CASPT2 and TD-DFT Study of Valence and Rydberg

ARTICLE pubs.acs.org/JPCA

CASSCF/CASPT2 and TD-DFT Study of Valence and Rydberg Electronic Transitions in Fluorene, Carbazole, Dibenzofuran, and Dibenzothiophene Ivan Ljubi
c* and Aleksandar Sablji
c Department of Physical Chemistry, Ru{er Boskovi
c Institute, Bijenicka cesta 54, P.O. Box 180, HR-10002, Zagreb, Republic of Croatia

bS Supporting Information ABSTRACT: A combination of multireference CASSCF/ CASPT2 and time-dependent DFT (TD-B3P86) theoretical treatments was employed to test their predictions against recently proposed assignments of the vacuum-UV spectra of fluorene and its three heteroanalogues—dibenzofuran, carbazole, and dibenzothiophene—up to the ionization threshold. For the low-lying transitions, the theoretically based assignments are generally not problematic because of the wellresolved bands, although, even in this region, the two methods yield some opposing predictions. Further on toward the vacuum region, the assignments prove increasingly challenging because of predicted crowding of transitions, many of which exhibit significant intensity. Some of the transitions in this region and beyond—toward the ionization thresholds—are thus necessarily assigned only tentatively. Overall, the two methods are frequently found to complement each other well, and equivalent transitions usually appear as bracketed from the high- (CASPT2) and low- (TD-B3P86) energy sides.

’ INTRODUCTION Numerous possible applications of fluorene and its heteroanalogues (FHAs), namely, carbazole, dibenzofuran, and dibenzothiophene (Figure 1), making use of their photochemical features have been described in recent times.1
12 The advantageous electronic properties of these compounds, characterized primarily by extensive π conjugation, make them promising candidates for use in such diverse technologies as organic light-emitting diodes, thin-film transistors, chemical sensors, photovoltaics, photorefractives, holography, electronic data storage, and electroluminescent devices. As electron-donating functional units in copolymers,2 dendrimers,3 and smaller ambipolar systems,4 for example, they are typically combined with electron acceptors such as dibenzothiophene-S,S-dioxide2,4 and diester-substituted bithiophene5 to create compounds with finely tuned optoelectronic properties. Some examples of novel generations of FHA-based photoactive materials include donor
acceptor
donor systems with fluorene and carbazole as donors,4 highly pure and stable polyfluorenes as light emitters,6,7 dibenzothiophene-based semiconductors,8 poly(Nvinylcarbazole) as a host material for polymer light-emitting diodes,9 fluorene-based click polymers for dye-sensitized solar cells,10 liquidcrystalline polymers as field-effect transistors,11 and two-photon sensitizers with high quantum yields for use in photodynamic therapy.12 Gaining deeper insight into the higher electronic states of the FHA chromophores is therefore imperative in elucidating their roles in the photoelectronic properties of composite organic materials. FHAs are also important from the environmental standpoint where they are typically classified as heterocyclic derivatives of r 2011 American Chemical Society

polycyclic aromatic hydrocarbons (PAHs).13 Dibenzothiophene is thus notorious as a sulfur-containing PAH heterocycle contaminant that is refractory to biodegradation. It is abundant in heavier fractions of petroleum from where it must be eliminated for the fuel to meet the U.S. Environmental Protection Agency (EPA) and European Union (EU) required standards. Recently, its UV absorption properties have been of interest in the context of the treatment of products arising upon its biodegradation.14 Dibenzofuran is important in being a parent representative of a hazardous class of anthropogenic and naturally occurring pollutants, namely, polychlorinated dibenzofurans (PCDFs).15 These compounds display a high acute toxicity to humans and animals and are characterized by a substantially prolonged period of excretion, as well as tumor-promoting and teratogenic potential similar to that of polychlorinated dioxins.16 Several proposed methods for their real-time detection and deactivation in the environment rely on reliably distinguishing between the subtly different UV absorption properties of numerous congeners,17 which can differ vastly in their toxicity. Finally, carbazole is considered very toxic to aquatic organisms and is also a suspected carcinogen.18 Early vapor- and crystal-phase spectra of FHAs recorded by Pinkham and Wait19 and Bree et al.20
22 led to the assignment of a few of the lowest-energy electronic transitions. The former authors also used theoretical approaches, specifically semiempirical Received: February 18, 2011 Revised: April 1, 2011 Published: April 15, 2011 4840

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Figure 1. Representation of the four studied analogues and orientation of the molecule-fixed coordinate system. X = CH2 (fluorene), NH (carbazole), O (dibenzofuran), and S (dibenzothiophene).

configuration interaction, to corroborate their assignments. Tanaka extended the measurements of the single-crystal polarized reflection spectra of fluorene, carbazole, and dibenzofuran to the vacuum threshold and calculated the excitation energies using the semiempirical random-phase approximation.23 Among more recent efforts, Gudipati et al. measured the polarized excitation spectra of carbazole and fluorene up to the near vacuum-UV region (70000 cm
1) and employed the semiempirical CNDO/ S method.24 The groups of Thulstrup and Spanget-Larsen proposed assignments of the spectra of carbazole,25 dibenzofuran,26 and dibenzothiophene27 on the basis of the linear dichroism (LD) technique applied to samples uniaxially aligned in stretched polyethylene, as well as extensive time-dependent density functional theory calculations. Their most recent effort28 made use of synchrotron radiation and expanded the crystal and LD measurements of FHA spectra well beyond the ionization threshold (up to ∼80000 cm
1); they also performed TD-B3LYP/6-31G(d,p) calculations on the 150 lowest singlet roots. The agreement between the experimental excitation energies and the TD-B3LYP vertical transitions was in some cases reported as problematic, even for the low-lying states.28 Our aim here is to provide a purely theoretical perspective on a broad portion of FHA UV spectra, up to the ionization threshold, and to see how it compares against the freshly proposed assignments.28 For this purpose, we used two firmly established approaches to calculating electronic transitions and oscillator strengths: multireference perturbation theory to the second order (CASPT2)29 and time-dependent density functional theory (TD-DFT).30 The CASPT2 treatment, in particular, has been shown to be a widely applicable and accurate method for an extensive set of organic29 and inorganic31 molecules and is expected to be more reliable than TD-DFT. Apart from a previous study on the excited states of polychlorinated dibenzofurans, which was carried out up to the vacuum threshold,32 it has not yet been used in calculations on the electronic spectra of FHAs.

’ COMPUTATIONAL METHODS The geometries of fluorene (FL), carbazole (CZ), dibenzofuran (DBF), and dibenzothiophene (DBT) were optimized using the complete-active-space self-consistent-field (CASSCF) method and the standard Dunning’s correlation-consistent ccpVDZ basis set.33 The CASSCF active spaces included the 12 πbonding and -antibonding orbitals of the lateral benzene rings, as well as the lone pair of the bridging heteroatoms, resulting in (14,13) active spaces in line with our previous studies on the analogous compounds.32 Only in the case of fluorene, which lacks a bridging heteroatom and a π lone pair, was a smaller (12,12) active space used throughout. All optimizations were constrained to the C2v point group, which was subsequently confirmed by the harmonic frequency analyses as the true symmetry of the minima. Employing the standard orientation of the molecular coordinate frame (Figure 1), the active space includes seven molecular orbitals (MOs) of b1 symmetry and six

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of a2 symmetry. This constrains the wave functions to a1 and b2 symmetry and corresponds to the description of orbitally allowed π
π* transitions. State-averaged CASSCF (SA-CASSCF) calculations were performed in the optimized geometries over the 10 lowest equally weighted CASSCF roots. Additionally, geometric optimizations for the 2A1, 1B2, and 2B2 roots were carried out to determine the corresponding adiabatic excitation energies. The electron correlation was subsequently included in the formalism of the single-state multireference perturbation treatment to the second order (SS-CASPT2)29,34 with the 10 SA-CASSCF roots as the zeroth-order references. The vertical and adiabatic CASSCF/CASPT2 excitation energies were then calculated as the SS-CASPT2 energy differences between the ground and excited states. The vertical oscillator strengths, f, were derived from the CASSCF transition dipole moments35 (TDMs) in the length representation and the CASPT2 vertical excitation energies, ΔE, according to 2 f ¼ ðTDMÞ2 ΔE 3

ð1Þ

In the correlated CASPT2 calculations, we used the cc-pVDZ basis set, as the largest affordable basis set for molecular systems of this size. The standard contractions of 4s3p1d on S; 3s2p1d on C, O, and N; and 2s1p on H resulted in 232, 227, 222, and 226 basis functions in FL, CZ, DBF, and DBT, respectively. Because of the lack of basis functions for describing the core correlation, the inner shells of the constituent atoms were kept frozen. Shifting the denominators in the expression for the second-order perturbation energy proved mandatory because of the presence of intruder states, particularly in the case of the higher A1 states. We used the imaginary level shift (ILS) technique36 with the ILS parameter of 0.10 au, which proved satisfactory in our previous studies.32,37
39 Using this value of ILS, the reference weights in the excited states approach closely, to within 5%, those of the ground states, that is, around 0.65 for FL, CZ, and DBF and 0.64 for DBT. Finally, an ionization potential/electron affinity (IPEA) level shift of 0.10 au was employed to mitigate a systematic error of overestimating the stability of open-shell configurations by CASPT2.40 The underlying idea is that the CASPT2 energy denominators assume approximate values of either electron affinity (EA), or ionization potential (IP), depending on whether the excitation is into or from a partially occupied orbital. Here and previously,32,38,39 it was observed that the recommended value for the IPEA parameter of 0.25 au (deduced from the experimental sets of EAs and IPs)40 results in significantly overestimated CASPT2/cc-pVDZ valence excitation energies. Thus, we adopted a reduced value for the IPEA shift of 0.10 au. To put this modification on firmer grounds, we undertook an extensive study (see the Supporting Information) on the valence and Rydberg excited states of the relevant smaller molecules: benzene and thiophene. It was found that, for the cc-pVDZ results to mimic the benchmark results obtained with the larger basis set (cc-pVTZ þ a set of diffuse Rydberg functions), it is advantageous to adopt the smaller IPEA shift (0.10 au) for the valence states but to retain the recommended 0.25 au value when the Rydberg states are involved. This is reasonable because, within the two basis sets, the identical set of Rydberg functions is used, whereas cc-pVTZ with its 4s3p2d1f contractions on second-row atoms is much larger than cc-pVDZ and presumably nearly saturated with respect to the valence-type excitations. Thus, the use of the reduced IPEA shift could be justified by the deficiencies 4841

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The Journal of Physical Chemistry A in the herein-used cc-pVDZ basis set for the correlated calculations against cc-pVTZ as a benchmark set. In a previous study, we considered the UV spectra of polychlorinated DBFs that were not recorded beyond 200 nm.32 Because, in this case, we intended to compare our calculations with spectra covering a much broader range, well into the vacuum-UV region, it was necessary to include a description of Rydberg states. Currently, only the Rydberg transitions of the a1 and b2 symmetries were possible to calculate, which fortunately is sufficient for the present purposes. The reason is that these transitions, as the most intense, most importantly supplement the far-UV region of the spectra. To make the calculations feasible, the lowest-lying orbital, corresponding to the heteroatom lone pair in DBF, CZ, and DBT, had to be left out of the corresponding active spaces. This was assumed to affect the positions of the Rydberg transitions only negligibly. In addition, the number of the SA-CASSCF roots had to be reduced to six. For these calculations, the ccpVDZ basis set was augmented with a set of diffuse basis functions of the s, p, and d angular momenta, nine functions in all. The diffuse basis functions were themselves devised as contractions of eight diffuse Gaussians having universal exponents for an optimum representation of Rydberg states.41 For each compound, its own set of contracting coefficients was determined by averaging over equally weighted density matrices of the two monocations generated by eliminating the electrons from the highest occupied molecular orbital (HOMO) and HOMO
1. The centers of diffuse functions coincided with the centers of charge of the cations. Adding the three thus-generated Rydberg orbitals [3px (b1), 3dxy (a2), and 3dxz (b1)] to the (12,12) active spaces results in (12,15) active spaces, whereby large configuration spaces of dimension ∼3  106 are spanned. The centers, exponents, and contracting coefficients of the sets of diffuse Rydberg functions employed are provided in the Supporting Information. The vertical excitation energies were also calculated within the framework of time-dependent density functional theory30 employing the three-parameter hybrid B3P86 exchange-correlation functional42 (TD-B3P86) and the Pople-styled 6-311þG(2d,p) basis set.43 In describing the excited states of a set of small organic molecules, B3P86 showed the best performance of several tested functionals.44 The basis set was chosen as somewhat larger than usual to ensure sufficiently converged TD-B3P86 results. The 40 lowest singlet states were included. All multireference calculations were performed with MOLCAS 6.4,45 and the TD-B3P86 calculations were performed with the Gaussian 03 quantum chemistry package.46 Deconvolution of Spectra. To extract the specific data from the experimental absorption bands28 against which to compare the calculated excitation energies and oscillator strengths, we deconvoluted the spectra.47 In this procedure Lorentzian line shapes were assumed for the bands, and their number, positions, and intensities were varied to best re-create the observed spectral features, more specifically to minimize the root-mean-square errors between the fitted and experimental curves. The thus-obtained band centers and intensities are correlated with the excitation energies and oscillator strengths of notable neighboring CASSCF/ CASPT2 transitions, the likely candidates for a given absorption (please refer to Tables S1
S4 of the Supporting Information). This forms the basis of the here-proposed theoretical assignments, as well as providing better insight into the error bounds of the present CASSCF/CASPT2 treatment. More details of the deconvolution procedure are given in the Supporting Information.

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’ RESULTS AND DISCUSSION The vertical and adiabatic excitation energies and oscillator strengths of the valence and Rydberg transitions are reported in Tables 1
3. In Figures 2
5, the CASPT2 and TD-B3P86 theoretical spectra, as well as the experimental and fitted theoretical spectra, are depicted. Several general remarks can be made about the performance of the two methods. The general shapes of the spectra are more or less recognizably reproduced by the calculations. This means that, for the most part, it is possible to determine which transitions contribute to which band system. Much more demanding is establishing a correct detailed ordering of transitions within compound bands, as well as their intensities, because doing so depends on many factors presently beyond the scope of our calculations, most notably the vibronic interaction. In the majority of cases, the CASPT2 values require a red shift, and the TD-B3P86 ones require a blue shift, to bring them into closer agreement with the experimental data. Generally, the higher the CASPT2 excitation energies, the more of a red shift they require, which can be attributed to the growing need for diffuse functions in the basis set when describing the higher excited states. The two theoretical approaches often act in a complementary fashion in providing a firmer support for either assigning underlying transitions to a particular band system or removing them from the same system. For the multireference calculations, a systematic serious discrepancy is assigning CASSCF oscillator strengths that are far too low to the lowest of the A1- and B2-symmetry transitions, which is similar to the problem encountered in dioxins.37,38 Furthermore, these lowest-lying bands exhibit rather complex structures characterized by some remarkably sharp vibronic progressions that tend to accumulate much of the band’s oscillator strength. They are consequently poorly described by the Lorentzian line shapes. It is also of interest to note that, in a few cases, the multistate (MS) version of CASPT2 (with the same IPEA shift) gives better values for the low-energy transitions than SS-CASPT2. The MSCASPT2 approach, however, seems to become increasingly unreliable toward higher excitation energies, so in general, we found no reason to prefer it over SS-CASPT2. Focusing on the problem of oscillator strengths, the CASSCF y-polarized transitions (B2) are generally much more intense than the z-polarized (A1) ones. In fact, in all four analogues, only a single A1 transition has a CASSCF oscillator strength larger than 0.1. Thus, some evidently strong A1 transitions in the spectra remain unexplained, so an efficient vibronic intensity borrowing mechanism must be postulated.48 In contrast, with TD-B3P86, the A1 transitions accumulate a fair share of the total oscillator strength and are not much inferior to the B2 transitions. The possible shortcomings of the largest affordable basis set, cc-pVDZ, for the CASPT2 calculations here are discussed elsewhere.39 For TD-B3P86, it was observed that the addition of diffuse functions on the heavier atoms systematically lowered the excitation energies by 0.1
0.2 eV over the whole spectral range. It is reasonable to assume that a similar effect would be observed with CASPT2. For instance, in FL, the introduction of extra diffuse Rydberg basis functions (using an IPEA shift of 0.10 au for comparison) red shifts the valence 2A1 and 3A1 excitation energies by 0.08 and 0.04 eV, respectively. To consistently compare the two methods over the whole spectral range, one would need to establish an approximate correspondence between the CASPT2 and TD-B3P86 sets of excited states. This, however, becomes increasingly difficult toward higher 4842

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Table 1. Vertical and Adiabatic (0
0) Excitation Energies and Oscillator Strengths (in Length Representation) of Valence Transitions at the SS-CASPT2/cc-pVDZ Levela FL state

a

Ev/nm (eV)

CZ f

2A1

266

(4.66)

2A1(0
0)

279

(4.45)

3A1 4A1

196 187

(6.32) (6.62)

5A1

186

6A1

186

7A1

Ev/nm (eV)

0.0004

DBF f

307

(4.04)

325

(3.81)

0.0012 0.3325

232 198

(5.35) (6.27)

(6.67)

0.0129

194

(6.67)

0.0038

191

179

(6.91)

0.0049

8A1

164

(7.58)

9A1

158

10A1

Ev/nm (eV)

0.0164

283

(4.38)

298

(4.16)b

0.0827 0.0014

208 194

(5.96) (6.39)

(6.39)

0.1154

188

(6.47)

0.0469

187

189

(6.57)

0.0253

0.0060

168

(7.40)

(7.85)

0.0161

155

155

(7.97)

0.0495

1B2 1B2(0
0)

278 291

(4.46) (4.26)

2B2

253

(4.90)

2B2(0
0)

271

(4.57)

3B2

207

(5.98)

4B2

193

(6.41)

5B2

181

6B2

DBT f 0.0078

Ev/nm (eV)

f

298

(4.16)

316

(3.93)

0.0122

0.0983 0.0039

239 218

(5.20) (5.69)

0.0369 0.1796

(6.59)

0.1393

198

(6.26)

0.0044

(6.61)

0.0369

189

(6.56)

0.0860

182

(6.80)

0.0120

188

(6.59)

0.0086

0.0020

172

(7.23)

0.0276

164

(7.55)

0.0312

(7.98)

0.0264

152

(8.16)

0.0790

161

(7.70)

0.0702

152

(8.16)

0.0592

150

(8.24)

0.0005

161

(7.72)

0.0052

0.0125

264 276

(4.69) (4.49)

0.0815

262 275

(4.73) (4.51)b

0.0533

265 276

(4.69) (4.49)

0.0128

0.4822

250

(4.97)

0.0004

241

(5.14)

0.3043

0.1069

252

(4.92)

251

(4.94)b

0.0274

218

(5.68)

0.5854

194

(6.39)

0.8739

201

(6.16)

0.5509

191

(6.48)

(6.85)

0.0360

191

(6.50)

0.2398

188

175

(7.10)

0.1283

178

(6.98)

0.2234

7B2 8B2

170 159

(7.30) (7.80)

0.0038 0.0507

172 165

(7.22) (7.51)

9B2

148

(8.39)

0.0158

162

10B2

145

(8.52)

0.0158

155

257

(4.83)

268

(4.63)

0.3228

218

(5.68)

0.4822

0.5522

207

(5.99)

0.0158

(6.60)

0.1994

199

(6.24)

0.4037

183

(6.76)

0.0593

185

(6.71)

0.4394

0.0000 0.0880

170 155

(7.27) (8.01)

0.4132 0.0027

176 166

(7.05) (7.45)

0.2133 0.0295

(7.64)

0.2588

153

(8.12)

0.0264

164

(7.55)

0.2889

(8.01)

0.0176

151

(8.19)

0.0124

149

(8.32)

0.0001

CASSCF(14,13) references [CASSCF(12,12) in the case of FL]; imaginary level shift and IPEA shift of 0.10 au were used. b From ref 32.

Table 2. Vertical Excitation Energies and Oscillator Strengths (in Length Representation) of Rydberg Transitions at the SSCASPT2/cc-pVDZ Levela FL state A1

Ev/nm (eV)

CZ f

Ev/nm (eV)

DBF f

Ev/nm (eV)

DBT f

Ev/nm (eV)

f

b

4b1 f 3px

181

(6.86)

0.0181

216

(5.75)

0.0068

188

(6.58)

0.0219

200

(6.21)

0.0045

3a2 f 3dxy 4b1 f 3dxz

190 169

(6.54) (7.32)

0.0004 0.0057

192 198

(6.47) (6.25)

0.0001 0.0033

183 177

(6.77) (6.99)

0.0010 0.0013

183 186

(6.79) (6.66)

0.0010 0.0120

B2b 3a2 f 3px

202

(6.14)

0.0066

206

(6.02)

0.0221

194

(6.39)

0.0059

192

(6.46)

0.0083

3a2 f 3dxz

188

(6.58)

0.0070

190

(6.52)

0.0003

181

(6.85)

0.0036

180

(6.87)

0.0218

4b1 f 3dxy

165

(7.52)

0.0036

198

(6.25)

0.0034

178

(6.95)

0.0282

182

(6.82)

0.0117

2a2 f 3px

173c

(7.15)

0.1476

173c

(7.18)

0.0839

166

(7.46)

0.0472

167

(7.41)

0.0642

a CASSCF(12,15) references; imaginary level shift of 0.10 au and IPEA shift of 0.25 au were used. b In the case of DBT, the orbital label 4b1 should be replaced by 5b1. c Multistate (MS-CASPT2) excitation energies with an IPEA shift of 0.25 au were used.

energies. Only a few of the lowest transitions can be compared clearly with respect to the dominant excitations, whereas for the higher-lying states, the relevant excitations become many and the importance of double excitations in CASSCF rises. In principle, the latter aspect also implies a diminished accuracy of the TDDFT approach in such cases.49 The electronic structure, specifically the ordering of the equivalent Kohn
Sham (K
S) orbitals by energy, exhibits interesting differences in the four analogues. With highly electronegative N and O heteroatoms, the inductive effects are

comparable. Therefore, it is the mesomeric effect due to the heteroatom lone-pair interaction with the π density of the lateral benzene rings that chiefly affects the energies of the b1-symmetry K
S orbitals.50 The lone-pair density has the opposite phase to the π densities of the rings and, being more disperse in the case of the N and S atoms, interacts more unfavorably, thus lifting the energy of the corresponding b1 orbitals to the level of the HOMO in CZ and DBT. In particular, the CZ HOMO is raised by this effect, and as a result, this analogue has, by far, the lowest ionization potential (IP). The very high-intensity broad bands at 4843

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Table 3. Vertical Excitation Energies and Oscillator Strengths (in Length Representation) of Valence Transitions at the TDB3P86/6-311þG(2d,p) Level FL state

Ev/nm (eV)

CZ f

DBF

Ev/nm (eV)

f

Ev/nm (eV)

DBT f

Ev/nm (eV)

f

2A1

253

(4.89)

0.0080

306

(4.05)

0.0286

277

(4.47)

0.0277

305

(4.06)

0.0240

3A1

223

(5.56)

0.0166

235

(5.28)

0.0343

226

(5.48)

0.0715

250

(4.97)

0.0131

4A1

213

(5.82)

0.1107

213

(5.82)

0.0605

208

(5.97)

0.0249

225

(5.51)

0.1570

5A1

205

(6.06)

0.0072

201

(6.16)

0.0644

200

(6.20)

0.0676

202

(6.13)

0.0276

6A1 7A1

195 188

(6.35) (6.59)

0.0253 0.0629

195 180

(6.34) (6.87)

0.0397 0.0040

194 178

(6.39) (6.96)

0.0226 0.0043

196 186

(6.33) (6.67)

0.0043 0.0244

8A1

171

(7.25)

0.0256

175

(7.08)

0.0318

166

(7.45)

0.0177

184

(6.75)

0.0701

9A1

165

(7.49)

0.7096

170

(7.30)

0.0670

163

(7.59)

0.2838

175

(7.10)

0.0010

165

(7.53)

0.1292

159

(7.80)

0.1610

170

(7.31)

0.1582

10A1 1B2

273

(4.54)

0.1678

277

(4.48)

0.1444

269

(4.61)

0.3049

272

(4.56)

0.1027

2B2

262

(4.74)

0.2812

238

(5.20)

0.4398

234

(5.29)

0.0341

250

(4.97)

0.0222

3B2

224

(5.53)

0.0041

230

(5.38)

0.1399

221

(5.62)

0.2575

238

(5.20)

0.5521

4B2 5B2

212 192

(5.84) (6.44)

0.1364 0.8045

210 203

(5.91) (6.12)

0.5969 0.0687

208 194

(5.97) (6.38)

0.4592 0.0890

218 209

(5.69) (5.93)

0.1812 0.0574

6B2

184

(6.75)

0.0273

180

(6.89)

0.2718

183

(6.76)

0.4439

195

(6.36)

0.4904

7B2

174

(7.11)

0.0002

173

(7.16)

0.0004

170

(7.30)

0.0003

175

(7.07)

0.0129

8B2

167

(7.41)

0.0200

169

(7.34)

0.0063

164

(7.55)

0.0145

165

(7.53)

0.0149

9B2

164

(7.56)

0.0350

164

(7.57)

0.0041

Figure 2. Fluorene theoretical (CASPT2/CASSCF and TD-B3P86) and fitted spectra superimposed on the experimental28 vapor spectrum (80 °C). The black lettering corresponds to the proposed theoretical assignments.

the high-energy ends of the four spectra correspond to the onset of ionization processes, and the positions of the vertical rules in Figures 2
5 agree with the previously reported first IPs.51 The spectral features in this region and beyond are outside the scope of the present theoretical framework.

The unfavorable mesomeric interaction is less pronounced in DBF, which is more similar to the parent FL in this respect, as both retain the a2-symmetry HOMO. All four analogues have an equivalent b1 LUMO in common, whereas the ordering of the next few virtual orbitals again shows some differences. Here, DBF 4844

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The Journal of Physical Chemistry A is more similar to CZ, as is DBT to FL. The partial electronic configurations are (3b1)2(2a2)2(4b1)2(3a2)2(5b1)0(6b1)0(4a2)0 for FL, (3b1 )2 (2a 2 )2 (3a 2 )2 (4b1 )2 (5b1 )0 (4a 2 )0 (6b 1 )0 for CZ, (3b1 )2 (2a 2 )2 (4b 1 )2 (3a 2 )2 (5b1 )0 (4a 2 )0 (6b 1 )0 for DBF, and (4b1)2(2a2)2(3a2)2(5b1)2(6b1)0(7b1)0(4a2)0 for DBT. These subtle differences in the electronic structures of the four analogues bring about some major differences in the relative positions and intensities of the electronic transitions. An interesting consequence is that the lowest excited state is 1B2 in FL, but 2A1 in DBF, CZ, and DBT. Also, the two lowest states, 2A1 and 1B2, lie much closer in DBF than in CZ or DBT (Tables 1 and 3). The predictions of the two methods for the individual analogues, including the proposed assignments of their UV spectra, are discussed in detail in the following sections. The experimental data used for comparison against the theoretical predictions are chiefly based on the most recent vacuum-UV gas-phase spectra of ref 28, which are reproduced in Figures 2
5. Fluorene. Interestingly, even for the lowest-lying states of the parent analogue, the two methods give conflicting predictions. Thus, unlike TD-B3P86 (Table 3), CASPT2 verifies the observed23,28 sequence of the three lowest transitions (1B2, 2A1, and 2B2; Table 1), although the vertical intensities of 1B2 and 2A1 transitions seem considerably underestimated. The CASPT2 adiabatic 1B2 transition is close to the sharp 296-nm origin of the first band characterized by several notable progressions. The location and intensity of the vertical 2B2 transition agrees nicely with the 255-nm summit of the broad intense band with complex vibronic structure extending through the 230
270-nm region (Figure 2). TD-B3P86 assigns significant intensities to both the 1B2 and 2B2 transitions, but places them too close. Consequently, in the simulated TD-B3P86 spectrum (Figure 2), the 1B2 transition appears as a shoulder to the more intense 2B2 transition, and the sharp peak at 296 nm remains unexplained. The two methods also disagree about the excitations that contribute dominantly to these states. This is reflected in dissimilarities of the spectra, which is actually most pronounced in FL. The 2A1 transition predicted as negligible by both methods and seen as a small bump in the LD and crystal polarized spectra28 probably gains intensity through vibronic interaction. The medium-intensity shoulder at 212 nm coincides nicely with the fairly strong TD-B3P86 4A1 and 4B2 transitions, both with nearly the same oscillator strengths (Table 3). In contrast, CASPT2 does not place a single A1 transition in the region, and in the vicinity of the shoulder, only the low-intensity 3B2 transition appears at 207 nm (Table 1). Because CASPT2 predicts one A1 transition of medium intensity (3A1 in DBF, CZ, and DBT; Table 1) to precede the region of the strongest absorptions in the remaining analogues, the absence of such a transition here represents a peculiar exception. The proposed assignment is B2,28 and so the shoulder is correlated with the CASPT2 3B2 transition [Table S1 (Supporting Information), Figure 2]. Further on follows an exceptionally strong band system covering the 180
210-nm region distinguished by two sharp peaks at 198 and 203 nm. Similar well-defined high-intensity bands around 200
230 nm, where the strongest of the B2 and A1 transitions overlap, are typical for the FHA spectra. According to CASPT2, in FL, this might be primarily due to 4A1 and 4B2 transitions, with the latter having, by far, the largest oscillator strength in all four analogues. In terms of position and intensity, these two transitions probably best correspond to the TD-B3P86 4A1 and 5B2 transitions, although a direct comparison between the states is difficult for the reasons given above. It was proposed that the

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high-intensity peaks match two distinct A1 transitions,28 but this is not unequivocally supported by the calculations. Because it is also indicative that both methods predict a remarkably intense B2 state in the region, the shape of this region is best correlated with the CASPT2 4A1 and 4B2 transitions (Table S1, Supporting Information). A trend in the CASPT2 predictions in FHA is that the strongest bands are followed by several lower-intensity, but still fairly strong B2 transitions. In FL, these are the 5B2 and 6B2 transitions, which might correspond to the shoulder at 187 nm and the distinct bump at 178 nm. Of noteworthy intensity nearby is also the 2a2 f 3px Rydberg transition at 173 nm. Its oscillator strength could be suspicious, however, as the corresponding CASSCF root is plagued by strong valence
Rydberg mixing (Table 2). The last two assignments are only speculative, as they were omitted from the experimental spectra, although an analogy can perhaps be drawn with the B2 bands observed in the equivalent region of the DBT spectrum.27,28 Instead, it was the 195-nm shoulder that was assigned to a B2 state.28 TD-B3P86 places a few low-intensity A1 and B2 transitions in the vicinity (7A1, 8A1, 6B2, and 7B2), but nothing as convincing. Further on follows yet another exceptionally strong band peaking at 162 nm. Whereas CASPT2 is ambiguous here in predicting nearby only 10A1 and 8B2 transitions of equally toolow intensity, TD-B3P86 is definite in assigning this to the second strongest transition, 9A1, although some weaker B2 transitions (8B2 and 9B2) are also predicted in the vicinity. This concurs with experiment, as this region of the polarized spectra is dominated by the strongest A1 absorption, although the LD B2-polarized spectrum also indicates a clear rising trend at its high-energy end.24,28 Just prior to the ionization threshold, one finds a shoulder at 152 nm, which was left unassigned. We tentatively propose a B2 state, because here CASPT2 indicates combined 9B2 and 10B2 transitions (Table 1). Carbazole. The CASPT2 and TD-B3P86 spectra of CZ are more comparable than are those of FL (Figure 3). Unlike that of FL, the lowest excited state of CZ, DBF, and DBT is predicted to be of A1 symmetry, which agrees with the experimental and other theoretical findings.23,24,28 Both CASPT2 and TD-B3P86 give the dominant contribution of the HOMO f LUMO excitation and virtually the same vertical excitation energy for 2A1 (307 and 306 nm; Tables 1 and 3), which is far from the most prominent peak (327 nm)28 of this low-intensity band. However, the CASPT2 adiabatic 2A1 transition nicely coincides with the peak, which is reminiscent of the situation in DBF (see below), so this likely corresponds to the 000 origin. Turning to the broad medium-intensity band in the 260
290nm region with the sharp origin at 284 nm, this is unambiguously assigned to the 1B2 transition. In the leading configurations, the equivalent CASSCF and K
S orbitals are involved, with the principal contribution to this state arising from the (HOMO
1) f LUMO excitation. The CASPT2 adiabatic transition at 276 nm and the TD-B3P86 value of 277 nm lie close to the band origin, but the latter method is more accurate in predicting an oscillator strength that is nearly twice as large. The next absorption, which appears as the 235
250-nm shoulder, is more difficult to assign by theory alone, owing to the different predictions by the two methods. Selecting simply the nearest transition of a significant intensity is ambiguous in that the shoulder could be assigned to either an A1 transition (3A1 at 232 nm by CASPT2, Table 1) or a B2 transition (2B2 by 238 nm TD-B3P86, Table 3). Also, the correspondence between 4845

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Figure 3. Carbazole theoretical (CASPT2/CASSCF and TD-B3P86) and fitted spectra superimposed on the experimental28 vapor spectrum (125 °C). The black lettering corresponds to the proposed theoretical assignments.

CASSCF and TD-B3P86 already for these low-lying states is not straightforward. For instance, although the CASSCF and TD-B3P86 2B2 roots are dominated by the equivalent excitations, the signs of their corresponding coefficients are opposite in the former. This brings about a negligible CASSCF oscillator strength, in contrast to a very strong one calculated by TD-B3P86. Also, the nearby high-intensity CASPT2 3B2 transition has a principal contribution from a double excitation and, thus, has no analogue within the TD-B3P86 states. The ambiguities are resolved by the current and previous experiments, which concur in assigning the 243-nm gas-phase absorption to a y-polarized (2B2) transition (c band in the LD spectrum).25 Thus, the CASSCF intensity for the 2B2 transition is erroneously predicted as negligible, whereas TD-B3P86 assigns the second strongest oscillator strength to 2B2 (Table 3), which seems exaggerated. The 235
250-nm shoulder in fact appears wide and more complex, and the calculations suggest that it is actually built of the two near-lying transitions. The CASPT2 and TD-B3P86 3A1 transitions at 232 and 235 nm are of small to moderate intensity and most likely correspond to the z-polarized d band of the LD spectrum25 centered at 237 nm.25 The neighboring transition is the high-intensity band (comprising the e and f bands)25 characterized by the two strong peaks at 225 and 218 nm. The former corresponds to the strong 3B2 transition bounded from the high- and low-energy sides by the CASPT2 and TD-B3P86 values of 218 and 230 nm, respectively. The latter peak is assigned to A1 on the basis of the strong absorption in the LD and crystal spectra at ∼215 nm, but this assignment by theory seems inconclusive because of considerably differing predictions regarding the A1 transitions. In particular, whereas TD-B3P86 gives a medium-strength 4A1 transition at

213 nm, CASPT2 indicates a negligibly weak 4A1 transition and a strong 5A1 transition, both toward significantly higher energies. Although this corresponds to the largest uncertainties in the positions of A1 transitions of the CASPT2 spectrum, the reconstructed band is jointly correlated to 3B2, 4A1, and 5A1 [Table S2 (Supporting Information), Figure 3]. In contrast, the mediumstrength well-defined band peaking at 204 nm and also clearly defined in the polarized LD spectrum can be unambiguously assigned to 4B2, whose intensity and location are predicted as very similar by the two methods. Closer to the ionization threshold, assignments are increasingly difficult owing to the predicted crowding of A1 and B2 transitions, many of which exhibit non-negligible intensities. We propose the following tentative assignments, which are in reasonable agreement with both methods. The medium-intensity 5B2 transition probably emerges on the high-energy side of 4B2 around 195 nm followed by 6A1, which could account for the visible bump at 187 nm. Although an unevenness in the LD A1 spectrum is clearly visible in this region, it was left unassigned.28 Next follows a high-intensity band whose onset near 180 nm is most likely due to a B2 transition: 6B2 nicely coincides in location and intensity by both methods. This agrees with the proposed assignment.28 Onward, the polarized spectra indicate the building up of strong A1 transitions. Of noteworthy transitions, TDB3P86 here suggests 8A1, 9A1, and 10A1, which could match the CASPT2 9A1 and 10A1 transition, but further modeling of the spectra was not attempted because of the large uncertainties near the ionization threshold. Dibenzofuran. Previous multireference calculations32 encompassed the six lowest roots per symmetry species, whereas, here, four more roots were included. This extension of the state-averaging 4846

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Figure 4. Dibenzofuran theoretical (CASPT2/CASSCF and TD-B3P86) and fitted spectra superimposed on the experimental28 vapor spectrum (80 °C). The black lettering corresponds to the proposed theoretical assignments.

space was found to have only a small effect on the previously calculated excitation energies and oscillator strengths.32 On the whole, the differences in predictions between the two theoretical methods appear the smallest in DBF. In the two lowest states, 2A1 and 1B2, the leading contributions are analogous to those in CZ, but as the ordering of the K
S orbitals is changed, the second-lowest state, 1B2, is now dominated by the HOMO f LUMO excitation. The vertical excitation energies are predicted by both methods as being blueshifted relative to the peaks observed at 298 and 282 nm.28 Still, both peaks are likely better compared to the adiabatic transitions. For instance, the 000 origin is seen as fairly strong in the laserinduced fluorescence spectra,52 and the CASPT2 adiabatic 2A1 transition matches the 000 experimental value to within 0.01 eV.32 TD-B3P86 correctly predicts larger intensities of the 2A1 and 1B2 transitions, unlike CASSCF, which is already identified as a recurrent problem of the latter approach. Conversely, for the third lowest band, the medium-intensity 2B2 transition, distinguished by the sharp spike at 243 nm, TD-B3P86 predicts too low of an intensity, and CASSCF predicts a rather strong one. In addition, the CASPT2 vertical 2B2 transition lies much closer to the spike (Table 1, Figure 4). Turning to the region of the most intense absorptions (180
230 nm), the two methods predict some similar features corresponding to equivalent transitions. Thus, by the predicted positions and intensities, the shoulder at 222 nm can be assigned to 3A1, and the one at 210 nm can be assigned to 3B2. For these two states, CASPT2 gives values that are too blue-shifted, and TDB3P86 gives ones that are too red-shifted, although the latter lie significantly closer to the observed absorptions. The analogous analysis applies to the 4A1 state, to which an another high-intensity shoulder at 204 nm is assigned.28 Here, the CASPT2 value at 194 nm is again far from the TD-B3P86 one at 208 nm,

but the two methods similarly predict oscillator strengths that are too small. Thus, the 4A1 transition probably gains its considerable intensity by a vibronic mechanism through mixing with the nearlying B2 states, 3B2 or 4B2. The summit of the band at 201 nm is most likely due to the 4B2 transition. This is again bracketed from the high- and low-energy sides by the CASPT2 and TD-B3P86 values (191 and 208 nm) and also predicted as the most intense by both methods (Tables and 3). Toward the high-energy side of this complex (180
230-nm band), there are more uncertainties. Although no assignments were proposed here,28 some characteristically intense transitions are predicted in the vicinity, notably 5A1 and 5B2. It should be stressed that, in the fitted spectrum, these additional absorptions are required to give the band the required thickness. Although the strong 5B2 transition can appear as merged with 4B2 in the LD B2 polarized spectra, several mild bumps in the A1 LD and crystal spectrum can also be seen around 200 nm and beyond.28 For these reasons, 5A1, 6A1, and 5B2 are correlated to the features observed on the high-energy side of the 180
230-nm band [Table S3 (Supporting Information), Figure 4]. According to CASPT2, these can be followed by 6B2, although an equivalent B2 transition is missing from the TD-B3P86 spectrum. The 5A1, 5B2, 6B2, and 6A1 assignments are thus only tentative, although they seem to find some rationale in the experiment. The CASPT2 7B2 and TD-B3P86 6B2 are both predicted as rather strong and well separated from the preceding bands (Figure 4). These could correspond to the distinct mediumintensity absorption at 174 nm, which they bracket nicely. This absorption was also left unassigned, however.28 This can be followed by a weaker A1 band because the 8A1 states have some intensity and are placed sufficiently close by the two methods. This is speculative, as the A1-symmetry LD spectra exhibit only a very mild unevenness in the region.28 Much more credible is the 4847

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Figure 5. Dibenzothiophene theoretical (CASPT2/CASSCF and TD-B3P86) and fitted spectra superimposed on the experimental28 vapor spectrum (95 °C). The black lettering corresponds to the proposed theoretical assignments.

assignment of the remarkably intense peak at 157 nm to A1, which clearly matches the principal crystal-phase absorption and is corroborated by theory. This peak is correlated here to the combination of the close-lying CASPT2 9A1 and 10A1 transitions [Tables 1 and S3 (Supporting Information)]. Dibenzothiophene. From the onset of absorption up to the vacuum threshold, the present calculations support the earlier and recent assignments based on the LD and crystal UV spectra of DBT, as well as the TD-B3LYP results.27,28 The assignment of spectra in this region can thus be considered practically definite. Once more, several of the low-lying transitions are predicted as too blue-shifted and the corresponding oscillator strengths as too low. The missing oscillator strength is evidently redistributed to higher energies, notably to the 180
205-nm region (Figure 5), where a host of transitions is predicted, some of them apparently too intense for what is seen in the spectrum. The lowest state is 2A1, shifted to the similarly low energy as in the case of CZ, owing to the above-discussed role of the mesomeric effect in raising the energy of the HOMO. The CASPT2 value for the adiabatic excitation energy of 316 nm is close to the most prominent peak of the 2A1 band observed at 321 nm. The two methods equally describe this state as arising principally from the HOMO f LUMO and (HOMO
1) f (LUMOþ2) excitations, with the former having a weight in the wave function that is nearly 3 times larger. The second-lowest is the 1B2 band in the 270
285-nm region distinguished by the sharp absorption at 282 nm, which is well approached by the CASPT2 adiabatic excitation energy of 276 nm (Table 1). Beyond 265 nm, the absorption intensity gradually builds up: At first, two close medium-intensity bands are present, 2B2 and 3A1, having peaks at 258 and 250 nm,

respectively. These first four bands are similarly positioned by the two methods and are comparatively well-defined in the spectrum, so that assignments by theory alone are possible. However, starting from the strongest-intensity bands (∼210 nm) onward, some major uncertainties are present as a result of conflicting predictions by the two methods. The strongest absorption of the 210
240-nm region, distinguished by the peak at 228 nm and the shoulder at 224 nm, is due to the transitions to the 3B2 and 4A1 states, respectively. For these states, CASPT2 rightly predicts virtually the same excitation energies and large oscillator strengths (Table 1), while TDB3P86 is here less definite, separating 3B2 and 4A1 by more than 0.3 eV (Table 2). The clear-cut assignments from the earlier LD spectra27 end with the 4B2 peak observed at 207 nm, at the verge of the vacuum region. This coincides with the position of the CASPT2 4B2 transition, although the CASSCF intensity of this transition is too low. Quite the opposite, the TD-B3P86 4B2 state placed at 218 nm is too red-shifted, but has an appreciably strong intensity. For the difficult 160
200-nm region of the spectrum, we propose the following assignments, which reasonably well reconcile the predictions of the two methods and the proposals based on the LD spectra.28 Several of the predicted moderate- to high-intensity transitions can be used to faithfully reconstruct the spectrum up to the ionization threshold [Figure 5, Table S4 (Supporting Information)]. It is seen that the two methods agree in attributing the majority of the intensity to the B2 transitions, in particular CASSCF/CASPT2. Of the B2 bands in this region of the LD spectra, a medium-intensity bump with a high-energy shoulder is observed around 200 nm.28 Here, the CASPT2 5B2, 6B2, and 7B2 transitions are correlated to the Lorentzians used in 4848

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The Journal of Physical Chemistry A modeling this band (Table S4, Supporting Information). The equivalent TD-B3P86 states are not far off and are also of noteworthy intensity, particularly the second strongest transition 6B2. Also, the prominent 6A1 transition of similar location and intensity by both methods is assumed to lie between 5B2 and 6B2. The distinct peak at 176 nm is tentatively assigned to 8B2, although some similar-intensity A1 bands, such as 8A1, are also predicted in the vicinity. The last strong absorption (155
165 nm) prior to the ionization threshold is correlated to a combination of notable A1 and B2 transitions predicted in the region, 8A1, 9A1 (i.e., 10A1 by TD-B3P86), and 9B2 (Table S4, Supporting Information). In the LD spectra, a buildup in the A1 intensity is observed so the 162-nm peak was proposed to be solely due to an A1 transition.28 Rydberg Transitions. The Rydberg transitions have not been modeled so far in this set of compounds. The six calculated SA-CASSCF(12,15) roots were found to include three Rydberg states of A1 symmetry and four of B2 symmetry to complement the sets of valence transitions. Their assignments with regard to the dominant excitations are given in Table 2. The influence of the basis set and active space, which are now extended by the Rydberg basis functions, on the positions of the low-lying valence states included in these six roots (two of them per symmetry) is small. Some problems in modeling Rydberg states can be caused by interactions between valence and Rydberg CASSCF roots, which need to be resolved for the results to be accurate. In FHAs, these mixings are observed to occur only in the 2a2 f 3px state of CZ and FL, so in this case, the multistate (MS-CASPT2) excitation energies were used instead. On the other hand, in DBF and DBT, the SS- and MS-CASPT2 results differ negligibly because all of the states have either a clear valence or Rydberg character (i.e., no mixing occurs). The calculated Rydberg states are located in the region of the most intense bands. Thus, because they are typically of modest intensity, they are not expected to affect the appearance of the FHA spectra significantly. The 2a2 f 3px transition might be a notable exception; however, in CZ and FL, where it appears as the most intense, the corresponding CASSCF roots exhibit pronounced valence
Rydberg mixings, which makes the corresponding CASSCF oscillator strengths somewhat doubtful. The onset of the Rydberg transitions is the earliest in CZ (2a2 f 3px at 216 nm), which anticipates the lowest ionization potential of this analogue.

’ CONCLUSIONS Multireference CASSCF/CASPT2 and time-dependent density functional theory (TD-B3P86) treatments were employed in calculating vertical and adiabatic excitation energies and oscillator strengths of fluorene and its heteroanalogues. A principal intention was to test the predictions of the advanced theoretical treatments against the most recently proposed experimental assignments.28 The two methods complement each other frequently, and equivalent transitions usually appear as bracketed from the high- (CASPT2) and low- (TD-B3P86) energy sides. At lower energies, the bands are few and normally well-separated and, thus, can mostly be assigned through theory alone. Toward the vacuum threshold and beyond, the gradual crowding of electronic transitions is predicted, and many bands start to overlap, all of which greatly complicates the spectral features, making the assignments increasingly difficult. With the exception

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of the few lowest transitions, where occasional strong vibronic progressions result in pronouncedly asymmetrical bands, the use of Lorentzians proved effective in reconstructing the spectral features over a wide range of energies. This deconvolution procedure enabled the extraction of data directly comparable to the calculations, helped assess the errors of the present CASSCF/ CASPT2 treatment, and assured that no important transition was overlooked in the analysis.

’ ASSOCIATED CONTENT

bS

Supporting Information. Details of the deconvolution procedure; calculated and fitted CASPT2 vertical transitions and CASSCF oscillator strengths; CASPT2 results of testing the effects of change in basis sets and IPEA shift in smaller model compounds (benzene and thiophene); CASSCF equilibrium geometries; centers, exponents, and contraction coefficients of the generated Rydberg basis functions. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: þ385-1-45-61-089. Fax: þ385-1-46-80-245. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the Ministry of Science, Education and Sport of the Republic of Croatia under Project 0980982915-2944. The authors thank Dr. Lina Nakhimovsky for useful discussions and for providing the vacuum-UV gas-phase and crystal spectra of the studied compounds. ’ REFERENCES (1) Qi, T.; Liu, Y.; Qiu, W.; Zhang, H.; Gao, X.; Liu, Y.; Lu, K.; Du, C.; Yu, G.; Zhu, D. J. Mater. Chem. 2008, 18, 1131–1138. (2) Chowdury, M. A. H.; Monkman, A. P.; Chawdhury, N. J. Polym. Res., published online Jan 7, 2011, http://dx.doi.org/10.1007/s10965010-9542-x. (3) Lo, S.-C.; Burn, P. L. Chem. Rev. 2007, 107, 1097–1116. (4) Moss, K. C.; Bourdakos, K. N.; Bhalla, V.; Kamtekar, K. T.; Bryce, M. R.; Fox, M. A.; Vaughan, H. L.; Dias, F. B.; Monkman, A. P. J. Org. Chem. 2010, 75, 6771–6781. (5) Lee, K.-H.; Morino, K.; Sudo, A.; Endo, T. Polym. Bull., published online Aug 26, 2010, http://dx.doi.org/10.1007/s00289-010-0372-0. (6) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed. 1998, 37, 402–428. (7) Bernius, M.; Inbasekaran, M.; Woo, E.; Wu, W.; Wujkowski, L. J. Mater. Sci.: Mater. Electron. 2000, 11, 111–116. (8) Park, J.-W.; Dong, H. L.; Chen, J.; Bae, M.-H.; Kang, M.-S.; Kim, Y.-H.; Pyo, S.; Yi, M. H.; Kwon, S.-K. Curr. Appl. Phys. 2010, 10, e152–e126. (9) Ye, T.; Chen, J.; Ma, D. Phys. Chem. Chem. Phys. 2010, 12, 15410–15413. (10) Karim, Md. A.; Cho, Y.-R.; Park, J. S.; Kim, S. C.; Kim, H. J.; Lee, J. W.; Gal, Y.-S.; Jin, S.-H. Chem. Commun. 2008, 1929–1931. (11) Heeney, M.; Bailey, C.; Giles, M.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Zhang, W.; McCulloch, I. Macromolecules 2004, 37, 5250– 5256. (12) Belfield, K. D.; Corredor, C. C.; Morales, A. R.; Dessources, M. A.; Hernandez, F. E. J. Fluoresc. 2006, 16, 105–110. (13) Wassenberg, D. M.; Nerlinger, A. L.; Battle, L. P.; Di Giulio, R. T. Environ. Toxicol. Chem. 2005, 24, 2526–2532. 4849

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