J . Phys. Chem. 1988, 92, 5619-5627 Figures 1 and 2 show the calculated gas-phase infrared spectra of H3P0 and H,PS, respectively. For H3P0, v1 and v4 overlap strongly, and the very weak v3 band is buried under the much stronger v5 band, while v2 and V 6 are well-separated bands. Second-order Coriolis effects are not expected to change the appearance of the overlapping bands in H3P0 too much due to the small coupling between v I and v4 = 0.02) and the low intensity of v3 (see Table V, 15;51= 0.43). In the case of H3PS, all bands overlap pairwise (v1/v4, v2/v5,v3/v6)so that second-order Coriolis effects may be more important, especially for the latter two pairs (1Cl41 = 0.01, lc251= 0.57, = 0.23). When judging the simulated spectra in Figures 1 and 2, one has to keep in mind that our frequency predictions are normally accurate to only about 20 cm-’ (see Table V and associate discussion). In spite of this limited accuracy we hope that these simulated spectra may be a useful guide to experimental gas-phase work. Note Added in Proof. A theoretical study of H 3 P 0of at the RHF/6-31G** level has been p~blished.’~The calculated vi-
(lr141
1c361
(75) Person, W. B.; Kwiatkowski, J. (THEOCHEM) 1987, 157, 237.
s.;Bartlett, R. J. J . Mol. Struct.
5619
brational frequencies, intensities, and force constants are similar to our results. Acknowledgment. This work was supported by the Deutsche Forschungsgemeinschaft through the Sonderforschungsbereich (SFB) 42 and by the Fonds der Chemischen Industrie. The calculations were carried out by using the Perkin-Elmer 3242 computer of SFB 42, the Cyber 175 computer of the Universitat Wuppertal, and the Cray X-MP 48 computer of NASA Ames Research Center. We are grateful to Prof. H. Burger for many discussions, for making the simulation program available, and for recording the Raman spectrum of liquid trimethylphosphine sulfide. Registry NO. H,P, 7803-51-2;H,FQ, 13840-40-9;H3PS, 35280-73-0; F,P, 7783-55-3; F3P0, 13478-20-1;F,PS, 2404-52-6;(CH3),P, 594-09-2; (CH,),PO, 676-96-0; (CH,),PS, 2404-55-9. Supplementary Material Available: Tables listing isotopic frequency shifts, potential energy distributions, centrifugal distortion constants, and Coriolis constants for phosphine oxides and sulfides (4 pages). Ordering information is given on any current masthead page.
Tlme-Resolved Resonance Raman Spectroscopy Applled to the Photochemistry of the Sulfonated Derivatives of 9,lO-Anthraquinone John N. Moore,+ David Phillips,* Davy Faraday Research Laboratory, The Royal Institution, 21 Albemarle Street, London, W1X 4BS UK
and Ronald E. Hester Department of Chemistry, University of York, Heslington, York, YO1 5DD UK (Received: September 4, 1987; In Final Form: March 18, 1988)
Several sulfonated derivatives of 9,lO-anthraquinone have been studied by using both steady-state and time-resolved Raman spectroscopy. Using a two laser pumpprobe technique and varying the probe laser wavelength and chemical conditions, we observed the resonance Raman spectra of five different species following photolysis of anthraquinone-2,6-disulfonate (AQ26DS), these species being the triplet state, the semiquinone radical anion and neutral radical, and two complexes formed by reaction of the triplet with water and termed transients B and C. The assignment of the observed bands of the ground-state parent molecule and the transient species has allowed a comparison of the different C = O and C-C stretching frequencies of these species, thereby yielding information on their structures. Transients B and C have been assigned as water-carbonyl and water-benzenoid ring complexes, respectively. Comparison of the transient spectra observed on photolysis of AQ26DS with those observed on photolysis of the other “strong” sensitizers studied here indicates that these molecules undergo structural changes very similar to those undergone by AQ26DS. By contrast, the transient bands observed on photolysis of the “weak” sensitizer anthraquinone-1,s-disulfonateare different from those observed for AQ26DS, and this may be attributed to their respective AT* and na* TI states, previously assigned by other work.
Introduction The original interest in the photophysics and photochemistry of 9,lO-anthraquinone (AQ) and its derivatives arose because of the possibility of their use as model compounds for the study of the phototendering action of anthraquinonoid vat dyes on cellulosic materials.’+ Several of the simple anthraquinone derivatives were found to be highly reactive on photolysis and could be used as photosensitizers, for example, in reactions driven by solar radiat i ~ n . ~The . ~ prineipal experimental technique used in those studies has been that of the observation of UV-visible absorption spectra following photolysis or radiolysis. Recent studies using this technique,’,* including work from this laboratory,9*10have led to the proposal of a full mechanism for the reactions of sulfonated anthraquinones (AQS) on photolysis. The AQS compounds have Present address: Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104.
0022-3654/88/2092-5619$01 .50/0
previously been classed into two different categories; “strong” and “weak” sensitizers, corresponding to the photochemically reactive and unreactive compounds, respectively. This difference in reactivity has been attributed to the nature of the lowest energy (1) Venkataraman, K. In The Chemistry of Synthetic Dyes; Ed. K. Venkataraman, K., Ed.; Academic: New York, 1952; Vol. 11. (2) Bolland, J. L.; Cooper,H. R. Proc. I?. Soc. London,A 1954, 225,405. (3) Bridge, N. K.; Porter, G. Proc. R. Soc. London, A 1958, 244, 259. (4) Evans, N. A.; Stapleton, W. In The Chemistry of Synthetic Dyes; Venkataraman, K., Ed.; Academic: New York, 1978; Vol. 111. (5) Scharf, H. D.; Weitz, R. Tetrahedron 1979, 35, 2255. (6) Okura, I.; Kim-Thuan, N. Chem. Lett. 1980, 1569. (7) Loeff, I.; Treinin, A.; Linschitz, H.J . Phys. Chem. 1983, 87, 2536. (8) Loeff, I.; Treinin, A.; Linschitz, H. J. Phys. Chem. 1984, 88, 4931. (9) Moore, J. N.; Phillips, D.; Nakashima, N.; Yoshihara, K. J . Chem. SOC.,Faraday Trans. 2 1986, 82, 745. (10) Moore, J. N.; Phillips, D.; Nakashima, N.; Yoshihara, K. J . Chem. SOC.,Faraday Trans. 2 1987,83, 1487.
0 1988 American Chemical Society
Moore et al.
5620 The Journal of Physical Chemistry, Vol. 92, No. 20, 1988 (AQS----H,O)C++
'AQS
f 2
AOS
( 3 A Q S - . - - H,O) B b -.
&nor
a - AQSOH+ p - A Q S O H
AQS
H*
A Q S ~e AQSH'
I
t
O2
AQS
Figure 1. Reaction scheme for AQS photochemistry.
triplet states, these being nr* for the strong sensitizers and ?rr* for the weak sensitizers.1° All reported photochemistry proceeds via 3AQS, populated by intersystem crossing from the short-lived excited singlet state, and the features of the mechanism that are relevant to the studies reported here are given in Figure 1. The strong sensitizers undergo all the reactions given in Figure 1, while the weak sensitizers undergo neither of the reactions of 3AQS with water and react with electron donors to produce the semiquinone species in relatively low yield. The transient absorption studies have established the mechanistic details of AQS photochemistry and the electronic absorption spectra of the various species involved. However, little is known of the structural changes occurring in the molecules during reaction, and, in particular, nothing is revealed of the structures of the two triplet-water complexes, transients B and C. This information is of interest because the formation of B and C constitutes undesirable side reactions in any potential application of AQS compounds as sensitizers, especially transient C, since this results in the production of hydroxylated derivatives (AQSOH) that are photochemically unreactive. Time-resolved resonance Raman (TR3) spectroscopy is a technique that has been used to study the vibrational spectroscopy of a variety of transient species."J2 This method may be particularly useful for the study of a system containing a mixture of species with overlapping absorption spectra since the resonance effect may allow the selective enhancement of Raman bands of only one species, by appropriate choice of Raman probe wavelength. The studies reported here use TR3 spectroscopy in an attempt to elucidate the structural changes occurring in the AQS compounds following photolysis. Ground-state AQS spectra and transient spectra observed on photolysis under a variety of experimental conditions are reported for several AQS compounds. The observations of the strong sensitizer anthraquinone-2,6-disulfonate, which was studied in most detail in the transient absorption work,g are described in most detail here, and the results on the other AQS compounds compared with these. Preliminary reports of this work have been given e l ~ e w h e r e . ' ~ JIt~ is hoped that the detailed results of the application of the TR3 technique to anthraquinone photochemistry, perhaps of limited general interest, will nevertheless serve to illustrate the potential of the TR3 technique.
TABLE I: Experimentally Observed Raman Bands of Ground-State Sulfonated Anthraquinone Molecules under Conditions of Nonresonance" wavenumber/cm-' assignAQ2S
AQ26DS
AQZMS
AQl5DS
AQlB2MS
mentb
1668 (10)
1669 (8)
1667 (IO)
1686 (10)
1665 (10)
1597 ( 5 )
1596 (4) 1481 1420 1281 (1)
1607 (5)
1582 (4) 1462 1434 1313 (1)
1587 (3)
aE,C=O SYm a8
1274 (1) 1207 (2) 1187 (7)
ag
1297 (1) 1177 (8) 1162 (2) 1085 (2) 1039 (3) 961 (1)
1177 (10)
1263 (1) 1211 (3) 1180 (4)
1096 (2) 1038 (1) 957 (3)
1108 1043 (2) 989 (1)
820 717 687 (1)
738 (1) 688 (1)
776 (1) 726 692 (1)
1183 (6) 1149 (1) 1074 (6) 1041 (3) 917 (1) 804 (1) 761
1119 (1) 1039 (2) 991 (2) 773 (1) 589 (1)
492 (3) 452 (2)
465 (4)
484 (4)
515 (2) 499 (2) 472 456 408 (1)
b,, aB
488 (2)
a8 a,
4Probe laser wavelength at 514.5 nm. Numbers in parentheses indicate relative intensity; where no value is given, the signal is very weak. bsym = symmetric.
combined collinearly and focused below a flowing sample solution, the scattering collected at 90" to the beams and analyzed by using a triple spectrometer (Spex Triplemate). A gated, intensified diode array (Princeton Applied Research 1420/1218/1211/OMA-2) was used as the detector. The timing of the laser pulses and the detector gate was achieved by using pulse and delay generators such that each could be varied independently, and the repetition rate was set at 10 Hz. Typically, 2400 pumpprobe pulse pairs were used to obtain each time-resolved spectrum. The spectral and temporal accuracies of the system were zt6 cm-I and f 1 0 ns, respectively. Raman spectra of the parent AQS molecules were obtained by using continuous wave (CW) lasers at wavelengths of 514.5 (argon laser) and 356.4 nm (krypton laser), and conventional scanning spectrometer/photomultiplier tube detection. The spectral accuracy was f 2 cm-'. The sodium salts of anthraquinone-2-sulfonate (AQZS), anthraquinone-2,6-disulfonate(AQ26DS), and anthraquinone- 1,5disulfonate (AQ15DS) (Aldrich) were recrystallized three times from water. Anthraquinone-2-methylsulfonate(AQ2MS) and anthraquinone- 1-bromo-2-methylsulfonate(AQ1B2MS) were used as obtained from Professor H. D. Scharf.ls These AQS compounds were studied at a concentration of 5 X M in aqueous solution and, where present, sodium nitrite at a concentration of 0.1 M. Sodium hydroxide and sulfuric acid were used to achieve pH adjustments, and the samples were oxygen-saturated to avoid any possible accumulation of intermediates.
Experimental Section The experimental procedures were as described p r e v i o ~ s l y . ' ~ , ~ ~ Results An excimer laser was used as the photolysis source (Lambda Ground-State Spectra. The ground-state spectra of the parent Physik EMG201 or EMGl50) at 351 nm (XeF; 15-11s pulse width AQS molecules were initially recorded by using CW probe lasers fwhm, 30 mJ/pulse) or 337 nm (N2;8-11s pulse width fwhm, 8 and single-channel detection. The normal Raman spectra are mJ/pulse). Raman spectra were obtained by using an excimershown in Table I. Resonance Raman spectra, obtained in respumped dye laser (Lambda Physik EMGlOl and FL2002) as the onance with the TP* electronic absorption band centered at 330 probe laser in the range 370-580 nm ( 8 4 s pulse width fwhm, nm,s were given in Table 2 of ref 14. The assignments given will 0.5-10 mJ/pulse, line width >1250 cm-I. Hence, the bands at 1277 cm-l and lower wavenumber values are probably assignable to C-C stretches, and the C = O stretch to one of the bands observed at 1608, 1570, or 1440 cm-', the others again probably being assignable to C-C stretches since these are expected to be the only other vibrations observed in the region 1200-1700 crn-'. That there are several bands assignable to C-C stretches in the 3AQ15DS spectrum is not surprising since the change in the molecule on going from the ground state to this ar* state is mostly associated with the aromatic ring system. In the case of AQlBZMS, no TR3 spectra were observed on photolysis that could be identified as different from the groundstate spectrum of this compound. This may be ascribed to the very weak transient absorption observed for 3AQ1BZMS, which was assigned to a low triplet quantum yield.1° The lack of any observable TR3 spectrum is therefore considered to be due to a triplet yield too low for the observation of triplet-state resonance Raman scattering with the experimental arrangement used here. (75) Turro, N. J. Modern Molecular Photochemistry; Benjamin/Cummings, 1978.
The Journal of Physical Chemistry, Vol. 92, No. 20, 1988 5627
Conclusion Time-resolved Raman spectra of all the transient species observed by transient absorption spectroscopy have been obtained for the AQ26DS compound. The spectra assignable to the different species have been obtained by variation of the chemical conditions, probe laser wavelength, and time delays at which the TR3 spectra are observed. The depolarization ratios of the bands have also been obtained. The observed bands have been assigned to particular modes, and their frequencies compared with those of the corresponding vibrations in the ground state of the parent molecule and the other transients. From the assignments and comparisons, the structural changes occurring in the molecule on going to each of the transients has been determined. The two triplet-water complexes, transients B and C, have been confirmed to be strong charge-transfer complexes, with significant charge transferred to the anthraquinone from the water in the exciplexes, by comparison of their Raman spectra with that of the radical anion. Additionally, the absorption of a photon by these transients has been found to result in their ionization. The structure of B has been assigned as a water-carbonyl adduct, and that of C as a water-benzenoid ring adduct, leading to the hydroxylated products. Both complexes are formed by nucleophilic attack of the aqueous oxygen on the triplet state. The TR3 spectra of the transients of the other strong sensitizers investigated here have been found to be similar to those of AQ26DS, and this confirms their similar structures. A comparison of the triplet state TR3 spectra of AQ26DS and the weak sensitizer AQl5DS showed that the differences between their spectra were greater than those between all the triplet-state spectra of the strong sensitizers. This supports the previous assignment of the T1 states of the strong and weak sensitizers as na* and AX* states, respectively. The results obtained here may be useful in the consideration of the use of AQS molecules as photosensitizers. The principal disadvantage to their use is the formation of hydroxylated species, which are not sensitizers, as permanent products on irradiation. Since the mechanisms for these processes are knowng and from the results given here the structures of the transients leading to these products are also known, it may be possible to use AQS molecules at a particular pH range and/or with appropriate sites blocked to minimize the occurrence of these side reactions. However, it should be noted that further substitutions onto the side rings may result in a reordering of the triplet states, with the possibility of T1 becoming an inactive ra* state. Acknowledgment. We thank Professor G. H. Atkinson and Dr. P. M. Killough for their close involvement in the preliminary stages of this work. The experiments were mainly carried out at the Laser Support Facility of the Rutherford Appleton Laboratory, and we thank the staff for their assistance and SERC for provision of facilities and financial support.
