On the molecular conformation of bisaromatic systems the case of 2-phenyl-2H-benzotriazoles

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Chemical Physics 340 (2007) 32–42 www.elsevier.com/locate/chemphys

On the molecular conformation of bisaromatic systems the case of 2-phenyl-2H-benzotriazoles Javier Catala´n

b

a,*,1

, Pilar Pe´rez

a,1

, Rosa Marı´a Claramunt Vladimir Bobosik b,2,3

b,*,2

, Dolores Santa Marı´a

b,2

,

a Departamento de Quı´mica Fı´sica Aplicada, Universidad Auto´noma de Madrid, Cantoblanco, 28049 Madrid, Spain Departamento de Quı´mica Orga´nica y Bio-Orga´nica, Facultad de Ciencias, UNED, Senda del Rey 9, 28040 Madrid, Spain

Received 29 May 2007; accepted 17 July 2007 Available online 10 August 2007

Abstract 2-Phenyl-2H-benzotriazole exhibits a planar molecular conformation both in its ground electronic state (S0) and its first excited singlet (S1) and triplet state (T1). However, introducing one or two methyl groups in the ortho positions of the phenyl ring causes the aromatic systems in the compound to lose their coplanarity in both S0 and T1 electronic states. On the other hand, 2-(2-methylphenyl)-2H-benzotriazole regains such coplanarity in its first excited singlet state S1, giving rise to population inversion that could be used to generate stimulated radiation around 350 nm. As shown in this work, the effectiveness of the ISC process in these compounds is markedly dependent on the twisting angle, h, of the structure; accordingly, ISC occurs to a negligible extent in a planar compound such as 2-phenyl-2H-benzotriazole, where h = 0. This evidence supports the assumption that planar molecular forms of the TIN-P photoprotectors are more photostable than non-planar ones due to the non effective generation via ISC of their triplet states.  2007 Elsevier B.V. All rights reserved. Keywords: 2-Arylbenzotriazoles; Absorption spectra; Emission spectra; Theoretical calculations

1. Introduction The tendency of aromatic systems to adopt a planar molecular structure is ascribed to such a structure ensuring the greatest possible stabilization by resonance of its p electron system. Thus, an aromatic system is usually viewed as a rigid molecular structure that only departs from coplanar equilibrium by effect of changes altering hybridization in some sites contributing to its p electron system. Therefore, in the absence of a specific hindrance, a molecular system

*

Corresponding authors. E-mail addresses: [email protected] (J. Catala´n), rclaramunt@ ccia.uned.es (R.M. Claramunt). 1 Tel.: +34 91 4974263. 2 Tel.: +34 91 3987322; fax: +34 91 39883722. 3 Present address: Synkola, Mlynska´ dolina, Area´l PrivF UK, 84215 Bratislava, Slovak Republic. 0301-0104/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2007.07.048

consisting of two aromatic systems connected via a typical single bond (i.e. a bisaromatic system) also tends to adopt an equilibrium conformation with the two aromatic systems lying coplanarly in order to facilitate further delocalization of their p electrons. Two aromatic systems connected via a single bond can rotate about it and adopt special molecular conformations corresponding to the highest or lowest energy values for the molecular system in the electronic state concerned. The energy balance for the system at an energy minimum will reflect a situation in between delocalization of p electrons and electrostatic or steric non-bonding interactions between molecular sites adjacent to the connecting bond. Changes in such interactions can be expected to alter the twisting angle corresponding to the equilibrium situation and hence change the equilibrium molecular structure. The interest aroused by the twisting mechanism that describes motion around the bond connecting two cyclic

J. Catala´n et al. / Chemical Physics 340 (2007) 32–42

aromatic systems in the past few decades has produced a wealth of documentation for compounds such as 9-phenylanthracene [1,2], 2-phenylindole [3–5], biphenyl [6–8], 9,9bianthryl [9], 2,2 0 -dipyrimidine [10] and 2-phenylindene [11–14]. Their structures possess two salient features, namely: (a) the bond connecting the two aromatic systems involves two sp2 carbon atoms each of which contributes one electron to the p system, which allows the adoption of planar neutral forms conjugated to some extent in the ground electronic state; and (b) the structures exhibit substantial non-bonding interactions between sites of the same nature in the two aromatic systems, the interaction being of the lone electron pair/lone electron pair type in 2,2 0 -dipyrimidine and between neighboring protons in all other compounds. Introducing appropriate substituents at sterically sensitive ortho positions in these bisaromatic systems is known to cause changes in their UV–vis absorption spectra; according to the hypothesis put forward by O’Shaughnessy and Rodebush [15] long ago, the changes are due to decreased conjugation through the bond connecting the aromatic systems, which reduces coplanarity [16]. Based on the positions of the torsional minima in these bisaromatic systems, Forbes [17] classified the behavior of their UV–vis absorption bands and rationalized the hypsochromic shifts and hypochromic effects usually caused by steric hindrance in them. The structure of 2-phenyl-2H-benzotriazoles departs from that typical of bisaromatic compounds in some interesting respects. Thus, the single bond connecting the two aromatic systems retains its character throughout a torsional movement in the ground state as it is formed by an sp2 carbon atom that contributes one p electron to the p system and an sp2 nitrogen atom that contributes two electrons; this precludes their conjugating without producing a charged, unstable form in the ground electronic state. In addition, 2-phenyl-2H-benzotriazole exhibits interactions between protons next to the carbon involved in the bond connecting the phenyl ring and lone electron pairs in the nitrogen atoms next to that involved in such a bond. Finally, 2-phenyl-2H-benzotriazole can be made sterically hindered by introducing appropriate substituents in ortho positions of the phenyl ring. In this work, we studied the behavior of the 2-aryl-2Hbenzotriazoles 1–4 relative to 2-methyl-2H-benzotriazole (5). R6 N

N

N

R4

N

CH3

N

N R2 R2 = R4 = R6 = H R2 = CH3; R4 = R6 = H R2 = R6 = CH3; R4 = H R2 = R4 = CH3; R6 = H

1

5

2 3 4

Compound 5 was used to study the spectroscopic behavior of a benzotriazole bearing a non-conjugated substituent

33

at position 2, and compound 1 that of a 2-aryl-2H-benzotriazole subject to no steric hindrance. Finally, compounds 2–4 were used to examine the behavior of sterically hindered benzotriazoles. In this work, we focussed on the absorption, fluorescence emission and phosphorescence emission of these compounds. This required identifying the equilibrium structure of each compound in both the ground electronic state (S0) and the first excited singlet (S1) and triplet (T1) in order to rationalize the spectroscopic behavior of these bisaromatic systems from potential conformational changes caused by electronic excitation. A tentative explanation for such conformational changes based on the molecular orbitals involved in the electronic transition is provided and, whether the steric hindrance [18–21] posed by a methyl group is independent of the particular electronic state or if these compounds can interact via nonclassical hydrogen bonds of the N:  H–C type between a methyl group on the phenyl ring and a lone electron pair on the nitrogen atom in the benzotriazole ring are discussed. Also, one should bear in mind that 2-phenyl-2H-benzotriazole structures are specially significant because they constitute the molecular backbone for such important photoprotectors as Ciba-Geigy’s TIN-P, the efficiency of which seems to be strongly dependent on planarity retention in the 2-phenyl-2H-benzotriazole system. Worth special note here is the widespread assumption that nonplanarity in TIN-P fosters the generation of photolabile triplet states, thereby facilitating its photodegradation. Finally, one interesting, as yet unresolved question, is whether, by analogy with 2-phenylindole, 2-phenylbenzotriazoles can undergo population inversion by effect of the conformational changes caused by electronic excitation. 2. Experimental 2.1. General methods The UV–vis spectra for compounds 1–4 were recorded on a Cary-5 spectrophotometer, using Suprasil quartz cells of 1 cm light path with solutions more diluted than 105 M in 2-methylbutane (2MB), or cells of 10 cm light path for samples in the gas phase. The temperature of the solutions was controlled by means of an Oxford DN1704 cryostat equipped with an ITC4 controller and interfaced to the spectrophotometer. The cryostat used for the liquid samples was purged with dried nitrogen over 99.99% pure. The temperature of the gaseous samples was maintained at 80.0 ± 0.2 C by means of a Fisons Haake GH thermostat. The 2-methylbutane used was Merck Uvasol-grade and contained less than 0.005% moisture. All emission spectroscopic measurements were made in cylindrical quartz cells of 3 mm light path from Suprasil. The temperature of the solutions was controlled by means of an Optistat DN Oxford cryostat equipped with an ITC-

34

J. Catala´n et al. / Chemical Physics 340 (2007) 32–42

502 controller and interfaced to the spectrophotometer. Consequently, the path length of to the cell center was less than 1.5 mm, the average length ranging from 0 to 1.5 mm. Corrected emission and excitation spectra were obtained by using a precalibrated Aminco-Bowman AB2 spectrofluorimeter. Phosphorescence was generated by excitation with light from a 7 W pulsed xenon lamp, using a delay of 250 ls to detect phosphorescence. Phosphorescence lifetimes were measured by tuning light to the 0–0 component, using a slit width of 8 nm in the excitation monochromator and a continuous lamp. Once a stationary signal was obtained, the excitation shutter was closed and the emission shutter kept open in order to monitor the emission intensity until it fell to negligible levels. The reported lifetime values are the averages of 12 measurements each. Theoretical computations for compounds 1–4 in their ground and first triplet states were done within the framework of the density functional theory (DFT), using the Gaussian 98 software package [22]. Full geometry optimization of the ground state and triplet states was achieved by using the hybrid functional B3LYP [23,24] in conjunction with the 6-31G** basis set [25]. Full geometry optimization of the first excited singlet state was done with the CIS method [26] in combination with the 6-31G** basis set. All structures thus identified were found to correspond to true minima – they had all real vibrational frequencies. The energies of the 0–0 components of the phosphorescence transitions, T1 ! S0, were estimated with provision for the energy difference between the equilibrium structures in the two states as corrected for the corresponding zeropoint energies [27–29]. Melting points were determined with a ThermoGalen hot stage microscope and are given uncorrected. Elemental analyses for carbon, hydrogen and nitrogen were performed by the Microanalytical Service of the Complutensian University of Madrid, using a Perkin–Elmer 240 analyzer. Column chromatography was conducted on silica gel (Merck 60, 70-230 mesh). Rf values were measured on aluminum-backed TLC plates of silica gel 60 F254 (Merck, 0.2 mm) with the indicated eluent. NMR spectra were recorded at 300 K (9.4 T, 400.13 MHz for 1H and 100.62 MHz for 13C). Chemical shifts (d, in ppm) are given from internal solvent CDCl3 (7.26 for 1H and 77.0 for 13C) and DMSO-d6 (2.49 for 1H and 39.5 for 13C). 2D gs-COSY (1H–1H) and 2D inverse proton detected heteronuclear shift correlation spectra [gs-HMQC (1H–13C) and gsHMBC (1H–13C)] were obtained by using the standard pulse sequences to assign 1H and 13C signals.

ing phosphorus pentachloride to give N-(2-nitrophenyl)benzimidoyl chloride, which was then treated with sodium azide in dry N,N 0 -dimethylformamide [30]. The 2 0 -nitrobenzanilides 6–9 were readily prepared by reacting an appropriate benzyl chloride with 2-nitroaniline [31]. Finally, 2-methyl-2H-benzotriazole (5) was obtained by methylation of benzotriazole with dimethyl sulfate in an alkaline medium [32] and separated from 1-methyl-1Hbenzotriazole by column chromatography.

2.2. Syntheses (see Scheme 1)

2.2.3. 5-(2,6-Dimethylphenyl)-1-(2-nitrophenyl)tetrazole (12) It was obtained in 66% yield from 2,6-dimethyl-N-(2nitrophenyl)benzamide (8) as described above for 10. M.p. 147–149 C (crystallized from ethanol:water). 1H NMR (DMSO-d6): d (ppm) 8.20 (dd, 1H, H-300 , J 300 ;400 ¼ 7:5, J 300 ;500 ¼ 2:1), 7.86 (ddd, 1H, H-500 , J 500 ;600 ¼ 7:3, J 500 ;400 ¼ 7:5), 7.83 (ddd, 1H, H-400 , J 400 ;600 ¼ 2:0), 7.49 (dd,

2-Phenyl-2H-benzotriazoles 1–4 were prepared by thermolysis of the corresponding 1-(2-nitrophenyl)-5-phenyltetrazoles 10–13, using a previously reported method [30] for 2-phenyl-2H-benzotriazole (1) in nitrobenzene as solvent. The tetrazoles 10–13 were obtained by heating the corresponding 2 0 -nitrobenzanilides 6–9 in toluene contain-

2.2.1. 1-(2-Nitrophenyl)-5-phenyltetrazole (10) A mixture of 2 0 -nitrobenzanilide (6) (1.52 g, 6.29 mmol) and phosphorus pentachloride (1.31 g, 6.29 mmol) in 20 mL of toluene was heated under reflux for 1 h, after which the hot solution was filtered and the solvent removed under reduced pressure. The resulting N-(2-nitrophenyl)benzimidoyl chloride oil was dissolved in dry DMF (20 mL) and added dropwise to a vigorously stirred mixture of sodium azide (0.82 g, 12.58 mmol) and dry DMF (10 mL) over a period of 45 min, the temperature being kept below 25 C throughout. The suspension thus obtained was heated at 80 C for 1 h and then cooled. Next, enough water to dissolve any inorganic salts present and then cause turbidity was added and the mixture stored refrigerated at 5 C for one day. The crystals thus formed were filtered off and washed with water to obtain 0.98 g (80%) of 10 m.p. 166–169 C (lit [33]. 166–168 C). 1H NMR (DMSO-d6): d (ppm) 8.35 (ddd, 1H, H-300 , J 300 ;400 ¼ 8:0, J 300 ;500 ¼ 1:3, J 300 ;600 ¼ 0:5), 8.04 (ddd, 1H, H-500 , J 500 ;600 ¼ 8:0, J 500 ;400 ¼ 6:6), 8.01 (ddd, 1H, H-600 , J 600 ;400 ¼ 2:7), 7.97 (ddd, 1H, H-400 ), 7.56 (m, 1H, H-4 0 ), 7.53 (m, 2H, H-2 0 /H-6 0 ), 7.47 (m, 2H, H-3 0 /H-5 0 ). 13C NMR (DMSO-d6): d (ppm) 154.5 (C5), 144.1 (C200 ), 135.6 (C500 ), 133.0 (C400 ), 130.1 (C600 ), 131.7 (C4 0 ), 129.2 (C3 0 / C5 0 ), 128.4 (C2 0 /C6 0 ), 126.7 (C100 ), 126.5 (C300 ), 122.4 (C1 0 ). 2.2.2. 5-(2-Methylphenyl)-1-(2-nitrophenyl)tetrazole (11) It was prepared in 82% yield from 2-methyl-N-(2-nitrophenyl)benzamide (7) as described above for 10. M.p. 176– 177 C. 1H NMR (DMSO-d6): d (ppm) 8.22 (dd, 1H, H-300 , J 300 ;400 ¼ 8:2, J 300 ;500 ¼ 1:2), 7.98 (ddd, 1H, H-500 , J 500 ;600 ¼ 8:0, J 500 ;400 ¼ 6:4), 7.94 (dd, 1H, H-600 , J 600 ;400 ¼ 2:7), 7.88 (ddd, 1H, H-400 ), 7.41 (m, 1H, H-4 0 ), 7.38 (br d, 1H, H-3 0 ), 7.18 (m, 2H, H-5 0 /H-6 0 ), 2.27 (s, 3H, CH3). 13C NMR (DMSO-d6): d (ppm) 154.2 (C5), 143.8 (C200 ), 138.1 (C200 ), 135.3 (C500 ), 132.7 (C400 ), 131.4 (C4 0 ), 131.1 (C3 0 ), 129.9 (C6 0 /C600 ), 126.2 (C300 ), 126.0 (C5 0 /C100 ), 121.6 (C1 0 ), 19.3 (CH3).

J. Catala´n et al. / Chemical Physics 340 (2007) 32–42

R4

R2

C R6

R4

NH

O

R4

R2

PCl5

NaN3

toluene

DMF C R6

O2N R2 = R4 = R6 = H R2 = CH3; R4 = R6 = H R2 = R6 = CH3; R4 = H R2 = R4 = CH3; R6 = H

35 3' 4'

R2

2'

N

1'

5'

4 6'

N

5

R6

Cl O2N

1

N

6''

2''

4''

7

NO2

3''

8

N

1''

5''

6

R2 = R4 = R6 = H R2 = CH3; R4 = R6 = H R2 = R6 = CH3; R4 = H R2 = R4 = CH3; R6 = H

9

3N

2

10 11 12 13

reflux nitrobenzene R6

4 3a

5

N N

6

7a

6'

5'

1' 4'

N

2'

7

R4

3'

R2 R2 = R4 = R6 = H R2 = CH3; R4 = R6 = H R2 = R6 = CH3; R4 = H R2 = R4 = CH3; R6 = H

1 2 3 4

Scheme 1. Synthesis of tetrazoles 10–13 and benzotriazoles 1–4.

1H, H-600 ), 7.36 (dd, 1H, H-4 0 , J 40 ;50 ¼ J 40 ;30 ¼ 7:6), 7.17 (d, 2H, H-3 0 /H-5 0 ), 1.97 (s, 6H, CH3). 13C NMR (DMSO-d6): d (ppm) 153.3 (C5), 143.5 (C200 ), 138.0 (C2 0 /C6 0 ), 134.9 (C500 ), 132.4 (C400 ), 131.4 (C4 0 ), 128.1 (C3 0 ), 127.7 (C600 ), 126.4 (C300 ), 125.3 (C100 ), 121.5 (C1 0 ), 19.3 (CH3).

sponding N-(2-nitrophenyl)benzimidoyl chlorides, which were not isolated. Finally, thermolysis of the tetrazoles afforded compounds 1–4 in quantitative yields. All compounds were completely characterized by 1H and 13C NMR spectroscopies.

2.2.4. 5-(2,4-Dimethylphenyl)-1-(2-nitrophenyl)tetrazole (13) It was prepared in 74% yield from 2,4-dimethyl-N-(2nitrophenyl)benzamide (9) as described above for 10. M.p. 111–112 C (crystallized from ethanol:water). 1H NMR (DMSO-d6): d (ppm) 8.22 (dd, 1H, H-300 , J 300 ;400 ¼ 8:2, J 300 ;500 ¼ 1:2), 7.97 (ddd, 1H, H-500 , J 500 ;600 ¼ 7:3, J 500 ;400 ¼ 6:9), 7.94 (dd, 1H, H-600 , J 400 ;600 ¼ 2:1), 7.87 (ddd, 1H, H-400 ), 7.20 (s, 1H, H-3 0 ), 7.05 (d, 1H, H-6 0 , J 60 ;50 ¼ 7:9), 6.98 (d, 1H, H-5 0 ), 2.26 (s, 3H, CH3-4 0 ), 2.24 (s, 3H, CH3-2 0 ). 13C NMR (DMSO-d6): d (ppm) 154.3 (C5), 143.8 (C200 ), 141.3 (C4 0 ), 137.9 (C2 0 ), 135.3 (C500 ), 132.7 (C400 ), 131.7 (C3 0 ), 129.9 (C600 ), 129.7 (C6 0 ), 126.7 (C5 0 ), 126.3 (C100 ), 126.2 (C300 ), 118.6 (C1 0 ), 20.8 (CH34 0 ), 19.2 (CH3-2 0 ).

2.2.6. 2-Phenyl-2H-benzotriazole (1) An amount of 0.90 g of 1-(2-nitrophenyl)-5-phenyltetrazole (10) and 4 mL of freshly distilled nitrobenzene were heated together under vigorous reflux for 1 h. After the reaction mixture was cooled, the nitrobenzene was removed by vacuum distillation and the residue purified by chromatography on silica gel (6:4 chloroform/hexane, Rf = 0.33), which afforded 0.59 g (90%) of 1. M.p. 108– 109 C (lit [30]. 108–109 C). 2.2.7. 2-(2-Methylphenyl)-2H-benzotriazole (2) It was prepared in 60% yield from 5-(2-methylphenyl)-1(2-nitrophenyl)tetrazole (11) as described above for 1. Rf = 0.28 (chloroform/hexane 6:4), 51–52 C (crystallized from ethanol:water 70:30) (lit [34]. 52 C).

2.2.5. Benzotriazole syntheses The 2-substituted-2H-benzotriazole derivatives 1–4, which were the targets of this study, were prepared following the procedure of Scheme 1 [30]. The starting anilides 6– 9[31] were converted into tetrazoles 10–13 via the corre-

2.2.8. 2-(2,6-Dimethylphenyl)-2H-benzotriazole (3) It was prepared in 65% yield from 5-(2,6-dimethylphenyl)-1-(2-nitrophenyl)tetrazole (12) as described above for 1. Rf = 0.24 (chloroform/hexane 6:4), 93–94 C (crystallized from ethanol:water 70:30). Anal. Calcd for C14H13N3: C,

36

J. Catala´n et al. / Chemical Physics 340 (2007) 32–42

75.31; H, 5.87; N, 18.82%. Found: C, 75.24; H, 6.08; N, 19.03%. 2.2.9. 2-(2,4-Dimethylphenyl)-2H-benzotriazole (4) It was obtained in 80% yield from 5-(2,4-dimethylphenyl)-1-(2-nitrophenyl)tetrazole (13) as described above for 1. Rf = 0.27 (chloroform/hexane 6:4), 72–73 C (crystallized from ethanol). Anal. Calcd for C14H13N3: C, 75.31; H, 5.87; N, 18.82%. Found: C, 75.35; H, 5.95; N, 18.73%. 2.2.10. 2-Methyl-2H-benzotriazole (5) To a stirred solution of benzotriazole (2.38 g, 20.0 mmol) in 75 mL of 20% NaOH was added 3 mL of dimethyl sulfate (32 mmol) over a period of 30 min, the mixture then being stirred at room temperature for 2 h. Next, the solution was extracted with chloroform (3 · 50 mL) and the organic layers were washed with water, dried over sodium sulfate and evaporated to dryness in order to obtain a crude mixture of the two N-methyl derivatives in a 1:2 (5-/1-methylbenzotriazole) ratio as calculated from the 1H NMR data. The two compounds were separated by column chromatography on silica gel, using 7:3 hexane/ethyl acetate as eluent, which afforded 390 mg of 5 as an oil (15% yield, Rf = 0.40) and 780 mg of 1-methylbenzotriazole (30% yield, Rf = 0.19). 3. Results and discussion This section starts with an analysis of the theoretical data available for the molecular structure of the target compounds in the ground electronic state, and in the first singlet and triplet excited states. It then compares such data with those obtained from an NMR analysis in the ground electronic state and from of the envelopes of the absorption spectra, and also of the fluorescence and phosphorescence emission spectra. Finally, the previous evidence is examined in the light of conformational changes in the compounds upon electronic excitation and its implications on their properties are discussed. 3.1. Calculated molecular structures Table 1 shows the most salient structural data for the equilibrium geometries of states S0, S1 and T1 in compounds 1–4. 2-Phenyl-2H-benzotriazole (1) exhibits a coplanar conformation (h = 0.0) in the three electronic states studied. However, the equilibrium geometry of its first excited singlet is of C1 symmetry since the optimized geometries obtained by forcing a higher symmetry in the compound are not equilibrium structures – in fact, they possess a higher energy and exhibit a non-real frequency. Interestingly, replacing the ortho proton in the phenyl group of compound 1 with a methyl group breaks the original coplanarity; thus, as can be seen from Table 1, 2-(2-methylphenyl)-2H-benzotriazole (2) in the ground electronic state exhibits a non-coplanar conformation

Table 1 Symmetry, bisaromatic torsion angles (h in degrees) and N2 –C10 bond ˚ ) of the geometries corresponding to the S0, S1 and T1 distances (in A minima of benzotriazole derivatives 1–4 Compound

S0

S1

T1

1

Symmetry h rN–C

C2v 0 1.426

C1 0 1.335

C2v 0 1.398

2

Symmetry h rN–C

C1 35 1.431

C1 0 1.344

C1 22 1.408

3

Symmetry h rN–C

C1 69 1.439

C1 61 1.419

C1 54 1.419

4

Symmetry h rN–C

C1 34 1.430

C1 0 1.341

C1 22 1.408

(h = 35); this suggests that the methyl group exerts steric hindrance even though it faces a lone electron pair of the nitrogen atom N2 in the benzotriazole ring in the coplanar form. Non-coplanarity in the aromatic systems of this compound decreases with the transition to the T1 state (h = 22) and then disappears (h = 0) when it reaches S1. This behavior is much interesting and cannot simply be explained on the basis of steric effects [18–21]; in fact, if such hindrance is possible in S0, the shorter bond connecting the two aromatic systems in S1 makes it even more feasible. 2-(2,6-Dimethylphenyl)-2H-benzotriazole (3) is strongly non-coplanar in the three electronic states. Surprisingly, the second methyl group in ortho also exerts some steric hindrance and increases twisting of the connecting bond, from 35 in 2 to 69 in 3 (i.e. in a nearly additive manner). In contrast to 2, 3 adopts no coplanar structure in S1, but rather exhibits virtually the same h value as S0. 2-(2,4-Dimethylphenyl)-2H-benzotriazole (4), which, like 3, possesses two methyl groups in resonant positions, exhibits a torsional behavior identical with that of 2. This clearly confirms that the methyl groups at positions 2 and 6 on the phenyl ring pose steric hindrance. As can be seen from Table 1, compound 4 is non-planar in its ground electronic state (h = 34), but planar (h = 0.0) in its first excited singlet state; similarly to what happens in compound 2. At this point, it should be noted that the distances of the N–C bond connecting the aromatic systems in compounds 1–4 (see Table 1) differ little in the ground electronic state ˚ , respectively); by contrast, (1.426, 1.431, 1.439 and 1.430 A the corresponding twisting angles are markedly different (0, 35, 69 and 34, respectively). This confirms that the bond acts as a single one throughout the torsional movement in the ground electronic state in these compounds by effect of the nitrogen involved in the bond contributing two p electrons and the carbon one – which makes conjugation difficult.

J. Catala´n et al. / Chemical Physics 340 (2007) 32–42

The difficulty in the N–C connecting bond acquiring the character of a double bond in the ground electronic state need not persist if electronic excitation of the compound involves one of the electrons in the p pair contributed by the N atom, as in the electronic transition undergone by these compounds, which is largely of the HOMO ! LUMO type. As can be seen in Scheme 2, the most notable results are the formation of a p bond between the atoms connecting the aromatic systems in the LUMO and the fact that the highest probability of finding the p electron is on the N2 atom in the HOMO. The molecular orbitals (MO) shown in Scheme 2 were drawn on 0.032 isosurfaces; if the values were raised to 0.14, then the p pair would be localized exclusively on N2. Consequently, the increased electron delocalization between the two p electron systems in compounds 1, 2 and 4 upon electronic excitation results in a markedly shortened connecting bond and in these compounds adopting a coplanar conformation. This is

37

not the case with compound 3 as its HOMO and LUMO are both localized on one aromatic system, which precludes electronic delocalization between them and hence the formation of double bond connecting the two aromatic fragments. The previous situation does not occur in the S0 ! T1 transition as the starting MO has its electron density delocalized over the three nitrogen atoms, whereas the other half-occupied MO cannot produce the p bond connecting the two aromatic systems (see Scheme 2). 3.2. NMR spectroscopy Tables 2 and 3 summarize the most relevant 1H and 13C NMR spectroscopic data for benzotriazoles 1–4. Full assignment of the NMR signals was achieved by careful analysis of the chemical shifts and coupling constants, and by comparison of such values with reported data [35]. Useful conclusions regarding the conformation of compounds 1–4 can be derived from the proton chemical shifts in the ortho protons in 1, 2 and 4. Thus, the chemical shift in the planar compound 1 is 8.37 ppm, whereas those in 2 and 4 are 7.71 and 7.59 ppm, respectively (or 7.83 ppm for both compounds if the effects of the methyl groups are considered). Therefore, the latter two compounds are sterically hindered. The extent of interannular conjugation in N-phenylazoles has also been studied by 13C NMR spectroscopy [36]. Both the 13C chemical shifts for the ortho-phenyl carbon d C-2 0 (or d C-6 0 ) and the chemical shift difference between the ortho- and meta-carbons, d C-3 0 -d C-2 0 (or d C-5 0 -d C-6 0 ), reflect that the bond is twisted and confirms the conformation of these bisaromatic systems in qualitative terms. Table 4 shows the variation of both parameters from the unhindered 2-phenyl-2H-benzotriazole (1) to the hindered 2-(2-methylphenyl)-2H-benzotriazole (2), 2-(2,6-dimethylphenyl)-2H-benzotriazole (3) and 2-(2,4-dimethylphenyl)2H-benzotriazole (4). Based on these data, compound 3 is the most strongly twisted; 2 and 4 are twisted to a similar extent and are in between 1 and 3 in this respect, which is quite consistent with the theoretical data of Table 1 for these compounds in their ground electronic state. 3.3. Ultraviolet spectra

Scheme 2. The most relevant molecular orbitals involved in the S0 ! S1 and S0 ! T1 of benzotriazole derivatives 1–4.

The conformations of compounds 1–4 in their ground and first excited singlet states (Table 1) show three different types of behavior as regards the envelope of their first UV absorption band (see Scheme 3). Thus, in compound 1, both singlets involved in the transition (S0 and S1) are planar, so the Franck–Condon transition between them will possess optically active vibronic structure in the bisaromatic compound. The first absorption-band spectrum and the fluorescence spectrum provide vibrational information from the S1 and the S0 states, respectively. The analyses of the vibronic progressions allow us to draw that in S0

J. Catala´n et al. / Chemical Physics 340 (2007) 32–42

38

Table 2 1 H NMR (d in ppm and J in Hz) of 2H-benzotriazoles 1–4 in CDCl3 H-4(7)

H-5(6)

H-2 0

H-3 0

H-4 0

H-5 0

H-6 0

CH3

7.94 (m) JH5 = 8.7 5 JH7 = 0.9

7.42 (m) 3 JH6 = 6.7 4 JH7 = 1.0

8.37 (m)

7.56 (m)

7.46 (m)

7.56 (m)

8.37 (m)

–

2

7.98 (m) 3 JH5 = 8.7 5 JH7 = 1.0

7.45 (m) 3 JH6 = 6.6 4 JH7 = 1.0

–

7.33–7.42 (m)

7.33–7.42 (m)

7.33–7.42 (m)

7.71 (m)

2.43 (s)

3

7.99 (m) 3 JH5 = 8.7 5 JH7 = 0.9

7.47 (m) 3 JH6 = 6.7 4 JH7 = 1.0

–

7.20 (d)

7.35 (dd) 3 J H30 ¼ 8:5 3 J H50 ¼ 6:6

7.20 (d)

–

1.98 (s)

4

7.96 (m) 3 JH5 = 8.7 5 JH7 = 0.9

7.44 (m) 3 JH6 = 6.8 4 JH7 = 1.0

–

7.20 (br s)

–

7.17 (br d)

7.59 (d) 3 J H50 ¼ 8:1

2.38 (s, CH3-2 0 ) 2.42 (s, CH3-4 0 )

Compound 1

3

Table 3 13 C NMR (d in ppm and J in Hz) of 2H-benzotriazoles 1–4 in CDCl3 C-3a(7a)

C-4(7)

C-5(6)

C-1 0

C-2 0

C-3 0

C-4 0

C-5 0

C-6 0

CH3

145.0 J = 3J = 9.4 2 J = 4.1

118.4 1 J = 165.4

127.1 1 J = 160.8 3 J = 8.2 2 J = 1.8

140.4

120.6 1 J = 166.0

129.4 1 J = 162.5 3 J H50 ¼ 8:2

128.9 1 J = 162.4 3 J = 3J = 7.7

129.4

120.6

–

2

144.7 3 J = 3J = 9.4 2 J = 4.3

118.3 1 J = 165.2

126.8 1 J = 160.4 3 J = 8.4

140.2

133.2

131.6 1 J = n.m. 3 J H30 ¼ 8:3 3 JMe = 5.2

129.5 1 J = 161.3 3 J = 8.1

126.6 1 J = 161.0 3 J = n.m.

125.9 1 J = 164.8 3 J = 6.1

18.8 1 J = 129.8

3

144.5 3 J = 3J = 9.3 2 J = 4.0

118.4 1 J = 165.0

126.7 1 J = 160.5 3 J = 8.4

140.0

135.2 3 J = 6.4 2 JMe = 6.4

128.2 1 J = 160.7 3 J H50 ¼ 7:6 3 JMe = 4.9

129.9 1 J = 160.3

128.2 1 J = 160.7 3 J H30 ¼ 7:6 3 JMe = 4.9

135.2 3 J = 6.4 2 JMe = 6.4

17.1 1 J = 128.1 3 J = 4.8

4

144.7 3 J = 3J = 9.2 2 J = 4.6

118.3 1 J = 164.1

126.7 1 J = 160.2 3 J = 8.4

138.0

132.9

132.3 1 J = 158.0 3 J H50 ¼ 6:1 3 JMe = 4.6

139.6 3 J = 6.1 2 JMe = 6.1

127.2 1 J = 161.0 3 J H30 ¼ 7:6 3 JMe = 4.6

125.8 1 J = 162.5

18.7 (CH3-2 0 ) 1 J = 128.8 3 J = 4.6 21.1 (CH3-4 0 ) 1 J = 127.3 3 J = 4.6

Compound 1

3

Table 4 13 C NMR chemical shifts of 2H-benzotriazoles 1–4 with the methyl-induced chemical shifts correction on the C-ortho and C-meta atoms (Z1 9.2, Z2 0.7, Z3 0.1, Z4 3.0) [37] Compound

dC-ortho

dC-ortho

dC-meta

1 2 3 4

120.6 125.9 + 0.1 = 126.0 133.2 – 9.2 = 124.0 135.2 + 0.1  9.2 = 126.1 125.8 + 0.1 + 0.1 = 126.0 132.9  9.2 + 0.1 = 123.8

120.6 125.0 126.1 124.9

dC-meta  dC-ortho

Degree of interannular conjugation

129.4 130.25

8.8 5.25

Extensive Hindered

126.1 130.2

4.4 5.3

Hindered Hindered

dC-meta

(average)

(average)

129.4 126.6 + 3.0 = 129.6 131.6  0.7 = 130.9 128.2 + 3.0  0.7 = 130.5 127.2 + 3.0  0.7 = 129.5 132.3  0.7  0.7 = 130.9

the fundamental vibration yields 1140 cm1, and the S1 state involves a lower energy vibration of 798 cm1. On the other hand, compounds 2 and 4, where S0 is non-planar (h = 35) and S1 is planar, will exhibit a Franck–Condon transition the envelope for which must lack vibronic struc-

ture, be hypsochromically shifted and exhibit a hypochromic effect with respect to 1. Finally, in compound 3, which is clearly twisted (by 69) in both S0 and S1, the transition between these two states must exhibit an optically active vibronic structure typical of one of the aromatic

J. Catala´n et al. / Chemical Physics 340 (2007) 32–42

a

b

39

1

c S1

S0

θ

0

2

1

Absorbance

S0

θ

3

S1

S1

S0

0

θ

0

0

Scheme 3. Spectral behavior of benzotriazole derivatives: (a) compound 1; (b) compounds 2 and 4; (c) compound 3.

280

300

320

340

360

λ abs in nm

Fig. 2. UV absorption spectra for benzotriazoles 1–3 dissolved in 2MB at 293 K as normalized at the maximum.

1

3

2 1

Absorbance

fragments and hence a marked hypochromic effect, albeit only as a result of the aromatic fragments having much smaller extinction coefficients than a coplanar compound. Fig. 1 shows the UV spectra for compounds 1–3 in the gas phase following equilibration in the air at 353 K. The envelopes of these spectra are consistent with the previous description based on Scheme 3. Fig. 2 shows the same spectra as obtained in 2MB at 293 K and normalized at their maxima. Interestingly, compound 2 is hypsochromically shifted with respect to 1, but both have the same onset. Fig. 3 shows the spectra obtained at lower temperatures (e.g. 175 K) in the same solvent; as can be seen, compounds 1 and 3 are even more structured than before, and compound 2 continues to exhibit no structure based on Scheme 3. One other interesting spectroscopic result is that compound 3, with a strongly twisted molecular structure (h = 69), must behave as two different chromophores (viz. a 2-alkyl-2H-benzotriazole and a 1,3-dimethyl-2-alkylbenzene). Fig. 4 shows the spectrum obtained by combining those for 2-methyl-2H-benzotriazole (5) and 1,2,3trimethylbenzene (TMB) in 2MB after correction for their

260

0 260

280

300

320

340

360

λ abs in nm

Fig. 3. UV absorption spectra for benzotriazoles 1–3 dissolved in 2MB at 175 K as normalized at the maximum.

1 0.25

Absorbance

Absorbance

1

0.20

0.15

2 0.10

3 0.05

0 260

0.00 260

280

300

320

340

360

λ abs in nm

Fig. 1. UV absorption spectra for benzotriazoles 1–3 in the gas phase in equilibrium with crystalline samples at 353 K.

280

300

320

340

360

λ abs in nm

Fig. 4. Spectral envelope obtained as a combination of those for 2-methyl2H-benzotriazole (5) and 1,2,3-trimethylbenzene (TMB) in 2MB at 293 K as corrected for their respective absorption coefficients.

J. Catala´n et al. / Chemical Physics 340 (2007) 32–42

40 1.0

1

3

0.8

1

2

3 5 TMB Iem

Iem

0.6

0.4

0.2

0.0 300

350

400

450

0

500

300

350

400

λ em in nm

450

500

λ em in nm

Fig. 5. Fluorescence spectra for benzotriazoles 1–3 dissolved in 2MB at 298 K, respectively excited at 290 nm, 290 nm and 274 nm.

Fig. 6. Fluorescence spectra for benzotriazoles 3 and 5, and 1,2,3trimethylbenzene, dissolved in 2MB at 298 K, respectively excited at 274 nm, 274 nm and 260 nm.

extinction coefficients (viz. 9550 for the former [34] and 138 for the latter [38]); as can be seen, the resulting spectrum is acceptably consistent with that for compound 3. Based on the foregoing, irradiating compound 3 with light in this spectral region results in its benzotriazole half absorbing roughly 97% of all photons; therefore, compound 3 must behave largely as an excited 2-alkylbenzotriazole.

3.5. N:  H–C interactions

As can be seen form Fig. 5, the fluorescence spectra for compounds 1–3 in 2MB at 298 K are all structured. However, their most salient features are the strong similarity between the spectra for 1 and 2, and the fact that the spectrum for 2 is red-shifted by 3 nm. This suggests that compound 2, where S1 is planar, may emit on the planar system in S0, which may be consistent with the abovedescribed fact that the onset of its absorption spectrum coincides with that for 1. In other words, when compound 2 absorbs light it seemingly behaves as a body of structures dominated on average by one twisted by 35 but including fully planar forms as well. The planar forms in the S1 state cause population inversion that conveniently used could generate laser radiation about 350 nm. Fig. 6 shows the emission spectra for compound 3, and the emissions for 2-methyl-2H-benzotriazole and TMB; obviously, the emission of 3 corresponds to its benzotriazole fragment. It should be noted that the emission of 3 is structured, whereas that of 2-methyl-2H-benzotriazole (5) in 2MB is not; in principle, this can only be ascribed to 2-methyl-2H-benzotriazole occurring as a mixture of Cs and C1 conformations –the contributions of which may cause the vibronic structure of the compound to collapse – adopted by twisting of its methyl group. The unique stable form for 2-methyl-2H-benzotriazole that has all vibrational frequencies real is the one that corresponds to the conformation with one of the hydrogens 90 up with respect to the benzotriazole plane.

0.4

95 K 135 175 215 255 295 K

0.3

Absorbance

3.4. Fluorescence spectra

The fact that the spectral envelope for compound 2 has no vibronic structure in the gas phase or in 2MB at 353 K suggests that the potential interaction between the lone electron pair in N1 and a proton of the methyl group in ortho on the phenyl ring is not strong enough to produce a planar conformation. In principle, the aza effect of the two adjacent nitrogens may decrease the basicity of the electron pair [39] substantially enough to prevent a strong interaction with the hydrogen atom; also, the methyl group is known to rotate continuously, which will further hinder such an interaction. Although the reduction of the basicity of the nitrogen atom cannot be directly examined, one can delay rotation of the methyl group by lowering the temperature in order to obtain a glass [40] in 2MB. Fig. 7 shows the absorption spectra for compound 2 in 2MB at temperatures from 295 to 95 K. As can clearly be concluded, no structure is

0.2

2 0.1

0.0 260

280

300

320

340

360

λ abs in nm

Fig. 7. UV absorption spectra for compound 2 in 2MB as obtained over the temperature range from 295 to 95 K.

J. Catala´n et al. / Chemical Physics 340 (2007) 32–42

Table 5 Absolute energies (DET in cm1), zero point energies (DZPE in cm1), 0–0 components (in cm1) and phosphorescence life-times (sp in seconds) corresponding to the theoretical and experimental T1 ! S0 of benzotriazole derivatives 1–4

0.07

0.06

115 K

0.05

0.04 295 K

Iem

41

0.03

0.02

Compound

DET

DZPE

0–0theor

0–0exp

sp

1 2 3 4

20,370 20,698 21,152 20,812

1216 1208 1165 1220

19,154 19,490 19,987 19,592

20,538 20,747 21,098 20,871

0.94 0.56 0.83 0.53

0.01

0.00 300

350

400

450

500

0.6

λ em in nm

produced at any temperature, so no hydrogen-bonding interaction leading to a coplanar structure can be assumed. Fig. 8 shows the corresponding emission spectra at 295 and 115 K. As can be seen, the compound gains in structure as the temperature is lowered. Also, as withdrawn from the absorption spectra shown in Fig. 9, compound 4, where the second methyl group is at position 4 on the phenyl ring, cannot adopt a coplanar form either.

Table 5 lists the theoretical predictions of the 0–0 components of the phosphorescence signals for the studied compounds as estimated from the structures involved in the phosphorescent transition T1 ! S0. As with the S0 ! S1 transition, the 0–0 component for the unsubstituted bisaromatic compound appears at the lowest energy position and a blue shift is observed with increase in twisting angle of the compound. These theoretical predictions are quite consistent with available experimental evidence (see Fig. 10 and Table 5); thus, the theoretical and experi-

0.8

95 K 135 175 215 255 295 K

Absorbance

0.6

4

0.2

0.0 260

280

300

3

0.4

0.2

1

4

2

0.0 400

500

600

700

λ em in nm

Fig. 10. Phosphorescence spectra for benzotriazoles 1–4 in 2MB at 77 K as obtained with a delay of 250 ls, respectively excited at 290 nm, 290 nm, 274 nm and 290 nm.

3.6. Phosphorescence spectra

0.4

IPhos

Fig. 8. Emission spectra for compound 2 in 2MB obtained at 295 and 115 K, excited at 290 nm.

320

340

360

λ abs in nm

Fig. 9. UV absorption spectra for compound 4 in 2MB as obtained over the temperature range from 295 to 95 K.

mental 0–0 components fit with r = 0.995. More important, the data reveal that (a) the phosphorescence of these compounds exhibits a long lifetime (0.53–0.94 s, Table 5) and (b) the proportion of triplets produced by ISC increases markedly with increasing twisting angle and is therefore very low in compound 1, with h = 0. In fact, as shown in Fig. 10, the phosphorescence measured with a cell of 1 cm light path in a solution of the compound in 2MB exhibits an absorbance of 0.24 at the excitation wavelength. Also, the phosphorescence signal for compound 1 in 2MB (Fig. 10) is very likely to be due to the presence in solution of a fraction of compound 1 in non-planar forms resulting from solvation by moisture traces present in the solvent. In fact, the intensity of the signal increases strongly with the addition of small amounts of ethanol to the solution and slightly reducing the concentration of 1 it causes the signal to disappear. The triplet states of TIN-P appear to be their photolabile forms [41], and they are more easily generated from non-planar structures. Accordingly, the hydroxyl group in these compounds, that helps to preserve planarity via the hydrogen bond that anchors the two aromatic halves, confers them photoprotecting properties. 4. Conclusions The spectral behavior of 2-(2-methylphenyl)-2H-benzotriazole (2) cannot be merely rationalized on the steric hin-

42

J. Catala´n et al. / Chemical Physics 340 (2007) 32–42

drance as a bulk effect resulting from the volume occupied by a methyl group on the phenyl ring, as a methyl group cannot change appreciably in volume from the ground electronic state to the first excited singlet. The equilibrium structures of a single molecule depend on the total forces acting in every atom and it is necessary to take into account all of them. The envelopes of the absorption and emission bands for 2-phenyl-2H-benzotriazoles provide a reasonable explanation for the conformational changes undergone by these compounds upon electronic excitation. The 2-phenyl-2H-benzotriazoles that adopt a planar conformation in their ground electronic state seem to be inefficient in generating their triplet states, the opposite being true for the compounds with a non-planar conformation. This finding sheds some useful light on the mechanism by which TIN-P compounds acquire photostability. Acknowledgement Thanks are given to MEC of Spain for finacial support (Project Numbers CTQ2005-03052 and CTQ2006-02586). References [1] D.W. Werst, W.R. Gentry, P.F. Barbara, J. Phys. Chem. 89 (1985) 729. [2] D.W. Werst, W.F. Londo, J.L. Smith, P.F. Barbara, Chem. Phys. Lett. 118 (1985) 367. [3] W.E. Sinclair, H. Yu, D. Phillips, J.M. Hollas, J. Chem. Phys. 106 (1997) 5797. [4] J.M. Hollas, J. Chem. Soc., Faraday Trans. 94 (1998) 1527. [5] J. Catala´n, E. Mena, F. Fabero, F. Amat-Guerri, J. Chem. Phys. 96 (1992) 2005. [6] A. Karpfen, C.H. Choi, M. Kertesz, J. Phys. Chem. A 101 (1997) 7426. [7] H.S. Im, E.R. Bernstein, J. Chem. Phys. 88 (1988) 737. [8] Y. Takei, T. Yamaguchi, Y. Osamura, K. Fuke, K. Kaya, J. Phys. Chem. 92 (1988) 577. [9] K. Yamasaki, K. Arita, O. Kajimoto, Chem. Phys. Lett. 123 (1986) 277. [10] V. Barone, P. Cristinziano, Chem. Phys. Lett. 215 (1993) 40. [11] A.A. Heikal, J.S. Baskin, L. Ban˜ares, A.H. Zewail, J. Phys. Chem. A 101 (1997) 572. [12] J. Qian, S.L. Schultz, J.M. Jean, Chem. Phys. Lett. 233 (1995) 9. [13] G.B. Dutt, W. Konitsky, D.H. Waldeck, Chem. Phys. Lett. 245 (1995) 437. [14] C. Muller, M. Kloppel-Riech, F. Schroder, J. Schroder, J. Troe, J. Phys. Chem. A 110 (2006) 5017. [15] M.T. O’Shaughnessy, W.H. Rodebush, J. Am. Chem. Soc. 2 (1940) 2906.

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