Infrared and Electronic Spectra of Radicals Produced from 2-Naphthol

Article pubs.acs.org/JPCA

Infrared and Electronic Spectra of Radicals Produced from 2‑Naphthol and Carbazole by UV-Induced Hydrogen-Atom Eliminations Masahiko Sekine,† Hiroshi Sekiya,‡ and Munetaka Nakata*,† †

Graduate School of BASE (Bio-Applications and Systems Engineering), Tokyo University of Agriculture and Technology, Naka-cho, Koganei, Tokyo 184-8588, Japan ‡ Department of Chemistry, Faculty of Sciences, Graduate School of Molecular Chemistry, Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan S Supporting Information *

ABSTRACT: The photoreaction mechanisms of 2-naphthol and carbazole in lowtemperature argon matrices have been investigated by infrared and electronic absorption spectroscopy with aids of density-functional-theory (DFT) and timedependent DFT (TD-DFT) calculations. When the matrix samples were irradiated upon UV light, 2-naphthoxyl and N-carbazolyl radicals were produced by the elimination of the H atom in the O−H group of 2-naphthol and in the N−H group of carbazole, respectively. The observed IR and electronic absorption spectra of the radicals were reproduced satisfactorily by the quantum chemical calculations. To understand a role of the radicals in the excited-state proton transfer (ESPT), the fluorescence and excitation spectra of 2-naphthol and carbazole were measured in aqueous solution at room temperature as well as in the low-temperature argon matrices. It was found that the intensity of the fluorescence emitted from carbazole anion in aqueous solution decreased when oxygen gas was blown into the solution.

1. INTRODUCTION It is known that dual fluorescence, that is, two kinds of fluorescence from two excited species by exciting one species, is observable in some organic compounds having acidic hydrogen atoms in, for example, hydroxy groups. Förster and Weller1,2 first proposed that one of the dual fluorescence originates from the excited-state proton transfer (ESPT), and the acidity of organic compounds in the electronic excited states is different from that in the ground state. They suggested that one of the dual fluorescence is caused by the electronic transition from an excited state to the ground state of neutral species without ESPT and the other is due to an anion or a tautomer produced by ESPT. One of the typical molecules to show dual fluorescence is 7-hydroxyquinoline (7-HQ), which has been investigated under various experimental conditions such as gas phase,3 solution,4 and low-temperature solid matrices.5−9 The dual fluorescence of 7-HQ is observed at room temperature; one is due to an enol-type isomer and the other is due to a keto-type isomer, which is tautomerized from the enol-type isomer by the migration of the H atom in the OH group to the N atom in the quinoline ring. On the other hand, the third fluorescence of 7-HQ was observed in a low-temperature hydrocarbon matrix by Nagai et al,6 besides those of the enol and keto tautomers. To explain the third fluorescence, we investigated the photoreaction mechanism of 7-HQ in lowtemperature argon matrices by infrared and electronic absorption and fluorescence spectroscopy10,11 and found that © 2012 American Chemical Society

the quinolinoxyl radical was produced by the elimination of the H atom in the OH group. As shown in Scheme 1, the radical yielded a five-membered ring-ketene compound (we call it ketene for simplicity hereafter) by the additional elimination of an H atom in the neighboring C−H bonds and a keto-type tautomer by the attachment of the dissociated H atom to the N atom in the quinoline ring. Because the quinolinoxyl radical exists stably upon UV irradiation, the question arises to us Scheme 1

Received: July 22, 2012 Revised: August 20, 2012 Published: August 21, 2012 8980

dx.doi.org/10.1021/jp307259f | J. Phys. Chem. A 2012, 116, 8980−8988

The Journal of Physical Chemistry A

Article

mixed gas was expanded through a stainless steel pipe of 1/8 in. in diameter and deposited on a CsI plate, cooled at 15 K by a closed-cycle helium-refrigerator unit (CTI Cryogenics, model M-22), in a vacuum chamber, kept at better than 1 × 10−5 Pa by rotary and molecular-turbo pumps. UV radiation from a super-high-pressure mercury lamp (Ushio, HB-50106AA-A) was used to induce photoreaction, where a water filter was used to remove thermal reactions and cutoff optical filters, UV30 (λ > 290 nm) and UV32 (λ > 310 nm), to choose the irradiation wavelength. The IR spectra of the matrix samples were measured with an FTIR spectrophotometer (JEOL, model JIR-7000). The band resolution was 0.5 cm−1, and the number of accumulation was 64. UV−visible absorption spectra and fluorescence spectra were measured with conventional spectrophotometers, JASCO, model V-550, and SHIMADZU, RF-5300PC, respectively. The band resolution for absorption spectra was 0.5 nm. The data interval for fluorescence spectra was 1 nm. To examine the stability of radicals and anions in aqueous solutions, oxygen gas, purchased from Taiyo Toyo Sanso, or nitrogen gas, generated by a nitrogen-gas generator (Iwatani, GN20), was used to blow into the solutions. Other experimental details were reported elsewhere.10,11 The DFT calculations with the 6-31++G** basis set were carried out using the Gaussian03 program.24 Beck’s threeparameter hybrid density functional,25 in combination with the Lee−Yang−Parr correlation functional (B3LYP),26 was used to optimize the geometrical structures and to obtain the relative energies and IR spectral patterns. TD-DFT calculations were carried out to estimate the vertical transition energies and the oscillator strengths from the electronic ground state to the excited states of reactants, radicals, and anions.27,28

whether the radical is possibly produced even in solution at room temperature by UV irradiation like in low-temperature argon matrices and play an important role in ESPT or not. To understand the role of radicals in ESPT, we have investigated the photoreaction mechanisms of two related molecules, 2-naphthol and carbazole, by low-temperature matrix-isolation spectroscopy. It is well-known that both 2naphthol and carbazole show dual fluorescence in aqueous solutions by UV irradiation like 2-HQ,12−15 where the dual fluorescence originates from the neutral and anion species in electronically excited states. However, there is the possibility that 2-naphthoxyl and N-carbazolyl radicals, which are easily produced by UV irradiation, as shown in Scheme 2, play an important role in ESPT. Scheme 2

The 2-naphthoxyl radical was previously studied by electronic spectroscopy and electron spin resonance (ESR), where the radical was produced from 2-naphthol using oxidizing agents.16−20 On the other hand, Ottolenghi et al. reported that the radical was produced from 2-naphthol by UV irradiation,21 where 2-naphthoxyl radical is directly produced from 2-naphthol in aqueous solution by exposure to the UV light coming from a cadmium lamp. The N-carbazolyl radical was also confirmed to be produced from carbazole in organic solvents upon UV irradiation by electronic absorption and Raman spectroscopy.22,23 In the present work, we measured the IR spectra of the photoproducts of 2-naphthol and carbazole in low-temperature argon matrices and assigned the observed IR bands to the corresponding radicals by comparison with the calculated spectral patterns obtained by the density-functional-theory (DFT) calculation. Based on this identification, we analyzed the electronic absorption spectra of the photoproducts and compared with the previous reports and with the results obtained by the time-dependent DFT (TD-DFT) calculation. Then we measured the fluorescence and fluorescence excitation spectra in low-temperature argon matrices and in aqueous solution to discuss about the role of radicals in ESPT.

3. RESULTS AND DISCUSSION IR Spectra in Low-Temperature Argon Matrices. Two rotational isomers are possible around the C−O bond axis of 2naphthol. We call them Outer and Inner hereafter. The H atom in the OH group for Outer is far from the molecular long axis, shown in Scheme 2, while that for Inner is close. An observed matrix-isolation IR spectrum of 2-naphthol before UV irradiation is shown in Figure 1 and compared with the calculated spectral patterns of Outer and Inner obtained by the DFT calculation, where the scaling factors of 0.98 and 0.95

2. EXPERIMENTAL AND CALCULATION METHODS The samples of 2-naphthol (purity >99.0%) and carbazole (purity >95.0%) were purchased from Wako Purity Ltd., which were used after vacuum distillation to remove water and impurities. Each sample placed in a deposition nozzle with a heating system was vaporized at 303 and 343 K for 2-naphthol and carbazole, respectively. Pure argon (Taiyo Toyo Sanso, 99.9999%) was flowed over the sample, and the flow rate of argon gas was adjusted to obtain a sufficient isolation. The

Figure 1. Observed and calculated IR spectra of 2-naphthol. (a) Spectral pattern of Inner, (b) spectral pattern of Outer, calculated at the DFT/B3LYP/6-31++G** level, where scaling factors of 0.98 and 0.95 are used in the regions lower and higher than 3500 cm−1, and (c) observed matrix-isolation spectrum before UV irradiation. The splitting of O−H stretching band is due to the matrix effect and Fermi resonance. 8981



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were used in the lower and higher than 3500 cm−1 regions, respectively.29,30 It is found that the strong bands observed at 1634, 1610, 1184, 1162, and 1120 cm−1 correspond to the calculated values of Outer, 1644, 1618, 1182, 1168, and 1120 cm−1, respectively. On the other hand, the strong C−O stretching mode of Inner, calculated at 1275 cm−1, corresponds to no band in the observed spectrum. Because the observed spectrum in the region between 700 and 1000 cm−1 is consistent with the calculated spectral pattern of Outer more than that of Inner, we concluded that Outer but not Inner exists in the low-temperature argon matrix. One may claim that two bands appear in the O−H stretching region around 3600 cm−1 and that both isomers could exist in the matrix. However, it is known that aromatic compounds involving hydroxy groups frequently show the splitting of the O−H stretching band due to the matrix effect or Fermi resonance.31,32 Therefore, we concluded that only Outer exists in the argon matrix. The observed and calculated wavenumbers and the relative intensities of Outer are summarized in Table 1, where the observed wavenumbers are consistent with the corresponding calculated values within 10 cm−1, except for the O−H stretching mode.

Figure 2. Observed spectral change and calculated IR spectra of 2naphthol. (a) Spectral patterns of 2-naphthoxyl radical (upper) and 2naphthol (lower), calculated at the DFT/B3LYP/6-31++G** level, where a scaling factor of 0.98 is used, and (b) difference spectrum of 2naphthol between spectra measured after minus before UV irradiation (λ > 290 nm) for 90 min. The strong band at 2120 cm−1 is due to the CCO stretching band of a small amount of ketene.

modes. To assign the observed bands in the region between 600 and 1800 cm−1, we calculated the spectral pattern of 2naphthoxyl radical and compared with the observed IR spectrum in Figure 2. The strong bands observed at 1582 and 1518 cm−1 are assignable to the naphthalene-ring and C− O• stretching modes, calculated at 1595 and 1502 cm−1, respectively. On the other hand, the bands observed at 856 and 750 cm−1 are assignable to the C−H out-of-plane bending modes, calculated at 853 and 748 cm−1, respectively. These findings lead to the conclusion that 2-naphthoxyl radical is mainly produced from 2-naphthol by UV irradiation and is stable in the low-temperature argon matrix. The observed and calculated wavenumbers and the relative intensities of 2naphthoxyl radical are compared in Table 2. It is found that they are consistent with each other within 20 cm−1. No band of

Table 1. Observed and Calculated Wavenumbers and Relative Intensities for Outer of 2-Naphthol calcda

obsd v/cm

−1

int.

3646 3640 3632

s m s

1634 1610 1593 1527 1471 1459 1390 1379 1370 1367 1286 1267 1226 1184 1162

s s w m m w m w w w w w w s s

b

v/cm

−1

calcda

obsd int.

c

3749

38

1644 1618 1593 1533 1478 1459 1394 1380

84 36 2 22 27 5 30 2

1373

10

1283 1263 1228 1182 1168

21 4 10 57 100

v/cm

−1

int.

1148 1138 1120 1021

w w s w

961

w

903

w

838 811

s w

745

m

718

w

623

w

b

v/cm−1

int.c

1150 1144 1120 1024 971 956 955 940 896 861 831 807 765 743 737 713 628 622

58 3 12 4 0 4 2 1 2 0 60 7 0 25 1 8 0 3

Table 2. Observed and Calculated Wavenumbers and Relative Intensities for 2-Naphthoxyl Radical calcda

obsd

a

Calculated at the DFT/B3LYP/6-31++G** level. Scaling factors of 0.98 and 0.95 are used in the regions lower and higher than 3500 cm−1, respectively. bLetters of s, m, and w denote strong, medium, and weak intensities, respectively. cRelative intensities.

Figure 2 shows an IR spectral change of 2-naphthol when the matrix sample was exposed to the UV light (λ > 290 nm) for 90 min. A strong band was observed at 2120 cm−1, which is assignable to the CCO stretching mode of ketene produced via a keto carbene by Wolff rearrangement, judging from the results on the UV irradiation of 7-HQ10,11 and 2halogeno phenols.29−36 Other IR bands of the ketene could be too weak to be observed, implying that the ketene is a minor photoproduct and its amount is very small; note the IR coefficient of the CCO stretching mode for ketene compounds is larger than 10 times that for normal vibrational

v/cm−1

int.

1585 1582

w w

1518 1462

m w

1384 1328

w w

1271 1218

w w

1114

w

b

calcda

obsd c

v/cm−1

int.

1605 1595 1537 1502 1468 1435 1397 1381 1340 1284 1262 1219 1181 1152 1133 1113

31 60 1 83 7 0 11 37 16 2 20 3 7 1 0 7

v/cm−1

int.

919 887

w w

856 797 767 750

m w w m

714

w

613

w

b

v/cm−1

int.c

1026 977 970 947 914 889 868 853 796 750 748 732 714 637 613

2 1 0 7 3 1 0 100 14 5 64 2 3 1 8

a Calculated at the DFT/B3LYP/6-31++G** level. A scaling factor of 0.98 is used. bLetters of s, m, and w denote strong, medium, and weak intensities, respectively. cRelative intensities.

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2-naphthol anion was observed because it was not stabilized in argon matrices unlike in aqueous solution. Similar to 2-naphthol, the photoreaction mechanism of carbazole in low-temperature argon matrices was investigated by IR spectroscopy. Figure 3 shows an IR spectral change of

Table 3. Observed and Calculated Wavenumbers and Relative Intensities for Carbazole calcda

obsd v/cm

Figure 3. Observed spectral change and calculated IR spectra of carbazole. (a) Spectral patterns of N-carbazolyl radical (upper) and carbazole (lower), calculated at the DFT/B3LYP/6-31++G** level, where a scaling factor of 0.98 is used, and (b) difference spectrum of carbazole between spectra measured after minus before UV irradiation (λ > 310 nm) for 30 min.

carbazole when the matrix sample was exposed to UV light (λ > 310 nm) for 30 min. Note the production of ketene via keto carbene is impossible for carbazole. It is compared with the calculated spectral patterns of N-carbazolyl radical (upper) and carbazole (lower). The increasing and decreasing bands correspond to the spectra of N-carbazolyl radical and carbazole, respectively. For example, the calculated bands of N-carbazolyl radical, 1562 cm−1 for the aromatic ring stretching mode, 1179, 1059, and 974 cm−1 for the vibrational modes in relation to the N atom, and 766 and 711 cm−1 for the C−H out-of-plane bending modes, correspond to the photoproduct bands observed at 1545, 1181, 1044, 958, 766, and 713 cm−1. Hiyoshi et al. observed a transient Raman spectrum of carbazole in acetonitrile solution by excitation at 308 nm and assigned nine Raman bands to N-carbazolyl radical.23 Among them, the 1379, 1172, and 1147 cm−1 bands correspond to 1369, 1181, and 1169 cm−1 bands appearing in the present IR spectrum, though their IR intensities are weak. The observed and calculated wavenumbers and the relative intensities of carbazole and Ncarbazolyl radical are summarized in Tables 3 and 4, respectively. It is found that the observed wavenumbers are consistent with the corresponding calculated values within 10 cm−1 for carbazole and within 20 cm−1 for N-carbazolyl radical. Therefore, we concluded that N-carbazolyl radical is produced from carbazole upon UV irradiation by the elimination of the H atom in the N−H group like that 2-naphthoxyl radical is produced from 2-naphthol by the elimination of the H atom in the O−H group. To our knowledge, no IR spectra of 2naphthoxyl and N-carbazolyl radicals have been reported until now. UV−Visible Absorption Spectra in Low-Temperature Argon Matrices. UV−visible absorption spectra of 2-naphthol in a low-temperature argon matrix were measured before and after UV irradiation. As shown in Figure 4, three absorption bands appeared at 220, 260, and 320 nm, as drawn by a solid

−1

int.

b

v/cm

−1

calcda

obsd int.

c

3512 3510 3507 3501

w m s w

3497

58

1637 1615 1609 1589

w w w w

1637 1622

6 38

1500 1496

w w

1592 1586 1502

3 0 40

1465 1453 1394 1349 1336 1327

m m w w w s

1491 1468 1452 1404 1354

1 29 33 9 8

1286 1238

w s

1203 1159

w w

1337 1314 1284 1238 1212 1209 1159

74 1 5 83 2 6 0

v/cm

−1

int.

1150 1123 1120 1111 1017

w w w w w

1004

w

925 866

w w

844 841

w w

751

m

727 726 645 619

m m w w

566

w

b

v/cm−1

int.c

1152 1119

7 8

1107 1023 1018 996 960 958 923 922 869 845 845 840 767 748 742 737 724

4 6 1 9 0 0 0 2 0 0 2 1 0 78 0 0 78

654 617 574 565

1 6 0 14

a

Calculated at the DFT/B3LYP/6-31++G** level. A scaling factor of 0.98 is used. bLetters of s, m, and w denote strong, medium, and weak intensities, respectively. cRelative intensities.

line. The 320 nm band corresponds to the 333 nm band of 2naphthol in aqueous solution previously reported by Tsutsumi.13 The red shift of 13 nm seems to be caused by the intermolecular interaction between 2-naphthol and water molecules in the solution. To confirm this assignment, we carried out the quantum chemical calculations for the vertical transition energies and the oscillator strengths of 2-naphthol at the TD-DFT/B3LYP/6-31++G** level. The calculated values are summarized in Table 5, and the calculated absorption spectral pattern is compared with the observed spectrum in Figure 4. The observed strong band at 220 nm corresponds to the calculated absorption bands at 218 and 215 nm with oscillator strengths of 0.827 and 0.343, respectively. The bands observed around 260 and 320 nm seem to correspond to the calculated bands at 233 and 301 nm with oscillator strengths of 0.034 and 0.046, respectively. The dotted and broken lines drawn in Figure 4 show the spectra measured after 4 and 30 min UV irradiation (λ > 290 nm), where four new absorption bands appeared at 230, 260, 350, and 460 nm. These bands are assignable to 2-naphthoxyl radical and ketene, judging from the analysis of IR spectra. The 350 and 460 nm absorption bands correspond to those of 2naphthoxyl radical reported previously.16−18 Our TD-DFT calculations showed that the 230 nm band corresponds to the two calculated bands of 2-naphthoxyl radical at 230 nm with oscillator strengths of 0.513 and 0.147, while the 260 nm band corresponds to the calculated 279 nm band with an oscillator strength of 0.038. A broad band around 350 nm seems to correspond to the calculated ones of 2-naphthoxyl radical, 336, 8983



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Table 4. Observed and Calculated Wavenumbers and Relative Intensities for N-Carbazolyl Radical calcda

obsd v/cm

−1

int.

1566 1545

w s

1426 1369 1304 1292 1254

w w w w w

1181 1169

w w

1044 1016 1013

m w w

b

v/cm

−1

1605 1588 1581 1562 1479 1459 1428 1421 1388 1304 1296 1261 1232 1179 1176 1146 1138 1090 1059 1015

calcda

obsd int.

c

10 0 27 64 9 13 1 8 4 1 1 12 1 37 3 0 1 0 57 15

−1

v/cm

int.

963 958

m m

853

w

766

s

713 711

m m

b

v/cm−1

int.c

1011 974

0 26

964 963 934 933 870 863 861 858 766 752 746 735 711

0 0 0 6 0 2 0 0 83 0 0 0 60

656 608

0 0

a

Calculated at the DFT/B3LYP/6-31++G** level. A scaling factor of 0.98 is used. bLetters of s, m, and w denote strong, medium, and weak intensities, respectively. cRelative intensities. Figure 4. UV−visible absorption spectra of 2-naphthol measured before (a solid line) and after 4 min (a dotted line) and after 30 min (a broken line) UV irradiation (λ > 290 nm). The expanded spectra are given in the inset. Calculated spectral patterns of 2-naphthol, 2naphthoxyl radical, and ketene were obtained at the TD-DFT/B3LYP/ 6-31++G** level. The width of calculated bars is arbitral.

352, and 375 nm. The observed band around 460 nm may be assigned to the 415 nm band of 2-naphthoxyl radical with an oscillator strength of 0.031, though the wavelength is slightly far. From the above-mentioned analyses of IR and UV−visible absorption spectra, we concluded that 2-naphthoxyl radical is produced from 2-naphthol in low-temperature argon matrices by UV irradiation. The isosbestic point was not clearly observed in the region shorter than 250 nm because the light scattering due to solid argon depends on irradiation time. No electronic absorption bands of ketene were clearly found because the amount of ketene was too small to be detected, as mentioned in the previous section. Similar to 2-naphthol, UV−visible absorption spectra of carbazole were measured before and after a 10 min UV irradiation, as shown in Figure 5. The 280 and 320 nm strong bands observed before UV irradiation (a solid line) are consistent with the calculated values, 280 and 308 nm, obtained by the TD-DFT calculation, as listed in Table 6. New absorption bands appeared at 360, 544, and 592 nm in the spectrum measured after UV irradiation (a broken line). The 360 nm absorption band corresponds to the calculated 343 nm band of N-carbazolyl radical, listed in Table 6. The 544 and 592 nm absorption bands are consistent with those of N-carbazolyl radical in acetonitrile, 564 and 613 nm, reported by Hiyoshi et al.,23 and in cyclohexane solution, 550 and 600 nm, reported by Martin et al.22 The energy difference between 544 and 592 nm is calculated to be about 1490 cm−1, which is close to the vibrational energy of a ring stretching mode. Therefore, we assume that the 544 and 592 nm absorption bands are assignable to the calculated 535 nm band and its vibrationally excited band of N-carbazolyl radical. The observed and calculated vertical transition energies and the oscillator

Table 5. Calculated Vertical Transition Energies and Oscillator Strengths of 2-Naphthol, 2-Naphthoxyl Radical, Ketene, and 2-Naphthol Aniona 2-naphthol

2-naphthoxyl radical

ketene

2-naphthol anion

λ/nm

I

λ/nm

I

λ/nm

I

λ/nm

I

301 276 233 218 215 208 201

0.046 0.013 0.034 0.827 0.343 0.173 0.074

662 415 375 352 336 279 241 230 230 220 210 208

0.010 0.031 0.023 0.042 0.012 0.038 0.047 0.513 0.147 0.201 0.054 0.054

300 276 257 233 219 206

0.101 0.057 0.062 0.396 0.013 0.016

423 321 318 305 304 273 258 256 245 238 233 229 219 205 205

0.065 0.012 0.045 0.013 0.089 0.023 0.113 0.028 0.422 0.050 0.102 0.095 0.120 0.386 0.046

a

Bands with wavelength longer than 200 nm and with an oscillator strength larger than 0.01 are listed.

strengths of carbazole and N-carbazolyl radical are summarized in Table 6. 8984



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absorption of the first photon, which is relaxed to the T1 state by intersystem crossing, and then N-carbazolyl radical is produced from the T1 state by the absorption of the second photon. On the other hand, Martin et al. reported that Ncarbazolyl radical was produced by the 529 nm laser irradiation after a 265 nm laser excitation,22 but not by only the 592 nm irradiation. However, our IR and UV−visible absorption spectroscopic studies in low-temperature argon matrices showed that the N-carbazolyl radical is produced by the exposure of the matrix sample to the UV light coming from super-high-pressure mercury lamp. Because the UV light used in our experiments is much weaker than the laser lights used in the previous experiments,22,23 it should be difficult that the species in the S1 state absorb the second photon to produce Ncarbazolyl radical. Therefore, we concluded that N-carbazolyl radical can be directly produced from vibrationally excited states of carbazole in the S1 state by one-photon absorption. Fluorescence and Excitation Spectra in Low-Temperature Argon Matrices. The fluorescence and excitation spectra of 2-naphthol measured before (a solid line) and after (a broken line) a 4 min UV irradiation in low-temperature argon matrices are shown in Figure 6. The fluorescence Figure 5. UV−visible absorption spectra of carbazole measured before (a solid line) and after (a broken line) a 10 min UV irradiation (λ > 310 nm). The expanded observed spectra are given in the inset. Calculated spectral patterns of carbazole and N-carbazolyl radical were obtained at the TD-DFT/B3LYP/6-31++G** level. The width of calculated bars is arbitral.

Table 6. Calculated Vertical Transition Energies and Oscillator Strengths of Carbazole, N-Carbazolyl Radical, and Carbazole Aniona carbazole

N-carbazolyl radical

carbazole anion

λ/nm

I

λ/nm

I

λ/nm

I

308 280 242 238 234 229 215 213 206 204 203

0.031 0.140 0.455 0.039 0.140 0.012 0.059 0.572 0.080 0.068 0.011

535 343 279 276 259 246 234 230 229 222 220

0.085 0.029 0.039 0.011 0.390 0.356 0.109 0.022 0.118 0.096 0.058

395 334 277 255 247 240 234 234 228 226 219 218 213 211 207 206 203 202

0.022 0.010 0.684 0.396 0.038 0.046 0.077 0.121 0.023 0.025 0.167 0.010 0.033 0.021 0.017 0.018 0.094 0.070

Figure 6. Fluorescence spectrum excited at 300 nm and fluorescence excitation spectrum monitored at 360 nm of 2-naphthol. Solid and broken lines represent spectra measured before and after a 4 min UV irradiation. The expanded spectra are given in the inset.

spectrum was obtained by excitation at 300 nm, whereas the excitation spectrum was obtained by monitoring at 360 nm. No new bands were observed in both the fluorescence and the excitation spectra measured after UV irradiation, though the intensities were decreased slightly, meaning that no bands of the photoproducts appeared in the spectra. The fluorescence spectrum of 2-naphthol in aqueous solution was reported by Tsutumi et al.,13 who showed the dual fluorescence due to neutral and anion species at 357 and 417 nm, respectively. The former band, due to neutral 2-naphthol, was observed in our fluorescence spectrum at 360 nm, however, the latter band, due to 2-naphthol anion, was not observed around 417 nm. To confirm the assignment of the 2-naphthol anion reported by Tsutumi et al, we carried out the TD-DFT calculation for 2naphthol anion as well as for neutral 2-naphthol and 2naphthoxyl radical. The vertical transition energies and the oscillator strengths are summarized in Table 5, where the oscillator strength of the 423 nm band of 2-naphthol anion, 0.065, is stronger than those of the 301 nm band of 2-naphthol, 0.046, and of the 415 nm band of 2-naphthoxyl radical, 0.031, implying that the 423 nm band of 2-naphthol anion could appear when its amount is enough. Therefore, the assignment of Tsutsumi et al. seems to be true; the 417 nm band observed

a

Bands with wavelength longer than 200 nm and with an oscillator strength larger than 0.01 are listed.

As described before, we concluded from the analysis of IR and electronic absorption spectra that N-carbazolyl radical can be produced from carbazole by UV irradiation. Hiyoshi et al. measured the electronic absorption and Raman spectra of Ncarbazolyl radical produced by 308 nm laser irradiation.23 They claimed that carbazole is excited to the S1 state by the 8985



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anion in the argon matrix without solvation, as described in the section of UV−visible absorption spectra. Because Bonesi et al. assigned the 400−500 nm emission observed in a glass solid of propanol and ethyl ester at 77 K to the phosphorescence of carbazole,38 we assumed that the 400−500 nm emission is due to the phosphorescence of neutral carbazole. This assumption can be supported by the fact that the similar excitation spectra of neutral carbazole were obtained by monitoring at both 341 and 433 nm. It can be also supported by the TD-DFT calculation that the vertical transition energy from the S0 state to the T1 state, 312 kJ mol−1 (384 nm), is close to the shortest wavelength in the phosphorescence band of 400−500 nm. Role of Radicals in ESPT. We concluded from the abovementioned results that 2-naphthoxyl and N-carbazolyl radicals were easily produced from 2-naphthol and carbazole in lowtemperature argon matrices by UV irradiation like 7-HQ. This finding suggests that the radicals play an important role in ESPT in aqueous solution. Then, we examined the dependence of the fluorescence intensity of anions on the concentration of oxygen molecules in aqueous solution to discuss the possibility that the radicals are related to ESPT even in solution at room temperature. Figure 8a shows fluorescence spectra of carbazole in aqueous solution at room temperature. The solid and dotted lines

in aqueous solution, but not observed in low temperature argon matrices, is due to 2-naphthol anion. Our DFT calculation resulted in that the relative energy of 2-naphthol anion is about 1420 kJ mol−1 higher than that of neutral 2-naphthol in the ground state. Because the 2-naphthol anion can be stabilized by solvents, its fluorescence could be observed at 417 nm in aqueous solution. On the other hand, no stabilization due to solvents can be expected in low-temperature argon matrices, and no fluorescence of 2-naphthol anion was observed in our spectrum. The weak broad band appearing at 450−550 nm in our expanded spectrum inset in Figure 6 is assignable to the phosphorescence of 2-naphthol because it is similar to that reported in aqueous solution.37 Then we concluded that the fluorescence and phosphorescence of 2-naphthol were observed in low-temperature argon matrices, but no emission of the 2naphthoxyl radical and 2-naphthol anion. To confirm our assignment, we tried to measure a fluorescence excitation spectrum of 2-naphthol. There are two bands around 270 and 320 nm in the excitation spectrum monitored at 360 nm, as shown in Figure 6, which correspond to the absorption bands of 2-naphthol shown in Figure 4, 260 and 320 nm, respectively. The excitation spectrum obtained by monitor at 475 nm shows the similar excitation spectrum of 2-naphthol, though the intensities of the bands are weak. Therefore, we assumed that the 360 nm and 450−550 nm bands are due to the fluorescence and phosphorescence of neutral 2-naphthol, but not due to photoproducts. Figure 7 shows fluorescence and excitation spectra of carbazole in low-temperature argon matrices measured after a

Figure 8. Fluorescence spectra of carbazole in NaOH solutions. (a) Solid and dotted lines represent spectra measured in 0.0 and 0.1 mol L−1 NaOH solutions, respectively, and (b) solid and dotted lines represent spectra measured in N2 and O2 saturated solutions.

represent the spectra measured in 0.0 and 0.1 mol L−1 NaOH solutions, respectively. When NaOH is not contained in the solution, only the fluorescence of neutral carbazole was observed at 320−370 nm. On the other hand, the intensity of the fluorescence of neutral carbazole decreased vigorously in the 0.1 mol L−1 NaOH solution, and a strong emission appeared at 400−500 nm, which is consistent with the previous report.15 Because it is unlikely that strong phosphorescence from the T1 state of neutral carbazole was enhanced by the addition of a small amount of NaOH, we assumed that the 400−500 nm band originates from the carbazole anion. To confirm this assignment, we examined the intensity changes of the fluorescence of carbazole when the oxygen concentration in the solution was changed. If the emission band is due to the phosphorescence of neutral carbazole, it should disappear completely by the energy transfer from the T1 state of neutral carbazole to oxygen molecules. The fluorescence spectrum of carbazole in the oxygen-gas saturated NaOH solution is

Figure 7. Fluorescence spectrum excited at 310 nm (a solid line) and fluorescence excitation spectrum monitored at 341 nm (a broken line) and at 433 nm (a dotted line) of carbazole.

10 min UV irradiation, which was mostly unchanged from that measured before the irradiation. The fluorescence spectrum (a solid line) was measured by excitation at 310 nm, while the excitation spectra were monitored at 341 nm (a broken line) and 433 nm (a dotted line). The broad emission bands at 320− 370 nm and 400−500 nm with a series of peaks were observed at an excitation of the 310 nm absorption band. Chattopadhyay observed the dual fluorescence of carbazole in aqueous solution at 330−400 nm and 400−500 nm, where the former is due to neutral carbazole and the latter is due to carbazole anion.15 One may assign the 400−500 nm band in our fluorescence spectrum to carbazole anion. However, it is ruled out because the energy higher than 1400 kJ mol−1 is required to produce a carbazole 8986



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ground state is accelerated by the interaction with oxygen molecules.

compared with that in the nitrogen-gas saturated NaOH solution in Figure 8b. The intensity of the 400−500 nm band of carbazole anion decreased slightly when oxygen gas was saturated. Therefore, we assume that the 400−500 nm band is assignable to the fluorescence of carbazole anion but not the phosphorescence of neutral carbazole. This band seems to correspond to the calculated band of 395 nm with an oscillator strength of 0.022 of carbazole anion, listed in Table 6. The similar experiment for 2-naphthol was not carried out in the present study because the 2-naphthol anion can be easily produced in basic solution on the potential surface in the ground state without ESPT. It is important that the intensity of the fluorescence of the carbazole anion slightly decreased by blowing oxygen gas into the aqueous solution. One possibility to explain this phenomenon is that radicals are produced by the elimination of a H atom upon UV irradiation in aqueous solutions as well as in the low-temperature argon matrices. The H atoms are highly reactive and would be expected to react first with naphthol and carbazole, as it is known that H atoms react with various aromatic molecules with rates between 108 and 109 mol−1 s−1 in aqueous solution.39 When the oxygen gas is saturated into the solution, the oxidation of the radicals to produce peroxidic compounds occurs, resulting in the decrease of the intensity of the fluorescence due to anions. Another possibility is that the radiationless transition of anions in the electronically excited states to the ground state is accelerated by the interaction with oxygen molecules. When nitrogen gas was blown into the oxygen-gas saturated solution satisfactorily, the intensity of the fluorescence of anions slightly decreased. This finding implies that a small amount of peroxidic compounds is no longer related in the dual fluorescence, and the former possibility is more reasonable than the latter. Further experimental and theoretical studies are required to confirm it.

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ASSOCIATED CONTENT

S Supporting Information *

Full description of ref 24. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +81-42-388-7349. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Professor Kazuhiko Shibuya (Tokyo Institute of Technology) and Professor Nobuyuki Akai (Tokyo University of A&T) for their helpful discussions.

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REFERENCES

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4. CONCLUSION From the analysis of matrix-isolation IR and UV−visible absorption spectra, we concluded that 2-naphthoxyl and Ncarbazolyl radicals are produced from 2-naphthol and carbazole by the elimination of the H atom in the OH group and in the NH group upon UV irradiation, respectively, similar to the photoreaction of 7-HQ. The observed spectra of the radicals were reproduced satisfactorily by the calculated spectral patterns obtained by the DFT and TD-DFT methods. In the electronic emission spectrum of 2-naphthol, the fluorescence and phosphorescence of neutral 2-naphthol were observed around 360 and 450−550 nm in the low-temperature argon matrices, but no emission of 2-naphthol anion, previously reported in aqueous solution by Tsutsumi, was observed. A similar result was obtained in the electronic emission spectrum of carbazole in the low-temperature argon matrices, where the fluorescence and phosphorescence of neutral carbazole were observed around 320−370 nm and 400−500 nm, respectively, but no fluorescence of carbazole anion. We concluded that these findings are understandable by the different solvation in low-temperature argon matrices and aqueous solutions. The intensity of the fluorescence of the carbazole anion was found to decrease slightly when the oxygen gas was blown into the aqueous solution. We proposed two possibilities: one is that the detached H atoms react with carbazole and the radicals react with oxygen molecules to produce peroxidic compounds in aqueous solution; another possibility is that the radiationless transition of anions in the electronically excited states to the 8987



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Infrared and Electronic Spectra of Radicals Produced from 2-Naphthol

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