Article pubs.acs.org/IECR
New ESIPT-Inspired Photostabilizers of Two-Photon Absorption Coumarin−Benzotriazole Dyads: From Experiments to Molecular Modeling Yulong Gong,† Zhenqiang Wang,† Shengtao Zhang,† Ziping Luo,*,†,‡ Fang Gao,*,†,‡ and Hongru Li*,†,‡ †
College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China National-municipal Joint Engineering Laboratory for Chemical Process Intensification and Reaction, Chongqing University, Chongqing 400044, China
‡
S Supporting Information *
ABSTRACT: In this work, a variety of new coumarin− benzotriazole dyads are synthesized to study the photostabilities under the irradiation of broad UV light as well as near-infrared (near-IR) laser respectively inspired by efficient excited state intramolecular proton transfer (ESIPT). The corresponding esterified dyads and the coumarin dyes are prepared as the references. The intramolecular hydrogen bond in the target dyads is confirmed by 1H NMR spectra and UV/ visible absorption spectra. The internal proton transfer in the excited states of the target dyads under single photon excitation and near-IR laser irradiation is demonstrated by the measurements of one-photon fluorescence spectra and two-photon upconverted fluorescence spectra, respectively. The photostabilities of the target dyads under broad UV light irradiation and near-IR laser irradiation in various organic solvents with saturated air, oxygen, or argon are studied. The theoretical evidence of ESIPT of the target dyads is provided by the molecular geometry optimization of enol, keto, and transition-state forms in the ground and excited states as well as the potential energy curves of enol−keto phototautomerization.
■
INTRODUCTION Due to high energy, ultraviolet light (UV light) is much harmful to lives in land and ocean including plants, bacteria, animals, and human beings.1 For instance, malignant melanoma is the representative fatal skin tumor caused by the excess absorbance of ultraviolet light.2 Furthermore, without the photostabilizing additives, the extended exposure of the polymer backbone to ultraviolet light irradiation often leads to photodegradation and photooxidation of polymeric materials.3,4 Hence, unless this energy can be absorbed and consumed, a whole host of photochemical reactions occur at molecular level, which is a definite serious detriment to us. In order to eliminate the burning caused by the exposure of UV light, the use of UV stabilizers is an efficient strategy in the industry.5−7 An essential requirement of UV absorbers is that they possess a unique ability for the rapid dissipation of the absorbed high energy via an appropriate intramolecular or intermolecular rearrangement.8 A number of organic dyes with intramolecular hydrogen bond such as o-hydroxy-phenyl-benzotriazole and its simple derivatives are commercially employed as the UV absorbers,9,10 for example, the additives for the sunscreen cream or the inhibitors for the polymer aging, due to various advantages including the intensive absorption in UV light region, the short excited state lifetime as well as the excellent photostability. It is considered that the excited state intramolecular proton transfer © 2016 American Chemical Society
(ESIPT) of the o-hydroxy-phenyl-benzotriazole derivatives through a four cycle enol−keto phototautomerization (E → E* → K* → K → E, E, enol, K, keto) could be the predominant pathway to dissipate the absorbed energy.11−14 It is found that the simple o-hydroxy-phenyl-benzotriazole derivatives possess the less absorption in UVA region (320− 420 nm), and thus the application potentials as the UVabsorbers are limited. In particular, these derivatives are lacking near-infrared (near-IR) two-photon absorption, so they are not the qualified candidates as the photostabilizers under near-IR laser. Hence, the synthesis of new photostable o-hydroxyphenyl-benzotriazole derivatives with intramolecular proton transfer in the excited state under broad UV light (UVA and UVB lights, 275−420 nm) and near-IR laser irradiation through rational molecular preconstruction is a great challenge to organic dye chemists. In this study, we utilize α,β-unsaturated carbonyl group as a linker to build coumarin−benzotriazole dyads. The UV absorption and near-IR two-photon absorption of the target molecules could be improved by the coumarin part. Furthermore, the electron-accepting ability of the linker is Received: Revised: Accepted: Published: 5223
January 15, 2016 April 10, 2016 April 22, 2016 April 22, 2016 DOI: 10.1021/acs.iecr.6b00195 Ind. Eng. Chem. Res. 2016, 55, 5223−5230
Article
Industrial & Engineering Chemistry Research Scheme 1. Chemical Structures of the Organic Dyes Studied in This Work
favorable for the increase of intramolecular hydrogen bond strength in o-hydroxy-phenyl-benzotriazole part. The corresponding esterified dyads and the coumarin dyes are prepared as the references. Hence, it is considered the target coumarin− benzotriazole dyads could exhibit internal proton transfer in the excited state under broad UV light exposure and near-IR laser irradiation, and the photostability application could be enhanced accordingly. To the best of our knowledge, this is the first successful attempt to synthesize new photostable benzotriazole-based organic dyes under broad UV light irradiation as well as near-IR laser irradiation, respectively.
were employed for the spectroscopic measurement. Onephoton ultraviolet/visible absorption spectra of the target dyes were recorded by a TU1901 spectrophotometer from Beijing PUXI General Equipment Limited Corporation. Shimadzu RF531PC spectrofluorophotonmeter was used to determine onephoton fluorescence emission spectra. The fluorescence quantum yields of the samples were measured by using quinine sulfate in 0.5 mol/L H2SO4 (Φ, 0.546) as the reference.15−17 In order to minimize the experimental errors, the absorption at the excited wavelength is guaranteed below 0.1. Fluorescence quantum yields were calculated by eq 115−17
2. EXPERIMENTAL SECTION 2.1. Materials and Characterization. Organic solvents were purchased from Aldrich Chemical Corporation, and the target dyads were prepared in our own laboratory (Scheme 1). Bruker 400 MHz and 500 MHz nuclear magnetic resonance (NMR) apparatus were employed to determine 1H and 13C NMR spectra of the samples in the deuterated solvents at room temperature. 1H and 13C chemical shifts in NMR spectra of the samples were detected by using tetramethylsilane (TMS) as the internal standard. Elemental analysis was obtained by CE440 elemental analysis meter obtained from Exeter Analytical Inc. The melting points of the samples were detected by Beijing Fukai melting point apparatus. Fourier transform infrared spectrometer was employed to measure IR spectra of the target molecules. 2.2. One-Photon Absorption Spectra and Two-Photon Absorption Optical Properties. The spectral grade reagents
Φf =
Φ0f
n2A0 ∫ If (λf )dλf n02A ∫ If0(λf )dλf
(1)
wherein n0 and n show the refractive indices of the solvents, A0 and A represent the optical densities at the excitation wavelength, Φf and Φf0 are the fluorescence quantum yields of the sample and reference, respectively, and the integrals are the fluorescence emission spectral areas of the reference and the sample, respectively. The pumped Ti:sapphire femtosecond laser (Tsunami modelocked, 80 MHz, < 130 fs, the power ≤ 700 mW, SpectraPhysics Ltd.) tuning at the step of 10 nm in the range of 700− 880 nm was utilized to determine near-IR two-photon excited fluorescence spectra. TPA emission spectra were recorded by an Ocean Optics USB2000 CCD camera with the detecting range of 180 to 880 nm. Up-conversion fluorescence method was used to measure TPA cross sections by using 5 × 10−4 5224
DOI: 10.1021/acs.iecr.6b00195 Ind. Eng. Chem. Res. 2016, 55, 5223−5230
Article
Industrial & Engineering Chemistry Research
Figure 1. 1H NMR spectra of C1 and C13 detected in DMSO-d6 (500 MHz).
mol/L fluorescein in 0.1 mol/L solution of sodium hydroxide as the reference. All the determined samples were bubbled with high purity nitrogen for half an hour to remove oxygen before the determination. TPA cross sections of the samples were calculated by the following equations:18 σ=
σ TPE ΦF
TPE σ TPE = σcal
The synthesis and characterization of the studied molecules are shown in Supporting Information.
3. RESULTS AND DISCUSSION 3.1. Target Dyads. Scheme 1 shows that the studied molecules include two groups: (i) the target dyads bearing ohydroxy-phenyl-benzotriazole segment (GI, C1−C6) and (ii) the esterified dyads and the coumarin dyes (GII, C7−C12, C13−C18). This means that G1 could show internal proton transfer in the excited state, whereas it is impossible to produce ESIPT for GII. Hence, it is anticipated that intramolecular proton transfer in the excited state and the photostability of the target dyads could be tuned by varying the substituted groups and the substituted positions in the coumarin moiety at molecular level. As presented in Scheme 1, the new target coumarin− benzotriazole dyads have successfully been prepared through the convergent methods using aldol condensation reactions which involved the nucleophilic addition of a ketone enolate to an aldehyde, and an α,β-unsaturated carbonyl linking bond was formed as soon as a molecule of H2O was lost. The satisfied purification yields of the target dyads were obtained at the final step (∼40%). 3.2. Intramolecular Hydrogen Bond Based on 1H NMR Analysis. A sensitive measure of hydrogen bond energy is 1H NMR chemical shift of the bridge proton. Due to the deshielding effect, the conventional hydrogen bond leads as a rule to an increase of chemical shift.21,22 Figure 1 shows that the 1 H NMR chemical shift of phenolic hydroxy in C1 moves to the lower field comparing to that of C13 (C1, 11.240 ppm; C13, 10.143 ppm), which implies that the hydrogen of the phenolic hydroxy in C1 is more deshielding owning to the stronger intramolecular hydrogen bond (OH···N). Furthermore, it is found that the 1H NMR chemical shift of phenolic hydroxy in the equal molar mixture of C7/phenol or C13/benzotriazole is almost the same as that of phenol and C13 correspondingly (Figure S1, Supporting Information, SI), indicating that there is no intermolecular hydrogen bond between C7 and phenol or between C13 and benzotriazole. It is also observed that the other target dyads and the corresponding coumarin dyes show the similar contrasting 1H NMR spectral character as C1. Figure 2 shows that 1H NMR chemical shift of the hydroxy increases with the increase of the Hammett parameters of the substituents in the coumarin part. It is known that a greater Hammett constant (αp) of the group means a stronger electron-withdrawing ability. Hence, the deshielding effect on the hydrogen atom is strengthened by the electron-accepting
(2)
ccal ncal S c n Scal
(3)
wherein σ shows the TPA cross section, σTPE represents the two-photo excited cross-section, ,c are the concentrations of reference (ccal) and sample, respectively, n is the refractive index of the solvent, S stands for the two-photo up-conversion fluorescence intensity, and cal represents a reference. 2.3. Photostability. For the photostability under UV light irradiation, the samples in solvents with saturated air, oxygen, or argon were irradiated by a 150 W Hg lamp for 2 h, respectively. The irradiation was filtered by a band-pass filter (Hamamatsu, filter # a4538-03, 280 nm < λ < 400 nm), which allows no passage of light less than 280 nm or above 400 nm. The servo manipulator carried the quartz cells including the samples in solvents with saturated air, or oxygen or argon. The cells were driven to move evenly and freely both in horizontal and vertical direction under the irradiation of Hg light or focused near-IR femtosecond laser at 730 nm. The samples were irradiated under near-IR laser for 10 min. The UV/visible absorption spectra of the samples were recorded before and after the irradiation to check the photostability. Photobleaching rates (ν) of the samples could be calculated by the following equation: ν=
Aλ0max − Aλi max Aλ0max ·t
× 100% (4)
A0λmax
wherein t is the UV or near-IR laser irradiation time, and Aiλmax are the absorption at peak wavelength of target molecules before and after the irradiation, respectively. 2.4. Computational Details. The molecular geometries of the target dyads in the ground state and the excited state were calculated, and the detailed information on quantum chemical calculation is shown in Supporting Information. 2.5. Synthesis. 3-(2H-Benzotriazol-2-yl)-4-hydroxy-benzaldehyde (S1) and 3-(2H-benzotriazol-2-yl)-4-acetoxy-benzaldehyde (S2) were first prepared in our laboratory.19 The preparation of 3-acetyl-7-cyano-2H-chromen-2-one (S3) was carried out as described elsewhere with some modifications.20 5225
DOI: 10.1021/acs.iecr.6b00195 Ind. Eng. Chem. Res. 2016, 55, 5223−5230
Article
Industrial & Engineering Chemistry Research
The greatly polar solvents could favor the increase of intramolecular hydrogen bond strength in C1.23 The further study shows that the equal molar mixtures of C7/phenol or C13/benzotriazole do not display the new long-wavelength absorption band in various solvents (Figure S2, SI), which indicates that this absorption band is generated by the intramolecular interaction rather than intermolecular interaction. The other target dyads C2−C6 also exhibit the similar absorption spectral phenomena as C1 comparing to the corresponding references. The above studies further demonstrate that there is the presence of intramolecule hydrogen bond in the target dyads. It is known that the protic solvents such as methanol possess the greater polarity than DMSO and DMF. Nevertheless, the UV/visible absorption spectra of the target dyads such as C1 show the weaker new long-wavelength absorption band in various alcohol solvents (such in MeOH, Figure S3, SI). This could be due to intermolecular hydrogen bond between the target dyads with the protic solvents.24−26 This perturbation transforms the normal enol form into the solvated enol form, which decreases intramolecular hydrogen bond strength. 3.4. ESIPT Based On One-Photon Absorption. The target dyad C1 and its references C7 and C13 are employed as the examples to illustrate one-photon emission spectral properties. The representative linear emission parameters of C1, C7, and C13 in various organic solvents are provided in Table S2. The results show that the emission maxima of C7 and C13 are not dramatically varied in different solvents. The typical fluorescence emission spectra of C1, C7, and C13 in 1,4dioxane and DMF are presented in Figure 4. It is found that C1 shows the similar one-photon emission spectra as C7 and C13 in 1,4-dioxane, whereas in DMF, C1 produces the dual emission bands, in contrast, C7 and C13 still display single emission band. It is further found that the equal molar mixture of C7/benzotriazole or C13/phenol also exhibit almost the identical emission to C7 and C13 correspondingly in various solvents (Figure S4, SI). The emission maximum of the first emission band in C1 is quite close to the maximal emission wavelength of C7 and C13 in DMF, whereas the second emission band possesses a remarkably large Stokes shift (∼170 nm). Hence, the fluorescence emission of C7 and C13 in 1,4dioxane and DMF is yielded by the normal decay of the excited state. The first emission band of C1 in DMF is produced by the decay of the tautomer enol* to enol as well, whereas the second emission band is generated by the decay of keto-tautomer,
Figure 2. Variation of 1H NMR chemical shift of phenolic hydroxy group with Hammett constants of the substituents in comarin part of the target dyads C1−C5.
groups, leading to a downshift of 1H NMR peak of phenolic hydroxy group. As a consequence, intramolecular hydrogen bond increases. It is further noticed that 1H NMR chemical shift of hydroxy group in C5 switches to the higher magnetic field comparing to that C6, which is due to the greater conjugacy of C6. 3.3. Effect of Internal Hydrogen Bond on Single Photon UV/Visible Absorption Spectra. The one-photon ultraviolet/visible absorption spectra of the target dyads were determined in various solvents. C1 and its references C7 and C13 are used as the representative examples to show the absorption spectral properties. The typical UV/visible absorption spectra of these molecules in 1,4-dioxane and DMF are presented in Figure 3. The representative one-photon absorption spectral parameters of these molecules are given in Table S1. Figure 3 shows that the UV/visible absorption shapes, the absorption maxima and the molar extinction coefficients of C1 and C7 are close with each other, suggesting that coumarin part plays the important roles in UV/visible absorption spectra of the target dyads. The results demonstrate that the target dyad C1 shows the well-extended UV absorption from 280−400 nm in various organic solvents. We also observe that C1 produces a new long-wavelength absorption band from 450 to 650 nm in the strong aprotic polar solvents such as DMSO and DMF. In contrast, the references C7 and C13 are absent of this new absorption band in various solvents (Figure 3b). It is considered that the new absorption band of C1 is attributed to the intramolecular hydrogen bond between the triazole ring and the phenolic hydroxy group.
Figure 3. UV/visible absorption spectra of C1, C7, and C13 in 1,4-dioxane (a) and DMF (b); the concentration is 1 × 10−5 mol/L. 5226
DOI: 10.1021/acs.iecr.6b00195 Ind. Eng. Chem. Res. 2016, 55, 5223−5230
Article
Industrial & Engineering Chemistry Research
Figure 4. One-photon fluorescence emission spectra of C1, C7, and C13 in 1,4-dioxane (a) and DMF (b); the concentration is 1 × 10−5 mol/L; excited wavelength is 350 nm.
Figure 5. TPA upconverted fluorescence emission spectra of C1 and C7 in 1,4-dioxane (a) and DMF (b) under near-IR femtosecond 730 nm laser; the concentration is 5 × 10−4 mol/L.
which is the product of internal proton transfer in the excited state. In strong protic solvents such as methanol, the red-shifted second emission band of C1 is weaker (Figure S5, SI). The IE*/ IN* value of C1 (the ratio of the maximal internal proton transfer emission intensity to the maximal normal emission intensity) in MeOH is much smaller than in DMF and DMSO (DMF, 1.69; MeOH, 0.68). The enol form of C1 could be protonated by the alcohols via intermolecular H-bond, resulting in the rise of the tautomer enol* emission band at the cost of tautomer keto* emission. The other target dyads also show the similar absorption spectral phenomena as C1 comparing to the corresponding references except C5 (Figure S6, SI). It is further observed that IE*/IN* increases with the electron-withdrawing ability of the substituents (such as in DMF: C1, 1.69; C2, 1.12; C3, 0.71; C4, 0.46; C5, 0; C6, 0.39). It is noticed that there is only one substituted position difference of methoxyl group between C5 and C6, whereas C6 displays much more remarkable ESIPT fluorescence emission band in DMF, DMSO as well as in protic solvents than C5 (Figure S6, SI). The worst conjugative effect of methoxyl group at 7-position could increase the occurrence of ESIPT. 3.5. ESIPT Based On Near-Infrared Laser-Induced Two-Photon Excitation. Near-IR Ti:sapphire femtosecond laser system was utilized to determine TPA optical properties of the target and reference chromophores in various solvents. Typical TPA upconverted fluorescence emission spectra of C1 and C7 in 1,4-dioxane (Figure 5a) and DMF (Figure 5b) are shown in Figure 5. It is found that C1 exhibits the dual TPA
emission bands in the range of 400−600 nm in both 1,4dioxane and DMF solutions, but the reference molecule C7 still shows single TPA emission band. The peak wavelengths of the dual TPA emission bands of C1 in 1,4-dioxane are located at 450 and 520 nm, respectively. The first emission band of C1 is close to that of the maximal wavelength of C7, indicating that it is produced by the tautomer enol form. It is found that the second TPA emission band of C1 possesses a great Stokes shift in various solvents (∼170 nm). Hence, the second emission band in the longer wavelength region is generated by the decay of the tautomer keto* form, which is the product of intramolecular proton transfer in the excited state. It is also found that the other target dyads C2−C6 show the similar phenomena under near-IR femtosecond laser irradiation, which demonstrates that the target dyads can undergo internal proton transfer in the excited state under near-IR twophoton absorption. It is shown that the IE*/IN* ratio in TPA fluorescence emission spectra is much larger in one-photon emission spectra (such as in DMF solution: C2, 1.72; C3, 0.93). The above discoveries suggest that ESIPT emission becomes the primary deactivation route of the excited states of the target dyads under near-IR femtosecond laser irradiation. TPA cross sections of the studied molecules were further determined based on TPA fluorescent methods. The target molecules possess the larger near-IR TPA cross sections than the corresponding references (see Table S3). The extent of the internal charge transfer could be increased by intramolecular Hbond in the target molecules, and thus the TPA absorption sections are improved27 (such as C1, 139.6 GM; C7, 12.8 GM). 5227
DOI: 10.1021/acs.iecr.6b00195 Ind. Eng. Chem. Res. 2016, 55, 5223−5230
Article
Industrial & Engineering Chemistry Research 3.6. Photostability in Organic Solvents Under UV Light and Near-IR Laser Irradiation. After 2 h of UV light irradiation (280−400 nm, UVA and UVB light), the absorption spectra of C1, C2, C3, C4, and C6 exhibit the small changes and the maximal absorbance was decreased by less than 5% in saturated air DMF solution (Figure S7, SI). It is further found that the decrease in the absorption spectra of C5 is above 18% at the same experimental conditions. The photobleaching rates of the dyads are listed in Table 1 and Table S4, respectively. The data suggest that C1 undergoes
We further measured the variations of molecular weights (Mn, number-average molecular weight; Mw, weight-average molecular weight) of PMMA during the UV light irradiation monitored by a Shimadzu GPC instrument with THF. It is found that the Mn of PMMA shows the small variations in air saturated DMF C1 solution after the UV light irradiation (Mn, before UV light irradiation, 5.65 × 105 and after UV light irradiation, 5.57 × 105; Mw, before UV light irradiation, 1.26 × 106 and after UV light irradiation, 1.20 × 106), indicating that the doping of C1 could efficiently protect the polymer from the photodegrading under UV light irradiation. 3.7. Molecular Geometry Optimization. 3.7.1. Transition State Forms of Phototautomerization of the Target Dyads. Because proton transfer is an actual chemical reaction, thus the transition state of enol−keto phototautomerization of the target dyads could be presence. Unfortunately, it is demonstrated that the energy of the keto form in the ground states of the target dyads is too high to get the stable structure of keto form. This means that the internal proton transfer in the ground state could not occur due to the absence of keto form. However, it in turn favors the occurrence of four-level cycle of ESIPT (E → E* → K* → K → E) because no energy barrier is required for the transformation of keto to enol forms at the final step of ESIPT. In contrast, the stable keto form is obtained in the excited states of these dyads. In addition, the transition state forms of enol−keto phototautomerization in the excited states are acquired. This implies that the internal proton transfer in the excited states of the target dyads may be feasible. The optimized structures of enol, keto, and transition state forms in the excited state of C1 are shown in Figure S8 (SI), and the corresponding structural parameters involving in ESIPT are listed in Table S8 (SI). The geometrical data show that the distances of HO in the transition states are between those of enol and keto forms in S1 states of these dyads (such as to C1, 0.956 Å in enol form, 1.196 Å in transition state form, 1.846 Å in keto form), indicating that intramolecular proton transfer reactions could be required to be through an energy-barrier pathway. The distance of ON involving in the hydrogen bond in the transition state form is much shorter than that in enol and keto forms, respectively (C1 in S1 state, 2.638 Å in enol form, 2.358 Å in transition state form, 2.595 Å in keto form, Table S8), which suggests that internal hydrogen bond in the transition state form is stronger. The ON length in the reference molecules is much longer than those of enol and keto forms in the excited states of the corresponding target ones (such as enol in S1; C7, 2.848 Å; C1, 2.638 Å), further suggesting that the presence of intramolecular hydrogen bond in both enol and keto forms of the target dyads. It is noticed that the enol forms in the excited target dyads possess the larger OH distance and the smaller NH and NO distance than those in the ground states (such as C1, in the ground state: OH, 0.951 Å; NH, 1.869 Å; NO, 2.667 Å and in the excited state, OH, 0.956 Å; NH, 1.831 Å; NO, 2.638 Å; Table S8), which shows that the internal hydrogen bond is stronger in the excited state than that in the ground state. As a consequence, internal proton transfer in the excited states could be easier for these target dyads. The important dihedral angle of ∠C1O1H1N1 involving in ESIPT reaction is also calculated. The presence of the internal hydrogen bond is evidenced by the small dihedral angle of ∠C1O1H1N1 in enol and keto forms in both the ground and
Table 1. Photobleaching Rates, ν, of the Target Dyads in Various Solvents with Saturated Air under UV Light Irradiation ν, h−1 DMF 1,4-dioxane methanol
C1
C2
C3
C4
C5
C6
1.47 5.19 3.37
1.85 5.34 3.41
1.97 5.59 3.78
2.15 5.91 3.97
4.98 12.62 9.67
2.47 6.23 4.25
the minimal photobleaching rates among the target dyads in organic solvents with saturated air (such as in DMF: C1, 1.47 h−1; C2, 1.85 h−1; C4, 2.15 h−1). This could be attributed to the stronger ESIPT of C1. In addition, the target dyads display the more photostabilities in DMF comparing to in 1,4-dioxane and methanol. Furthermore, the data suggest that he photobleaching rates of target dyads in saturated air organic solution are much lower than those of the esterified dyads (such as in DMF: C1, 1.47 h−1; C7, 9.76 h−1; C13, 13.44 h−1), which suggests that the ESIPT dissipates efficiently the absorbed energy in the excited states of the target dyads.28 It is also observed that the UV light burning in saturated oxygen solvents produces the larger photobleaching rates for all the target dyads comparing to that in air saturated solvents (Table S5 and S6, SI). However, the target dyads show the less photobleaching under the UV light irradiation (280−400 nm, UVA and UVB light) in solvents with saturated argon. Furthermore, the photobleaching rates of all the target dyads under near-IR 730 nm laser irradiation were determined and listed in Table 2 and Table S7 (SI). The results suggest that Table 2. Photobleaching Rates, ν, of Target Dyads in Various Solvents under 730 nm near-IR Laser ν, h−1 DMF 1,4-dioxane methanol
C1
C2
C3
C4
C5
C6
4.59 11.59 8.32
5.17 12.17 8.94
5.56 12.38 9.94
5.91 13.04 10.21
8.12 16.39 11.94
6.13 13.21 11.00
these dyads exhibit the quite photostabilities under near-IR laser irradiation. On the other hand, the photobleaching rates of the target dyads in various solvents under near-IR laser irradiation are higher than those under UV light irradiation (such as in DMF. Under UV light irradiation: C1; 1.47 h−1; C2, 1.85 h−1. Under near-IR laser irradiation: C1, 4.59 h−1; C2, 5.17 h−1). This could be mainly due to much smaller two-photon absorption cross sections comparing to one-photon absorption cross sections. Thus, some photons could not be absorbed, and as a result, the energy could not be consumed. 5228
DOI: 10.1021/acs.iecr.6b00195 Ind. Eng. Chem. Res. 2016, 55, 5223−5230
Article
Industrial & Engineering Chemistry Research the excited states (enol form of C1 in ground state, −0.155°; in excited state, −0.027°; Table S8). Because of the internal hydrogen bond, the dihedral angel ∠C1C2N2N1 tends to be 0° in the target dyads, whereas for the references, this angle significantly increases by the steric effect of the esterified groups due to the absence of the intramolecular hydrogen bond (such as ∠C1C2N2N1; C1, −0.045°; C7, 43.962°; in the excited state). In addition, the dihedral angles ∠C1O1H1N1 and ∠C1C2N2N1 of transition state form are much smaller than those of E and K forms in S1 state of C1 (such as ∠C1O1H1N1 in C1: −0.0270° in enol form, −0.0004° in transition state, 0.0329° in keto form, Table S8). 3.7.2. Energy Barrier of Phototautomerization of the Target Dyads. A critical evaluation of the process on the perspective of ESIPT potential energy curve (PEC) is of significant importance. Herein, the PEC for ESIPT of the target dyads is calculated by specifying a reaction coordinate of the distance from O atom to H atom. The minimum-energy paths connecting the two structures of C1 in each electronic state are computed to identify the major coordinates involved in internal proton transfer process, which is shown in Figure 6.
characteristic of HOMO and LUMO orbitals in the ground and excited states (typically shown in Figure S9, SI). In HOMOs of the target dyads such as C1, the electron cloud density is mainly distributed in α,β-unsaturated carbonyl group and the neighboring aromatic ring, whereas there is a small part of electron cloud density in the triazole ring. However, as C1 is excited from HOMO to LUMO orbitals, the electron cloud density is shifted to the coumarin segment. The occurrence of electron transfer from HOMO to LUMO leads to the larger charge density distribution in the phenolic ring, which is the driving force for the internal proton transfer in S1 state of C1.29 The increase of negative charge distribution in N atom (such as C1, from −0.462 e in the ground state to −0.476 e in the excited state, Table S10) together with a decrease in O atom of the OH group (C1, from −0.667 e in the ground state to −0.651 e in the excited state) also suggest the favorable translocation of the proton in the excited surface. Hence, C1 may undergo internal proton transfer in the excited state.
4. CONCLUSIONS In closing, the new coumarin−benzotriazole dyads are prepared to efficiently increase intramolecular proton transfer in the excited state. The internal H-bond effect is demonstrated in these new benzotriazole-coumarin dyads by 1H NMR spectra and the UV/visible absorption spectra. The characteristic onephoton and two-photon ESIPT emission bands of the target dyads are well detected by UV irradiation and near-IR laser excitation, respectively. It is demonstrated that the target dyads show the good photostabilities under broad UV light irradiation and near-IR laser irradiation respectively by the consuming the absorbed energy via ESIPT process. The optimized structural parameters of enol, keto, and transition state forms of the target dyads are favorable for ESIPT occurrence. In particular, the small energy barriers of enol−keto phototautomerization suggest the great possibility of internal proton transfer in the excited states of the target dyads. The results shown in this study would guide us to design and synthesize new photostabilizers under UV light irradiation and near-IR laser irradiation, respectively.
Figure 6. Internal proton transfer reaction potential energy curves relaxed along the OH distance in the ground and the first single excited state of C1.
■
The instability of the K-form in the ground states of the target dyads indicates the nonviability of the ground state intramolecular proton transfer. As seen in Figure 6, the energy barrier of excited state is small (8.34 kJ/mol), suggesting the great possibility of the intramolecular proton transfer in excited state of C1. In addition, the energy of the keto form is lower than that of enol form, suggesting the internal proton transfer in the excited state is exothermic process. The other target dyads also show the similar energy potential properties as C1. Table S9 lists the energy barriers for the enol forms to the transition state forms in excited states of the target dyads. The data show that C1 possesses the lower energy barrier than the other dyads (such as C1, 8.34 kJ/mol; C4, 14.98 kJ/mol). The energy barrier decreases in the order as C1 < C2 < C3 < C4 < C5. Hence, ESIPT of the target dyads is enhanced by the electron-withdrawing substituents in coumarin segments. In addition, the energy barrier of C6 is smaller than that of C5, suggesting that the inhibitory effect on ESIPT by electrondonating groups in the 7-position of coumarin segment is more dramatic. 3.7.3. Frontier Orbitals and Mulliken Atomic Charge of Tautomer Enol of the Target Dyads in S0 and S1 States. The target and reference molecules display good π-symmetric
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b00195. Computational details and characterization (including 1H NMR, 13C NMR, infrared spectrum, and elemental analysis) of C1−C18, 1H NMR spectra, absorption spectra and fluorescence emission spectra of equal molar mixture of C7/phenol and C13/benzotriazole, absorption spectra and fluorescence emission spectra of C1 in MeOH, the linear absorption and fluorescence emission spectra of C1−C7 in DMF, the absorption spectra of C1 and after 2 h of UV irradiation in DMF, the optimized geometries of enol, TS and keto forms in excited state of C1, frontier molecular orbital of C1 (Figures S1−S9). UV and fluorescence spectral parameters of C1, C7, and C13 in various solutions, two-photon cross sections of target and reference compounds, photobleaching rates of reference compounds in various solvents with saturated air under UV light irradiation and near-IR laser, photobleaching rates of all compounds in various solvents with saturated oxygen under UV light, the 5229
DOI: 10.1021/acs.iecr.6b00195 Ind. Eng. Chem. Res. 2016, 55, 5223−5230
Article
Industrial & Engineering Chemistry Research
■
transfer in an ultraviolet-filter molecule. Chem. Phys. Lett. 2000, 325, 171−175. (12) Chudoba, C.; Riedle, E.; Pfeiffer, M.; Elsaesser, T. Vibrational coherence in ultrafast excited state proton transfer. Chem. Phys. Lett. 1996, 263, 622−628. (13) Otterstedt, J. E. A. Photostability and molecular structure, Photostability and molecular structure. J. Chem. Phys. 1973, 58, 5716− 5725. (14) Crawford, J. C. 2(2-Hydroxy-phenyl)-2H-benzotriazole ultraviolet stabilizers. Prog. Polym. Sci. 1999, 24, 7−43. (15) Eaton, D. Reference materials for fluorescence measurement. Pure Appl. Chem. 1988, 60, 1107−1114. (16) Maus, M.; Rettig, W.; Bonafoux, D.; Lapouyade, R. Photoinduced intramolecular charge transfer in a series of differently twisted donor-acceptor biphenyls as revealed by fluorescence. J. Phys. Chem. A 1999, 103, 3388−3401. (17) Fischer, M.; Georges, J. Fluorescence quantum yield of rhodamine 6G in ethanol as a function of concentration using thermal Lens spectrometry. Chem. Phys. Lett. 1996, 260, 115−118. (18) Albota, M. A.; Xu, C.; Webb, W. W. Two-photon fluorescence excitation cross sections of biomolecular probes from 690 to 960 nm. Appl. Opt. 1998, 37, 7352−7356. (19) Lu, Y.; Gong, Y. L.; Ma, H. Y.; Wang, Z. Q.; Li, H. L.; Luo, Z. P.; Li, H. R.; Gao, F. 4-formyl-2-(2H-benzotriazol-2-yl-)-phenol: an ESIPT chromophore. Chin. Sci. Bull. 2014, 59, 4741−4751. (20) Wang, T.; Wu, F. P.; Shi, M. Q.; Gao, F.; Yang, Y. Y. Synthesis of a Novel Ketocoumarin Visible-Light Photosensitive Dye and Its Initiating Properties. Acta Chim. Sin. 2004, 62, 527−531. (21) Sobczyk, L.; Grabowski, S. J.; Krygowski, T. M. Interrelation between H-bond and Pi-electron delocalization. Chem. Rev. 2005, 105, 3513−3560. (22) Schneider, H. J.; Hacket, F.; Rüdiger, V.; Ikeda, H. NMR studies of cyclodextrins and cyclodextrin complexes. Chem. Rev. 1998, 98, 1755−1786. (23) Nazir, H.; Yildiz, M.; Yilmaz, H.; Tahir, M.; Ulku, D. Intramolecular hydrogen bonding and tautomerism in Schiff bases. Structure of N-(2-pyridil)-2-oxo-1-naphthylidenemethylamine. J. Mol. Struct. 2000, 524, 241−250. (24) Van Benthem, M. H.; Gillispie, G. D. Intramolecular hydrogen bonding. 4. Dual fluorescence and excited-state proton transfer in 1,5dihydroxyanthraquinone. J. Phys. Chem. 1984, 88, 2954−2960. (25) Kasha, M. Proton-transfer spectroscopy. Perturbation of the tautomerization potential. J. Chem. Soc., Faraday Trans. 2 1986, 82, 2379−2392. (26) Strandjord, A. J. G.; Barbara, P. F. The proton-transfer kinetics of 3-hydroxyflavone: solvent effects. J. Phys. Chem. 1985, 89, 2355− 2361. (27) Zeng, L.; Chen, S.; Xia, T.; Zhong, C.; Liu, Z. Designing twophoton fluorescent probes based on the target-induced enhancement of the absorption cross-section. Chem. Commun. 2014, 50, 11139− 11142. (28) Okazaki, T.; Hirota, N.; Terazima, M. Thermalization process after the relaxation of electronically excited states: Intramolecular proton transfer systems studied by the transient grating method. J. Chem. Phys. 1999, 110, 11399−11410. (29) Chen, K. Y.; Hsieh, C. C.; Cheng, Y. M.; Lai, C. H.; Chou, P. T. Extensive spectral tuning of the proton transfer emission from 550 to 675 nm via a rational derivatization of 10-hydroxybenzo [h] quinolone. Chem. Commun. 2006, 42, 4395−4397.
important bond lengths (angstroms) and dihedral angel (degrees) associated with ESIPT of C1 and C4, the energy barrier of phototautomerization of the target dyads, mulliken charge of N and O atom of C1−C6 (Tables S1−S10). (PDF)
AUTHOR INFORMATION
Corresponding Authors
*Tel./Fax: 86-23-65102531. E-mail:
[email protected]. *E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors greatly thank the warm support from Municipal Natural Science Foundation of Chongqing (Grant Nos. CSTC2012jjB50007 and CSTC2010BB0216). H.L. is grateful to the China Postdoctoral Science Foundation for financial supporting (Grant Nos. 22012T50762 and 2011M501388). We also appreciate the warm support from National Natural Science Foundation of China (Grant No. 21376282).
■
REFERENCES
(1) Cordero, R. R.; Roth, P.; Georgiev, A.; DaSilva, L. Climatology of surface ultraviolet-radiation in Valparaiso, Chile. Energy Convers. Manage. 2005, 46, 2907−2918. (2) Menichini, G.; Alfano, C.; Marrelli, M.; Toniolo, C.; Provenzano, E.; Statti, G. A.; Nicoletti, M.; Menichini, F.; Conforti, F. Hypericum perforatum L. subsp. perforatum induces inhibition of free radicals and enhanced phototoxicity in human melanoma cells under ultraviolet light. Cell Proliferation 2013, 46, 193−202. (3) Gandra, N.; Frank, A. T.; Gendre, O. L. Possible singlet oxygen generation from the photolysis of indigo dyes in methanol, DMSO, water, and ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate. Tetrahedron 2006, 62, 10771−10776. (4) Rosu, D.; Rosu, L.; Cascaval, C. N. Effect of ultraviolet radiation on vinyl ester network based on bisphenol A. J. Photochem. Photobiol., A 2008, 194, 275−282. (5) Samper, M. D.; Fages, E.; Fenollar, O.; Boronat, T.; Balart, R. The potential of flavonoids as natural antioxidants and UV light stabilizers for polypropylene. J. Appl. Polym. Sci. 2013, 129, 1707− 1716. (6) Allmer, K.; Hult, A.; Rånby, B. Surface modification of polymers. IV. UV initiated degradation of polymers with stabilizers grafted onto the surface. J. Polym. Sci., Part A: Polym. Chem. 1989, 27, 3419−3427. (7) Crawford, J. C. 2 (2-Hydroxyphenyl) 2H-benzotriazole ultraviolet stabilizers. Prog. Polym. Sci. 1999, 24, 7−43. (8) Farkas, R.; Lhiaubet-Vallet, V.; Corbera, J.; Törincsi, M.; Gorchs, O.; Trullas, C.; Jiménez; Miranda, O. M. A.; Novak, L. Synthesis of New 2-(2′-Hydroxyaryl)benzotriazoles and Evaluation of Their Photochemical Behavior as Potential UV-Filters. Molecules 2010, 15, 6205−6216. (9) Waiblinger, F.; Keck, J.; Stein, M.; Fluegge, A. P.; Kramer, H. E. A.; Leppard, D. Light-induced opening of the intramolecular hydrogen bond of UV absorbers of the 2-(2-hydroxyphenyl)-1, 3, 5-triazine and the 2-(2-hydroxyphenyl) benzotriazole type. J. Phys. Chem. A 2000, 104, 1100−1106. (10) Goeller, G.; Rieker, J.; Maier, A.; Stezowski, J. J.; Daltrozzo, E.; Neureiter, M.; Port, H.; Wiechmann, M.; Kramer, H. E. Deactivation processes of ultraviolet stabilizers of the 2-(hydroxyphenyl) benzotriazole class with intramolecular hydrogen bonds. J. Phys. Chem. 1988, 92, 1452−1458. (11) Fournier, T.; Pommeret, S.; Mialocq, J. C.; Deflandre, A.; Rozot, R. Fentosecond laser studies of excited state intramolecular proton 5230
DOI: 10.1021/acs.iecr.6b00195 Ind. Eng. Chem. Res. 2016, 55, 5223−5230
