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A strategy of integrating ultraviolet absorption and crosslinking in a single molecule: DFT calculation and experimental
Journal of Molecular Structure 1107 (2016) 249e253
Contents lists available at ScienceDirect
Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc
A strategy of integrating ultraviolet absorption and crosslinking in a single molecule: DFT calculation and experimental Mingli Shan a, Yujing Liu a, Shuwei Xia a, Qunwei Tang b, Liangmin Yu a, * a b
Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao 266100, China Institute of Materials Science and Engineering, Ocean University of China, Qingdao 266100, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 September 2015 Received in revised form 16 November 2015 Accepted 19 November 2015 Available online 2 December 2015
Creation of advanced ultraviolet light absorbers having crosslinking ability has been persistent objective for anti-ultraviolent aging polymers. We present here the integration of 2, 4-dihydroxybenzophenone (UV-0) and N-methylol acrylamide (NMA) for novel ultraviolet absorber namely (3,5-dimethacryl amide-2,4-dihydroxyphenyl) (phenyl)methanone (UV-CA), which is subsequently utilized as a crosslinking agent after suffering FriedeleCrafts reaction. The preliminary results demonstrate that quantum chemical calculations (DFT) is a promising avenue in demonstrating the optimized geometry, charges, energy levels and UV electronic absorption bands of the UV-CA in the singlet (steady and excited states). The structure parameters and natural band orbital (NBO) calculations suggest that the intramolecular hydrogen bond (IMHB) in the UV-0 group is significantly enhanced in comparison to that between UV0 and NMA groups. The acrylic acid polymers functionalized with UV-CA yield high crosslinking degree and robust UV absorbing performance. The impressive results demonstrate that quantum chemical calculations are promising in organic synthesis to develop advanced compounds. © 2015 Elsevier B.V. All rights reserved.
Keywords: UV absorber Crosslinking agent Quantum chemical calculations
1. Introduction Organic polymer materials are widely used due to their low manufacturing costs, easy applied and high stability. The organic polymer materials are mainly applied in outdoor environment, consequently their long-term stability and service life would be deteriorated after suffering from humidity and temperature variations, oxygen, and sunlight [1e4]. Ultraviolet (UV) light is believed to be crucial in depending the lifetime of these functional organic polymers because the corresponding chemical bonds can be destroyed under high energy irradiations. In this fashion, UV absorbers seem crucial in protecting the organic polymers from being damaged in practical applications [5e7]. Among various UV absorbers, the absorbers having gigantic intramolecular hydrogen bridges have attracted growing interests to prevent polymers from UV aging [8]. One of the traditional strategies of designing anti-UV polymers is to add UV absorbers as nonreactive additives. A remaining issue for this technique is the unsatisfactory operational lifetime, arising from facile dissolution of UV absorber from organic polymers when suffering specific
* Corresponding author. E-mail address: [email protected] (L. Yu). http://dx.doi.org/10.1016/j.molstruc.2015.11.049 0022-2860/© 2015 Elsevier B.V. All rights reserved.
atmosphere. Since the first study on reactive UV absorber for robust anti-UV performances [9], many efforts have placed their focuses on developing novel reactive UV absorbers by compolymerizing them with the monomers of organic polymers [10e12]. Great effort is currently being devoted to the design of multifunction compounds to facilitate the applications [13]. The commonly utilized UV absorbers have only UV-absorbing ability with unsatisfactory crosslinking performance [14]. A rising challenge for UV agents is to combine anti-UV performance with crosslinking ability in a single species because a reasonable crosslinking degree is always prerequisite for functional polymers [15]. By addressing this issue, we present here the experimental realization of dual-functionalized UV absorbers by FriedeleCrafts reaction based on the quantum chemistry calculation theory-guided design. The optimized geometry and energy levels in the singlet (steady and excited states) are obtained by quantum chemistry calculations, while natural band orbital (NBO) analysis demonstrates the content of intramolecular hydrogen bonds (IMHBs) in the resulting molecule has an beneficial effect on photostability. The preliminary results suggest that the resultant (3,5dimethacrylamide-2,4-dihydroxyphenyl) (phenyl)methanone (UV-CA) has superior anti-UV and crosslinking performances in functionalized acrylic acid polymers.
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M. Shan et al. / Journal of Molecular Structure 1107 (2016) 249e253
Scheme 1. Synthetic route of UV-CA.
Waters 2695 GPC using DMF as eluent. Elemental analysis was performed on a Vario EL Elementar. 2.3. Synthesis of UV-CA The synthesis processes for UV-CA is shown in Scheme 1. In details, a mixed solution of alchlor (0.08 g) in acetone (5 mL) was added dropwise to an acetone solution containing UV-0 (0.01 mol) and NMA (0.01 mol) under vigorous agitation at 35 C. After reaction for 48 h, the brown precipitates were filtered and rinsed with 0.01 M hydrochloric acid aqueous solution and dried in air. Subsequently, the as-synthesized intermediate products were purified by recrystallization to obtain the white UV-CA. Yield: 83%. FTIR (KBr)/cm1: 3270(OH), 3081(NH), 1656, 1623(C]O) (Fig. S1.); 1H NMR (600 MHz, DMSO)/ppm: d 13.04 (s, 1H, OeH), 11.86 (s, 1H, OeH), 9.33 (t, 1H, NeH), 8.59 (t, 1H, NeH), 7.61e7.66 (m, 3H, AreH), 7.54 (t, 2H, AreH), 7.40 (s, 1H, AreH), 6.09e6.23 (m, 4H, ¼CH2), 5.74 (t, 1H, eCH ¼ ), 5.62 (t, 1H, eCH ¼ ), 4.38 (d, 2H, eCH2-), 4.21 (d, 2H, eCH2-) (Fig. S2.); 13C NMR (150 MHz, DMSO): 199.7, 167.8, 165.7, 163.0, 162.0, 138.0, 133.9, 132.2, 131.5, 130.4, 129.2, 128.9, 127.8, 126.2, 118.7, 113.0, 111.7, 38.1, 32.4 (Fig. S3.); Mass (ESI) found m/z: 381.1445 (MH, 100); Element analysis: Found (%): C, 66.29, H, 5.28, O, 21.09, N, 7.36; Calcd (%): C, 66.31, H, 5.26, O, 21.05, N, 7.36.
Fig. 1. The S0 optimized structure of UV-0 at B3LYP/6-31G(d,p) level. (red: O atom, white: H atom, gray: C atom.)
2. Experimental 2.1. Materials and reagents UV-0 (analytical reagent, 99%), N-methylol acrylamide, dimethyl sulfoxide (analytical reagent, 99%), AlCl3 (Chemical pure, 99%), acetone (analytical reagent, 99.5%), concentrated hydrochloric acid (chemical pure, 37%) and ethanol (Chemical pure, 99.5%) were purchased from Sinopharm Chemical Reagent Beijing Co. Ltd. (Beijing, PR China). Styrene (St, chemical pure, 99%), butyl methacrylate (BMA, chemical pure, 99%), ethyl acrylate (EA, Chemical pure, 99%) were purchased from Aladdin Reagent Co. Ltd (Shanghai, China).
2.4. Synthesis of UV-CA crosslinked acrylic acid polymers The UV-CA crosslinked acrylic acid polymers were synthesized using the following procedures: A mixture of St (11 g), BMA (11 g), EA (11 g), and stoichiometric UV-CA was prepared by dissolving in 50 g dimethyl benzene. Before adding 0.5 g 2, 2-Azobisisobutyronitrile (AIBN), the above-mentioned solution was degassed and refilled with argon. After vigorous agitation at 90 C for 8 h, the transparent polymers were dissolved in DMF to measure their UV absorption spectras.
2.2. Characterizations The UV absorption spectrum of the compound and functionalized acrylic acid polymers were performed on a Beckman Coulter DU-800 ultraviolet/visible spectrometer. Infrared spectrum (KBr pellets) of solid sample was recorded in a region of 4000e400 cm1 on a Perkine Elmer Spectrum-One FT-IR spectrophotometer. 1H and 13 C NMR spectrum was recorded on AVANCE III 600 MHz NMR Spectrometer with tetramethylsilane (TMS) as internal standard in Dimethyl sulfoxide. ESI mass spectra were measured on a micro mass Q-TOF (waters) mass spectrometer by utilizing an acetonitrile/formic acid matrix. GPC measurements were conducted by
2.5. Calculations The optimized geometry in the ground state and UV absorption spectra were determined by time dependent (TD) and density functional theory (DFT) calculations until the vibrational frequencies were true minima. The solvent (DMF) effect was analyzed by means of Polarizable Continuum Model (PCM) [16]. The geometrical parameters, frontier molecular orbital energies,
Table 1 Natural charge of UV-0 calculated from B3LYP-TD/6-31G(d,p).a Atom
C1b
C2
C3
C4
C5
C6
C7
C8
Natural Charge (e)
0.38326
0.32383
0.17694
0.22169
0.23692
0.22038
0.23588
0.19269
a b
The calculation was carried out based on the S0 optimized structure of UV-0 as shown in Fig. 1. The number of the carbon atoms are labeled in Fig. 1.
M. Shan et al. / Journal of Molecular Structure 1107 (2016) 249e253
Fig. 2. The S0 optimized structure of UV-CA at B3LYP/6-31G (d) level. Bond length in Angstrom, red: O atom, blue: N atom, white: H atom, gray: C atom.
251
Fig. 3. UVevis absorption spectra of UV-0 and UV-CA in 2 105 M DMF.
second-order interaction energies from NBO analysis of the title compound were calculated based on the optimized structure at B3LYP/6-31G(d,p) level. All the DFT calculations in this paper were carried out in the Gaussian09 package on the personal computer [17].
1.6759 Å, bond angle: 148.6)]. Based on gas monomolecular model, there are two strong UV absorption bands in UV-CA with the calculated maximum molar absorption coefficient 1,2500 and 1,3750 L mol-1 cm-1, respectively. Therefore, we can make a conclusion from quantum chemistry calculation that the resultant UV-CA molecule is efficient in absorbing UV radiation.
3. Results and discussion
3.2. Absorption spectrum
3.1. Theory-guided design
The quantum chemistry calculation results including electronic absorption bands, electric transition orbitals, oscillator strength (f) and the maximum absorption wavelength of UV-CA based on gas monomolecular model are summarized in Table 2. Among the calculated electronic transitions, two characteristic absorption bands (S0/S1, S0/S4) located at >270 nm are induced by electric transition with the oscillator strength (f) of 0.1596 and 0.1753, respectively. As shown in Table 2 and Fig. 3, the simulated (S0/S1,S0/S4) electronic transition bands of the UV-CA are matching to the experimental spectra (15,000 L mol1 cm1) at 295 nm and 12,000 L mol1 cm1 at 339 nm. The solvatochromic behavior of UV-CA was analyzed by means of the Polarizable Continuum Model (PCM) and the results were summarized in Table 3. No distinct differences can be found between the maximum absorption wavelength calculated from gas monomolecular model (335, 297 nm) and Polarizable Continuum Model (336, 299 nm). This is possibly related to the presence of a strong intramolecular hydrogen bond in the UV-CA [20]. Fig. 4 displays the three orbitals (99 HOMO-1, 100 HOMO and 101 LUMO) associated with the S0/S1 and S0/S4 electronic transitions. It indicates that orbital 99, 100, and 101 are p orbitals, and the electron density of orbitals (99, 100 and 101) are mainly
FriedeleCrafts reaction is one of the widely utilized method in transferring CeC bonds to aromatic rings [18]. Substituent group will attack the electron-enriching atoms to form carbocation species, subsequently acting as electrophile in an electrophilic aromatic substitution. In this fashion, the molecular structure can be extracted by analyzing the electron distribution of this aromatic compound by natural population analysis (NPA) technique. The S0 optimized geometry of UV-0 is shown in Fig. 1 and the corresponding parameters calculated by NBO analysis are summarized in Table 1. From the extracted data, one can find that C1 and C2 are similar in electron distribution and have richer charges in comparison to other carbon atoms, allowing for being facilely attacked to form carbocation and therefore decorated by two substituent groups. Quantum chemistry calculation, such as TD DFT, has been utilized to simulate the parameters of UV absorption successfully [19]. Fig. 2 shows the optimized structure of the resultant UV-CA, illustrating that there are two intermolecular hydrogen bonds, O H … O in the UV-0 group (bond length: 1.6759 Å, bond angle: 141.7) and O H … O bond between the UV-0 and NMA groups (bond length:
Table 2 Transition states, character, transition orbitals, electronic transition energies, and oscillator strengths (f) for UV-CA calculated from B3LYP-TD/6-31G (d,p). Transition state
Character
S0/S1 S0/S2 S0/S3 S0/S4 S0/S5 S0/S6 S0/S7
1
Transition orbitals
Transition energy (eV) Calc.
(p, 1 (p, 1 (p, 1 (p, 1 (p, 1 (p, 1 (p,
p *) p *) p *) p *) p *) p *) p *)
100 100 100 99 99 100 98
/ / / / / / /
101(0.66) 102(0.52) 102(0.47) 101(0.64) 102(0.69) 103(0.45) 101(0.61)
3.69(335 3.93(314 3.94(312 4.17(297 4.28(289 4.37(283 4.40(281
Oscillator strength (f) Expt. (nm)
nm) nm) m) nm) nm) nm) nm)
339
295
0.1596 0.0019 0.0003 0.1753 0.0512 0.005 0.0169
252
M. Shan et al. / Journal of Molecular Structure 1107 (2016) 249e253
Table 3 Transition states, character, transition orbitals, electronic transition energies, and oscillator strengths (f) for UV-CA calculated from B3LYP-TD/6-31G (d,p) based on PCM. Transition state
Character
(p, (p, 1 (p, 1 (p, 1 (p, 1 (p, 1 (p, 1
S0/S1 S0/S2 S0/S3 S0/S4 S0/S5 S0/S6 S0/S7
1
p*) p*) p*) p*) p*) p*) p*)
Transition orbitals
100/101(0.66) 97/101(0.49) 99/101(0.65) 100/102(0.70) 98/101(0.65) 100/103(0.54) 95/101(0.54)
Transition energy (eV)
Oscillator strength (f)
Calc.
Expt. (nm)
3.68(336nm) 3.99(310nm) 4.11(299m) 4.29(289nm) 4.43(280nm) 4.50(275nm) 4.52(274nm)
339
0.2230 0.0033 0.2681 0.0018 0.0134 0.0095 0.0102
295
Fig. 4. Molecular orbitals (99, 100 and 101) for the strong absorption bands of UV-CA above 275 nm region at B3LYP-TD/6-31G (d,p) level.
localized at the UV-O group and only a small amount of electron density are localized on the NMA groups. To orbital 99, the electronic clouds are localized at the UV-O group and the two NMA groups, electronic clouds of orbital 100 are localized at the UV-O group and one of the NMA groups, while orbital 101's electronic clouds are only delocalized on the UV-O group. According to the classic view, the intense absorption bands at 339 nm and 295 nm are assigned to p/p* transition. Obviously, S0/S1 (100 / 101) transition is mainly derived from the UV-0 group and NMA groups, and S0/S4 (99 / 101) transition comes from the UV-0 group and one of the NMA groups. With NMA groups participating in electronic transitions, the absorption bands of the UV-CA shift to long wavelength compared with UV-0, as shown in Fig. 3.
ð2Þ DEij
2 F ∅j ∅i b ¼ εi εj
where b F is the Fock operato, εj correspond to the energy eigenvalues of the donor molecular orbital ∅i and the acceptor molecular orbital ∅i. NBO analysis indicates the sum DE(2) ij of Oe H / O formed between UV-0 part and NMA part is 31.98 kcal/mol, while the Oe H / O in UV-0 part is 51.69 kcal/mol. These calculation results clearly Table 4 Structural parameters, NBO analysis for IMHB in UV-CA at B3LYP/6-31G (d,p) level.a r(O H)b
IMHB
r(O H / O)b
:(O H / O)c
3.3. IMHB interaction e
The parameters calculated from Fig. 2 show that the UV-CA absorber has two IMHB structures. The conclusion can be crosschecked by NBO analysis, an efficient avenue of giving insights conjugative interactions in molecular systems. Analysis of the NBO second-order interaction energies DE(2) ij of these IMHBs can estimate the relative strength of hydrogen-bonding interaction [21]. DE(2) ij is calculated as
O H / O O H / Of
1.004 0.997
2.517 2.661
141.7 148.6
NBOd
fi
fj
D E(2) ij
O O
*
31.98 51.69
s O H s* O H
a NBO analysis is performed by 6e31G(d,p) method based on the B3LYP/631G(d,p) optimized structureb Interatomic distances is in Angstrom unit. r(O H) is the length of the H-bond. r(O H / O) is the distance from O to O / H in the O H / O hydrogen bondsc:(O H / O) angle is in degreed NBO donor orbitalsfi; acceptor orbitalsfj and their corresponding secondorder interaction energie D E(2) ij in kcal/mole O H / O formed between UV-0 group and one of the NMA groupf O H / O in the UV-0 group.
M. Shan et al. / Journal of Molecular Structure 1107 (2016) 249e253 Table 5 Effect of UV-CA dosage on the Mn of polymer. No.
P1 P2 P3 P4
Monomer composition (wt.%) St
BMA
EA
UV-CA
33.33 33.03 32.73 32.44
33.33 33.03 32.73 32.44
33.33 33.03 32.73 32.44
0.009 0.017 0.265
Solids content (wt.%)
103Mn
39.7 40.0 40.2 40.4
7.314 233.675 421.126 933.219
253
property with crosslinking ability in UV-CA molecule under the guideline of quantum chemistry calculations. After careful characterizations, the excellent absorption of UV-CA is derived from two electronic transition bands: S0/S1 and S0/S4. Structural parameters and NBO analysis indicate that IMHB in UV-0 part is stronger than that formed between UV-0 and NMA species. The UV-CA crosslinked acrylic acid polymers show both high crosslinking degree and robust UV absorbing performances. This work represents a significant step forward, as it realizes the integration of UV absorbing and crosslinking capabilities in a single molecule for advanced anti-UV polymers. Acknowledgement National Ocean commonweal project (No.201005028-2), Natural Science Foundation of China (50673085), Ministry of education doctoral special fund (20130132130001). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.molstruc.2015.11.049. References
Fig. 5. UV absorption spectrum of UV-CA-functionalized- acrylic acid polymers in DMF.
show that IMHB in UV-0 part is stronger than that formed between UV-0 part and NMA part, as shown in Table 4. 3.4. Effect of UV-CA dosage on the Mn of crosslinked acrylic acid polymers The dependence of number-average molar mass (Mn) of UV-CA crosslinked acrylic acid polymers on UV-CA crosslinker dosage is determined, as shown in Table 5. The Mn of P2, P3, and P4 with UVCA crosslinker are much larger than that of P1 without UV-CA. Moreover, an increase in UV-CA dosage results in an elevated Mn value for the polymer. Till now, we can make a conclusion that UVCA is an efficiency crosslinking agent for improving the Mn of functional polymers. Fig. 5 displays the UV absorption spectrum of UV-CA-functionalized-acrylic acid polymers in DMF, indicating their excellent UV absorbing performance. 4. Conclusions In summary, we have successfully combined the anit-UV
[1] J.M. Taguenang, A. Kassu, P.B. Ruffin, C. Brantley, E. Edwards, A. Sharma, Opt. Commun. 281 (2008) 2089e2092. [2] F. Bennet, P.J. Barker, T.P. Davis, A.H. Soeriyadi, C. Barner-Kowollik, Macromol. Chem. Phys. 211 (2010) 2034e2052. [3] P. Costa, S. Ribeiro, G. Botelho, A.V. Machado, S. Lanceros Mendez, Polym. Test. 42 (2015) 225e233. [4] J. Jin, S. Chen, J. Zhang, Polym. Degrad. Stab. 95 (2010) 725e732. [5] R.R. Cordero, P. Roth, A. Georgiev, L. DaSilva, Energy Convers. Manag. 46 (2005) 2907e2918. [6] D. Rosu, L. Rosu, C.N. Cascaval, J. Photochem. Photobiol. A Chem. 194 (2008) 275e282. [7] V.B. Bojinov, N.I. Georgiev, N.V. Marinova, Sensors Actuators B Chem. 148 (2010) 6e16. [8] K. Pei, Z. Cui, W. Chen, J. Mol. Struct. 1032 (2013) 100e104. [9] A.I.G.a.S. J. Fertig, Appl. Polym. 10 (1966) 9. [10] W. Czajkowski, J. Paluszkiewicz, R. Stolarski, M. Ka zmierska, E. Grzesiak, Dyes Pigments 71 (2006) 224e230. [11] J. Akrman, J. Prikryl, J. Appl. Polym. Sci. 108 (2008) 334e341. [12] J. Mamnicka, W. Czajkowski, Cellulose 19 (2012) 1781e1790. [13] F. Pina, M. Maestri, V. Balzani, Chem. Commun. (1999) 107e114. [14] V.B. Bojinov, I.P. Panova, D.B. Simeonov, N.I. Georgiev, J. Photochem. Photobiol. A Chem. 210 (2010) 89e99. [15] C.G. Koukiotis, M.M. Karabela, I.D. Sideridou, Prog. Org. Coatings 75 (2012) 106e115. [16] M. Almandoz, M. Sancho, P. Duchowicz, S. Blanco, Spectrochimica Acta Part A Mol. Biomol. Spectrosc. 129 (2014) 52e60. [17] R.A. Gaussian09, Inc, Wallingford CT, (2009). € m, H. H€ [18] Z. Bohstro arelind, K. Holmberg, J. Mol. Catal. A Chem. 366 (2013) 171e178. [19] Z. Cui, X. Li, X. Wang, K. Pei, W. Chen, J. Mol. Struct. 1054e1055 (2013) 94e99. [20] C. Marrassini, A. Idrissi, I. De Waele, K. Smail, N. Tchouar, M. Moreau, A. Mezzetti, J. Mol. Liq. 205 (2015) 2e8. [21] E.D. Glendening, J. Badenhoop, F. Weinhold, J. Comput. Chem. 19 (1998) 628e646.
Contents lists available at ScienceDirect
Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc
A strategy of integrating ultraviolet absorption and crosslinking in a single molecule: DFT calculation and experimental Mingli Shan a, Yujing Liu a, Shuwei Xia a, Qunwei Tang b, Liangmin Yu a, * a b
Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao 266100, China Institute of Materials Science and Engineering, Ocean University of China, Qingdao 266100, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 September 2015 Received in revised form 16 November 2015 Accepted 19 November 2015 Available online 2 December 2015
Creation of advanced ultraviolet light absorbers having crosslinking ability has been persistent objective for anti-ultraviolent aging polymers. We present here the integration of 2, 4-dihydroxybenzophenone (UV-0) and N-methylol acrylamide (NMA) for novel ultraviolet absorber namely (3,5-dimethacryl amide-2,4-dihydroxyphenyl) (phenyl)methanone (UV-CA), which is subsequently utilized as a crosslinking agent after suffering FriedeleCrafts reaction. The preliminary results demonstrate that quantum chemical calculations (DFT) is a promising avenue in demonstrating the optimized geometry, charges, energy levels and UV electronic absorption bands of the UV-CA in the singlet (steady and excited states). The structure parameters and natural band orbital (NBO) calculations suggest that the intramolecular hydrogen bond (IMHB) in the UV-0 group is significantly enhanced in comparison to that between UV0 and NMA groups. The acrylic acid polymers functionalized with UV-CA yield high crosslinking degree and robust UV absorbing performance. The impressive results demonstrate that quantum chemical calculations are promising in organic synthesis to develop advanced compounds. © 2015 Elsevier B.V. All rights reserved.
Keywords: UV absorber Crosslinking agent Quantum chemical calculations
1. Introduction Organic polymer materials are widely used due to their low manufacturing costs, easy applied and high stability. The organic polymer materials are mainly applied in outdoor environment, consequently their long-term stability and service life would be deteriorated after suffering from humidity and temperature variations, oxygen, and sunlight [1e4]. Ultraviolet (UV) light is believed to be crucial in depending the lifetime of these functional organic polymers because the corresponding chemical bonds can be destroyed under high energy irradiations. In this fashion, UV absorbers seem crucial in protecting the organic polymers from being damaged in practical applications [5e7]. Among various UV absorbers, the absorbers having gigantic intramolecular hydrogen bridges have attracted growing interests to prevent polymers from UV aging [8]. One of the traditional strategies of designing anti-UV polymers is to add UV absorbers as nonreactive additives. A remaining issue for this technique is the unsatisfactory operational lifetime, arising from facile dissolution of UV absorber from organic polymers when suffering specific
* Corresponding author. E-mail address: [email protected] (L. Yu). http://dx.doi.org/10.1016/j.molstruc.2015.11.049 0022-2860/© 2015 Elsevier B.V. All rights reserved.
atmosphere. Since the first study on reactive UV absorber for robust anti-UV performances [9], many efforts have placed their focuses on developing novel reactive UV absorbers by compolymerizing them with the monomers of organic polymers [10e12]. Great effort is currently being devoted to the design of multifunction compounds to facilitate the applications [13]. The commonly utilized UV absorbers have only UV-absorbing ability with unsatisfactory crosslinking performance [14]. A rising challenge for UV agents is to combine anti-UV performance with crosslinking ability in a single species because a reasonable crosslinking degree is always prerequisite for functional polymers [15]. By addressing this issue, we present here the experimental realization of dual-functionalized UV absorbers by FriedeleCrafts reaction based on the quantum chemistry calculation theory-guided design. The optimized geometry and energy levels in the singlet (steady and excited states) are obtained by quantum chemistry calculations, while natural band orbital (NBO) analysis demonstrates the content of intramolecular hydrogen bonds (IMHBs) in the resulting molecule has an beneficial effect on photostability. The preliminary results suggest that the resultant (3,5dimethacrylamide-2,4-dihydroxyphenyl) (phenyl)methanone (UV-CA) has superior anti-UV and crosslinking performances in functionalized acrylic acid polymers.
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M. Shan et al. / Journal of Molecular Structure 1107 (2016) 249e253
Scheme 1. Synthetic route of UV-CA.
Waters 2695 GPC using DMF as eluent. Elemental analysis was performed on a Vario EL Elementar. 2.3. Synthesis of UV-CA The synthesis processes for UV-CA is shown in Scheme 1. In details, a mixed solution of alchlor (0.08 g) in acetone (5 mL) was added dropwise to an acetone solution containing UV-0 (0.01 mol) and NMA (0.01 mol) under vigorous agitation at 35 C. After reaction for 48 h, the brown precipitates were filtered and rinsed with 0.01 M hydrochloric acid aqueous solution and dried in air. Subsequently, the as-synthesized intermediate products were purified by recrystallization to obtain the white UV-CA. Yield: 83%. FTIR (KBr)/cm1: 3270(OH), 3081(NH), 1656, 1623(C]O) (Fig. S1.); 1H NMR (600 MHz, DMSO)/ppm: d 13.04 (s, 1H, OeH), 11.86 (s, 1H, OeH), 9.33 (t, 1H, NeH), 8.59 (t, 1H, NeH), 7.61e7.66 (m, 3H, AreH), 7.54 (t, 2H, AreH), 7.40 (s, 1H, AreH), 6.09e6.23 (m, 4H, ¼CH2), 5.74 (t, 1H, eCH ¼ ), 5.62 (t, 1H, eCH ¼ ), 4.38 (d, 2H, eCH2-), 4.21 (d, 2H, eCH2-) (Fig. S2.); 13C NMR (150 MHz, DMSO): 199.7, 167.8, 165.7, 163.0, 162.0, 138.0, 133.9, 132.2, 131.5, 130.4, 129.2, 128.9, 127.8, 126.2, 118.7, 113.0, 111.7, 38.1, 32.4 (Fig. S3.); Mass (ESI) found m/z: 381.1445 (MH, 100); Element analysis: Found (%): C, 66.29, H, 5.28, O, 21.09, N, 7.36; Calcd (%): C, 66.31, H, 5.26, O, 21.05, N, 7.36.
Fig. 1. The S0 optimized structure of UV-0 at B3LYP/6-31G(d,p) level. (red: O atom, white: H atom, gray: C atom.)
2. Experimental 2.1. Materials and reagents UV-0 (analytical reagent, 99%), N-methylol acrylamide, dimethyl sulfoxide (analytical reagent, 99%), AlCl3 (Chemical pure, 99%), acetone (analytical reagent, 99.5%), concentrated hydrochloric acid (chemical pure, 37%) and ethanol (Chemical pure, 99.5%) were purchased from Sinopharm Chemical Reagent Beijing Co. Ltd. (Beijing, PR China). Styrene (St, chemical pure, 99%), butyl methacrylate (BMA, chemical pure, 99%), ethyl acrylate (EA, Chemical pure, 99%) were purchased from Aladdin Reagent Co. Ltd (Shanghai, China).
2.4. Synthesis of UV-CA crosslinked acrylic acid polymers The UV-CA crosslinked acrylic acid polymers were synthesized using the following procedures: A mixture of St (11 g), BMA (11 g), EA (11 g), and stoichiometric UV-CA was prepared by dissolving in 50 g dimethyl benzene. Before adding 0.5 g 2, 2-Azobisisobutyronitrile (AIBN), the above-mentioned solution was degassed and refilled with argon. After vigorous agitation at 90 C for 8 h, the transparent polymers were dissolved in DMF to measure their UV absorption spectras.
2.2. Characterizations The UV absorption spectrum of the compound and functionalized acrylic acid polymers were performed on a Beckman Coulter DU-800 ultraviolet/visible spectrometer. Infrared spectrum (KBr pellets) of solid sample was recorded in a region of 4000e400 cm1 on a Perkine Elmer Spectrum-One FT-IR spectrophotometer. 1H and 13 C NMR spectrum was recorded on AVANCE III 600 MHz NMR Spectrometer with tetramethylsilane (TMS) as internal standard in Dimethyl sulfoxide. ESI mass spectra were measured on a micro mass Q-TOF (waters) mass spectrometer by utilizing an acetonitrile/formic acid matrix. GPC measurements were conducted by
2.5. Calculations The optimized geometry in the ground state and UV absorption spectra were determined by time dependent (TD) and density functional theory (DFT) calculations until the vibrational frequencies were true minima. The solvent (DMF) effect was analyzed by means of Polarizable Continuum Model (PCM) [16]. The geometrical parameters, frontier molecular orbital energies,
Table 1 Natural charge of UV-0 calculated from B3LYP-TD/6-31G(d,p).a Atom
C1b
C2
C3
C4
C5
C6
C7
C8
Natural Charge (e)
0.38326
0.32383
0.17694
0.22169
0.23692
0.22038
0.23588
0.19269
a b
The calculation was carried out based on the S0 optimized structure of UV-0 as shown in Fig. 1. The number of the carbon atoms are labeled in Fig. 1.
M. Shan et al. / Journal of Molecular Structure 1107 (2016) 249e253
Fig. 2. The S0 optimized structure of UV-CA at B3LYP/6-31G (d) level. Bond length in Angstrom, red: O atom, blue: N atom, white: H atom, gray: C atom.
251
Fig. 3. UVevis absorption spectra of UV-0 and UV-CA in 2 105 M DMF.
second-order interaction energies from NBO analysis of the title compound were calculated based on the optimized structure at B3LYP/6-31G(d,p) level. All the DFT calculations in this paper were carried out in the Gaussian09 package on the personal computer [17].
1.6759 Å, bond angle: 148.6)]. Based on gas monomolecular model, there are two strong UV absorption bands in UV-CA with the calculated maximum molar absorption coefficient 1,2500 and 1,3750 L mol-1 cm-1, respectively. Therefore, we can make a conclusion from quantum chemistry calculation that the resultant UV-CA molecule is efficient in absorbing UV radiation.
3. Results and discussion
3.2. Absorption spectrum
3.1. Theory-guided design
The quantum chemistry calculation results including electronic absorption bands, electric transition orbitals, oscillator strength (f) and the maximum absorption wavelength of UV-CA based on gas monomolecular model are summarized in Table 2. Among the calculated electronic transitions, two characteristic absorption bands (S0/S1, S0/S4) located at >270 nm are induced by electric transition with the oscillator strength (f) of 0.1596 and 0.1753, respectively. As shown in Table 2 and Fig. 3, the simulated (S0/S1,S0/S4) electronic transition bands of the UV-CA are matching to the experimental spectra (15,000 L mol1 cm1) at 295 nm and 12,000 L mol1 cm1 at 339 nm. The solvatochromic behavior of UV-CA was analyzed by means of the Polarizable Continuum Model (PCM) and the results were summarized in Table 3. No distinct differences can be found between the maximum absorption wavelength calculated from gas monomolecular model (335, 297 nm) and Polarizable Continuum Model (336, 299 nm). This is possibly related to the presence of a strong intramolecular hydrogen bond in the UV-CA [20]. Fig. 4 displays the three orbitals (99 HOMO-1, 100 HOMO and 101 LUMO) associated with the S0/S1 and S0/S4 electronic transitions. It indicates that orbital 99, 100, and 101 are p orbitals, and the electron density of orbitals (99, 100 and 101) are mainly
FriedeleCrafts reaction is one of the widely utilized method in transferring CeC bonds to aromatic rings [18]. Substituent group will attack the electron-enriching atoms to form carbocation species, subsequently acting as electrophile in an electrophilic aromatic substitution. In this fashion, the molecular structure can be extracted by analyzing the electron distribution of this aromatic compound by natural population analysis (NPA) technique. The S0 optimized geometry of UV-0 is shown in Fig. 1 and the corresponding parameters calculated by NBO analysis are summarized in Table 1. From the extracted data, one can find that C1 and C2 are similar in electron distribution and have richer charges in comparison to other carbon atoms, allowing for being facilely attacked to form carbocation and therefore decorated by two substituent groups. Quantum chemistry calculation, such as TD DFT, has been utilized to simulate the parameters of UV absorption successfully [19]. Fig. 2 shows the optimized structure of the resultant UV-CA, illustrating that there are two intermolecular hydrogen bonds, O H … O in the UV-0 group (bond length: 1.6759 Å, bond angle: 141.7) and O H … O bond between the UV-0 and NMA groups (bond length:
Table 2 Transition states, character, transition orbitals, electronic transition energies, and oscillator strengths (f) for UV-CA calculated from B3LYP-TD/6-31G (d,p). Transition state
Character
S0/S1 S0/S2 S0/S3 S0/S4 S0/S5 S0/S6 S0/S7
1
Transition orbitals
Transition energy (eV) Calc.
(p, 1 (p, 1 (p, 1 (p, 1 (p, 1 (p, 1 (p,
p *) p *) p *) p *) p *) p *) p *)
100 100 100 99 99 100 98
/ / / / / / /
101(0.66) 102(0.52) 102(0.47) 101(0.64) 102(0.69) 103(0.45) 101(0.61)
3.69(335 3.93(314 3.94(312 4.17(297 4.28(289 4.37(283 4.40(281
Oscillator strength (f) Expt. (nm)
nm) nm) m) nm) nm) nm) nm)
339
295
0.1596 0.0019 0.0003 0.1753 0.0512 0.005 0.0169
252
M. Shan et al. / Journal of Molecular Structure 1107 (2016) 249e253
Table 3 Transition states, character, transition orbitals, electronic transition energies, and oscillator strengths (f) for UV-CA calculated from B3LYP-TD/6-31G (d,p) based on PCM. Transition state
Character
(p, (p, 1 (p, 1 (p, 1 (p, 1 (p, 1 (p, 1
S0/S1 S0/S2 S0/S3 S0/S4 S0/S5 S0/S6 S0/S7
1
p*) p*) p*) p*) p*) p*) p*)
Transition orbitals
100/101(0.66) 97/101(0.49) 99/101(0.65) 100/102(0.70) 98/101(0.65) 100/103(0.54) 95/101(0.54)
Transition energy (eV)
Oscillator strength (f)
Calc.
Expt. (nm)
3.68(336nm) 3.99(310nm) 4.11(299m) 4.29(289nm) 4.43(280nm) 4.50(275nm) 4.52(274nm)
339
0.2230 0.0033 0.2681 0.0018 0.0134 0.0095 0.0102
295
Fig. 4. Molecular orbitals (99, 100 and 101) for the strong absorption bands of UV-CA above 275 nm region at B3LYP-TD/6-31G (d,p) level.
localized at the UV-O group and only a small amount of electron density are localized on the NMA groups. To orbital 99, the electronic clouds are localized at the UV-O group and the two NMA groups, electronic clouds of orbital 100 are localized at the UV-O group and one of the NMA groups, while orbital 101's electronic clouds are only delocalized on the UV-O group. According to the classic view, the intense absorption bands at 339 nm and 295 nm are assigned to p/p* transition. Obviously, S0/S1 (100 / 101) transition is mainly derived from the UV-0 group and NMA groups, and S0/S4 (99 / 101) transition comes from the UV-0 group and one of the NMA groups. With NMA groups participating in electronic transitions, the absorption bands of the UV-CA shift to long wavelength compared with UV-0, as shown in Fig. 3.
ð2Þ DEij
2 F ∅j ∅i b ¼ εi εj
where b F is the Fock operato, εj correspond to the energy eigenvalues of the donor molecular orbital ∅i and the acceptor molecular orbital ∅i. NBO analysis indicates the sum DE(2) ij of Oe H / O formed between UV-0 part and NMA part is 31.98 kcal/mol, while the Oe H / O in UV-0 part is 51.69 kcal/mol. These calculation results clearly Table 4 Structural parameters, NBO analysis for IMHB in UV-CA at B3LYP/6-31G (d,p) level.a r(O H)b
IMHB
r(O H / O)b
:(O H / O)c
3.3. IMHB interaction e
The parameters calculated from Fig. 2 show that the UV-CA absorber has two IMHB structures. The conclusion can be crosschecked by NBO analysis, an efficient avenue of giving insights conjugative interactions in molecular systems. Analysis of the NBO second-order interaction energies DE(2) ij of these IMHBs can estimate the relative strength of hydrogen-bonding interaction [21]. DE(2) ij is calculated as
O H / O O H / Of
1.004 0.997
2.517 2.661
141.7 148.6
NBOd
fi
fj
D E(2) ij
O O
*
31.98 51.69
s O H s* O H
a NBO analysis is performed by 6e31G(d,p) method based on the B3LYP/631G(d,p) optimized structureb Interatomic distances is in Angstrom unit. r(O H) is the length of the H-bond. r(O H / O) is the distance from O to O / H in the O H / O hydrogen bondsc:(O H / O) angle is in degreed NBO donor orbitalsfi; acceptor orbitalsfj and their corresponding secondorder interaction energie D E(2) ij in kcal/mole O H / O formed between UV-0 group and one of the NMA groupf O H / O in the UV-0 group.
M. Shan et al. / Journal of Molecular Structure 1107 (2016) 249e253 Table 5 Effect of UV-CA dosage on the Mn of polymer. No.
P1 P2 P3 P4
Monomer composition (wt.%) St
BMA
EA
UV-CA
33.33 33.03 32.73 32.44
33.33 33.03 32.73 32.44
33.33 33.03 32.73 32.44
0.009 0.017 0.265
Solids content (wt.%)
103Mn
39.7 40.0 40.2 40.4
7.314 233.675 421.126 933.219
253
property with crosslinking ability in UV-CA molecule under the guideline of quantum chemistry calculations. After careful characterizations, the excellent absorption of UV-CA is derived from two electronic transition bands: S0/S1 and S0/S4. Structural parameters and NBO analysis indicate that IMHB in UV-0 part is stronger than that formed between UV-0 and NMA species. The UV-CA crosslinked acrylic acid polymers show both high crosslinking degree and robust UV absorbing performances. This work represents a significant step forward, as it realizes the integration of UV absorbing and crosslinking capabilities in a single molecule for advanced anti-UV polymers. Acknowledgement National Ocean commonweal project (No.201005028-2), Natural Science Foundation of China (50673085), Ministry of education doctoral special fund (20130132130001). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.molstruc.2015.11.049. References
Fig. 5. UV absorption spectrum of UV-CA-functionalized- acrylic acid polymers in DMF.
show that IMHB in UV-0 part is stronger than that formed between UV-0 part and NMA part, as shown in Table 4. 3.4. Effect of UV-CA dosage on the Mn of crosslinked acrylic acid polymers The dependence of number-average molar mass (Mn) of UV-CA crosslinked acrylic acid polymers on UV-CA crosslinker dosage is determined, as shown in Table 5. The Mn of P2, P3, and P4 with UVCA crosslinker are much larger than that of P1 without UV-CA. Moreover, an increase in UV-CA dosage results in an elevated Mn value for the polymer. Till now, we can make a conclusion that UVCA is an efficiency crosslinking agent for improving the Mn of functional polymers. Fig. 5 displays the UV absorption spectrum of UV-CA-functionalized-acrylic acid polymers in DMF, indicating their excellent UV absorbing performance. 4. Conclusions In summary, we have successfully combined the anit-UV
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