Solvatochromic Fluorescence Emission of an Anthranol Derivative

Article pubs.acs.org/JPCC

Solvatochromic Fluorescence Emission of an Anthranol Derivative without Typical Donor−Acceptor Structure: An Experimental and Theoretical Study Jian Zhang,† Chongyang Zhao,‡ Heng Liu,† Yanlin Lv,† Rongji Liu,§ Shuangshuang Zhang,§ Hui Chen,*,‡ Guangjin Zhang,*,§ and Zhiyuan Tian*,† †

School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences (UCAS), Beijing 100049, P. R. China Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China § Key Laboratory of Green Process and Engineering, Beijing Key Laboratory of Ionic Liquids Clean Process, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P.R. China ‡

ABSTRACT: An anthranol derivative (DTDDP) and an anthraquinone derivative diastereomer (DDDP) without typical donor−acceptor (D−A) structural features were synthesized from 1,4-diaminoanthraquinone via a one-step reaction. The absorption and fluorescence emission properties of DTDDP and DDDP were investigated in common organic solvents with different polarity. In spite of a lack of apparent D−A structure, DTDDP exhibited apparent solvatochromic fluorescence emission with shift >80 nm in aprotic media. The linear correlation of the Stokes shift of DTDDP in a Lippert−Mataga plot was observed, and the change in the dipole moment of DTDDP upon excitation, 10.7 D, was determined. Time-dependent density functional theory (TDDFT) calculation confirmed the marked hole−electron separation of the excited DTDDP molecule as compared to that of the excited DDDP molecule. The experimental results and the theoretical calculation revealed the excited-state intramolecular charge-transfer nature of the emitting state of DTDDP molecules, which is responsible for the observed solvatochromic fluorescence emission features. The photooxidation-mediated facile conversion of DTDDP to its anthraquinone form counterpart DDDP was also confirmed.

■

INTRODUCTION Conjugated organic compounds possessing intramolecular charge-transfer (ICT) features have been applied to many areas such as the construction of low band gap materials for optoelectronic devices,1−7 nonlinear optics,8−10 fluorescent chemosensors,11−14 and microenvironmental monitoring.15 Such types of compounds typically possess a π-conjugate bridge connecting an electron donor (D) and an acceptor (A) to facilitate ICT processes.16 Very recently, Adachi et al.17reported dual ICT fluorescence features derived from a phenothiazine-triphenyltriazine derivative and demonstrated the crucial role of the donor unit and the conformational heterogeneity of the molecules in the photophysical processes with thermally activated delayed fluorescence (TADF) characteristics involved. By integrating electron-donating and electron-accepting moieties into the molecule skeleton, Tang et al.18−20developed a series of ICT-based narrow band gap materials that possess the aggregation-induced emission (AIE) property with emission wavelength ranging from yellow to the NIR region for the construction of light-emitting diodes (LEDs). It is well-established that the ICT compounds typically exhibit solvatochromism features attributable to the change in the local molecular microenvironment that originates from the variation of solvent polarity.21,22 Additionally, solvatochromism © 2015 American Chemical Society

of ICT compounds is usually positive-going; namely, the emission feature undergoes a bathochromic (red) shift upon increasing solvent polarity, though emission features in negative correlation to the solvent polarity were also observed occasionally. The positive-going solvatochromism could be attributable to the solvent dipole relaxation around the excitedstate solvated chromophore and therefore the energetic stabilization of the excited-state chromophore in polar solvents. 23 For example, Jovin et al. 21 reported 7-(4(dimethylamino)phenyl)-3-hydroxy-4H-chromen-4-one (7AHC) displaying superior solvatochromic properties (i.e., >170 nm emission shift in aprotic media) originating from the change of orientation of the excited-state dipole moment upon excitation. It is noted that the change of dipole moment between the excited state and the ground state upon excitation, Δμ, is crucial to the typical ICT process,24 and the extension of the π-conjugation in the D−π−A system is expected to lead to an increase in the Δμ term and therefore substantial redshift of the fluorescence spectrum. Owing to their sensitive responses to the solvent polarity, compounds possessing ICT characterReceived: November 10, 2014 Revised: December 29, 2014 Published: January 12, 2015 2761

DOI: 10.1021/jp511264r J. Phys. Chem. C 2015, 119, 2761−2769

Article

The Journal of Physical Chemistry C Scheme 1. Synthetic Route for DTDDP and DDDP

Figure 1. (A) Normalized UV−vis absorption and fluorescence emission spectrum of DTDDP in THF (10 μM) and solid powder, λex = 405 nm. (B) Normalized UV−vis absorption and fluorescence emission spectrum of DDDP in THF (10 μM), λex = 600 nm.

molecules investigated in the present work, i.e., the anthranol derivative (3,4-diphenyl-2,3,5,6-tetrahydro-H,4H,7H,14H6a,14a-diaza-8,13-dioxa-naphtho[1,2,3,4-ghi] perylene, denoted by DTDDP) and the anthraquinone derivative diastereomer (1,2,3,4,5,6,7,8,9,14-decahydro-9,14-dioxo-4,5diphenylnaphtho[2,3-f ]-[4,7] phenanthroline-[(4α,5β)-and (4α,5α)-], denoted by DDDP), were synthesized starting from 1,4-diaminoanthraquinone, styrene, and formaldehyde, as illustrated in Scheme 1. It deserves mentioning that for the DTDDP product only trans-DTDDP was obtained via the neutral alumina column chromatography strategy. In contrast, both trans- and cis-DDDP diastereomer products were obtained with the ratio of trans- to cis-DDDP being ∼3.4:1. All the compounds were purified and characterized by NMR and mass spectroscopy (Experimental Section). These as-prepared compounds had favorable solubility in common organic solvents such as THF, dichloromethane, and toluene. Optical Properties. As shown in Figure 1A, the dilute solution of DTDDP in THF exhibited a broad absorption spectrum with an absorption peak at 405 nm and two shoulders around 384 and 450 nm. Upon excitation at 405 nm, the DTDDP/THF dilute solution sample displayed a broad structureless emission band in the range of 500−780 nm with a peak around 580 nm, characteristic of intramolecular charge transfer (ICT), and a Stokes shift of 175 nm. The DTDDP solid powder sample showed similar absorption features with comparable peak positions to its counterpart spectrum of the solution sample, suggesting that DTDDP molecules retain their identities in the aggregated state. Moreover, the DTDDP solid powder sample displayed a fluorescence emission profile similar to that of the solution sample with an additional shoulder at the long wavelength region, possibly originating from the formation of excimer in the solid state. In contrast to the absorption features of DTDDP, the DDDP sample displayed finestructured absorption features in the relatively long-wavelength region with the absorption edge extended to 700 nm and showed three absorption peaks at 560, 600, and 650 nm,

istics can be used as potential candidates for monitoring microenvironmental polarity in biological systems. Arai et al.25 employed 2-[(1E)-2-(1H-pyrrol-2-yl)ethenyl]-quinoxaline (PQX), an ICT-based compound, as a fluorescent chromophore to detect the protein binding site polarity in view of its full-color solvatochromic fluorescence. Yang et al.26 reported an aminonaphthalene 2-cyanoacrylate (ANCA) derivative as a probe for fluorescently discriminating different types of amyloid deposits based on the capability of stabilizing the ground versus excited states of ANCA depending on the microenvironment polarity. In this work, an anthranol derivative (DTDDP) and an anthraquinone derivative diastereomer (DDDP) were synthesized from 1,4-diaminoanthraquinone (Scheme 1). Interestingly, DTDDP exhibits apparent solvatochromism effects that originate from ICT character, in spite of lack of typical D−A structural features. Spectral investigations reveal that photoinduced ICT occurs in DTDDP and results in solvatochromism, which are further supported by theoretical calculations. In addition, it was found that DTDDP could be photooxidized to its anthraquinone form counterpart DDDP in an irreversible manner, which was confirmed by UV−vis absorbance, fluorescence emission spectroscopy, 1H NMR, and MALDITOF MS, respectively. The fluorophore synthesized and investigated in the present work represents a new paradigm where a compound without an apparent D−π−A molecule skeleton exhibits remarkable photoinduced ICT features, suggesting the potential of such types of compounds for ICT-based applications such as optoelectronic devices and molecular probes for microenvironment.

■

RESULTS AND DISCUSSION Fluorogen Preparation. DTDDP and DDDP were synthesized following a previous procedure that Mellor et al.27described (Scheme 1). The experiment details and structure characterization of the model compounds were described in the Experimental Section. In brief, two target 2762

DOI: 10.1021/jp511264r J. Phys. Chem. C 2015, 119, 2761−2769

Article

The Journal of Physical Chemistry C Table 1. Spectroscopic Properties of DTDDP in Selected Solvents λmax (nm) solvent

polarity (H2O 100)

Δf

Abs

Em

Δυ (cm−1)

QY (%)

cyclohexane toluene diethyl ether 1,4-dioxane THF ethyl acetate chloroform DCM acetone DMF DMSO acetonitrile methanol

0.6 9.9 11.7 16.4 21 23 25.9 30.9 35.5 40.4 44.4 46 76.2

−0.003 0.016 0.167 0.025 0.210 0.200 0.148 0.218 0.284 0.274 0.262 0.306 0.309

403 406 404 405 405 404 404 405 406 407 408 405 402

535 565 562 577 583 584 596 600 603 612 618 610 598

6122 6932 6958 7360 7538 7629 7793 8024 8047 8230 8329 8298 8154

71.5 87.1 22.7 18.3 12.4 10.8 5.3 6.6 4.6 3.9 2.6 2.5 2.3

Figure 2. Absorption and emission spectrum of DTDDP (10 μM) in different solvents (A) and Lippert−Mataga plot for DTDDP (B).

red-shifted emission band with a peak at 565 nm and the highest fluorescence quantum yield (QY = 0.87). In contrast, DTDDP in solvents with medium polarity, such as 1,4-dioxane, chloroform, diethyl ether, ethyl acetate, tetrahydrofuran, and dichloromethane (DCM), showed progressive bathochromic fluorescence accompanying the remarkable attenuation in QY with an exception of chloroform, which was possibly attributable to the acidity of chloroform. In solvents with higher polarity such as acetone, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), acetonitrile, and methanol, DTDDP exhibited a fluorescence emission band in the lowerenergy region with the maximum wavelength >600 nm. Additionally, the consecutive drop of fluorescence quantum yield upon increasing the polarity of the solvents, from acetone to methanol, was clearly observed, as shown in Table 1. Thus, a solvatochromic effect in the fluorescence emission of DTDDP ranging from 535 nm in cyclohexane to 618 nm in DMSO, which endowed an emission wavelength shift >80 nm in aprotic media, was eventually observed. Generally, only fluorophores that are themselves polar present large sensitivity to solvent polarity, to which nonpolar molecules are much less sensitive. Taking this, along with the lack of apparent D−π−A molecular configuration of the DTDDP molecule, the observed solventinduced shift in emission wavelength herein represented an unusual example of dependence of the Stokes shift on solvent polarity. Figure 2A displayed the normalized absorption and fluorescence emission spectra of DTDDP in three types of

respectively. Upon excitation at 600 nm, DDDP showed an emission band with peak around 670 nm and a shoulder at 713 nm. It is noted that no noticeable disparity in the absorption and emission features of the trans- and cis-DDDP were observed. Thus, the DDDP sample showed absorption and emission features in the lower -energy region and small Stokes (∼20 nm) as compared to the counterpart properties of the DTDDP sample, indicating the difference between the electronic configuration of these two model molecules that will be discussed in the next section. Solvatochromic Effect. As discussed above, compounds possessing typical D−π−A electronic configuration feature inherently facilitated ICT processes. Unexpectedly, the model compound, in spite of a lack of typical D−A structural features, clearly exhibited typical solvatochromism effects that originate from ICT character that displayed the absorption and fluorescence emission properties of DTDDP in different solvents with varying polarities. It can be clearly seen that while the absorbance of DTDDP displayed a negligible change in different solvents from cyclohexane to THF and DMSO with increasing polarity the fluorescence emission derived from the ICT band exhibited a remarkable solvatochromism feature. It was also found that fluorescence quantum yield of DTDDP roughly showed negative correlation to the solvent polarity, the higher the polarity the lower the yield, with few exceptions. Upon excitation at 405 nm, DTDDP in cyclohexane with minimum polarity emitted strong green-yellow light with an emission peak at 535 nm, while DTDDP in toluene presented a 2763

DOI: 10.1021/jp511264r J. Phys. Chem. C 2015, 119, 2761−2769

Article

The Journal of Physical Chemistry C

Figure 3. ORTEP diagram (A), the crystal cell (B), the interlamellar spacing (C), and the stacking mode viewed down the reciprocal cell perpendicular to the bc plane (D) of DTDDP.

general, a fluorophore possesses a dipole moment in the excited state (μe) larger than that in the ground state (μg). To explore the underlying information regarding the general solvent effects that DTDDP displayed, a change in the dipole moment from the ground state to the excited state (i.e., Δμ, given by μe − μg) was calculated based on the Lippert−Mataga equation. Specifically, the Onsager cavity radius, ∼6.0 Å, and the ground-state dipole moment (μg) of the DTDDP molecule, ∼3.76 D, were determined based on the calculation from the B3LYP/6-311G* level. On the basis of such Onsager cavity radius value and the slope of the Lippert−Mataga plot, the difference of the dipole moment between the excited and ground state of DTDDP, 10.7 D, was determined. Consequently, an excited-state dipole moment (μe), 14.45 D, of the DTDDP molecule was estimated, which is comparable to that of the fluorophores that are widely used as polarity probes. Such μe is comparable to a unit charge separation of 3.0 Å, a distance consistent with the molecular modeling results as discussed in the next section. Thus, the dipole moment of a solvated DTDDP molecule remarkably increases upon photoexcitation, which facilitates the subsequent rearrangement of solvent dipoles around the excited DTDDP molecule. Such a solvent dipole relaxation process virtually gives rise to the energy decrease of the excited-state DTDDP molecule, from the Franck−Condon state to the relaxed state, which energetically enable stabilization of the excited state in the polar solvent milieu. Consequently, emission of the DTDDP molecule at lower energy or longer wavelength was observed. A single crystal of DTDDP, obtained via recrystallization from a saturated solution of DTDDP in DCM and petroleum ether (1:1) at room temperature in the dark, was determined in

typical solvent with various polarities, i.e., cyclohexane, THF, and DMSO, from which a pronounced solvatochromic effect in the fluorescence emission of DTDDP can be clearly observed. The solvent-dependent fluorescence features of DTDDP were also evaluated quantificationally using the Lippert−Mataga equation (eqs 1 and 2), which has been depicted as the Stokes shift (cm−1) of the absorption and emission maxima versus solvent orientation polarizability (Δf). Δv ≡ vab − vem =

2Δf

(μe − μg )2 + constant

(1)

ε−1 n2 − 1 − 2 2ε + 1 2n + 1

(2)

hca3

Δf = f (ε) − f (n2) ≈

where Δυ is the Stokes shift (in cm−1); h is the Planck constant; c is the velocity of light; a is the Onsager cavity radius; μe and μg represent the dipolar moments in the excited and ground states; and ε and n are the dielectric constant and refractive index of the solvent, respectively. Figure 2B showed a Lippert plot for DTDDP in organic solvents with different polarities, which can be fit adequately with a linear function with a slope of the linear fit up to 3862. The linearity of such a Lippert plot usually indicates the dominant importance of the general interactions between fluorophores and solvents rather than a specific interaction such as hydrogen bonding, preferential solvation, in the spectral shifts. Additionally, such a Lippert plot result definitely presents positive solvatochromism of DTDDP. It is known that the spectral shift and intensity variation (enhancement or attenuation) of fluorescence are indicative of the subtle perturbations of the ground and excited states. In 2764

DOI: 10.1021/jp511264r J. Phys. Chem. C 2015, 119, 2761−2769

Article

The Journal of Physical Chemistry C monoclinic space group P2(1)/c with four molecules in a unit cell (Z = 4) and the unit cell dimensions of a = 10.9758 Å, b = 12.7058 Å, and c = 18.1689 Å, β = 91.232°, U = 2533.2 Å. Figure 3A and 3B shows the ORTEP diagram and the packing of DTDDP molecules in a single crystal, respectively. On the basis of the single-crystal characterization result, several types of intermolecular interactions were identified. Specifically, the predominant intermolecular interactions include CH···π interactions existing in C33−H···C5, C8−H···C19, and C27− H···C6, respectively, and a hydrogen bond in C2−H···O2. The relatively weak intermolecular interactions within the crystal, i.e., C26−H···C30−H and C27···C1, were also identified. Additionally, the packing of the DTDDP molecule along the a axis in the lattice (Figure 3C) and the vertical distance between the adjacent anthracene planes, approximately 7.27 Å (Figure 3D), were determined from the single-crystal data. Another piece of information derived from the single-crystal data is the typical nonplanar configuration of the DTDDP molecule in the crystalline state. Specifically, the dihedral angle between the anthracene ring and the two pendant benzene rings, 89.9° and 84.1°, respectively, and the angle between the two benzene rings, 11.3°, were identified. It also deserves mentioning that C8, C9, C14, and C15 are out of the anthracene ring by 0.61, 0.17, 0.04, and 0.62 Å, respectively, while C10, C11, C16, and C17 locate at the opposite side of the anthracene plane with distance away from the plane of 0.67, 0.23, 0.11, and 0.62 Å, respectively. These specific conformation features of the DTDDP molecule in the crystalline state, for instance the methylene moieties that connect the nitrogen and oxygen atoms (C8 and C17) locating at the opposite sides of the anthracene plane, provide the basis for the following theoretical calculation study on the DTDDP molecule. Theoretical Calculation. To explore the molecular mechanism of the large Stokes shift that DTDDP displayed as compared to that of DDDP, time-dependent density functional theory (TDDFT) was used to investigate the ground and excited states of these two molecules. The M06 functional was used for geometry optimization of DTDDP and DDDP. It is noted that the maximum absorption of DTDDP, with experimental value of ∼400 nm, can be described more accurately by TD-B3LYP (389.27 nm) as compared to that by TD-M06 (377.21 nm). Taking this, photoexcitation and fluorescence emitting states of DTDDP and DDDP molecules were optimized by the B3LYP functional in the present work. Figure 4 shows the optimized geometries of the ground state and fluorescence emitting states of the DTDDP molecule. It can be clearly seen that the methylene moieties connecting the nitrogen and oxygen atom locate at the opposite sides of the anthracene plane, perfectly consistent with the abovementioned single-crystal characterization data. Additionally, the computed maximum absorption at the B3LYP/6-311G* level is 389.27 nm with the oscillator strength of 0.5443, comparable to the counterpart experimental value of ∼400 nm, and the computed emitting peak of DTDDP in DMSO is 692.11 nm which is comparable to the experimental value of 618 nm. Upon absorption of a photon, an electron is excited from the HOMO into the LUMO, leaving behind a localized positively charged hole. The overlap of the electron and the hole in space represents the extent of the charge separation. To gain insight into the charge separation of the model compounds, the hole− electron analysis about the molecular orbitals (MOs) of DTDDP and DDDP molecules involving the excitation was

Figure 4. (A) and (C): Optimized geometries of DTDDP at the ground state viewed from different directions. (B) and (D): Optimized geometries of DTDDP at the excited state viewed from different directions.

carried out with results shown in Figure 5 and Table 2. It can be clearly seen from Figure 5 that the DTDDP molecule at the

Figure 5. Charge distribution of hole (blue)−electron (green) of DTDDP (A) and DDDP (B) at excited states.

Table 2. Important Parametric: Overlap Integral and Distance of the Hole and Electron overlap integral of hole−electron distribution distance between centroid of hole and electron/Å

DDDP

DTDDP

0.503 0.959

0.423 2.358

excited state displayed hole−electron separation with extent remarkably larger than that of the DDDP molecule. Specifically, the overlap integral of the hole−electron distribution of the DTDDP molecule at the excited state was identified as 0.423, while that of the DDDP molecule was 0.503. Such discrepancy in the overlap integral of the hole−electron distribution is indicative of the marked difference in the charge separation of DTDDP and DDDP molecules at their excited state. Additionally, a significant discrepancy in the distance between the centroid of hole and electron of these two types of model molecules at their excited state, namely, 0.959 Å for DDDP and 2.358 Å for DTDDP, was identified which firmly confirms the significant charge separation of the DTDDP molecule at its excited state. It is also noted that such an extent of charge separation in the DTDDP case is consistent with the aforementioned result of charge separation obtained from the excited-state dipole moment, suggesting that the photoinduced charge transfer in the DTDDP molecule is robust. Taking this, 2765

DOI: 10.1021/jp511264r J. Phys. Chem. C 2015, 119, 2761−2769

Article

The Journal of Physical Chemistry C

the exposure time. It can be clearly seen that as time elapsed the absorption band in the region of 500−700 nm attributable to the anthraquinone form underwent a noticeable increment at the expense of the absorption band centered at 404 nm originating from anthranol form. Informatively, such an absorption spectrum evolution revealed an isosbestic point at ∼500 nm, indicative of a clean one-to-one conversion between two forms. Figure 6B depicted the absorption features of DTDDP (10 μM) solution sample as a function of the illumination time, suggesting that the conversion nearly reached completion after 1500 s of illumintion. Owing to such conversion between the absorption bands in distinct regions, the DTDDP solution clearly changed from slightly yellowish to ink blue, as shown in Figure 6B. Such conversion from DTDDP to DDDP resulted in significant changes in fluorescence emission features. As shown in Figure 6C, the emission band centered at 565 nm attributable to the ICT band of DTDDP showed a remarkable decrease as the exposure time elapsed in toluene. However, unlike the evolution of absorption features, the remarkable decrease in the ICT emission band of DTDDP upon photooxidation was not accompanied by a noticeable increase in the emission band of DDDP centered at ∼670 nm. Such evolution of fluorescence emission features was attributable to the significant disparity between the fluorescence quantum yields of DTDDP (0.87) and DDDP (0.0014), which makes the feeble fluorescence signal of DDDP nearly completely obscured by that of DTDDP. By plotting the emission intensity at a much lower intensity scale, a gradual increase in the emission band of DDDP centered at ∼670 nm with exposure time was clearly observed, as illustrated in Figure 6D. Specifically, it can be seen from the evolution of emission bands centered at 565 and 670 nm, respectively, against the

it is reasonable to assign the emitting state of the DTDDP molecule to the charge-transfer excited state. Table 3 displayed the comparison of the calculated results of the absorption and fluorescence emission features of DTDDP Table 3. Calculated and Experimental Value of DTDDP and DDDP, Respectively DTDDP excitation/nm emission/nm

DDDP

calcd results

exptl results

calcd results

exptl results

389.27 692.11

400 618

619.46 710.21

600, 650 700

and DDDP compounds with their counterpart experimental results. It can be seen that the calculated absorption bands of DTDDP and DDDP compounds are roughly consistent with the corresponding experimental results. Additionally, the calculated emission maximum of the DDDP compound (710 nm) is in well accordance to the experimental result of 700 nm, while the calculated emission peak of the DTDDP compound (692 nm) is comparable to the counterpart experimental result of 618 nm. Noteworthy information deriving from Table 3 also includes the large Stokes shift that the DTDDP compound displayed as compared to that of the DDDP compound, in calculated and experimental results, suggesting the chargetransfer nature of the DTDDP molecule at the excited state. Photooxidation of DTDDP. Another noticeable feature of the model compounds investigated in the present work is the conversion from DTDDP to DDDP via a photooxidation process in an irreversible manner (Scheme 1). Figure 6A shows the absorption spectrum evolution of DTDDP solution in toluene (10 μM) upon illumination from a solar simulator over

Figure 6. Evolution of absorption (A) and fluorescence (C) spectra (λex = 405 nm) of DTDDP upon photooxidation in toluene and that of the absorbance peaks (C) and emission peaks (D) as a function of exposure time, respectively. Inset in (B) and (D): photographs of the DTDDP sample upon photooxidation with incremental exposure time under room light (B) and 365 nm light (D) illumination. 2766

DOI: 10.1021/jp511264r J. Phys. Chem. C 2015, 119, 2761−2769

Article

The Journal of Physical Chemistry C

Figure 7. Partial 1H NMR spectra of DTDDP in deuterated DMSO upon photooxidation with various exposure time at low (A) and high field (B). These spectra are displayed vertically for clarity.

that the proton signals exclusively attributable to DTDDP disappeared after 15 min photo-oxidation reaction, as shown in Figure 7A, and signals attributable to DDDP species and two broad featureless peaks around 7.0 and 7.25 ppm, respectively, that cannot be assigned to DDDP clearly appeared in the NMR spectra. Upon increasing the exposure time, the proton signals attributable to DDDP exhibited progressive enhancement at the expense of the two broad featureless peaks, which disappeared until 10 h exposure of light. It was speculated that an intermediate species, probably the species in the excited state or a metastable adduct derived from DTDDP and O2, was involved in the photooxidation-based conversion from DTDDP to DDDP. Unfortunately, the effort to separate and identify the intermediate failed. The conversion from DTDDP to DDDP was also corroborated by the MALDI-TOF MS characterization results. Specifically, the DTDDP sample prior to and after 3 h illumination presented m/z peaks at 496.2 and 470.1, respectively, which is in well accordance to the characteristic mass-to-charge ratio values of DTDDP to DDDP compounds.

exposure time that the photooxidation process reached completion after 1500 s of exposure, similar to the absorption features shown in Figure 6B. Owing to such a significant change in the fluorescence emission, fluorescence of the sample became extremely faint from bright yellow color before photooxidation-based conversion, as shown in the inset in Figure 6D. To investigate the influence of solvent on the conversion, the behavior of photooxidation of DTDDP was further investigated in two other typical organic solvents, namely, THF with medium polarity and DMSO with strong polarity. It was found that the photooxidation reaction in these two kinds of solvents displayed the time-elapsed absorption features similar to that in toluene. However, the solvent polarity exerted an obvious influence on the conversion ratethe stronger the polarity of the solvents, the lower the reaction rate of the photooxidation. Specifically, while the photooxidation in toluene nearly reached completion after ∼25 min illumination, it took 100 min and 180 min for the accomplishment of photooxidation in THF and DMSO, respectively. It was established that the dipole moment of the DTDDP molecule at the excited state is larger than that at the ground state. Taking this, the solvent with stronger polarity was expected to exert a more prominent effect on the stabilization of excited-state DTDDP species, which gives rise to the lower reaction rate of the photooxidation toward the photochemical product. 1 H NMR spectroscopy was carried out to confirm the photooxidation-based conversion from DTDDP to DDDP in deuterated DMSO solution. As illustrated in Figure 7A, exposure of DTDDP in deuterated DMSO solution to light clearly resulted in the emergence of proton signals at the low field with δ = 11.78, 8.31, 7.80, 7.37, 7.30, and 7.11 ppm. These proton signals are ascribed to the DDDP compound and exhibited progressive enhancement upon increasing the exposure time. These changes in the proton signals at the low field were echoed by the evolution of the proton signals at the high field upon photooxidation. Specifically, upon photooxidation-enabled conversion from DTDDP to DDDP, proton signals with δ = 4.06, 3.46, 2.97, 1.96, and 1.83 ppm affiliated to DDDP species clearly emerged in step with the appearance of the above-mentioned proton signals at the high field (Figure 7B). It is noted that 1H NMR characterization results revealed that the trans-DDDP diastereomer was the exclusive product of such photooxidation of trans-DTDDP, indicating that the stereochemical configuration of the starting DTDDP species most likely plays a vital role in determining the stereochemical features of the photooxidation product. It deserves mentioning

■

CONCLUSION In conclusion, an anthranol derivative (DTDDP) and an anthraquinone derivative diastereomer (DDDP) were synthesized from 1,4-diaminoanthraquinone via a one-step reaction, and their optical properties were investigated in the present work. In spite of a lack of typical D−A structural feature, the DTDDP compound exhibited apparent solvatochromism effects with emission wavelength shift >80 nm in aprotic media. It was demonstrated that the dipole moment increases from 3.76 to 14.45 D upon photoexcitation, and such a remarkably increased dipole moment facilitates the dipole relaxation of solvent molecules around the excited DTDDP molecules, which eventually resulted in an energy decrease of the excited-state DTDDP molecules and emission of DTDDP molecule with longer wavelength. Theoretical calculation results based on time-dependent density functional theory indicated that the excited-state DTDDP molecules possess hole−electron separation with extent remarkably larger than that of DDDP molecules. The theoretical calculation was in good accordance to the experimental results, and they revealed the photoinduced intramolecular charge-transfer nature of the emitting state of DTDDP molecules and the underlying mechanism for the observed solvatochromic fluorescence emission features. The photooxidation-mediated facile conversion of DTDDP to its anthraquinone form counterpart DDDP was confirmed by UV−vis absorption, fluorescence 2767

DOI: 10.1021/jp511264r J. Phys. Chem. C 2015, 119, 2761−2769

The Journal of Physical Chemistry C emission spectroscopy, characterization.

1

■

H NMR, and MALDI-TOF MS

Article

AUTHOR INFORMATION

Corresponding Authors

■

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected].

EXPERIMENTAL SECTION Materials and Measurements. All chemicals were purchased from Acros or Energy and used as received. 1H and 13C NMR spectra were measured on a Bruker Avance 600 MHz NMR spectrometer in deuterated DMSO with tetramethylsilane (TMS; δ = 0 ppm) as the internal standard. MALDI-TOF mass spectra were recorded on a Carlo-Erba1106 instrument. PL spectra were recorded on a spectrofluorophotometer (F-7000, Japan). UV absorption spectra were taken on Shimadzu UV2550 UV−vis spectrophotometers. The ground-state geometries were optimized with the B3LYP/6311G* program, using the Gaussian 03 package. Relative ΦF values were estimated by using Rhodamine B in ethanol (ΦF = 70%) as the standard. The single-crystal data were collected on a Rigaku Saturn diffractometer with CCD area detector. Synthesis of DTDDP and DDDP. To the solution of 2 equiv of trifluoroacetic acid (20 mmol) in acetonitrile (80 mL) under nitrogen atomosphere, 1,4-diaminoanthraquinone (10 mmol) was added, and the deep purple mixture was heated to 80 °C. Subsequently, styrene (40 mmol) mixed with 37% formalin solution (20 mmol) was added to the mixture dropwise, and the mixture was heated at reflux for 5 h. After the completion of the reaction, acetonitrile was removed, and then dichloromethane was added to the residual which resulted in a deep color solution. The solution was washed with sodium bicarbonate 3 times and distilled water 3 times, respectively, and then dried with anhydrous sodium sulfate. After solvent removal, the crude product was purified by neutral alumina column chromatography that yielded DTDDP as reddishbrown powder 0.68 g (14%), trans-DDDP 1.90 g (40%), and cis-DDDP 0.55 g (12%) both as deep blue crystalline solid, respectively. For DTDDP, 1H NMR (600 MHz, DMSO-d6) δ 8.159− 8.175 (2H, m), 7.487−7.504 (2H, m), 7.237−7.337 (6H, m), 7.003 (4H, d, J = 7.2 Hz), 5.220 (2H, d, J = 6.6 Hz), 4.891 (2H, d, J = 6.6 Hz), 3.630 (2H, d, J = 3.6 Hz), 3.091 (2H, d, J = 10.2 Hz), 2.730 (2H, t, J = 10.8 Hz), 2.026 (2H, m), 1.785 (2H, d, J = 12 Hz); 13C NMR (150 MHz, DMSO-d6) δ 146.27, 140.33, 130.39, 129.55, 128.35, 127.75, 126.19, 124.70, 120.90, 112.07, 109.14, 80.69, 38.45, 37.53, 29.47. MALDI-TOF MS, m/z calcd for C34H28N2O2, 496.2, found, 496.2 (M+). For trans-DDDP, 1H NMR (600 MHz, DMSO-d6) δ 11.781 (2H, d, J = 4.2 Hz), 8.300−8.315 (2H, m), 7.798−7.813 (2H, m), 7.374 (4H, t, J = 7.2 Hz), 7.297 (2H, t, J = 7.2 Hz), 7.109 (4H, d, J = 7.2 Hz), 4.058 (2H, m), 3.450−3.473 (2H, m), 2.960−3.010 (2H, m), 1.935−2.015 (2H, m), 1.798−1.857 (2H, m); 13C NMR (150 MHz, DMSO-d6) δ 179.27, 144.56, 141.94, 134.12, 134.01, 131.92, 128.84, 127.40, 126.90, 125.57, 106.68, 38.21, 35.29, 27.51. MALDI-TOF MS, m/z calcd for C32H26N2O2, 470.2, found, 470.4 (M+). For cis-DDDP, 1H NMR (600 MHz, DMSO-d6) δ 12.033 (2H, d, J = 4.2 Hz), 8.311−8.326 (2H, m), 7.793−7.808 (2H, m), 6.823−6.835 (6H, m), 6.677−6.690 (4H, m), 4.768 (2H, m), 3.477−3.499 (2H, m), 3.022−3.028 (2H, m), 2.116−2.175 (2H, m), 1.909−1.949 (2H, m); 13C NMR (150 MHz, DMSOd6) δ 178.73, 145.16, 134.42, 134.01, 131.71, 127.43, 127.32, 125.46, 105.86, 37.95, 35.20, 27.64. MALDI-TOF MS, m/z calcd for C32H26N2O2, 470.2, found, 470.4 (M+).

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (Grant No. 21173262, 21373218, 21290194, and 21221002) and the “Hundred-Talent Program” of CAS to ZT are acknowledged.

■

REFERENCES

(1) Müller, C. D.; Falcou, A.; Reckefuss, N.; Rojahn, M.; Wiederhirn, V.; Rudati, P.; Frohne, H.; Nuyken, O.; Becker, H.; Meerholz, K. Multi-Colour Organic Light-Emitting Displays by Solution Processing. Nature 2003, 421, 829−833. (2) Zhou, Y.; Liu, W. J.; Ma, Y. G.; Wang, H. L.; Qi, L. M.; Cao, Y.; Wang, J.; Pei, J. Single Microwire Transistors of Oligoarenes by Direct Solution Process. J. Am. Chem. Soc. 2007, 129, 12386−12387. (3) Goushi, K.; Yoshida, K.; Sato, K.; Adachi, C. Organic LightEmitting Diodes Employing Efficient Reverse Intersystem Crossing for Triplet-to-Singlet State Conversion. Nat. Photonics 2012, 6, 253−258. (4) Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C. Highly Efficient Organic Light-Emitting Diodes from Delayed Fluorescence. Nature 2012, 492, 234−238. (5) Rao, A.; Chow, P. C. Y.; Gelinas, S.; Schlenker, C. W.; Li, C. Z.; Yip, H. L.; Jen, A. K. Y.; Ginger, D. S.; Friend, R. H. The Role of Spin in the Kinetic Control of Recombination in Organic Photovoltaics. Nature 2013, 500, 435−439. (6) Yang, L. Q.; Yan, L.; You, W. Organic Solar Cells Beyond One Pair of Donor-Acceptor: Ternary Blends and More. J. Phys. Chem. Lett. 2013, 4, 1802−1810. (7) Pastore, M.; Fantacci, S.; De Angelis, F. Modeling Excited States and Alignment of Energy Levels in Dye-Sensitized Solar Cells: Successes, Failures, and Challenges. J. Phys. Chem. C 2013, 117, 3685− 3700. (8) Bhaskar, A.; Ramakrishna, G.; Lu, Z. K.; Twieg, R.; Hales, J. M.; Hagan, D. J.; Van Stryland, E.; Goodson, T. Investigation of TwoPhoton Absorption Properties in Branched Alkene and Alkyne Chromophores. J. Am. Chem. Soc. 2006, 128, 11840−11849. (9) Kim, K. S.; Noh, S. B.; Katsuda, T.; Ito, S.; Osuka, A.; Kim, D. Charge Transfer Induced Enhancement of Near-IR Two-Photon Absorption of 5,15-Bis(Azulenylethynyl)Zinc(Ii) Porphyrins. Chem. Commun. 2007, 24, 2479−2481. (10) Stewart, D. J.; Dalton, M. J.; Swiger, R. N.; Fore, J. L.; Walker, M. A.; Cooper, T. M.; Haley, J. E.; Tan, L. S. Symmetry- and SolventDependent Photophysics of Fluorenes Containing Donor and Acceptor Groups. J. Phys. Chem. A 2014, 118, 5228−5237. (11) Thomas, S. W.; Joly, G. D.; Swager, T. M. Chemical Sensors Based on Amplifying Fluorescent Conjugated Polymers. Chem. Rev. 2007, 107, 1339−1386. (12) Li, X.; Zhang, S.; Cao, J.; Xie, N.; Liu, T.; Yang, B.; He, Q. J.; Hu, Y. Z. An ICT-Based Fluorescent Switch-on Probe for Hydrogen Sulfide in Living Cells. Chem. Commun. 2013, 49, 8656−8658. (13) Pan, H.; Fu, G. L.; Zhao, Y. H.; Zhao, C. H. Through-Space Charge-Transfer Emitting Biphenyls Containing a Boryl and an Amino Group at the O,O′-Positions. Org. Lett. 2011, 13, 4830−4833. (14) Lin, W. C.; Fang, S. K.; Hu, J. W.; Tsai, H. Y.; Chen, K. Y. Ratiometric Fluorescent/Colorimetric Cyanide-Selective Sensor Based on Excited-State Intramolecular Charge Transfer-Excited-State Intramolecular Proton Transfer Switching. Anal. Chem. 2014, 86, 4648− 4652. (15) Vazquez, M. E.; Blanco, J. B.; Imperiali, B. Photophysics and Biological Applications of the Environment-Sensitive Fluorophore 62768

DOI: 10.1021/jp511264r J. Phys. Chem. C 2015, 119, 2761−2769

Article

The Journal of Physical Chemistry C N,N-Dimethylamino-2,3-Naphthalimide. J. Am. Chem. Soc. 2005, 127, 1300−1306. (16) Ajayaghosh, A. Donor-Acceptor Type Low Band Gap Polymers: Polysquaraines and Related Systems. Chem. Soc. Rev. 2003, 32, 181− 191. (17) Tanaka, H.; Shizu, K.; Nakanotani, H.; Adachi, C. Dual Intramolecular Charge-Transfer Fluorescence Derived from a Phenothiazine-Triphenyltriazine Derivative. J. Phys. Chem. C 2014, 118, 15985−15994. (18) Shen, X. Y.; Wang, Y. J.; Zhao, E. G.; Yuan, W. Z.; Liu, Y.; Lu, P.; Qin, A. J.; Ma, Y. G.; Sun, J. Z.; Tang, B. Z. Effects of Substitution with Donor-Acceptor Groups on the Properties of Tetraphenylethene Trimer: Aggregation-Induced Emission, Solvatochromism, and Mechanochromism. J. Phys. Chem. C 2013, 117, 7334−7347. (19) Shen, X. Y.; Yuan, W. Z.; Liu, Y.; Zhao, Q. L.; Lu, P.; Ma, Y. G.; Williams, I. D.; Qin, A. J.; Sun, J. Z.; Tang, B. Z. Fumaronitrile-Based Fluorogen: Red to Near-Infrared Fluorescence, Aggregation-Induced Emission, Solvatochromism, and Twisted Intramolecular Charge Transfer. J. Phys. Chem. C 2012, 116, 10541−10547. (20) Yuan, W. Z.; et al. Efficient Solid Emitters with AggregationInduced Emission and Intramolecular Charge Transfer Characteristics: Molecular Design, Synthesis, Photophysical Behaviors, and OLED Application. Chem. Mater. 2012, 24, 1518−1528. (21) Giordano, L.; Shvadchak, V. V.; Fauerbach, J. A.; Jares-Erijman, E. A.; Jovin, T. M. Highly Solvatochromic 7-Aryl-3-Hydroxychromones. J. Phys. Chem. Lett. 2012, 3, 1011−1016. (22) Do, J.; Huh, J.; Kim, E. Solvatochromic Fluorescence of Piperazine-Modified Bipyridazines for an Organic Solvent-Sensitive Film. Langmuir 2009, 25, 9405−9412. (23) Marini, A.; Munoz-Losa, A.; Biancardi, A.; Mennucci, B. What Is Solvatochromism? J. Phys. Chem. B 2010, 114, 17128−17135. (24) Jiang, W.; Tang, J. N.; Qi, Q.; Wu, W. B.; Sun, Y. M.; Fu, D. W. The Synthesis, Crystal Structure and Photophysical Properties of Three Novel Naphthalimide Dyes. Dyes Pigments 2009, 80, 11−16. (25) Kudo, K.; Momotake, A.; Kanna, Y.; Nishimura, Y.; Arai, T. Development of a Quinoxaline-Based Fluorescent Probe for Quantitative Estimation of Protein Binding Site Polarity. Chem. Commun. 2011, 47, 3867−3869. (26) Cao, K.; Farahi, M.; Dakanali, M.; Chang, W. M.; Sigurdson, C. J.; Theodorakis, E. A.; Yang, J. Aminonaphthalene 2-Cyanoacrylate (ANCA) Probes Fluorescently Discriminate between Amyloid-Beta and Prion Plaques in Brain. J. Am. Chem. Soc. 2012, 134, 17338− 17341. (27) Gregoire, P. J.; Mellor, J. M.; Merriman, G. D. Reaction of Imines of Aminoanthraquinones with Formaldehyde and Alkenes. Tetrahedron 1995, 51, 6133−6144.

2769

DOI: 10.1021/jp511264r J. Phys. Chem. C 2015, 119, 2761−2769