J. Phys. Chem. A 1997, 101, 7735-7741
7735
Van der Waals Complexes of Jet-Cooled Aromatic Thiones with Noble Gases: The Phosphorescence Excitation Spectrum of 4H-Pyran-4-thione A. A. Ruth* Max-Planck-Institut fu¨ r Biophysikalische Chemie, Abteilung Spektroskopie und Photochemische Kinetik, Am Fassberg 11, D-37077 Go¨ ttingen, Federal Republic of Germany
F. J. O’Keeffe,† M. W. D. Mansfield,† and R. P. Brint‡ Departments of Physics and Chemistry, National UniVersity of Ireland, UniVersity College, Cork, Ireland ReceiVed: March 31, 1997; In Final Form: July 17, 1997X
The solvent spectral shifts of the S0 f T1 origin bands of jet-cooled 4H-pyran-4-thione (PT) and 4H-1benzopyran-4-thione (BPT) were observed in phosphorescence excitation spectra. Van der Waals complexes (1:1) of seed molecules with the carrier gas were observed in several rare gases. In all cases, the combination of a dispersive red-shift and a dipole-induced dipole blue-shift produced a total red-shift, up to a maximum of δν˜ ) -21 cm-1 for PT-Xe and δν˜ ) -30 cm-1 for BPT-Xe. Complexes of 1:n (n > 1) were not observable owing to the weakness of the direct S0 f T1 transition. The inductive contributions to the shifts were calculated using values of excited-state dipole moments estimated from solution Stokes shifts; the dipole moments of both molecules in T1 were found to be close to zero, significantly smaller than their ground-state dipole moments. The phosphorescence excitation spectrum of isolated PT is discussed in some detail. The electronic origin of T1 at 16 844 cm-1 is assigned to the transition S0,0 f T1z,0; the transitions S0,0 h T1(xy),0 are too weak to be detected with this method. Vibronic transitions S0,0 f T1z,V in PT were analyzed up to excess energies of ∼hc1450 cm-1.
1. Introduction Van der Waals (vdW) complexes of aromatic molecules with rare gas atoms have been reported by many authors in recent years.1-10 For the most part, the solvent spectral shift (SSS) due to microsolVation has been detected in strongly allowed transitions, where weak sidebands next to the main excitation band of the (unsolvated) molecule are observable. The formation of vdW complexes in an adiabatic expansion is dependent only on the expansion conditions (i.e. the type and pressure of the carrier gas, the nozzle diameter, the distance of the excitation zone from the nozzle, etc.) and is independent of the subsequent excitation process; the same fraction of seed molecules will be microsolvated prior to a forbidden process, as for an allowed one. Since S0 f T1 transitions are generally very weak, detection of an even weaker sideband (by a factor of ∼10-2 or even less) presents obvious experimental challenges. It is not evident a priori whether a measurable solvent spectral shift of the triplet absorption energy of a thione is produced by microsolvation with a rare gas atom; the SSS could lie within the rotational envelope of the unsolvated band. vdW complexes involving aromatic thiones have been reported for the SSS of the strongly allowed S0 f S2 transition of xanthione and 4H-1-benzopyran4-thione (BPT).9-11 Microsolvation shifts of spin-forbidden transitions have never been reported for thione compounds. In this paper an investigation of solvent spectral shifts in the T1 f S0 phosphorescence excitation spectra of jet-cooled BPT and 4H-pyran-4-thione (PT) are presented, using direct S0 f T1 excitation. * Corresponding author. Present address: Department of Physics, National University of Ireland, University College, Cork, Ireland. † Department of Physics. ‡ Department of Chemistry. X Abstract published in AdVance ACS Abstracts, September 1, 1997.
S1089-5639(97)01120-1 CCC: $14.00
In the case of molecules with a permanent dipole moment, the total shift δν˜ consists of two components, a dispersiVe shift δν˜ disp and a dipole-induced dipole shift δν˜ ind:
δν˜ ) δν˜ disp + δν˜ ind
(1)
The magnitudes of these two components are generally calculated using a perturbation model originally developed by Longuet-Higgins and Pople12 to explain spectral shifts in liquid solvents but which has also been applied to vdW complexes:3,9
δν˜ disp )
(
)
3EMiR′M |M0fi|2 -R′Ar-6 ηIMIA + (2a) hc IM + IA - EMi 2(IM + IA) 4π0(IA - EM ) i δν˜ ind ) -
(
)
R′Ar-6 |µi|2 - |µ0|2 2hc 4π0
(2b)
Here, R′ is the polarizability volume13 (i.e. R′ ) R/4π0, where R is the polarizability). The indices M and A refer to the seed molecule and carrier gas atom, respectively. r is the equilibrium separation of the atom and molecule in the ground state. EMi is the energy of the relevant molecular transition. IA and IM are the ionization energies of the atom and molecule, and M0fi is the dipole moment of the electronic transition. µ0 and µi are the permanent dipole moments14 of the molecule in its ground and excited state (i), respectively. η is an empirical factor, of © 1997 American Chemical Society
7736 J. Phys. Chem. A, Vol. 101, No. 42, 1997
Ruth et al.
order unity, which is included to compensate for the inaccuracy involved in using IM and IA as average excitation energies in deriving eq 2a; this issue has been discussed at some length by Kettley et al.3 The dispersive shift δν˜ disp is always negative (red-shift) and depends essentially on the transition moment, M0fi, of the excitation process. The sign of the dipole-induced dipole shift δν˜ ind depends on the change in the permanent dipole moment on excitation. For an allowed transition, δν˜ disp is usually considerably larger4 than δν˜ ind, and the total shift δν˜ is therefore negative. For a forbidden singlet-triplet transition, however, with a much smaller transition moment, this is not necessarily true. In the S0 f T1(n,π*) excitation of thiones one would expect a significant transfer of electron density from the S atom to the CdS bond and consequently a large reduction of the permanent dipole moment. Therefore the dipole-induced dipole shift is likely to make a significant contribution; the relative magnitudes of δν˜ disp and δν˜ ind determine the direction of the resulting SSS. It is convenient to split expression 2a for the dispersive shift into two components, one dependent on the polarizability of the solute molecule, R′M, the other on the transition dipole moment, M0f1:
δν˜ disp ) δν˜ (1) ˜ (2) disp + δν disp
(3)
with
δν˜ (1) disp ) -
|M0fi|2 ηIMIA R′Ar-6 (3a) hc IM + IA - EMi 4π0(IA - EM ) i
δν˜ (2) disp
3EMiR′M ηIMIA R′Ar-6 )hc IM + IA - EMi 2(IM + IA)
(3b)
The relative magnitude, β, is given by
βt
δν˜ (1) disp δν˜ (2) disp
)
|M0fi|2 IA + IM 6π0EMiR′M IA - EMi
(4)
For an allowed transition, β is on the order of unity and both terms are significant.15 In contrast, for singlet-triplet transition -4 of aromatic thiones,16 δν˜ (2) disp dominates and β ≈ 10 . (Relevant atomic and molecular parameters in eqs 3 and 4 are listed in Table 4 for PT, BPT, and the rare gases.) All of the solvation effects treated here are due to complexes of either PT or BPT with noble gas atoms. Complexes of 1:n, with n solvent atoms surrounding a solute molecule, are known for values of n up to 55,17 with correspondingly (but not necessarily linearly) increased values of δν˜ .18 In this paper, however, only the n ) 1 case will be discussed, as higher order complexes were not detected in the phosphorescence excitation spectra. The phosphorescence excitation spectrum of PT will be treated before the discussion of vdW complexes in section 3.
Figure 1. Normalized phosphorescence excitation spectrum of jetcooled PT in the region of the S0 f T1 absorption. The carrier gas is helium at a stagnation pressure of 1000 mbar. Wavelengths and wavenumbers are vacuum-corrected; relative intensities are not corrected with respect to the excitation fluence.
laser beam. The emission was detected with a gated photomultiplier (Philips XP2254/B) in the photon-counting mode. The home-built gate of the PMT has a rise time of J 3 µs and a width of 300 µs. The pulses from the photomultiplier were accumulated in a time window between 3 and 30 µs by a digital photon-counter (SRS-400), which was interfaced to a computer for the final data acquisition. Typical sampling rates were 100200 shots per data point. The synthesis and purification of the zone-refined PT and BPT are described in ref 20. For each of the two compounds different rare gases (He, Ne, Ar, Kr, and Xe) were used in turn as the carrier gas. The distance between the nozzle and the zone of excitation was varied between ∼10 and ∼30 mm so that the laser pulse would intercept the jet at different stages of the expansion. In this way the temperature and density of molecules were varied. The stagnation pressure of the carrier gases was in the range between 0 and 3 bar for He, Ne, and Ar and between 0 and 1 bar for Kr and Xe. The vapor pressure of the seed compounds was varied by adjusting the reservoir temperature between 20 and 35 °C for PT and between 70 and 90 °C for BPT, in order to change the relative proportions of seed molecules and carrier gas atoms in the jet. No clear set of experimental conditions for 1:1 complex formation could be established. For each carrier gas, a different combination of backing pressure and position of the nozzle provided the best conditions for bringing the very weak complex peaks above the noise. Care was taken to distinguish solvation complex features from a difference hot band located 20 cm-1 below the S0,0 f T1z,0 transition in the BPT excitation spectrum (cf. section 3.2 in ref 19), particularly when the cooling conditions were not optimum (e.g. when He was used as the carrier gas at low pressures and the excitation zone was close to the nozzle). In most cases, due to the weakness of the solvation peaks, heavy saturation of the 0-0 peak (using maximum laser pulse energies) was necessary to make complex features detectable.
2. Experiment The experimental setup for the LIP excitation measurements has been described in detail elsewhere (Figure 1 in ref 19). A circular stainless steel nozzle with a diameter of 0.8 mm was used to create the pulsed adiabatic expansion (General Valve Corporation, Iota One nozzle driver). Molecules in the jet were excited with an excimer-pumped pulsed dye laser (Lumonics EX-700, XeCl; Lumonics HD-300; rhodamine B, rhodamine 6G, coumarine 153). The phosphorescence was collected with two lenses at right angles relative to the jet and to the excitation
3. Results and Discussion 3.1. Phosphorescence Excitation Spectrum of Jet-Cooled 4H-Pyran-4-thione. The T1 state of PT has been studied in solution and in rigid matrices by several groups in recent years.21-25 However, the T1 manifold of isolated PT has never been investigated. The phosphorescence excitation spectrum of jet-cooled PT is shown in Figure 1 in the region of the S0 f T1 absorption (535-595 nm). The spectrum is expected to possess many properties similar to that of BPT, which was
Excitation Spectrum of 4H-Pyran-4-thione
J. Phys. Chem. A, Vol. 101, No. 42, 1997 7737
TABLE 1: Comparison of the Fundamental Modes in T1 (Column 3) with Modes and Symmetries from the S0 Infrared Spectrum According to ref 29 (Column 2)a S0,V (IR) label ref 29
[cm-1] sym
origin V27 V22 V13 V26 V10 V21 V25 V9 V12 V24 V8 V11 V23 V7 V20 V6 V5 V19 V18 V17 V4 V16 V3
ref 29 94 b2* 300 b1m 400 a2m 415 b2* 420 a1m 648 b1m 685 b2m 722 a1 801 a2* 814 b2 918 a1 953 a2 963 b2 1018 a1 1021 b1 1168 a1 1226 a1 1232 b1 1301 b1 1399 b1 1426 a1 1548 b1m 1646 a1
T1z,V [cm-1]
V26 (b2) [cm-1]
V25 (b2) [cm-1]
V9 (a1) [cm-1]
V8 (a1) [cm-1]
16 844
17 230
17 508
17 541
17 743
459.6 (w) 385.7 (s) 471.9 (m) 673.0 (m) 663.5 (s) 697.3 (m) 793.8 (w) 815.0 (w) 899.0 (s) 958.4 (m) 995.2 (m) 1105.4 (s) 1110.5 (m) 1202.2 (s) 1212.0 (m) 1263.3 (m) 1337.7 (m) 1412.1 (m) 1416.8 (m) 1549.4 (m) (1719.1) (m)
459.9 (w) 384.8 (m) 470.0 (w) 673.1 (w) 660.6 (m) 698.4 (w)
459.6 (w) 382.8 (m) 473.5 (m)
459.4 (w) 386.8 (w) 471.4 (w)
461.6 (w) 385.3 (m) 474.8 (w)
664.3 (m) 697.1 (w) 794.7 (w) 815.8 (w) 899.5 (m)
663.3 (w)
664.0 (m) 698.9 (m)
900.6 (w)
899.4 (m) 960.0 (m)
898.6 (m) 956.8 (w) 995.4 (w) 1106.4 (m) 1111.1 (m) 1203.0 (w)
996.5 (w) 1107.1 (m)
V7 (a1) [cm-1] 17 950
386.7 (m) 470.1 (w) 672.1 (w) 665.2 (m) 695.7 (m)
1103.8 (m)
1204.6 (m)
1415.4 (m)
a * ) calculated, m ) melt. The most prominent progressions based on the vibronic origins V , V , V , V , and V are given in columns 4-8. 7 8 9 25 26 The difference between wavenumbers of fundamentals and combinations is j 1 cm-1. Band intensities are classified as (s) ) strong, (m) ) medium, or (w) ) weak.
communicated recently.19,26 The electronic origin at 16 844 cm-1 is assigned to the transition S0,0 f T1z,0, since the transitions to the other electronic triplet substrates S0,0 h T1(xy),0 are too weak to be detected with this method. The large zerofield splitting, which is known for PT in an n-pentane Shpolskii matrix (|D*| J 24 or 28 cm-1 depending on the trap site21) could not be investigated. Therefore the properties of the spectrum are entirely based on the triplet substrate T1z. Szymanski et al.27 estimated the energy origin of T1z to be ET1z = hc17100 cm-1 in a perfluoro-1,3-dimethylcyclohexane (PFDMCH) solution, giving a hypsochromic shift of ∆ν˜ hyp(PT) ≈ 256 cm-1. In comparison, the triplet states of BPT in a PFDMCH solution28 and in a supersonic jet expansion19 are separated by ∆ν˜ hyp(BPT) ≈ 262 cm-1. This near-identical solvent blue-shift of the two compounds is probably coincidental. The ObserVability of the S0 f S1 Transition in Jet-Cooled PT. For BPT in a supersonic jet the energy gap between S1,0 and T1z,0 was found to be similar to the value measured in solution as determined by a multiphoton excitation spectrum (∆ν˜ (S1,T1) ≈ 690 cm-1).26 In the single-photon excitation spectrum of BPT only an extremely weak feature was visible at the energy of S1,0.19 This observation was caused by the fact that no optical filtering was used in the experiment and a small fraction of blue S2,V′ f S0,V′′ fluorescence was detected following a resonance-enhanced multiphoton excitation process S0,0 f S1,w′′ f S2,w′.26 If the same behavior occurs in PT, one would expect the origin of S1 to lie ∼600 cm-1 above T1z,0 at ∼17 450 cm-1, since no optical filtering was used in the phosphorescence detection of the excitation spectrum in Figure 1 either. In fact a very weak feature may be observed in Figure 1 at 17 429 cm-1, which is 585 cm-1 above the T1 origin. This line was not assigned as a T1 mode because of its lack of intensity and because no combinations were found that could be based on it. However, assigning the line at 17 429 cm-1 as the S1 origin is very tentative, and further measurements are required to clarify this point. 3.2. Vibrational Assignments of Jet-Cooled PT in T1. The smaller size and the planar C2V symmetry of PT indicate that
its vibronic structure should be considerably simpler than that of BPT and other large aromatic thiones. An analysis of the structure can be attempted, particularly as there is a thorough analysis of the infrared spectrum and an assignment of all ground-state fundamentals.29 The labeling of vibrational modes in the following is taken from this ground-state analysis. PT is expected to possess 27 fundamental modes: 19 in-plane (10 a1 and 9 b2) and 8 out-of-plane (3 a2 and 5 b1) modes. Four of the in-plane modes are C-H stretches with energies around ∼hc3000 cm-1 and are outside the range of the excitation spectrum. Transitions involving a2 modes are symmetryforbidden, although they could gain intensity through mixing with the nearby S1 state of A2 symmetry (despite the low density of S1,V states at low excess energies). Their first overtones are expected to be more intense than their fundamental modes. Noting these limitations the spectrum was analyzed for repeated patterns in the energy separations of bands in comparison with energies of the ground-state vibrations from ref 29. In total 20 fundamental modes are identified in the excitation spectrum. Several of these form vibronic origins of remarkably similar progressions, listed in Table 1. We assume that there are no huge changes in vibrational energies between the S0 and T1 electronic states. PT is a reasonably large molecule and the nπ* electronic transition is most likely to effect vibrations localized in the CdS region, probably to an extent less than ∼15%. There is no evidence for T1 modes corresponding to the two lowest energy S0 modes at 94 and 300 cm-1. Three S0 and T1 modes have energies between 380 and 460 cm-1, well separated from others and therefore are assumed to correspond. One of the T1 modes, at 459 cm-1, is a weak transition in the spectrum with a strong overtone at 919 cm-1 and is therefore probably an a2 mode. One of the corresponding S0 modes at 400 cm-1 is of a2 symmetry, which supports this assignment, even though the energy change is ∼15%; the S0 value is purely calculational, which could account for this large change. The strong mode V26 at 386 cm-1 probably corresponds to a ground-state vibration at 415 cm-1 of b2 symmetry. Another argument for this
7738 J. Phys. Chem. A, Vol. 101, No. 42, 1997
Ruth et al.
TABLE 2: Spectroscopic Data on the Vibronic Transitions S0,0 f T1z,W of PT in an Expansion Using He as Carrier Gasa no.
λ(T1z,V) [nm]
ν˜ (T1z,V) [cm-1]
δν˜ (Vi - 0) [cm-1]
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 25 26 27 28 29 30 31 32 33 34 35 36 36 37 38 39 40
593.68 580.39 577.91 577.50 571.18 570.87 570.08 567.71 566.96 566.28 565.30 564.98 563.60 562.95 562.61 561.72 560.56 558.96 558.57 557.78 557.12 556.96 556.57 556.14 555.53 555.53 555.16 554.13 553.83 553.42 552.26 551.62 551.02 550.30 550.00 549.86 549.31 549.31 548.91 548.68 547.76 547.62
16 844.1 17 229.8 17 303.7 17 316.0 17 507.6 17 517.1 17 541.4 17 614.6 17 637.9 17 659.1 17 689.7 17 699.7 17 743.1 17 763.6 17 774.3 17 802.5 17 839.3 17 890.4 17 902.9 17 928.2 17 949.5 17 954.6 17 967.2 17 981.1 18 000.8 18 000.8 18 012.8 18 046.3 18 056.1 18 069.2 18 107.4 18 128.4 18 148.2 18 171.9 18 181.8 18 186.4 18 204.7 18 204.7 18 217.9 18 225.6 18 256.2 18 260.8
0.0 385.7 459.6 471.9 663.5 673.0 697.3 770.5 793.8 815.0 845.6 855.7 899.0 919.5 930.2 958.4 995.2 1046.3 1058.8 1084.1 1105.4 1110.5 1123.1 1137.0 1156.7 1156.7 1168.7 1202.2 1212.0 1225.1 1263.3 1284.3 1304.1 1327.8 1337.7 1342.4 1360.6 1360.6 1373.8 1381.5 1412.1 1416.8
|∆ν˜ | [cm-1]
