J. Phys. Chem. B 2010, 114, 16847–16853
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Investigation of Excited State Structural Dynamics of Bis(2-thienyl)ketone in the Condensed Phase Using Raman, IR, and UV-visible Spectroscopy Aided by Density Functional Theory Calculation Huigang Wang,* Shaosong Shen, Libo Wang, and Xuming Zheng* Department of Chemistry, Zhejiang Sci-Tech UniVersity, Hangzhou 310018, China, Engineering Research Center for Eco-dyeing and Finishing of Textiles, Ministry of Education of China, Zhejiang Sci-Tech UniVersity, Hangzhou 310018, China, and Key Laboratory of AdVanced Textile Materials and Manufacturing Technology, Ministry of Education of China, Zhejiang Sci-Tech UniVersity, Hangzhou 310018, China ReceiVed: September 26, 2010; ReVised Manuscript ReceiVed: NoVember 11, 2010
Detailed investigation on the vibrational and electronic spectra has been carried out in order to study various properties of bis(2-thienyl)ketone molecule in its ground and excited electronic states. To get insight into the structural and symmetry features of the molecule, resonance Raman spectra (RRs) of bis(2-thienyl)ketone have been acquired and the Raman excitation profiles of several normal modes have been analyzed, and density functional calculations were done to help the elucidation of the photo relaxation dynamics of A and B band electronic transitions. The RRs indicate that the photo relaxation dynamics for S0fS2 excited eletronic state is predominantly along the nominal the Ring breathing + νSynC(1)C(11) C(9) stretch, νCO + νC(2)C(3) + νC(8)C(9) stretch, and the γCH(I, II) + γOC stretch and simultaneously along the nominal γCH(II) relaxation processes, while that for S0fS5 is predominantly along the νCO + νC(2)C(3) + νC(8)C(9) stretch ν7 (1616 cm-1). The excited state short-time structural dynamics of bis (2-thienyl) ketone determined from RRs were interpreted with account of the Albrecht’s theory and Herzberg-Teller contributions. Introduction Resonance Raman spectroscopy (RRS) provides overwhelming superiorities over the other spectroscopy techniques with the electronic state-specific information.1 Critical study of RRS has the privilege to yield precious structural and conformational information of organic compounds with the aid of accurate quantum chemical calculation (QCC).2-6 Moreover, analyses of Raman excitation profiles may also be helpful in getting precious information such as symmetry properties, vibronic coupling, displacements of the potential energy minimum, and so forth of excited electronic states of molecules.7-11 The development of potential energy surface (PES) has long been recognized as a powerful knowledge for describing the dynamics of nuclear motion on electronic excitation following photoexcitation.12 Comparative studies on the PES of different electronic states with respect to that of the ground state are very supportive in this regard. The traditional sum-overstates picture of Raman scattering led early workers to focus on the relationship between resonance Raman intensities and the static excited state potential energy surface.13 It was the work of Lee and Heller,14 who interpreted the expression for the Raman amplitude in a time-dependent form, that first focused attention on the utility of resonance Raman intensities for probing vibrational dynamics in reactive excited states. This time-dependent view of the resonance Raman scattering process has led to resonance Raman spectroscopy being used to obtain a large amount of knowledge about the short-time dynamics of photodissociating small polyatomic molecules. Our group has concentrated on the * To whom correspondence should be addressed. (H.W.) E-mail:
[email protected]. Phone: 00186-571-8684-3627. Fax: 00186-571-86843627. (X.Z.) E-mail:
[email protected]. Phone: 86-571-86843699. Fax: 86-571-86843702.
resonance Raman intensity analysis of nitroalkanes and nitroaromatic compounds.11,15,16 FT-Raman is well-known for determining the molecular conformation in its ground state, while for resonance and near resonance Raman, contributions to the intensities of different Raman bands come from the resonant and its nearby electronic states. So the analyses of Raman excitation profiles of different Raman bands might produce valuable information on structural and other important properties of the molecules under investigation in those states. In the present investigation, a detailed exploration of the electronic spectra and the effects of excitation wavelength on the experimentally observed Raman intensity of bis(2-thienyl)ketone has been carried out and the Raman excitation profiles of different Raman bands obtained from different excitation wavelength have been compared and critically analyzed. Computational chemical methods may be carried out to better understand vibrational spectra. Herein, DFT calculations were carried out using the hybrid B3LYP functional to aid vibrational mode assignments. These studies are expected to be helpful in understanding the photophysical and photochemical characteristics of the molecule. Experimental and Computational Methods A. Resonance Raman Experiments. The methods and experimental apparatus used for the resonance Raman experiments have been described elsewhere,3,4,17 so only a short account will be given here. The harmonics of a nanosecond Nd:YAG laser and their hydrogen Raman shifted laser lines were used to generate the 252.7, 266.0, 282.4, 309.1 and 319.9 nm excitation wavelengths employed in the resonance Raman experiments. Spectroscopic grade solvents such as cyclohexane are purchased from Sigma. Concentrations of the bis(2thienyl)ketone (99% purity, Sigma) solutions are maintained from approximately 0.002 to 0.006 M for RRs and 0.0001 to
10.1021/jp109182h 2010 American Chemical Society Published on Web 11/30/2010
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0.0003 M for the electronic absorption spectra. A lower power was used during the resonance Raman measurements to avoid saturation effects and other problems associated with high peak powers. A backscattering geometry was used for sample excitation and for collection of the Raman scattered light by reflective optics that imaged the Raman scattered light through a polarizer and entrance slit of a 0.5 m spectrograph. The grating of the spectrograph dispersed the light onto a liquid nitrogencooled charge-coupled device (CCD) mounted on the exit of the spectrograph, and the CCD acquired the Raman signal for about 90-150s before being read out to an interfaced personal computer. The Raman shifts of the resonance Raman spectra were calibrated using the known vibrational frequencies of the solvent Raman bands. The solvent Raman bands were subtracted from the resonance Raman spectra using an appropriately scaled solvent spectrum. The Fourier transform (FT) IR and FT-Raman spectra of bis(2-thienyl)ketone in the neat solid phase were acquired to help assign the resonance Raman spectra. The spectra of an intensity calibrated deuterium lamp were used to correct the resonance Raman spectral intensities for the variation in detection efficiency as a function of wavelength and portions of the resonance Raman spectra were fitted to a baseline plus a sum of Lorentzian bands to find the integrated areas of the Raman bands. B. Computational Methods. Density functional theory (DFT)18,19 was done to determine the optimized geometry and vibrational frequencies as well as the electronic transition energies for the ground or excited electronic states of bis(2thienyl)ketone. Vibration wavenumber determination was computed by using the B3LYP/6-311G* level of theory for the ground state of bis(2-thienyl)ketone with a C1 symmetry, while the electronic transition energies were calculated using B3LYPTD/6-311G*. All of the DFT calculations made use of the Gaussian program software suite.20 Raman Excitation Profiles. The Raman excitation profiles (REPs) is a plot of Raman band intensities versus excitation wavelengths. Intensities were determined from the measured peak height ratio IN/IS for the bands of the sample (N) and internal standard (S) using
σN ) σS
( )(
IN ν0 - νS IS ν0 - νN
)( ) 4
CS CN
where σN and σS are the absolute Raman cross sections of the band being determined and of the internal standard band, respectively, CS/CN is the concentration ratio, ν0 is the laser excitation frequency, νN and νS are the vibrational frequencies of the sample and standard Raman bands, respectively. In this study, we mainly utilized cyclohexane as an internal standard. The σS value for the 802 cm-1 mode of cyclohexane was previously determined.21,22 Results and Discussion The molecule of our interest, bis(2-thienyl)ketone, belongs to the lowest symmetry C1 along with the two sulfur atoms placed asymmetrically with respect to the carbonyl group and the angle between two ring planes is about 40°. The loan pair electrons of sulfur atoms are responsible for electron transfer process in different types of photophysical and photochemical reactions. The DFT optimized structure of bis(2-thienyl)ketone molecule is shown in Figure 1. We have carried out DFT calculations for bis(2-thienyl)ketone in order to help elucidate the vibrational bands observed in the
Wang et al.
Figure 1. Structure and atom labeling scheme of bis(2-thienyl)ketone.
experimental FT-Raman and FTIR spectra of bis(2-thienyl)ketone as well as in the resonance Raman spectra of bis(2thienyl)ketone. The 18 atoms of bis(2-thienyl)ketone give rise to 48 normal modes of vibration. Obviously, all the vibrations are expected to be both Raman and IR active. Table 1 lists a comparison of the B3LYP/6-311G* calculated vibrational frequencies with experimental FT-Raman and FT-IR values. The notations and assignments of the vibrations are based on the visualization GAUSS VIEW 3.0 software and previous studied on the Raman spectra of 2,2-dipyridylketone were used as valuable references.23 The overall agreement between the linear regression scaled DFT calculated vibrational frequencies and the experimental values is good for bis(2-thienyl)ketone. The observed slight disagreement between the theory and the experiment may be attributed to anharmonicity. Besides, it is well-known that the general tendency of the quantum chemical methods is to overestimate the force constants at the exact equilibrium geometry.24 A. Absorption Spectrum. Figure 2 presents the absorption spectrum of bis(2-thienyl)ketone in cyclohexane solutions with the wavelengths for the resonance Raman experiments indicated above the spectrum. Table 2 lists the B3LYP-TD/6-311G* computed electronic absorption bands, the corresponding electric dipole transition orbitals, and the oscillator strengths for bis(2thienyl)ketone. Table 2 shows that among the calculated electronic transitions above 230 nm optical region there are two transition-allowed absorption bands at 302.62 and 270.06 nm (A- and B-band with the oscillator strength of f ) 0.29 and 0.07, respectively). This is in good agreement with the intense experimental absorption band at 301 and 267 nm (A- and B-band with the experimental oscillator strength of f ) 0.381 and 0.096, respectively). The A-band absorption of bis(2-thienyl)ketone at 301 nm is roughly separated from the B-band at 267 nm. The observed electronic absorption spectrum of bis(2-thienyl)ketone can be explained on the basis of three highest occupied and one lowest unoccupied molecular orbitals of the simple Huckel method. Figure 3 displays the three orbitals associated with the electronic transition of the calculated A-band and B-band absorption respectively. It shows that orbitals 48 (HOMO-2) are π orbitals with electron density being mainly delocalized on C(1)-C(2)-C(3)/C(4)-S(5) and C(11)-C(9)/C(6)-C(7) bonding, orbitals 49 (HOMO-1) are π orbitals mainly delocalized on S(5)-C(1)-C(2) and C(4)-C(3) /C(9)-C(8) bonding, orbitals 50(HOMO) are π orbitals mainly delocalized on S(10)-C(9)-C(8)/ C(3)-C(4)-S(5) and C(6)-C(7)/C(1)-C(2) bonding, while orbitals 51 (LUMO) are π orbitals mainly delocalized on C(1)-C(11)-C(9) and other π* orbitals with electron density being mainly localized on all atoms separately on the basis of our time-dependent density functional theory (TD-DFT) computations and natural orbital analysis. Thus the experi-
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TABLE 1: Experimental and B3LYP/6-311G* Computed Vibrational Frequencies of Bis(2-thienyl)ketonea modes A
ν1 ν2 ν3 ν4 ν5 ν6 ν7 ν8 ν9 ν10 ν11 ν12 ν13 ν14 ν15 ν16 ν17 ν18 ν19 ν20 ν21 ν22 ν23 ν24 ν25 ν26 ν27 ν28 ν29 ν30 ν31 ν32 ν33 ν34 ν35 ν36 ν37 ν38 ν39 ν40 ν41 ν42 ν43 ν44 ν45 ν46 ν47 ν48
FT-RAMAN
FT-IR
3101 3082
3097 3080
1616
1612
1514 1414
1514 1417
1352
1350 1325 1290 1120
1080 1049
1082 924
858
860
752
744 731
698 648
698 648 559 455
262 218 187
X: B3LYP/6-311G*
Yb
assignment
3241.5 3241 3228.4 3228 3205 3204 1684 1565 1560 1459 1458 1393 1386 1294 1264 1238 1134 1110 1100 1079 1053 930 913 884 867 865 849 830 753.4 753 746 722 714 681 622 580 570 523 461 451 381 286 211 202 154 115 49 41
3110 3109 3097.3 3097 3075 3074 1626 1513 1509 1413 1412 1350 1343 1256 1227 1203 1104 1081 1071 1051 1027 910 894 866 850 848 833 815 742 741.6 735 712 705 673 617 577 568 523 464 455 388 298 226 218 172 135 72 65
νC(6)H νC(4)H νCH(II) νCH(I) νCH(II) + βCC(II) νCH(I) + βCC(I) νCO + νC(2)C(3) + νC(8)C(9) βCH + νC(1)C(2) + νC(3)C(4) + νC(6)C(7) + νC(8)C(9) + νAsC(1)C(11) C(9) βCH(II) + νC(6)C(7) +νC(8)C(9) ring breathing + νSynC(1)C(11) C(9) ring breathing + νAsC(1)C(11) C(9) βCH(I,II) + νAsC(1)C(11) C(9) + νC(2)C(3) + νC(7)C(8) βCH(I,II) + νSynCS(I) βCH(II) + νAsC(1)C(11) C(9) βCH(I) βCH(II) βCH(I,II) + νSynC(1)C(11) C(9) βC(3)HβC(4)HβC(6)HβC(7)H βC(2)HβC(4)HβC(6)HβC(8)H βCH(I,II) βC(2)HβC(3)HβC(7)HβC(8)H + νSynC(1)C(11) C(9) γCH(II) γCH(I) ring deformation(I) + γCH(II) γCH(I,II) γCH(II) γCH(I) ring breathing + βCO γCH(I, II) + γOC CH out of plane bend + γOC CH out of plane bend + γOC + νAsynCS γCH(I) γCH(II) ring breathing(I) ring breathing(II) γCH(I,II) γCH(I,II) γCH(I,II) + βC(1)C(11) C(9) ring batterfly(I,II) ring batterfly(I,II) βCO δ(II) δ(I), γCC in CCS γCH(I,II) + CO out of plane bend δ(I,II) ΦCC in C(1)C(11)C(9) ΦCC in C(1)C(11)C(9)
a ν-stretching, β-in-plane bending, γ-wagging, δ-in-plane substitute bending; φ-torsion; I-S(5) ring, II-S(10) ring. b Scaling factors for DFT are Y ) 0.95043X + 25.9095 for the region (36-1589 cm-1) and Y ) 0.9594X for the region (3200-3300 cm-1).
mental 301 nm absorption band (A-band) (0.58 (50f51) orbital transition) is assigned as π(S(10)-C(9)-C(8)/ and π(C(6)-C(7)/ C(3)-C(4)-S(5))fπ(C(1)-C(11)-C(9)) C(1)-C(2))fπ*(C(6)-C(7)/C(1)-C(2)) transition, and the 267 nm absorption band (B-band) (0.54 (48f51)) is assigned as π(C(1)-C(2)-C(3)/C(4)-S(5))fπ*(C(1)-C(2)-C(3)/C(4)-S(5)) and π(C(11)-C(9)/C(6)-C(7))fπ*(C(11)-C(9)/C(6)-C(7)) transition. Our 319.9, 309.1 nm excitation wavelengths used in the resonance Raman experiments should be mostly on resonance with the A-band absorption of bis(2-thienyl)ketone while 266.0, 252.7 nm excitation wavelengths is in resonance with the B-band and 282.4 nm excitation wavelengths is in resonance with the overlap absorption of A and B band. Since the A-band transition (0.58 (50f51) orbital transition) transfers electron density from S(10)-C(9)-C(8)/C(1)-C(2) to C(11) of carbonyl (see Figure 3), a localized charge transfer nature is expectable for bis(2-thienyl)ketone in a short time upon absorbing 303 nm excitation.
Figure 2. Absorption spectrum of bis(2-thienyl)ketone in cyclohexane solution.
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TABLE 2: Experimental and Theoretical B3LYP-TD/6-311G* Computed Electronic Absorption Bands (nm) and Oscillator Strength f of Bis(2-thienyl)ketonea f singlets state C2v S1 S2 S3 S4 S5 S6 a
A A A A A A
main CI contributions 0.39 (46f51)-0.35 (49f51) 0.58 (50f51) 0.37 (49f51)-0.33 (47f51) 0.54 (47f51) 0.54 (48f51) 0.57 (49f52)
calc f f f f f f
) ) ) ) ) )
0.00 0.29 0.00 0.02 0.07 0.05
nm exp 0.381 0.096
calc 344.36 (3.60 302.62 (4.10 283.63 (4.37 274.98 (4.51 270.06 (4.59 233.27 (5.32
exp eV) eV) eV) eV) eV) eV)
301.0 267
HOMO ) MO 50, LUMO ) MO 51.
Figure 3. Three highest occupied and one lowest unoccupied orbitals for the A and B absorption band of bis(2-thienyl)ketone.
The molecular orbital coefficient analysis also supports the electron density redistribution. Similarly the B-band transition (0.54 (48f51)) transfers electron density from C(1)-C(2)-C(3)/ C(4)-S(5) to C11) of carbonyl upon absorbing 266.8 nm excitation (Figure 3). B. Resonance Raman Spectroscopy. We note first that since our laser line frequencies fall in the electronic absorption region (Figure 2) we may expect to see resonant Raman effects. To explore the exciting radiation wavelength dependent variation of Raman intensity, sum-overstates approach has been utilized as discussed earlier. In the framework of Albrecht’s theory, the Born-Oppenheimer approximation is employed to separate the vibronic states into products of electronic and vibrational states, and the transition dipole moments are expanded as a Taylor series in the nuclear coordinates. the Raman intensity, which are subject to enhancement when the incident radiation approaches a contour of an intense absorption band, get intensity contribution by either (i) Franck-Condon A-term, (ii) Herzberg-Teller B-term, or (iii) both. Nonzero A-term contribution implies a displacement of the potential energy minimum along the normal coordinate as between the ground and the excited electronic states. Moreover, the change in curvature of PES suggests that the concerned normal mode has different vibrational frequencies in the ground and in the excited states. Symmetry considerations require that such a displacement in the Franck-Condon enhancement mechanism can occur only for totally symmetric vibrational modes and no nontotally symmetric mode can be expected to undergo Franck-Condon scattering process. B-term involves the vibronic (Herzberg-Teller) coupling of the resonant excited state |er〉 to one other excited state |es〉. For the Herzberg-Teller integral ˆ e/∂Qk|er〉 to be nonvanishing, the irreducible factor hekser )〈es|∂H representation of the vibrational fundamental with normal coordinate Qk must be contained in the direct product of the irreducible representations of the states |er〉 and |es〉. Thus the B-term can be nonzero for both totally symmetric and nontotally symmetric fundamentals. On the other hand, the electric dipole moment selection rule affirms that both the |er〉r|eg〉 and |es〉r|eg〉 must be electric dipole allowed. Thus for a totally symmetric mode the states |er〉 and |es〉 must have the same symmetry and consequentially lead to their PESs repel. This repulsion gives rise to change in the shape of the respective PESs and the vibrational frequencies. According to the energy gap law, the strength of the coupling is inversely proportional
to the energy gap between the coupled states. In extreme cases, for very strong coupling the curvature of the PES of lower state may be inverted and double minima may appear and the B-term Herzberg-Teller expansion is not valid. Generally, the effect of the B-term is smaller than that of the A-term. The relative contribution of B-term is found to increase as the excitation wavelength is more and more away from the allowed electronic transition band. In the present case, as the 282.4 nm excitation is not in the region of resonance, the contributions of both the terms may be important. Also, their interference effect may sometimes be significant as discussed by Albrecht and co-workers.25,26 Figure 4 displays the comparison of the 252.7, 266, 282.4, 309.1, and 319.9 nm resonances Raman spectra of bis(2thienyl)ketone in cyclohexane solutions with the FT-Raman spectrum of solid bis(2-thienyl)ketone. The spectra shown in Figures 4 have been corrected for sample reabsorption as well as the wavelength dependence response of the detection system and the solvent Raman bands were removed from the spectra by subtracting an appropriately scaled solvent spectrum and regions of the solvent subtraction artifacts are indicated by asterisks. The dashed lines in Figure 4 indicate the correlation of nine fundamental vibrational modes labeled as ν7, ν9, ν10, ν12, ν14, ν19, ν20, ν29, and ν33 and several overtones and combination bands in 252.7, 266, 282.4, 309.1, and 319.9 nm resonance Raman spectra. The 319.9 nm laser frequency is below the lowest discrete molecular transition frequency and is termed preresonance Raman scattering. the 309.9 and 266 nm radiation is in the region of discrete vibronic transitions and is termed discrete resonance Raman scattering. The 282.4 nm incident radiation lies in which the continuum states of the potential surface of electronic state B overlap the discrete levels of state A, and 252.7 nm laser frequency lies above the B excited state in a continuum state and is termed continuum resonance Raman scattering. Figure 4 shows that while the vibrational modes in wavenumber and in description for different resonance Raman spectrum are very similar, the intensity patterns are very different. The most important difference between FT-Raman and A-band resonance Raman spectra is the relative enhancement of ν7, ν9 and ν29 for 319.9 nm, 309.1 and 282.4 nm resonance Raman spectra while for the difference between FTRaman and B-band resonance Raman spectra is the relative enhancement of ν7 and ν9 for 266 and 252.7 nm resonance Raman spectra. Clearly, the spectra obtained at different excitation wavelengths exhibit large variations in relative band intensity that reflects significant differences in the excited state structural dynamics. It is interesting to note that the most intense vibrational modes in the A-band resonance Raman spectrum is ν10 at 1414 cm-1, and this is noticeably different from the most intense vibrational modes ν7 at 1616 cm-1 in the B-band resonance Raman spectra. Apparently as the excitation wave-
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Figure 4. Comparison of the FT-Raman, 319.9, 309.1, 282.4, 266.0 and 252.7 nm resonance Raman spectra of bis(2-thienyl)ketone.
lengths move from 319.9 to 252.7 nm, the resonance Raman intensity of ν10 decreases, while that of ν7 and ν9 increases. This indicates that the 266 and 252.4 nm resonance Raman spectrum probes characteristically the excited state structural dynamics of B-band absorption, while the 309.1, and 319.9 nm resonance Raman spectra probe mostly that of A-band absorption. Figure 4 also presents the expanded overtones and combination bands views of the resonance Raman spectrum obtained with 319.9, 309.1, 282.4, 266, and 252.4 nm excitations with tentative vibrational assignments indicated above the spectra. Most of the A-band resonance Raman features in Figure 4 can be assigned to the fundamentals, overtones, and combination bands of about four Franck-Condon active vibrational modes based on the information provided in Table 1: the nominal ring breathing + νSynC(1)C(11) C(9) stretch ν10 (1414 cm-1), the nominal νCO + νC(2)C(3) + νC(8)C(9) stretch ν7 (1616 cm-1), the nominal γCH(I, II) + γOC stretch ν29 (752 cm-1), and the γCH(II) ν33 (698 cm-1). It appears that photoexcitation of bis(2-thienyl)ketone in the A-band absorption causes larger motions among the ring breathing + νSynC(1)C(11) C(9) stretch, νCO + νC(2)C(3) + νC(8)C(9) stretch, the γCH(I, II) + γOC stretch, and the γCH(II). Similarly most of the B-band resonance Raman features in Figure 4 can be assigned to the fundamentals, overtones, and combination bands of about three Franck-Condon active vibrational modes: the nominal νCO+νC(2)C(3)+ νC(8)C(9) stretch ν7 (1616 cm-1), the nominal Ring breathing+ νSynC(1)C(11) C(9) stretch ν10 (1414 cm-1), the nominal βCH(II) + νC(6)C(7) +νC(8)C(9) ν9 (1514 cm-1). The nominal νCO+νC(2)C(3)+ νC(8)C(9) stretch ν7 (1616 cm-1) forms the largest overtone progressions and combination bands with the other two Franck-Condon active modes (ν9 and ν10). This suggests that photoexcitation of bis(2-thienyl)ketone in the B-band absorption causes larger motions among the νCO + νC(2)C(3) + νC(8)C(9) stretch, ring breathing + νSynC(1)C(11) C(9) stretch, and βCH(II) + νC(6)C(7) + νC(8)C(9). Further inspection of Figure 4 reveals significant differences between the A- and B-band excited state structural dynamics of bis(2-thienyl)ketone. First, the electronic transition associated with the A-band absorption is featured by the localized π(S(10)-C(9)-C(8)/C(3)-C(4)-S(5))fπ(C(1)-C(11)-C(9))andπ(C(6)-C(7)/ C(1)-C(2))fπ*(C(6)-C(7)/C(1)-C(2)) transition. Since the former transition pumps an electron from S(10)-C(9)-C(8)/C(3)-C(4)-S(5) to C(11) of carbonyl group that strengthen the C(1)C(11)C(9) bond and the later transition weakens the C(6)-C(7)/C(1)-C(2) bond (see
orbitals 49 and 51 in Figure 3), the C(1)C(11)C(9) bond shortening and C(6)-C(7)/C(1)-C(2) bond lengthening is expected. This is consistent with the observed predominant overtone progressions of the nominal ring breathing + νSynC(1)C(11) C(9) stretch ν10 (1414 cm-1) in 319.9 and 309.1 nm resonance Raman spectra in Figure 4 and this also indicates that the molecule undergoes large excited state geometry structure change along C(6)-C(7)/C(1)-C(2) and C(1)C(11)C(9) reaction coordinate. The electronic transition associated with the B-band absorption is however very different from that of A-band absorption and is featured by the localized π(C(1)-C(2)-C(3)/C(4)-S(5))fπ*(C(1)-C(2)-C(3)/C(4)-S(5)) and π(C(11)-C(9)/C(6)-C(7))fπ*(C(11)-C(9)/C(6)-C(7)) transition that is the electron transition from C(1)-C(2)-C(3)/C(4)-S(5) to C(11) of carbonyl group. This agrees well with the observed most intense νCO+νC(2)C(3)+ νC(8)C(9) stretch mode at 1616 cm-1 accompanied modestly by the nominal ring breathing + νSynC(1)C(11) C(9) stretch ν10 (1414 cm-1) and the nominal βCH(II) + νC(6)C(7) + νC(8)C(9) ν9 (1514 cm-1). C. Raman Excitation Profiles (REPs). To quantitatively analyze the dependence of normal modes Raman intensity on excitation wavelength, the observed REPs of several normal modes of vibration of bis(2-thienyl)ketone molecule are calculated and presented in Figure 5. The observed profiles in cyclohexane solutions at room temperature (around 30 °C) are presented. From the Figure 5, the normal modes ν10 [breathing + νSynC(1)C(11)C(9) stretch] and ν7 [νCO + νC(2)C(3) + νC(8)C(9) stretch] get major intensity contribution from the 309.1 nm excitation (A-band absorption), which is in coincidence with the absorption spectrum. It could be inferred that they get major intensity enhancement from the diagonal A-term of the scattering tensor through the S2 electronic state (A-band). The diagonal A-term contributions from the relevant S2 states (A-band) for the above modes indicate that the relevant ring C-C bond, CdO bond and C(1)C(11)C(9) asymmetric bond distances undergo appreciable change due to excitation from the ground to the respective excited states. Thus, the electronic state S2 may be expected to have a good amount of charge transfer between the π-charge cloud of the ring and the charges of the ketone group. The normal modes ν29, ν33, and ν9 almost get no intensity contribution from the 266 nm excitation (B-band absorption, S5 state), and the intensity increase as the excitation wavelength is more and more away from the B-band electronic transition, which imply that their intensity contribution may comes from
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Figure 5. Comparison of the REPs for several vibrational modes and combination modes obtained from the 319.9, 309.1, 282.4, 266.0, and 252.7 nm resonance Raman spectra of bis(2-thienyl)ketone, respectively.
the Herzberg-Teller term from the pair of states A and B band. Herzberg-Teller term requires the symmetry of the product of S2 and S5 electronic wave functions is same as that of the vibrational modes ν29, ν33, and ν9. In other words, as the three modes get favorable off-diagonal contributions from the relevant pairs of states just mentioned, they are supposed to be effective in mixing those respective pairs of electronic states through vibronic coupling which indicates that the ring symmetries in the corresponding states remain the same. The normal modes ν29, ν33, and ν9 mainly concerns the ring breathing, it is a clear indication of appreciable change in the ring dimension in this excited electronic state. Figures 4 and 5 also show that the combination band ν7 + ν10, 2ν7, and ν10 + ν29 get their highest intensity in 282.4 nm resonance Raman spectroscopy. The 282.4 nm incident radiation lies in the overlap region of A-band and the discrete levels of B-band in which the adiabatic approximation is expected to fail and the mixing of vibronic levels through nonadiabatic interactions involving the nuclear kinetic energy operator becomes significant, which may lead to conical intersection between different potential energy surfaces and alter the photoreaction channel. This has already been verified by A. Marcelli et al. for time-resolved photoexcitation in the Soret band with pulses of femtosecond duration experiment.27 The 282.4 nm region gets the strongest vibronic coupling effects. At the same time, we notice that 282.4 nm radiation also resonance with the calculated S4 state. Apart from Herzberg-Teller contribution, there is also a possibility of A-term contribution involving S4 state. In fact, in the region of the experimentally observed somewhat broadband around 300 nm (corresponding to S2 state), two bands with appreciablefvaluesarecalculated:Botharisingfromπ(S(10)-C(9)-C(8)/ C(3)-C(4)-S(5))fπ(C(1)-C(11)-C(9)) and π(C(6)-C(7)/C(1)-C(2)) fπ*(C(6)-C(7)/C(1)-C(2)) transition, as discussed previously. Conclusion The purpose of the paper is to confirm theoretically the experimental findings of the electronic energy levels, vibrational signals and molecular geometry of bis(2-thienyl)ketone. The angle between the two ring planes is about 40° in the ground state as found from DFT calculation, although the geometry of carbonyl group (C1C11dOC9) is planar. DFT is not only found to give reasonably good singlet vertical excitation energies and
vibrational signatures of the molecule but are also compatible with the findings from the REP studies too. DFT method is used to aid the accurate description of the vibrational frequencies for the ground state. The 319.9, 309.1(A-band), 282.4, 266, and 252.7 nm (B-band) excitation wavelength RRs were acquired for bis(2-thienyl)ketone and the Raman effect was analyzed according to Herzberg-Teller (vibronic coupling) contributions. Our results indicate that the short-time S0fS2 photo relaxation dynamics of bis(2-thienyl)ketone have substantial multidimensional character mainly in the nominal the ring breathing + νSynC(1)C(11) C(9) stretch, νCO + νC(2)C(3) + νC(8)C(9) stretch, and the γCH(I, II) + γOC stretch with smaller contributions from the nominal γCH(II), while that for S0fS5 electronic state is predominantly along the νCO + νC(2)C(3) + νC(8)C(9) stretch ν7 (1616 cm-1). The overall picture of short-time dynamics for internal conversion and the vibronic coupling mechanisms are interpreted with the Albrecht’s theory. Acknowledgment. This work was supported by Grants from NSFC (Nos. 20703038 and 20803066), the National Basic Research Program of China (2007CB815203) and NSFZ (Nos. Y407245 and R405465). H.W. wishes to acknowledge the referees for their contributions in making this a better paper. References and Notes (1) Balakrishnan, G.; Ibrahim, M.; Mak, P. J.; Hata, J.; Kincaid, J. R.; Spiro, T. G. JBIC, J. Biol. Inorg. Chem. 2009, 14, 741. (2) Gao, P. C.; Wang, H. G.; Pei, K. M.; Zheng, X. M. Chem. Phys. Lett. 2007, 445, 173. (3) Wang, H. G.; Liu, B.; Wan, J. M.; Xu, J.; Zheng, X. M. J. Raman Spectrosc. 2009, 40, 992. (4) Wang, H. G.; Liu, B.; Zhao, Y. Y.; Zheng, X. M. J. Raman Spectrosc. 2009, 40, 1312. (5) Xu, J.; Wan, J. M.; Zhao, Y. Y.; Lv, M. Q.; Zheng, X. M.; Wang, G. D.; Wang, H. G. Spectrochim. Acta, Part A , 75, 1381. (6) Wang, H. G.; Xu, J.; Wan, J. M.; Zhao, Y. Y.; Zheng, X. M. J. Phys. Chem. B , 114, 3623. (7) Mishra, T.; De, A. K.; Chattopadhyay, S.; Mallick, P. K.; Sett, P. Spectrochim. Acta, Part A 2005, 61, 767. (8) Sett, P.; Misra, T.; Chattopadhyay, S.; De, A. K.; Mallick, P. K. Vib. Spectrosc. 2007, 44, 331. (9) Ruan, C. B.; Wang, H. G.; Zhu, H. L.; Zheng, X. M.; Phillips, D. L. J. Chem. Phys. 2008, 129. (10) Wang, Y. Q.; Wang, H. G.; Zhang, S. Q.; Pei, K. M.; Zheng, X. M.; Phillips, D. L. J. Chem. Phys. 2006, 125. (11) Zhang, S. Q.; Wang, H. G.; Pei, K. M.; Zheng, X. M.; Phillips, D. L. J. Chem. Phys. 2007, 126.
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