base contamination.

Figure 2. Normalized absorption spectra of compound 1 (DCM – 10 M; all other solvents – 15 M) in different solvents. DCM: dichloromethane (navy); ACN: acetonitrile (cyan); EtOH: ethanol (green); DMF: N,N-dimethylformamide (yellow); NMP: N-methyl-2-pyrrolidone (orange); and DMSO: dimethyl sulfoxide (red).


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A small degree of solvent dependency of absorption was observed (Figure 2). The trend did not follow a single solvent parameter such as dielectric constant or hydrogen bond (HB) basicity. For example, the least polar solvent dichloromethane (DCM) produced an absorption maximum that lied in the middle of the six spectra. In the most polar and HB-basic DMSO, the absorption wavelength maximum was the longest. Therefore, more than one factors determine the maximum of the absorption spectrum. DCM is the least polar solvent; yet because of that it allows the presence of an intramolecular HB that facilitates electronic delocalization on the HBO component. The collective effect of a low solvent polarity of and the intramolecular HB in DCM produces an absorption maximum in the middle of the distribution. The effect of HB-basic solvents on the absorption spectrum of 1 will be commented further in a later section.

Table 1. Spectroscopic data of 1 in various solvents, and 1-enolate and 1-MOM in DMSO. Solvent

abs (nm)

em (enol, nm)

em (keto, nm)

fa

DCM

346

-

542

0.32

ACN

343

416

543

0.06

EtOH

345

430

540

0.06

DMF

348

431

548

0.07

NMP

350

432

543

0.12

DMSO

350

445

550

0.16

DMSO (1-enolate)

457

585

-

0.78

DMSO (1-MOM)

346

426

-

0.71b

a. Average absolute quantum yield taken over λex = 320-390 nm (neutral); and λex = 390-490 nm (anion); b. measured using a relative method excited at 340 and 360 nm.


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Figure 3. (a) Normalized emission spectra of compounds 1 (15 M, solid lines) and 1-MOM (15 M, dashed lines) in different solvents excited at 340 nm; DCM (navy); ACN (cyan); EtOH (green); DMF (yellow); NMP (orange); DMSO (red). (b) Postulated ground state species that give either the enol (blue) or keto emission (green). S = hydrogen bond basic solvent.

(d) Emission spectra of 1 under a single excitation wavelength. Compound 1 was excited at 340 nm to produce emission spectra in a number of solvents (Figure 3a). Two emission bands – one centering at ~ 440 nm (the blue band) and the other centering at ~ 540 nm (the green band) – were observed, the relative intensity of which depended on solvent. The assignment of the blue band to the enol emission was supported by its resemblance to that of 1-MOM (Figure 3a, dashed lines) that is incapable of ESIPT, while the green band was attributed to the excited keto tautomer after the ESIPT. Based on the works on HBO,52,58 the conformational isomer of 1 (1-syn-enol, Figure


8

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3b) that contains an O-H···N hydrogen bond (HB), the linchpin of ESIPT, would account for the green keto emission. Conformer 1-anti-enol (Figure 3b) with an intramolecular O-H···O HB, which has been found to co-crystalize with 1-syn-enol in the solid state,59 and the solvent-bound species (Figure 3b) would give rise to the blue enol emission. In the weakly polar solvent DCM (solid navy line in Figure 3a), 1-syn-enol is the major species in solution to result in a predominant green keto emission band after an efficient ESIPT process. The intramolecularly hydrogen bonded 1-anti-enol, a less stable ground state conformer (Figure S20), would account for the minor normal enol emission in DCM. In a HB-basic solvent, the solvated species in place of 1-anti-enol would account for the blue enol emission. The ratio of the blue (~ 440 nm) and green (~ 550 nm) emission bands of 1 depended on HB-accepting ability of the solvent, which was ranked on the  scale.60 The intensity of the blue enol emission increased as the  value of the solvent grew (ACN 0.32; EtOH 0.48, DMF 0.74, and DMSO 0.88),61 because the ESIPT was disrupted in the solvated enol species, which lacked the intramolecular HB (Figure 3b).22,52,62-64 The spectra that were normalized at either enol or keto emission maximum are included as Figure S7. The emission of the enol form was more positively solvatochromic than the keto form, which suggested, with the support from computation presented in the ‘Discussion’ section, that the excited enol form had a more polar structure than the keto form. (e) The effect of base on absorption and emission of 1 in DMSO. The susceptibilities of the absorption spectrum of 1 to DBU (1,8-diazabicyclo[5.4.0]undec-7-ene, pKa of the conjugate acid is 18 in DMSO), a base, and TFA (trifluoroacetic acid, pKa = 3.4 in DMSO), an acid, in DMSO were examined. The addition of TFA did not significantly alter either absorption or emission spectra (Figure S8). The addition of DBU amplified the 457-nm band of the anion (1-enolate) at the expense of the 1-enol band (Figure 4a). An isosbestic point was developed in the course the


9


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titration experiment in the absorption mode, indicating that the neutral and anionic forms were the only two species interconverting in the ground state. Relevant to the current study, the ground state proton transfer of 3-hydroxyflavone, an intensely studied molecule known for its ESIPT, to HBbasic solvents to form the anion was reported by several groups.65-68 The phenol-containing green fluorescent protein chromophore has a population of anion in the ground state,69 a reflection of its high susceptibility to the basicity of its proximal microenvironment.


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Figure 4. The absorption (a) and emission (b) spectra of 1 (15 M) in the presence of DBU (blue: 0 molar equivalent, red: 1.1 molar equivalent; in DMSO). The arrows show the spectral change as [DBU] was increased. Inset in (b) is the photograph of neutral (left) and anionic (right) forms of 1 in DMSO irradiated by a handheld UV lamp (365-nm excitation).

Upon addition of DBU the emission maxima of compound 1 in a DMSO solution shifted from 436 nm (blue enol emission) to 590 nm (orange enolate emission, Figure 4b). Unlike the observations in the absorption mode, the lack of an isosbestic point was evident in the evolution of the emission spectrum upon increasing [DBU]. This was due to the presence of more than two emissive excited states, enol, keto, and enolate, that were interconverting during the course of the experiment. The photographs of the neutral and anionic forms in DMSO under the illumination of a handheld UV lamp (365 nm) are shown in the inset of Figure 4b. The change of emission wavelength covered a broad section of the visible spectrum from blue (436 nm) to orange (590 nm). Considering the wavelength maximum of the keto form, which is at 550 nm, a relatively even distribution in intensity of these three bands might afford white or close-to-white emission. (f) Excitation-dependent emission of 1. The emission spectra of 1 were acquired under the excitation at different wavelengths in N-methyl-2-pyrrolidone (NMP), in which both the enol and keto emissions were observed. The change of excitation wavelength within the window of 310380 nm altered the relative intensity of the enol and keto bands (Figure 5a). The excitation spectra taken at various emission wavelengths that cover both enol and keto bands revealed that longer excitation wavelengths favored the enol emission (Figure 5b). Similar excitation-dependent emission was observed in DMF (with 1% v/v AcOH to neutralize the adventitiously deprotonated


11


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compound 1, Figure S9). There appear to be two neutral ground state structures in a hydrogen bond-basic solvent that afford either enol or keto emission.

Figure 5. (top) Normalized emission spectra of 1 (15 M) in NMP. ex = 310 nm (violet), 345 nm (blue), 370 nm (green), 380 nm (red). (bottom) Normalized excitation spectra of 1 (15 M) in NMP. em = 424 nm (blue), 506 nm (green), 574 nm (red).


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Because compound 1 is a crossover of two structural components that are responsible individually for ESIPT or CT transitions in the excited state, we used HBO and compound 2 (Figure 1) as structural analogs of these two components and studied separately the effect of excitation wavelength on their emission. Compound 2 belongs to the arylvinylenebipy family of internal charge transfer fluorophores51 that show a distinctive emission solvatochromic effect (Figure S11). Different from 1, compound 2 did not show any discernable effect of excitation wavelength on the frequency and band shape of the emission (Figures S13-15). Meanwhile, the keto band intensity of HBO in several solvents was found to be amplified relative to the enol band as excitation wavelength increased (Figures S16-S18). While consistent with the reports on HBO by others,52,58 it is the opposite behavior from that of compound 1. When compound 1 was partially deprotonated, the mixture of neutral and anionic species would result in excitation-dependent emission. The major emission band of compound 1 in DMSO in its neutral form was centered at 445 nm (the enol emission), with a shoulder extending to 600 nm (the keto emission, Figure 6a). When 0.6 molar equivalents of DBU was added to this solution, the remaining neutral species could still be excited within the shorter wavelength region, while the enolate emission centering at 585 nm grew as the excitation wavelength increased (Figure 6b). At 1.7 molar equivalents of DBU, the major species in the sample was the enolate, the emission of which at 585 nm dominated the almost entire scanned excitation wavelength region (Figure 6c).


13


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Figure 6. The emission intensity contour plots of compound 1 (10 M) as a function of excitation (vertical, 360-400 nm) and emission (400-700 nm) wavelengths, measured in the presence of (a) 0, (b) 0.6, and (c) 1.7 molar equivalents of DBU.


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(g) Fluorescence quantum yields and lifetimes. The fluorescence quantum yields of compound 1 were measured in the six solvents listed in Table 1. Values larger than 10% were observed in DCM, which was assigned to almost exclusively the keto emission (Figure 3), and in HB-basic solvents NMP and DMSO. The emission in the latter two solvents primarily came from the enol form (Figure 3). The deprotonated form (1-enolate) had a high emission quantum yield of 78%. The increase in fluorescence quantum yield of aryl alcohol-based dyes upon deprotonation has been reported in many other cases, notably for fluorescent pH indicator molecules.70-72 Excited state intramolecular proton transfer (ESIPT) is expected to diminish fluorescence,73-75 which often results in quantum yield values of a few percentage points or less. Therefore, the emission yields of compound 1 in DCM, NMP, and DMSO exceeded that expectation. It should be noted that although rare, bright ESIPT-type of dyes, including HBO derivatives, are continuously being reported.76-78 In contrast to 1, the ESIPT-disabled 1-MOM has a high fluorescence quantum yield of 71% measured using a relative method79 at excitation wavelengths of 340 and 360 nm. Fluorescence lifetimes were measured using the time-correlated single photon counting (TCSPC) method. The neutral samples were excited at 370 nm using a LED source. The emission was observed over a wavelength range that covers both enol and keto emissions (420-650 nm). In DCM where little enol emission was observed, the coverage of emission was 500-600 nm. In all solvents (DCM, ACN, MeOH, DMF, and DMSO), two decay components were observed (Figures S1-5), the shorter one was sub-ns, and the longer one was in the range of 1-3 ns. The abundance of the longer component increased with increasing emission wavelength. In DCM where the keto emission dominated, t1 = 0.8 ns while t2 = 1.8 ns. The abundance of t1 decreased from 85% to 31% as the emission wavelength moves from 500 nm to 600 nm. HBO, in contrast, decays single exponentially in DCM53 or hexane52 with a time constant of 0.3 ns of the keto emission. Because


15


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the abundance of the longer component t2 of 1 increased as the emission wavelength becomes longer, t2 therefore should not be ascribed to the decay of the enol emission.52,58 Considering the relatively large dipole moment of the keto excited state (7.2 D as calculated), it is likely that t2 belongs to a solvent-stabilized population of the excited keto tautomer that carries a charge-transfer character, although more experimental work needs to be done to test this hypothesis. The anion of compound 1 in DMSO produced via deprotonation using DBU resulted in a monoexponential decay (Figure S6) when excited using a 460-nm LED source, with a time constant of 2.5 ns.

Discussion Fluorescence quantum yield and the composite emission color of HBO derivatives, or of any ESIPT fluorophores, determine their potential usefulness as energy-efficient organic light-emitting materials. These two parameters are known to be functions of solvent and substituent on the HBO core (Equation 1).41,80 What has not been as thoroughly studied is the dependency of those two properties on excitation wavelength, which may arise either from the solvent-dependent structural distribution81 or from proton transfer chemistry in the ground state.82 This work addresses the excitation-dependency of the emission of compound 1. 𝜙𝑓, 𝑐𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑒 𝑐𝑜𝑙𝑜𝑟 = 𝐹(𝑆𝑜𝑙, 𝑆𝑢𝑏, 𝜆𝑒𝑥, 𝑎𝑐𝑖𝑑, 𝑏𝑎𝑠𝑒)

(1)

f: fluorescence quantum yield; composite color results from the mixing of enol, keto, and enolate emission bands; Sol: solvent; Sub: substituent; ex: excitation wavelength.

In this section, we present the result of computation to explain the properties of compound 1 in the context of model compounds HBO and 2, and to offer support to the model illustrated in Figure 1. It should be noted that the excited state structures and energies of HBO have been calculated


16

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before using different methods.63,83,84 In addition to the results from computation, we apply the model in Figure 1 to produce white color emission from compound 1 using combinations of controlling factors (excitation wavelength, quantity of base, solvent). (a) Computational studies. The computational studies were conducted to understand how the vinylenebipy substituent para to the hydroxyl in 1 affected (1) absorption and emission, (2) the propensity of proton transfer on the ground and excited state surfaces, (3) the excitation-dependent emission in HB-basic solvents such as N,N-dimethylformamide (DMF), relative to those properties of HBO. Apart from a few noted cases, the calculations were done without consideration of solvation. Therefore, the absolute values of computed energies do not reliably reflect the experimental values; rather, the relative values within a series of controlled calculations may facilitate the understanding of experimental observations.


17


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Figure 7. Frontier molecular orbital diagrams of 1-enol (left) and 1-enolate (right) calculated at the DFT/B3LYP/def2-TZVP level of theory. The relative orbital energy values are listed.


18

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Ground state geometries and excitation energies. The ground state geometries of 1, 2, and HBO, as well as their conjugated bases were optimized using DFT at the B3LYP level of theory. The relevant computed structural and spectroscopic data are listed in Table 2. First, we considered the ground state properties of the conformers of 1 that contain an O-H···N hydrogen bond, which were the ones that would undergo ESIPT when excited. Of four such conformers generated from the rotation of the two bonds trans on the alkene, the one shown in Figure 1a was the most stable (also see Figure S20), which was selected as the subject of the ensuing calculations. The frontier molecular orbitals (FMOs) of compound 1 are shown in Figure 7. In both 1-enol and 1-enolate, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) primarily localized on the p-hydroxyphenylvinylenebipy portion of the molecule, although the difference of orbital occupancy between HOMO and LUMO was more apparent in the anion. The second notable feature was the involvement, or the lack thereof, of the benzoxazole component in the FMOs. In the neutral form, HOMO-1 and LUMO+1 extended to benzoxazole, while in the anion, benzoxazole was minimally involved in the FMOs. This observation is consistent with the hypothesis illustrated in Figure 1 – when compound 1 is deprotonated, the CT2 transition is amplified to dominate the electronic transitions of this compound (vide infra).

Table 2. Computed ground state structural and spectral data of most stable conformers.a compound

 (D)

1 (nm)/fb

S1 Dom. Contr. (TDDFT)

2 (nm)/fb

S2 Dom. Contr. (TDDFT)

1 (enol)

2.2

383/0.11 (dft) 332/0.26 (cc2)

HOMOLUMO+1: 51% HOMOLUMO: 45%

376/1.62 (dft) 324/1.95 (cc2)

HOMOLUMO: 52% HOMOLUMO+1: 45%

1 (anion)c

23

422/1.41

HOMOLUMO: 98%

399/0.33

HOMOLUMO+1: 98%

2

1.5

373/1.42 (dft) 323/1.70 (cc2)

HOMOLUMO: 98%

317/< 0.01 (dft) 273/0.09 (cc2)

HOMO-2LUMO: 94%


19


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2 (anion)c

33

440/1.28

HOMOLUMO: 84%

314/0.10

HOMOLUMO+1: 83%

HBO (enol)

2.1

317/0.38 (dft) 300/0.38 (cc2)

HOMOLUMO: 92%

279/0.34 (dft) 261/0.26 (cc2)

HOMO-1LUMO: 84%

HBO (anion)c

8.7

391/0.44

HOMOLUMO: 98%

327/< 0.01

HOMOLUMO+1: 99%

a. (TD)DFT/B3LYP/def2-TZVP level of theory. : dipole moment. The MO contributions to the electronic transitions are listed. The excitation energies () were also calculated using the coupled cluster method (cc2) with the resolution-of the-identity (RI) approximation;85-87 b. f: oscillator strength; c. DMSO ( = 47, n = 1.5) was simulated as the solvent using the COSMO model,88 and the excitation energies () were calculated using the ADC(2) level of theory.89

The first two excitation energies of the DFT-optimized geometries of 1, 2 and HBO, as well as their conjugated bases, were calculated using TDDFT/B3LYP and CC2 levels of theory (Table 2). The wavelength values resulted from CC2 were shorter than those from TDDFT by ~ 50 nm (~ 0.5 eV) for compounds 1 and 2, and ~ 20 nm (~ 0.2 eV) for HBO. The experimental values of the lowest energy absorption bands in all solvents fell between the values from TDDFT/B3LYP and CC2. There have been numerous discussions on the accuracies or the lack thereof between the density- and wave function-based methods in calculating excited state energy values. TDDFT/B3LYP has been noted specifically to underestimate the excitation energies of states with charge transfer characters,90-94 such as 1 and 2. Therefore, we do not interpret that the calculated values as quantitatively accurate; rather, the difference of these values between the molecules help formulate a qualitative photophysical model of these compounds. The values of oscillator strengths from these two methods were closer. Each of the first two electronic excitations of 1 involved both HOMOLUMO and HOMOLUMO+1 transitions, while HOMOLUMO+1 actually contributed more than the HOMOLUMO in the lowest excitation to the S1 state. The closeness in energy of LUMO and LUMO+1 (0.16 eV, Figure 7) was a major reason of the substantial involvements of both transitions in the first two excitations. This “dual-contribution”, however, was not seen in the model compounds HBO and 2. The electron


20

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density difference95 between the S0 and S1 states of HBO resembled that between the S0 and S1 states of 1, while the density shift of the lowest excitation of 2 matched the excitation to the S2 state of 1 (Figure 8a). Based on this analysis, it was to be expected that the S1 state of compound 1 in the neutral form has high propensity for ESIPT, which was experimentally observed. When compound 1 was deprotonated, the contribution of the benzoxazole component to the FMOs diminished (Figure 7). Consequently, the excitation to the S1 state was accompanied by an electron density shift that was mimicked by the S0 to S1 excitation of deprotonated 2 instead (Figure 8b). The electronic structural changes of the first two excitations were consistent with the design idea of 1, namely, it would exhibit the combined effect of two electronic transitions (ESIPT and CT2) that are orthogonal in space (Figure 1). Either transition can be switched on and off via proton transfer: ESIPT is on in the neutral form, while the CT2 takes over in the anion form.

Figure 8. Electron density difference plots95 of singlet excitations (S0 → S1 or S2) of 1, 2, and HBO (a) and their conjugated bases (b, isocontour value 0.002 au). The electron density increases are noted white while decreases are noted yellow.


21


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Figure 9. Minimal energy paths of HBO (a) and 1 (b) along the O−H coordinate (0.8−2.2 Å) on the ground (GS), first (XS1), and second (XS2) excited states. Minima and transition states are marked by red and blue arrows, respectively.

Minimal energy proton transfer paths of 1 and HBO. The energies of HBO and compound 1 along the OH coordinates on the ground and excited states were calculated on the DFT/def2-SV(P) and TDDFT/def2-SV(P) levels of theory, respectively. The minimal energy path calculation required to freeze one internal coordinate, in this case the distance of the OH bond, while all other internal coordinates were allowed to relax.75,96-104 The stationary points (minima and transition states as marked on Figure 9) along the OH coordinates were preliminarily identified. These stationary points were then optimized again without any frozen coordinate using the bigger basis sets def2-TZVP to produce the data listed in Table 3. Only the enol isomer was identified on the ground state surface of HBO, while both enol and the ESIPT product keto tautomer appeared on the S1 surface (Figure 9a). A barrier of 0.8 kcal/mol for the ESIPT of HBO on the S1 surface was determined (Table 3). To put this number into perspective, the same method, basis sets, and convergence criteria returned a value of 2.7 kcal/mol for the C-C rotational barrier of ethane. The dipole moment of the transition state was larger than


22

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those of both enol and keto tautomers. The keto tautomer was more stable than enol by 5.6 kcal/mol, and the calculated early transition state of ESIPT was consistent with the exothermicity.

Table 3. Relative energies (E) and geometries on different electronic levels. Compound

Dipole moment (D)

OH (Å)

NH (Å)

E (kcal/mol)

HBO (S0)

2.1

0.99

1.79

-

HBO-enol (S1)

2.7

1.04

1.60

X

HBO-TS (S1)

2.9

1.18

1.33

X + 0.8

HBO-keto (S1)

2.2

1.87

1.03

X – 5.6

1-enol (S0)

2.2

0.99

1.78

A

1-TS (S0)

4.8

1.47

1.10

A + 11

1-keto (S0)

5.2

1.62

1.06

A + 11

1-enol (S1)

16

1.01

1.67

B

1-TS (S1)

12

1.26

1.24

B + 3.2

1-keto (S1)

7.2

1.82

1.03

B – 0.2

1-enol (S2)

2.5

1.00

1.71

C

1-TS (S2)

9.9

1.40

1.14

C + 8.5

1-keto (S2)

11

1.70

1.04

C + 8.0

In the ground and the first two excited states of compound 1, both enol and keto tautomers were identified. In the ground state, the keto tautomer occupied a shallow minimum, with an energy that was almost identical to the energy of the transition state of proton transfer – 11 kcal/mol higher than the enol tautomer. Therefore, for all intents and purposes, the ground state of 1 adopted the enol structure. On the S1 surface, a barrier of 3.2 kcal/mol was identified, and the keto tautomer was 0.2 kcal/mol more stable than enol. The oscillator strength of the excited enol was smaller


23


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than that of excited keto tautomer (Table 4), which in part explains the fact that the keto emission dominated in non-polar solvents. The excited enol had a large dipole moment (16 D), consistent with the experimentally observed enol emission solvatochromism. The dipole moment decreased as the structure traversed the transition state (12 D) to the keto tautomer (7.2 D). Given the fact that the S1 transition state of 1 was relatively late on the reactive coordinate (i.e., relatively closer to the product) comparing to that of S1 of HBO (Table 3), this trend of dipole moment change can be explained by an early charge transfer event which maximizes the dipole moment, followed by proton transfer that neutralizes the built-up change. When compound 1 was excited to the S2 state, a minimal energy path similar to that of the ground state was observed. The enol tautomer was the dominant structure, and its energy was quite similar to that of the enol tautomer on the S1 state (Figure 9b). The closeness of enol energy in both S1 and S2 states can be attributed to the similar energies of the LUMO and LUMO+1 orbitals and suggests that a structural modification might cause the reaction path profiles of S1 and S2 to flip – i.e., to enable ESIPT to occur on S2 surface instead of S1.

Table 4. First singlet excited state (S1) dipole moments (), excitation energies (), oscillator strengths (f), and MO contributions to the S1 states. Compound

 (D)

 (nm)/f

S1 Dom. Contr. (DFT)

1 (enol)*

16

453/0.06 (dft) 383/0.13 (cc2)

HOMOLUMO: 74% HOMOLUMO+1: 24%

1 (keto)*

7.2

543/0.42 (dft) 492/0.62 (cc2)

HOMOLUMO: 98%

2*

5.7

423/1.54 (dft) 368/1.79 (cc2)

HOMOLUMO: 98%

HBO (enol)*

2.7

354/0.33 (dft) 338/0.36 (cc2)

HOMOLUMO: 93%


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HBO (keto)*

2.2

473/0.18 (dft) 471/0.30 (cc2)

HOMOLUMO: 99%

Excited state geometries and excitation energies. The excited state geometries of 1, 2, and HBO were optimized using TDDFT/B3LYP/def2-TZVP level of theory. The excitation energy values of the optimized structures were determined using theories of TDDFT and CC2 (Table 4). These numbers can be approximated to be the emission energy values. For HBO, the values that were calculated from both theories were close to each other and to the experimental values. Similar to the excitation energies of the ground state structures in Table 2, the results of compounds 1 and 2 from the two theories however diverged, where the predictions from TDDFT were 50-70 nm longer than those from CC2. The values of oscillator strengths were closer. The emission from the first excited state (S1) of 1 would heavily involve the HBO component, based on, first, the similarity between the oscillator strengths of the S1 excitations of 1 and those of HBO. Secondly, for all three compounds, the contribution to the S1 state came primarily from the HOMOLUMO transition (Table 4), and the LUMO of both the enol and keto forms of 1 (at the optimized excited state geometries) occupied to a significant degree the benzoxazole component (See Figure S21). Comparing to compounds 1 and HBO, the S1 excitation of compound 2 had a much larger oscillator strength, similar to the S2 excitation of compound 1. Taken together with the analysis of the absorption properties of 1, 2, and HBO (Table 2), the excitation energies of the excited states (i.e., the emission data) corroborated the conclusion that the S1 transitions (both absorption and emission) of 1 involved the ESIPT-capable HBO component, while the absorption to the S2 state involved primarily the CT2 fluorophore that was compound 2. ESIPT of HBO and 1. To study the similarity and differences between the ESIPT reactions of HBO and compound 1, the geometries of the starting materials (i.e., enol), transition states, and


25


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the products (i.e., keto) were optimized on the S1 surfaces (Table 3), and the electrostatic potential (ESP) maps of these structures were produced (Figures 10-12). Based on the ESP maps, the elevation of HBO from the ground (GSenol) to the excited (XSenol) states accompanied a noticeable electron density movement from the hydroxyphenyl to the benzoxazole moiety (Figure 10). The decrease of electron density of the hydroxy was consistent with the formation of a photoacid, which drives the proton transfer.105-107 In the transition state (XSTS), the nitrogen of the benzoxazole was being neutralized, yet the electron density distribution did not change much otherwise. When the proton transfer was complete (XSketo), the electron density of the benzoxazole component started to shift back to that of the ground state enol, setting up back-proton transfer as the compound returned to the ground state.

Figure 10. HBO with electrostatic potential mapped onto the electron density. The isocontour value for the density is 0.01, while the range of 0.1 (blue) to -0.04 (red) is used for the value of electrostatic potentials.


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The excitation of compound 2, which is a mimicry of the CT2 transition of compound 1, was accompanied by an electron density shift from the hydroxyphenylvinylene to the bipyridyl (Figure 11). Therefore, for both HBO and compound 2, electron density shift occurred upon electronic excitation, and the origin of the shift was the hydroxyphenyl moiety.

Figure 11. Compound 2 with electrostatic potential mapped onto the electron density. The isocontour value for the density is 0.01, while the range of 0.15 (blue) to -0.08 (red) is used for the value of electrostatic potentials.

Based on the analyses of the ESPs of HBO and compound 2, one might reasonably deduce that the hydroxyphenyl would be the electron donor for any electronic transition that involves charge transfer in compound 1, which is a hybrid of HBO and compound 2. Yet based on the ESPs of compound 1 (Figure 12), the charge transfer upon excitation was attributed primarily to the electron density shift from the vinylene part to the benzoxazole, while the hydroxyphenyl did not show as much change in density. The ESIPT of 1, therefore, was driven by the photobasicity of benzoxazole, rather than the photoacidity of hydroxyphenyl as in the case of HBO. The enol excited state of 1 exhibited the largest degree of charge separation, which explained its large dipole


27


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moment (16 D). The dipole moment was progressively decreasing as the proton transfer reaction traversed the transition state to the keto tautomer. The evolution of dipole moment on the reaction coordinate was consistent with the ESPs, on which the density was shifting back to the vinylenebipy component while the proton transfer was occurring.

Figure 12. Compound 1 with electrostatic potential mapped onto the electron density. The isocontour value for the density is 0.01, while the range of 0.15 (blue) to -0.08 (red) is used for the value of electrostatic potentials.

Excitation-dependent emission of 1. The last portion of the computation concerns with the subtle excitation dependence of the enol/keto emission ratio of compound 1 and HBO observed in NMP (Figure 5) and DMF (Figure S9). When the excitation wavelength was scanned from 310-380 nm, the enol emission band of 1 grew relative to the keto band. Yet the opposite was true for HBO when the excitation wavelength increased from 300-340 nm (Figures S16-18). These excitation dependencies could be assumed to have originated from the excitation of different solvated


28

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structures that produce preferentially enol or keto emissions. The structures of intramolecularly hydrogen bonded 1 and its DMF hydrogen-bonded form were optimized under the COSMO model ( = 37, n = 1.4). The DMF-solvated structure was only 1.3 kcal/mol (1.0 kcal/mol for HBO, Figure S22) higher in energy than the sum of the energies of the intramolecularly hydrogen bonded 1 and DMF (Figure 13). Therefore, we considered that the DMF-solvated compound 1, which would produce the emission of the enol form, existed in a significant portion in the sample. The calculated lowest excitation energy of DMF-solvated compound 1 was lower than the intramolecularly hydrogen bonded counterpart (Figure 13), consistent with the observed emission dependence on excitation – increasing excitation wavelength favored the enol emission, likely from the DMF-solvated structure. HBO on the other hand had a larger calculated excitation energy in the DMF-solvated form than the intramolecularly hydrogen bonded form (Figure S22). Therefore, the keto emission of HBO would be favored instead at a longer excitation wavelength, which was found to be consistent with the observations in Figures S16-18.

Figure 13. Simulated UV-VIS spectra from first 5 singlet excitation energies of compound 1, calculated at TDDFT/B3LYP/def2-TZVP level of theory under the COSMO model, in intramolecularly hydrogen bonded (blue) and DMF-solvated (red) forms.


29


N 1

HBODC, or diCN-HBO N (CT2)

(C T2 )

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

Page 30 of 56

O

NC

O CN

N

N HO

HO (CT1/ESPT)

8

(CT1/ESPT)

CN CN

O N HO

NBu2

HBBO O F3C

O

N HO

Figure 14. Molecular structures of compounds 1 (this work), HBODC108 (aka diCN-HBO),109 8,110 and HBBO.111

(b) The comparison between compounds 1, HBODC, and 8 (Figure 14). Many substituted HBO molecules have been reported;80 yet characterizing how excitation energy or the presence of acid or base on the emission properties of those compounds has not been a focus of study. Studying the effect of ground state factors on emission color and brightness is meaningful because excitation energy is one parameter that can be relatively easily altered in a molecularly based functional device. Three reported compounds are listed in this subsection (Figure 14) for their structural relevance to 1 and therefore their possible dependency of emission on excitation and acid or base. Compound 1 can undergo either ESIPT (on S1) or CT (on S2) upon excitation. The electron density shifts of 1 to the first two excited states (Figure 8) support this conclusion. The proton donor in ESIPT and the electron donor in CT coincide at the hydroxyphenyl group. Deprotonation abrogates the ESIPT while amplifies the CT process (also see Figure 8).


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In terms of having separate proton and electron acceptors yet a singular entity as the donor of both proton and electron, compound 1 is similar to HBODC (Figure 14) reported by Park and coworkers,108 which is also known as diCN-HBO in a detailed study on the spectroscopic properties of this compound.109 HBODC and compound 1 are different in the relative orientations of the CT and ESIPT transitions: in compound 1, the two transitions are divergent in the physical spaces they occupy, while in HBODC the two transitions occupy relatively the same molecular space and arguably reinforce each other. This difference is manifested in emission solvatochromism: for compound 1, the enol emission is more positively solvatochromic than the keto emission, while the opposite is true for HBODC. A more apt comparison with compound 1 should be compound 8 (Figure 14) that has an electron-withdrawing dicyanovinyl group para to the hydroxyl, reported as the signaling product of a Pd(II)-sensitive molecular probe by Pang and coworkers.110 In our view, compound 8 is relevant enough to warrant a rigorous photophysical study to determine the factors that impact the enol/keto emission ratio, fluorescence quantum yields, solvent effect, as well as the properties of the enolate. Another example of HBO that bears a vinylene-like substituent para to the hydroxyl is HBBO (Figure 14), reported by Ziessel and coworkers.111 That work focused on the effect of substituent on the composite emission colors of those HBO derivatives. HBBO, which carries an electrondonating substituent para to the hydroxyl, was the compound that affords close-to-white emission in film. Compounds in the same vein as HBBO have subsequently been published.76,77 In addition to the reported effect of substitution on emission, it would be interesting to characterize the excitation dependency of the emission, and also the emission of the deprotonated HBBO.


31


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Scheme 2. The model to explain the emission of compound 1. solvated enol* enolate*

bipy

N

enol*

bipy

N

O

O

N O

bipy

bipy

O N

H O

H O

H S

O

keto*

O

bipy

base

N O

bipy N

S

O O

enolate

O N

O

solvated enol

bipy

H O H S

enol

(c) A model of the multiple fluorescence of 1 (Scheme 2). Based on the wavelength- and solventdependent spectroscopic properties and calculated data of compound 1, a model of the electronic states of 1 that contribute to its multi-fluorescence is sketched in Scheme 2. In a non-polar environment, an intramolecular HB exists within compound 1, which upon excitation leads to the emission of the tautomeric keto form (green on the right).52 The emission of the intramolecularly hydrogen bonded form is attributed to the vinylene-substituted HBO component. In hydrogen bond-basic solvents such as DMF or DMSO, the solvated form is present in a significant portion, which would lead to the normal emission (blue in the middle) that originates from the hydroxyphenylvinylenebipy component. The charge transfer character of this component is amplified via deprotonation (left) to lead to a red-shifted emission (red on the left). The absorption profiles of the intramolecularly hydrogen bonded, solvated, and deprotonated forms differ, which results in excitation-dependent emission of 1.


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Figure 15. (a) Emission of compound 1 visualized by a handheld UV lamp (λex = 365 nm) in DCM, ACN, EtOH, DMF, DMSO, and DMSO + DBU from left to right; (b) Emission of compound 2 visualized by a handheld UV lamp (λex = 365 nm) in DCM, ACN, EtOH, DMF, DMSO, and DMSO + DBU from left to right. The photographs of HBO are not included because the overall emission is too weak comparing to that of 1 and 2, and the enol emission of HBO is in the UV range not perceptible by human eyes.

(d) The comparison of compound 1 with its structural components HBO and 2. A visual inspection of the emission of 1 and 2 under the illumination of a handheld UV lamp ( = 365 nm) in various solvents showed the sensitive solvatochromic effect: compound 1 (Figure 15) exhibited a green fluorescence in DCM, ACN, and EtOH attributable to the dominant keto tautomer emission. The fluorescence appeared pale in DMF and DMSO as the enol and keto bands were balancing out as perceived by human eyes. The solvent-dependent emission property of 1 resembled those of typical ESIPT-type fluorophores, such as HBO.52


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The deprotonated compound 1, however, revealed its departure in the electronic structure from HBO. When HBO was deprotonated, the emission shifted to the blue side of the keto tautomer.53 The emission of 1 on the other hand shifted to orange, longer than that of the keto tautomer. The emission color was similar to the deprotonated compound 2, which was consistent with the conclusion from computation that the CT2 transition, which HBO lacked, dominated in the deprotonated 1. This outcome lends support to the model in Figure 1, in which 1 is illustrated as having two electronic transitions that are divergent in the space they occupy. When compound 1 is deprotonated, the CT2 transition is amplified, which is supported by the comparison of the relative band positions of enol, keto, and enolate (anion) of 1, 2, and HBO (Table 5).

Table 5. Emission band positions of enol, keto, and enolate (anion) of 1, 2, and HBO. Enol (in DMSO)

Keto (in DCM)

Enolate (in DMSO)

1

436 nm

544 nm

590 nm

2

459 nm

n.a.b

618 nm

HBOa

370 nm

485 nm

450 nm

a. Data taken from reference #53; b. not applicable.


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Figure 16. (a) CIE1931 of 1 (0.5 M) in various solvents: DCM (navy), ACN (cyan), EtOH (green), DMF (yellow), NMP (orange), DMSO (red), DMSO + DBU (red diamond) excited at 330 nm (circles) and 400 nm (diamond);112 (b) CIE 1931 of 1 (0.5 M) in DMSO with 0.6 equiv. DBU (green) and 0.3 equiv. DBU (blue) excited from 364-394 nm and 354-402 nm respectively (Δλex = 2 nm). Perfect white light (0.33, 0.33) is marked by an “x” as a reference point, which was achieved at ex = 364 nm, 0.6 molar equivalent of DBU (photograph in inset of (b)).

Compound 1 could provide a white emission in DMSO with a contribution from 1-enolate, based on the fact that a straight line can be drawn on the CIE coordinates plot between the points of DMSO and DMSO + DBU shown in Figure 16a, and pass through (0.33, 0.33). DBU (0.6 molar equivalent) was added to 1 in DMSO, and the CIE coordinates were collected every 2 nm through the range of absorption (Figure 16b). White light emission (0.33, 0.33) was materialized when the sample was excited at 364 nm. The emission spectrum of this particular sample is shown in Figure


35


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S19. When 0.3 molar equivalent DBU was added, a coordinate of (0.32, 0.33) was achieved when excited at 360 nm.

Conclusion In summary, a fluorophore (1) capable of reaching triple emissive excited states is described. Compound 1 has the propensity to undergo excited state intramolecular proton transfer (ESIPT) and charge transfer (CT) processes over different regions of the fluorophore structure, and the switch between ESIPT and CT can be actuated via ground state acid/base chemistry. The ESIPT behavior of the neutral molecule mirrors that of HBO closely, while the excitation-dependent emission in polar solvents and the emission of the anion reveal the departure from the photophysics of HBO and the latent charge transfer property of the stilbenoid component of 1. Particularly noteworthy is the wavelength of the anion of 1, which is longer than those of both enol and keto emission bands. The anion emission represents a major difference from the property of HBO, the anion of which emits in a wavelength region that is in between those of enol and keto emission. The photophysical properties of 1 are supported by an electronic structural model of two electronic transitions (ESIPT and CT) that only shares the hydroxyphenyl moiety in space. This model and experimental observations are supported by computational studies. The product of this work is a molecule (compound 1) capable of multiple emission that can be controlled by applying excitation of different energies and the application of a base. The molecular design embodied in compound 1 may find utilities in applications requiring excitation-tunable multiple fluorescence. Toward this objective, and based on the structural-functional relationship uncovered in this work, the molecular structure of 1 will be modified to improve its emission quantum yield and to determine the factors that control the ratios of the normal and tautomer emission bands.


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Supporting Information. The method of calculations and additional figures, additional spectra, and 1H and 13C NMR spectra of new compounds. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT This work was supported by the National Science Foundation (CHE1566011).

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Excitation-Dependent Multiple Fluorescence of a Substituted 2-(2

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