5252
J. Phys. Chem. 1996, 100, 5252-5260
Resonance Raman, Surface-Enhanced Resonance Raman, Infrared, and ab Initio Vibrational Spectroscopic Study of Tetraazaannulenes Stephen Bell,† Joe A. Crayston,*,‡ Trevor J. Dines,*,† and Saira B. Ellahi† Department of Chemistry, The UniVersity, Dundee, Scotland, DD1 4HN, and School of Chemistry, UniVersity of St. Andrews, Fife, Scotland, KY16 9ST ReceiVed: October 13, 1995; In Final Form: January 3, 1996X
The IR and resonance Raman spectra of dibenzo[b,i][1,4,8,11]tetraaza[14]annulene (TAA) and 5,7,12,14tetramethyldibenzo[b,i][1,4,8,11]tetraaza[14]annulene (TMTAA) have been measured and compared to ab initio calculations of the vibrational wavenumbers, using the 3-21G basis set. An excellent fit is found between the experimental and calculated data, enabling precise vibrational assignments to be made. For the CC and CN stretching vibrations there is substantial separation between the vibrational motion of the diimine and benzene ring parts of the macrocycles. Surface-enhanced Raman spectra were obtained following adsorption on Ag electrodes, with potentials in the range -0.1 to -0.6 V Vs Ag/AgCl. The data indicate that both macrocycles adopt either a perpendicular edge-on or tilted orientation on the Ag electrode surface.
Introduction Naturally occurring macrocycles such as the porphyrins and phthalocyanines, and their metal complexes, have been extensively studied by Raman spectroscopy since the discovery of the resonance Raman (RR) effect.1,2 The RR technique provides both the sensitivity and the selectivity required for the study of the active chromophoric centers in biological systems at physiological, i.e., low, concentrations. Synthetic macrocycles are widely used as models although these studies have focused mainly on porphyrin and phthalocyanine complexes. Relatively little attention has been paid to the vibrational analysis of macrocycles such as dibenzo[b,i][1,4,8,11]tetraaza[14]annulene (TAA) and its metal complexes despite its simple, symmetrical structure (C2h geometry, Figure 1). A RR study of some metal complexes of 5,7,12,14-tetramethyldibenzo[b,i][1,4,8,11]tetraaza[14]annulene (TMTAA) was reported in 19763,4 and more recently a spectroelectrochemical study of [CoII(TAA)].5 Tetraazaannulene ligands are able to complex to a wide variety of metals; in particular, TMTAA binds to first- and second-row transition metals as well as to group 14 elements (Si, Sn).6 This allows for a systematic study of the effect of metal electronic configuration, charge and chemical parameters upon the structure, and properties of the macrocyclic ligand. A recent study of the electronic structure of TAA and [NiII(TAA)] has provided information regarding the electronic transitions in these systems.7 Like porphyrins, TAA ligands form tetradentate N4-coordinated metal complexes and their electronic spectra are dominated by in-plane π-π* transitions. With some important differences, the electronic absorption spectra bear some resemblance to those of porphyrins although the π system is localized in the phenyl and diimine regions of the molecule. This study and a subsequent one8 also highlighted the effect of π metal-macrocycle interactions being responsible for a substantial red-shift of the Soret and Q bands on complexation, which has not been observed for metalloporphyrin complexes. Furthermore, the Soret band is in the near-UV (unlike porphyrins that exhibit Soret bands in the visible) so that RR effects due * Authors for correspondence. † The University, Dundee. ‡ University of St. Andrews. X Abstract published in AdVance ACS Abstracts, March 1, 1996.
0022-3654/96/20100-5252$12.00/0
Figure 1. Structures of TAA and TMTAA.
to metal-centered vibrations are separable from those due to the ligand, facilitating the interpretation of metal effects. Interest in transition metal TAA complexes has focused upon their redox properties,9 which are similar to those of porphyrins. In fact ligand oxidation is a little easier in the TAA systems, rendering these complexes more useful than porphyrins in electrochemical reductions. The [CoII(TAA)] complex is even more active for O2 reduction than the porphyrin or phthalocyanine complexes, catalyzing the reduction of O2 to H2O2 at potentials 400 mV more positive than that of [CoIIPc].10 In common with porphyrins, [CoTAA] complexes bind reversibly with O2 in the presence of base. On account of their redox properties, TAA complexes have gained importance as electrocatalysts and are used in chemically modified electrodes. Surface-enhanced Raman scattering (SERS) has emerged as a powerful technique for studying species adsorbed on working electrodes. SERS occurs when molecules are adsorbed on certain metal surfaces, where Raman intensity enhancements of ca. 105-106 have been observed.11,12 The enhancement is primarily due to plasmon excitation at the metal surface. Thus, the effect is limited to copper, silver, gold, and a few other metals and semiconductors for which surface plasmons are excited by visible radiation. Chemisorption is not a necessary requirement for observation of SERS, but when it does occur there may be further enhancement of the Raman signal, since the formation of new chemical bonds and the consequent perturbation of adsorbate electronic energy levels can lead to a surface-induced RR effect. The combination of surface- and resonance-enhancement (SERRS) may occur when adsorbates have intense electronic absorption bands in the same spectral region as the metal surface plasmon resonance, yielding a total enhancement as large as 1010-1012. The sensitivity of SERRS © 1996 American Chemical Society
Tetraazaannulenes far exceeds that of any other technique that is capable of providing structural information. SERS and SERRS are extremely sensitive techniques for investigations of electrochemical reactions, in particular adsorption processes at modified electrodes. As part of a detailed study of the spectroelectrochemistry of TAA ligands and their metal complexes we have investigated the RR, SERRS, and FTIR spectra of the parent TAA ligand in order to (a) determine precise vibrational band assignments, (b) probe the nature of the resonant electronic transition via RR band intensities, and (c) elucidate the orientation of TAA adsorbed on a silver electrode. The high symmetry of TAA facilitates normal coordinate analysis and ab initio calculations, leading to precise vibrational assignments that are essential for interpretation of the RR and SERRS data. The only previous study of normal coordinate analysis on TAA complexes was reported in a RR study by Woodruff et al.3 on the saddle-shaped C2V TMTAA complexes in 1976. More recently, the RR and SERRS spectra of [CoII(TAA)] have been reported,5 although detailed band assignments were not given. In this paper we report the results from an FTIR, RR, and SERRS spectroscopic study of TAA and TMTAA, together with ab initio calculations of the vibrational band wavenumbers and potential energy distribution. Comparison of the calculations to the experimental data enabled precise vibrational assignments to be made. Experimental Section TAA and TMTAA were synthesized using literature methods. TAA was prepared by condensation of 1,2-diaminobenzene with propynal,13 and TMTAA was obtained by demetallation of the nickel(II) complex prepared by a template method.14 Both compounds were purified by recrystallization from N,N-dimethylformamide. X-ray powder diffraction patterns on TAA were run using a Sto¨e diffractometer. For IR, UV-visible, and RR measurements solid disks were prepared by grinding a mixture of TAA with KCl (ca. 1:100 mass) and subjecting this to a pressure of 24 MN m-2. For Raman measurements these disks were mounted on a spinning sample holder and rotated at ca. 2000 rpm. to minimize localized heating. Solution Raman measurements were obtained from samples of concentrations 10-4 to 10-3 mol dm-3 contained in capillary tubes. SERRS measurements were made using a home-made electrochemical cell with optically clear flat glass windows. The cell was fitted with a silver disk working electrode, an Ag/AgCl reference electrode, and a Pt wire counterelectrode. The potential was controlled by a µAutolab potentiostat with General Purpose Electrochemical System software. Before electrochemical roughening (ca. 10 oxidation-reduction cycles through -0.6 to +0.3 V), the silver electrode was polished with a series of diamond pastes of increasingly smaller grain size (coarse, medium, and fine). Solutions of TAA or TMTAA in either CHCl3 or tetrahydrofuran (ca. 6 µL of a solution containing 80 mg cm-3) were droplet-evaporated onto the roughened electrode surface. Raman spectra were recorded on a Spex 1403 spectrometer controlled by a DM1B data station, using argon ion laser excitation in the range 457.9-528.7 nm (Coherent Radiation Innova 90-6). Detection was by photon counting using a Hamamatsu R928 photomultiplier. All spectra were recorded with a spectral slit width in the range 2-3 cm-1 and corrected for the spectral sensitivity of the spectrometer. The spectrometer calibration was checked by reference to the neon emission spectrum. Depolarization measurements for solution samples were obtained by analysis of the scattered radiation with a Polaroid linear polarizer.
J. Phys. Chem., Vol. 100, No. 13, 1996 5253 IR spectra were recorded on a Nicolet 205 FTIR spectrometer, and UV-visible absorption spectra were measured on a PerkinElmer Lambda 16 spectrophotometer. Results and Discussion Ab Initio Calculations. Crystallographic studies have established a planar geometry for TAA although two different crystal modifications (polymorphs) have been identified.15,16 X-ray powder diffraction patterns on our own samples were consistent with the polymorph reported by Azuma et al.16 There are 108 vibrational modes, and although the molecular symmetry is strictly Ci, it is treated as C2h. It is appropriate to treat a molecule with a low barrier of inversion as having the symmetry of the saddle point (top of inversion barrier), since the vibrational wavefunctions must be specified relative to the symmetry of the higher point group. The vibrations of TAA are classified as follows:
Γ3N-6 ) 37Ag(R, ip) + 17Bg(R, op) + 17Au(IR, op) + 37Bu(IR, ip) where ip and op represent in-plane and out-of-plane, respectively. Although there are additional vibrations in TMTAA resulting from the substituent methyl groups, these were not observed in the spectra and are not therefore included in the ab initio calculations. In TMTAA steric hindrance between the methyl groups and benzene ring hydrogen atoms causes a buckling of the molecule, resulting in a saddle shape with approximately C2V symmetry.17 Semiempirical calculations of the vibrational spectrum of TAA were carried out using both the MNDO and AM1 methods, but these gave a very poor fit to the experimental data, yielding four negative frequencies and very high positive frequencies for the optimized planar C2h structure. Ab initio calculations were then performed using the Gaussian 92 program18 at the Hartree-Fock level with the STO-3G basis, but these yielded two negative frequencies for a planar-optimized structure. Further calculations using the 3-21G basis set gave only one very low, negative frequency for the planar structure; this corresponds well to there being a very low barrier to a single inversion mode for inversion between two nonplanar Ci conformations. The vibrational wavenumbers for TAA were then calculated at the 3-21G optimized geometry. It is usual to scale wavenumbers at the HF level19 using factors such as 0.9 for stretching force constants, 0.8 for bending force constants, and 1.0 for torsions.20 Scaling was done using the normal coordinate analysis programs derived from those of Schachtschneider.21,22 CH stretching wavenumbers are often still a little high with a scale factor of 0.9, and for CH stretching force constants alone a scale factor of 0.83 is suitable.20 However, for TAA, for heavy atom stretches a more severe scaling of 0.8 has been necessary, which may be a consequence of the ring structure of annulenes. The scale factors used in these calculations were as follows: (a) 0.83 for N-H and C-H stretches and (b) 0.8 for CC and CN stretches and all deformations. The unscaled and scaled vibrational wavenumbers are listed in Table 1, together with the potential energy distributions, determined using the scaled force field. The atom numbers used in the potential energy distribution are shown in Figure 2. Only symmetry coordinates contributing g10% to the potential energy distribution have been included in the list. Examination of the scaled wavenumbers and their assignments reveals that the CH stretching vibrations of the diimine and benzene ring chromophores are substantially mixed. Nevertheless, it is found that
5254 J. Phys. Chem., Vol. 100, No. 13, 1996
Bell et al.
TABLE 1: Ab Initio Calculations of the Vibrations of TAA and Their Assignments
Ag
Bg
Au
mode no.
ν (cm-1 unscaled)
ν (cm-1 scaled)
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 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42
3812 3412 3398 3392 3377 3372 3359 3258 1846 1781 1754 1689 1687 1645 1630 1562 1554 1467 1419 1372 1342 1314 1291 1266 1208 1165 1085 1008 941 779 709 657 587 355 349 201 151 1181 1171 1164 1129 1041
3473 3109 3095 3090 3077 3072 3060 2968 1651 1593 1569 1510 1509 1472 1458 1397 1390 1312 1269 1227 1201 1175 1154 1132 1080 1042 970 901 842 697 634 588 525 317 312 179 135 1056 1047 1041 1010 931
43 44 45 46 47 48 49 50
946 896 878 851 656 571 518 383
846 802 785 761 587 510 463 343
51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
249 229 113 41 1181 1171 1165 1131 1062 1028 899 880 864 659 601
222 205 101 37 1056 1047 1042 1011 950 920 804 788 773 590 538
66 67
498 361
445 322
68 69
324 196
290 175
assignmentsa νs(NH) [100%] νs(C4-H4) [50%], νs(C2-H2) [38%] νs(C7-H7) [72%], νs(C6-H6) [18%] 20a νs(C4-H4) [30%], νs(C2-H2) [16%], νs(C1-H1) [41%] νs(C7-H7) [15%], νs(C6-H6) [30%], νs(C5-H5) [30%], νs(C1-H1) [19%] νs(C6-H6) [16%], νs(C4-H4) [13%], νs(C2-H2) [31%], νs(C1-H1) [35%] νs(C6-H6) [34%], νs(C5-H5) [56%] 2 νs(C9-H8) [98%] νs(C9-N2) [41%], νs(C1-C2) [16%] νs(C6-C7) [16%], νs(C3-C4) [17%] 8b νs(C5-C6) [22%], νs(C4-C5) [12%], νs(C3-C8) [15%] 8a Fs(C3-C4-H4) [20%] νs(C9-N2) [15%], νs(C1-C2) [13%] νs(C9-C10) [10%], Fs(H1-C1-C2) [38%] Fs(C5-C6-H6) [28%], Fs(C4-C5-H5) [26%] 19a/19b νs(C9-N2) [21%], Fs(C10-C9-H8) [54%] δip(NH) [42%] νs(C3-C4) [10%], Fs(C8-C7-H7) [20%], Fs(C3-C4-H4) [24%] 14 νs(C2-N1) [11%], νs(C1-C2) [18%], Fs(C1-C2-H2) [26%], Fs(C2-C1-H1) [22%] νs(C3-N1) [28%], νs(C7-C8) [16%] νs(C3-C8) [12%], Fs(C5-C6-H6) [30%] νs(C8-N2) [33%], νs(C2-N1) [15%] νs(C2-N1) [12%], νs(C6-C7) [15%] νs(C7-C8) [10%], νs(C6-C7) [16%], νs(C5-C6) [15%], νs(C4-C5) [26%], νs(C3-C8) [11%] 1 νs(C2-N1) [11%] νs(C6-C7) [10%], νs(C5-C6) [43%], νs(C4-C5) [10%] 12 νs(C9-C10) [44%], δs(C8-N2-C9) [11%] νs(C3-C4) [10%], νs(C9-C10) [11%] νs(C8-N2) [15%], νs(C3-N1) [14%], δip(C6-H6) [20%] νs(C8-N2) [10%], νs(C3-C8) [13%] δs(C8-N2-C9) [15%] δip(C4-H4) [15%] F(N2-C8-C7) [22%] νs(C8-N2) [10%], νs(C9-C10) [10%], δs(C8-N2) [20%] νs(C3-N1) [12%] δip(C9-H8) [24%], δs(C8-N2-C9) [22%] δip(C1-H1) [26%], F(N2-C8-C7) [22%], F(N1-C3-C4) [24%] ωs(C6-H6) [40%], ωs(C5-H5) [43%], ωas(C4-H4) [15%] 5 ωs(C1-H1) [11%], ωas(C9-H8) [60%], τs(N2-C8) [15%], τs(C9-C10) [10%] ωs(C2-H2) [88%], ωs(C1-H1) [20%] ωs(C7-H7) [48%], ωs(C6-H6) [11%], ωs(C5-H5) [21%], ωs(C4-H4) [30%] 10a ωs(C7-H7) [18%], ωs(C6-H6) [14%], ωs(C5-H5) [10%], ωs(C4-H4) [28%], ωs(N1-C3-C8) [13%] 10b ωs(NH) [66%] ωs(NH) [12%], ωs(C5-H5) [11%], ωs(C4-H4) [16%], ωs(N2-C8-C3) [17%] ωs(C7-H7) [15%], ωs(C6-H6) [27%], ωs(C5-H5) [17%] 11 ωs(NH) [11%], ωs(C2-H2) [13%], ωs(C1-H1) [60%] ωs(N2-C8-C3) [11%], τs(C6-C7) [19%], τs(C5-C6) [38%], τs(C4-C5) [22%] 4 ωs(N1-C3-C8) [16%], τs(N2-C8) [13%], τs(C3-N1) [13%], τs(C6-C7) [11%] ωas(C9-H8) [12%], ωs(N2-C8-C3) [14%], τs(C1-C2) [62%] τs(N2-C8) [10%], τs(C6-C7) [15%], τs(C4-C5) [19%], τs(C3-C4) [12%], τs(C7-C8) [21%] 16a/16b τs(N2-C9) [19%], τs(C2-N1) [12%], τs(C3-C4) [16%], τs(C3-C8) [26%] ωas(C9-H8) [12%], τs(C9-C10) [52%], τs(C1-C2) [21%], τs(C3-N1) [10%] ωs(N2-C8-C3) [18%], τs(N2-C9) [21%], τs(C2-N1) [42%], τs(C3-N1) [11%] τs(N2-C8) [49%], τs(C3-N1) [38%] ωas(C7-H7) [19%], ωas(C6-H6) [40%], ωas(C5-H5) [44%], ωas(C4-H4) [15%] 5 ωs(C9-H8) [64%], τas(N2-C8) [21%] ωas(C2-H2) [82%], ωas(C1-H1) [31%] ωas(C7-H7) [48%], ωas(C6-H6) [11%], ωas(C5-H5) [19%], ωas(C4-H4) [30%] 10a ωas(NH) [74%], ωas(N1-C3-C8) [11%] ωas(NH) [27%], ωas(C7-H7) [13%], ωas(C6-H6) [17%], ωas(C4-H4) [28%] 10b ωas(C4-H4) [14%], ωas(N1-C3-C8) [17%] ωas(C7-H7) [12%], ωas(C6-H6) [36%], ωas(C5-H5) [29%], ωas(C4-H4) [12%] 11 ωas(C2-H2) [14%], ωas(C1-H1) [72%] ωas(N2-C8-C3) [17%], τas(C6-C7) [18%], τas(C5-C6) [34%], τas(C4-C5) [22%] 4 ωas(N1-C3-C8) [22%], τas(C9-C10) [12%], τas(C1-C2) [25%], τas(C2-N1) [11%], τas(C6-C7) [10%] ωas(N2-C8-C3) [17%], τas(C9-C10) [36%], τas(C4-C5) [11%], τas(C7-C8) [11%] ωas(N2-C8-C3) [10%], ωas(N1-C3-C8) [10%], τas(C6-C7) [15%], τas(C4-C5) [16%], τas(C3-C4) [16%], τas(C7-C8) [15%] 16a/16b τas(C9-C10) [38%], τas(C3-N1) [10%], τas(C3-C8) [20%] τas(N2-C9) [33%], τas(C9-C10) [11%], τas(C2-N1) [21%]
Tetraazaannulenes
J. Phys. Chem., Vol. 100, No. 13, 1996 5255
TABLE 1 (Continued)
Bu
mode no.
ν (cm-1 unscaled)
ν (cm-1 scaled)
assignmentsa
70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108
97 75 -26 3736 3412 3398 3391 3378 3371 3359 3258 1844 1784 1769 1755 1696 1632 1628 1586 1551 1470 1448 1392 1344 1325 1261 1255 1224 1161 1099 1045 887 850 696 616 528 421 413 195
87 67 -23 3404 3108 3095 3089 3077 3072 3060 2968 1649 1595 1583 1570 1517 1460 1456 1418 1387 1315 1295 1246 1202 1185 1128 1122 1095 1039 983 935 793 760 622 551 471 377 369 175
τas(N2-C9) [13%], τas(C9-C10) [12%], τas(C2-N1) [52%] τas(N2-C8) [20%], τas(C9-C10) [11%], τas(C2-N1) [21%] τas(N2-C8) [-63%], τas(C3-N1) [-34%] νas(NH) [100%] νas(C4-H4) [52%], νas(C2-H2) [37%] νas(C7-H7) [65%], νas(C6-H6) [17%] 20a νas(C7-H7) [15%], νas(C4-H4) [28%], νas(C2-H2) [15%], νas(C1-H1) [37%] νas(C7-H7) [11%], νas(C6-H6) [30%], νas(C5-H5) [30%], νas(C1-H1) [19%] νas(C6-H6) [17%], νas(C4-H4) [13%], νas(C2-H2) [33%], νas(C1-H1) [31%] νas(C6-H6) [34%], νas(C5-H5) [57%] 2 νas(C9-H8) [98%] νas(C9-N2) [29%], νas(C1-C2) [22%] νas(C9-N2) [30%], Fas(C3-N1-H3) [30%] νas(C7-C8) [12%], νas(C6-C7) [17%], νas(C4-C5) [14%], νas(C3-C4) [15%] 8b νas(C5-C6) [23%], νas(C3-C8) [15%] 8a Fas(C3-C4-H4) [20%] Fas(C5-C6-H6) [30%], Fas(C4-C5-H4) [24%] 19a/19b Fas(H1-C1-H2) [40%] νas(C9-N2) [12%], Fas(C3-N1-H3) [24%] Fas(C10-C9-H9) [50%] νas(C3-N1) [12%], νas(C3-C4) [12%], Fas(C3-C4-H4) [30%] νas(C2-N1) [27%], νas(C1-C2) [12%], Fas(C1-C2-H2) [38%] νas(C3-N1) [25%], νas(C7-C8) [17%] νas(C3-C8) [12%], Fas(C5-C6-H6) [24%], Fas(C4-C5-H5) [28%] νas(C8-N2) [18%], νas(C6-C7) [19%] νas(C8-N2) [10%], νas(C2-N1) [21%], νas(C7-C8) [12%] νas(C5-C6) [14%], νas(C4-C5) [31%], νas(C3-C8) [12%] 1 νas(C6-C7) [18%] νas(C6-C7) [13%], νas(C5-C6) [42%], νas(C4-C5) [11%] 12 νas(C9-C10) [42%], Fas(C8-N2-C9) [10%] δip(C1-H1) [17%] νas(C8-N2) [10%], νas(C3-C4) [15%], νas(C3-C8) [10%] νas(C8-N2) [11%], δip(C5-H5) [20%] δip(C7-H7) [26%], δip(C4-H4) [24%] Fas(C8-N2-C9) [13%] δip(C8-N2) [15%], δip(C3-N1) [15%] νas(C8-N2) [11%], Fas(C8-N2-C9) [20%] Fas(C8-N2-C9) [11%] δip(N1-H3) [22%], Fas(C8-N2-C9) [25%]
a Potential energy distributions refer to the scaled force field. ν ) stretch, δ ) deformation, F ) rock, ω ) wag, τ ) twist, s ) symmetric, as ) asymmetric, ip ) in-plane, and op ) out-of-plane. The numbers in bold type refer to the Wilson mode numbers for the benzene ring vibrations.
Figure 2. Atom numbering scheme for the vibrational mode descriptions of TAA.
many of the CC and CN stretches and deformations and the CH deformations can be clearly identified with either the diimine or benzene ring parts of the molecule. In the region below 1700 cm-1 there is less mixing of the symmetry coordinates. In fact many of the vibrations associated with the benzene rings are within the ranges expected for ortho-substituted benzene;23,24 these are labeled with their Wilson mode numbers. Electronic Spectra. The electronic absorption spectra of TAA and TMTAA in THF solutions and in the solid state are presented in Figures 3 and 4, and the band positions and assignments are listed in Table 2. Both macrocycles display an intense Soret band in the near-UV and a less intense Q band in the visible region. In TAA alone these bands appear as doublets, as is often the case in porphyrin spectra. However,
whereas in porphyrins such splitting is attributed to vibronic structure,25 this is not believed to be the case for TAA. The separations between the first pair of bands in the visible region (1380 cm-1) and the second pair in the near-UV (1620 cm-1) are not constant and vary markedly upon complexation. Furthermore, unlike porphyrins, several electric-dipole-allowed transitions are predicted. For these reasons the four bands are identified with four individual electronic transitions,7 all of which are polarized in the molecular plane. The solid state spectra for both TAA and TMTAA are redshifted with respect to the solution spectra. It is clear that in each case the resonant electronic transition, with visible laser excitation, is the lowest energy electric-dipole-allowed transition 1B r 1A (6b f 7a ). There are lower energy forbidden u g g u transitions (of the g-g type), but these are not expected to make any significant contribution to the RR band intensities. IR Spectra. The IR spectra of TAA and TMTAA are shown in Figure 5, and the band wavenumber positions and their assignments are listed in Table 3. The asymmetric NH stretch is clearly identified with a weak, broad band in the 3400 cm-1 region, in excellent agreement with the calculated position. The broadness of this band in both spectra is a consequence of intramolecular hydrogen bonding.16 CH stretching vibrations in both TAA and TMTAA give rise to weak bands, and only two of these could be definitively assigned. It is likely that several of the weak bands in this region are due to overtones
5256 J. Phys. Chem., Vol. 100, No. 13, 1996
Bell et al.
Figure 5. FTIR spectra of (a) TAA and (b) TMTAA in KCl disks. Figure 3. Electronic absorption spectra of TAA: (a) in a KCl disk; (b) in THF solution.
TABLE 2: Electronic Absorption Band Positions and Assignments ν˜ (cm-1) TAA
Figure 4. Electronic absorption spectra of TMTAA: (a) in a KCl disk; (b) in THF solution.
and combinations. Most of the CH stretching vibrations involve contributions from both the diimine and benzene ring parts of the molecule. The only two bands that gave a close fit to the scaled ab initio band wavenumbers and could be assigned are the benzene ring ν2 and the diimine νas(C9-H8) modes. Bands in the lower wavenumber region are more easily assigned; the majority of these are within 10 cm-1 of the calculated band wavenumbers and their relative intensities closely match the calculated intensity distribution. To a large extent these bands are associated with in-plane vibrations (Bu symmetry) and only a small number, in the region below 1050 cm-1, can be attributed to out-of-plane modes (Au symmetry).
TMTAA
solution
solid
solution
solid
22990 23980 27170 28250
21550 22940 26320 27930
23260
23260
29070
28570
assignment 1B u 1B u 1 Bu 1 Bu
r 1Ag (6bg f 7au) r 1Ag (6au f 8bg) r 1Ag (5bg f 7au) r 1Ag (6bg f 8au)
Furthermore, the out-of-plane vibrations yield only weak bands, with the sole exception of the ωas(CH) benzene ring ν11 mode. In the IR spectra of both macrocycles the most intense bands are predominantly associated with asymmetric CC and CN stretching vibrations of the diimine moiety, in particular the νas(CdC) and νas(CdN) band at 1646 cm-1 for TAA (1619 cm-1 for TMTAA), the νas(CsN) bands at 1286 and 1305 cm-1 (1278 and 1298 cm-1), and the νas(CsN) band at 764 cm-1 (773 cm-1). Of the bands that have been identified as localized within the benzene rings, most are of weak-to-medium strength; the only strong one is assigned to ν8a. In TAA this is the strongest IR band at 1547 cm-1. Vibrations associated with the methyl groups in TMTAA could not be identified. Methyl group C-H vibrations are expected in the ranges 2860-2885 (νs) and 2950-2975 cm-1 (νas).26 These are likely to be obscured by ν(CH) vibrations of the diimine and benzene ring moieties, since we were unable to locate any bands in this region that were not also present in the spectrum of TAA. Similarly, no additional bands could be found in the CH3 deformation region. It is likely that the only significant effect of the methyl groups is to cause the generally small differences between the band positions for TAA and TMTAA. The only large shift is that of the benzene ring ν11 band from 796 cm-1 in TAA to 850 cm-1 in TMTAA with considerably increased intensity. This is the most intense IR band of TMTAA, and the changes in the position and intensity of this band are ascribed to a substantial contribution from νas(C-CH3). RR Spectra. The RR spectra of the TAA and TMTAA in the solid state are shown in Figures 6 and 7, and the band wavenumber positions and their assignments are listed in Tables 4 and 5. The three excitation wavelengths (514.5, 488.0, and
Tetraazaannulenes
J. Phys. Chem., Vol. 100, No. 13, 1996 5257
TABLE 3: IR Band Wavenumber Positions (cm-1) for TAA and TMTAA, Calculated Band Wavenumbers (cm-1), and Assignments TAA 3430 m,br 3198 vw 3178 vw 3053 vw 3024 vw 2964 vw 2905 w 2874 vw 2850 w 2736 vw 1646 vs 1607 m 1547 vs 1517 sh 1458 w 1415 m 1393 w 1377 sh 1348 m
TMTAA
νas(NH) 2 × 1607 1547 + 1646 3060 vw νas(CH) benzene ring (2)
3425 w,br 3404 w 3200 m 3057 vw 2986 vw 2960 vw 2922 vw
2968 w
νas(C9-H8) diimine
1649 m 1595 vs 1570 w 1517 w 1456 m 1418 m 1387 vw
νas(CdC), νas(CdN) diimine νas(CdN), Fas(NH) diimine νas(CdC) benzene ring (8a) Fas(CH) benzene ring Fas(CH) diimine νas(CdN), Fas(NH) diimine Fas(CH) diimine
1315 w
νas(CsN), νas(CdC) Fas(CH) benzene ring νas(CsN), νas(CdC), Fas(CH) diimine νas(CsN), νas(CdC) benzene ring
2850 vw 1619 vs 1591 sh 1550 s 1511 m 1464 m 1439 m 1384 w 1364 m
1305 s
1298 m
1295 s
1286 s 1275 sh 1230 vw 1208 m 1182 m 1155 m
1278 m
1246 m
1111 vw 1044 vw 993 m 950 w 929 vw 881 m 858 sh 840 w 796 s
764 s
587 w 571 vw 527 w 484 vw a
assignmentsa
ab initio
1191 m 1159 w 1107 w 1047 sh 1029 m 1003 sh 947 w 927 vw 875 sh 850 vs 826 sh 793 vw 773 m 759 m 740 vw 697 vw 669 vw 557 s 535 vw 506 vw 479 vw
1202 w νas(CdC), Fas(CH) benzene ring 1185 vw νas(CsN), νas(CdC) benzene ring 1128 w νas(CsN) diimine, νas(CsN), νas(CdC) benzene ring 1122 w benzene ring br. (1) 1039 w νas(CdC) benzene ring (12) 1012 w ωas(CH) benzene ring (10a) 983 m νas(CsC) diimine 950 m ωas(NH) 920 w ωas(CH) benzene ring (10b)
788 m
ωas(CH) benzene ring (11)
773 w 760 w
ωas(CH) diimine νas(CsN), δip(CH) benzene ring
Figure 6. RR spectra of TAA in a KCl disk recorded with (a) 514.5, (b) 488.0, and (c) 457.9 nm excitation. Laser power ) 100 mW at the sample, spectral slit width ) 2-3 cm-1, integration time ) 1 s, the data interval was 1 cm-1, and the spectra were averaged over 12 scans.
590 vw benzene ring op def. (4) 551 vw δip(CNC) 538 w chelate ring op def 471 vw δip(CN)
Abbreviations as in Table 1.
457.9 nm) are all within the contour of the first absorption band of TAA (λmax ) 464 nm), and this electronic transition is therefore expected to dominate the RR scattering process. In the RR spectra of both macrocycles the relative band intensities show a slight variation with excitation wavelength. For TAA the 457.9 nm RR spectrum shows a relatively greater enhancement of the high wavenumber bands. In general bands below 1000 cm-1 exhibit their largest relative intensities with 488.0 nm excitation. The only exception is the 438 cm-1 band, assigned to the chelate ring deformation. Depolarization measurements established that for all Raman bands of TAA and TMTAA the depolarization ratios (F) were within the range 0.25-0.40, and they are therefore assigned to totally symmetric (Ag) vibrations. This is in contrast to Q-bandexcited RR spectra of porphyrins, which also display nontotally symmetric vibrations as relatively strong bands.27,28 This is a consequence of B-term scattering resulting from strong vibronic
Figure 7. RR spectra of TMTAA in a KCl disk recorded with (a) 514.5, (b) 488.0, and (c) 457.9 nm excitation. Laser power ) 100 mW at the sample, spectral slit width ) 2-3 cm-1, integration time ) 1 s, and the data interval was 1 cm-1.
coupling between the Q and Soret transitions, which are both r 1A1g. Although it could be posulated that the RR scattering in the tetraazaannulenes is dominated by an A-term scattering mechanism, this need not necessarily be the case. Since the next three lowest energy electronic transitions are all of the 1Bu r 1A1g type, vibronic coupling involving any of these would result in B-term scattering for totally symmetric modes.28 Hence, the RR scattering for TAA and TMTAA excited in the region of the lowest energy transition may have both A- and B-term contributions, with possible interference between them,29 1E u
5258 J. Phys. Chem., Vol. 100, No. 13, 1996 TABLE 4: RR and SERRS Band Wavenumber Positions (cm-1) for TAA, Calculated Band Wavenumbers, and Assignments RR 1618 m 1596 w 1581 w 1502 vs 1492 vs,sh 1478 m 1458 m 1409 w 1370 s 1351 s 1314 vw 1289 vw 1265 m 1223 s 1167 w 1153 w 1091 vw 1061 w 913 m 853 w 708 w 628 m 586 s 541 vw 491 vw 438 m 335 w 263 w a
ab SERRS initio
assignmentsa νs(CdC), νs(CdN) diimine νs(CdC) benzene ring (8b) νs(CdC) benzene ring (8a) Fs(CH) benzene ring νs(CdC), νs(CdN) diimine νs(CsC), Fs(CH) diimine Fs(CH) benzene ring (19a/19b)
1620 m 1597 w 1581 w 1501 vs 1493 sh 1480 m 1457 m
1651 1593 1569 1510 1509 1472 1458
1370 s 1351 s
1397 νs(CdN), Fs(CH) diimine 1390 δip(NH) 1312 Fs(CH) benzene ring (14)
1289 w 1265 w 1223 s 1168 w 1154 w
1269 1227 1175 1154 1132 1062 w 1080 914 m 901 853 w 842 734 m 708 w 697 668 w 628 m 634 586 s 588 448 m 387 w 337 w 290 w
νs(CsN), νs(CdC), Fs(CH) diimine νs(CsN), νs(CdC) benzene ring νs(CsN) νs(CdN) diimine, νs(CdC) benzene ring benzene ring br. (1) νs(CsN) νs(CsC) diimine, νs(CdC) benzene ring νs(CsN), δip(CH) benzene ring νs(CsN), νs(CdC) benzene ring δip(CNC) δip(CH) benzene
525
Fs(CCN)
317
δs(CNC)
179
δs(CNC)
Abbreviations as in Table 1.
although the A-term is expected to dominate. The measured F values are all close to the limiting value of 1/3 predicted for resonance with a single nondegenerate excited state. In general, the band positions in the RR spectra of both macrocycles are close to the scaled ab initio wavenumbers and can therefore be readily assigned. Although it was possible to estimate relative Raman intensities using the Gaussian 92 program, these are calculated on the assumption of a ground state transition polarizability. The calculated intensities are not therefore valid under resonance conditions, and they are not listed in the tables. Nevertheless, the ab initio calculations predict that the strongest band for TAA is the one at 1510 cm-1, which in fact corresponds to the strongest band for both TAA and TMTAA. Most RR bands are associated with vibrations that are localized within either the diimine or benzene ring moieties. Furthermore, the strongest bands are those ascribed to diimine vibrations, with only a few exceptions. The benzene ring F(CH), ν14, and δip(CH) modes each yield strong bands; otherwise, the benzene ring vibrations give only weak bands. It follows that the principal geometric changes attendant upon excitation to the resonant electronic state are within the diimine chromophore and that the electronic transition is mainly localized within this region. From the relative RR band intensities it appears that the major geometric changes in the excited state involve the diimine CdC, CsC, CdN, and CsN bond lengths. SERRS Spectra. The SERRS spectra of TAA and TMTAA, measured with Ag electrode potentials in the range -0.1 to -0.6 V Vs Ag/AgCl and using 514.5 nm excitation, are shown in Figures 8 and 9, and the band wavenumbers are listed together with the RR data and assignments in Tables 4 and 5. Band
Bell et al. TABLE 5: RR and SERRS Band Wavenumber Positions (cm-1) for TMTAA, Calculated Band Wavenumbers, and Assignments RR
SERRS
1620 w 1597 m 1575 w 1512 vs 1512 vs 1477 m 1478 m 1441 m 1441 m 1383 w 1338 s 1340 s 1309 vw 1307 vw 1284 w 1284 w 1263 vw 1264 w 1224 w 1227 w 1177 w 1176 w 1160 w 1160 w 1104 w 1103 w 1048 w 1050 w 1030 m 1028 m 938 m 940 m 876 w 878 w 808 w 762 w 762 w 735 w 664 vw 666 w 651 w 652 vw 611 m 611 m 572 w 573 w 556 m 556 m 534 w 536 m 520 w 522 w 478 s 479 s 432 w 431 w 387 vw 351 w 353 m 293 vw 1592 m
a
ab initio
assignmentsa
1651 1593 1569 1510 1472 1458 1397 1390 1312
νs(CdC), νs(CdN) diimine νs(CdC) benzene ring (8b) νs(CdC) benzene ring (8a) Fs(CH) benzene ring νs(CsC), Fs(CH) diimine Fs(CH) benzene ring (19a/19b) νs(CdN), Fs(CH) diimine δip(NH) Fs(CH) benzene ring (14)
1269 1227 1175 1154 1132 1080
νs(CsN), νs(CdC), Fs(CH) diimine νs(CsN), νs(CdC) benzene ring νs(CsN) νs(CdN) diimine, νs(CdC) benzene ring benzene ring br. (1) νs(CsN)
901 νs(CsC) diimine, νs(CdC) benzene ring 842 νs(CsN), δip(CH) benzene ring 697 νs(CsN), νs(CdC) benzene ring
634 δip(CNC) 588 δip(CH) benzene
525 Fs(CCN) 317 δs(CNC)
Abbreviations as in Table 1.
Figure 8. SERRS spectra of TAA in the potential range -0.1 to -0.6 V Vs Ag/AgCl. λ0 ) 514.5 nm, laser power ) 100 mW at the sample, spectral slit width ) 2-3 cm-1, integration time ) 1 s, the data interval was 1 cm-1, and the spectra were averaged over five scans.
positions in all SERRS spectra are within 5 cm-1 of those in the RR spectra, except for the 438 cm-1 band of TAA, which
Tetraazaannulenes
J. Phys. Chem., Vol. 100, No. 13, 1996 5259 for neutral adsorbates (i.e., the particles would tend to desorb into the electrolyte). Conclusion
Figure 9. SERRS spectra of TMTAA in the potential range -0.1 to -0.6 V Vs Ag/AgCl. λ0 ) 514.5 nm, laser power ) 100 mW at the sample, spectral slit width ) 2-3 cm-1, integration time ) 1 s, the data interval was 1 cm-1, and the spectra were averaged over six scans.
shifts to 448 cm-1 and has reduced intensity. These observations indicate that the molecules are physisorbed on the Ag surface rather than chemisorbed and that they retain C2h geometry. The surface enhancement is due entirely to the electromagnetic mechanism. In such large physisorbed molecules it is likely that any perturbation of the molecular geometry and its electronic structure will be small and will lead to only very slight changes of band positions and intensities. Such behavior has been witnessed in SERRS studies of metal bipyridine complexes adsorbed on Ag electrodes30,31 and colloids32 but contradicts the observations reported for SERRS studies of porphyrins. The interaction of porphyrins with Ag surfaces involves chemisorption with coordinated surface Ag atoms.33-35 The relative band intensities in the SERRS spectra are also similar to those in the RR spectra, suggesting that there is comparable enhancement of all observed totally symmetric modes. According to surface selection rules proposed by Creighton,36,37 a flat orientation of the adsorbed molecule, with the molecular plane parallel to the surface, would lead to preferential enhancement of out-of-plane vibrations. Conversely, a perpendicular edge-on orientation would favor inplane modes. If the adsorbate were to adopt a tilted orientation, then both in-plane and out-of-plane vibrations would have components in the direction perpendicular to the surface, and this would result in surface enhancement for both types of vibrations. Nevertheless, the out-of-plane vibrations are not subject to resonance-enhancement, so they would not be expected to appear in the SERRS spectrum. We therefore believe that our experimental data indicate either a perpendicular edge-on or tilted adsorbate orientation. The lack of any significant potential dependence of the SERR spectra indicates that the orientation of the adsorbed molecules does not change with potential. This is consistent with adsorbates that are insoluble in the electrolytesin general it is believed that a charged surface would have a decreased coverage at potentials further removed from the potential of zero charge
In this study we have reported the first measurements of the RR spectra of TAA and TMTAA. The vibrational band assignments of both the RR and IR spectra were facilitated by comparison to ab initio calculations using the 3-21G basis set. This approach was necessary because the RR effect is selective and not all vibrations can be observed, therefore precluding a normal coordinate analysis based upon experimental data alone. Notwithstanding the modest basis set employed in the calculations, which was necessary because of the large size of these molecules, there is excellent agreement between the experimental and calculated data. SERRS spectra of both macrocycles showed little differences from their RR spectra. This can be attributed to (a) physisorption of these molecules with no lowering of symmetry upon adsorption, (b) surface-enhancement via the electromagnetic mechanism alone, and (c) a tilted adsorbate orientation. The results of this study are a promising indication that the combination of SERRS data with IR, RR, and ab initio calculations will be fruitful in providing new insights into the behavior of transition metal TAA complexes adsorbed at electrode surfaces. The results of SERRS investigations of CoII and NiII TAA and TMTAA complexes will be reported in a future paper. Acknowledgment. The authors thank the EPSRC for a research grant. References and Notes (1) Felton, R. H.; Yu, N.-T. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. 3, p 347. (2) Spiro, T. G.; Czernuszewicz, R. S. Coord. Chem. ReV. 1990, 100, 541. (3) Woodruff, W. H.; Pastor, R. W.; Dabrowiak, J. C. J. Am. Chem. Soc. 1976, 98, 7999. (4) Nafie, L. A.; Pastor, R. W.; Dabrowiak, J. C.; Woodruff, W. H. J. Am. Chem. Soc. 1976, 98, 8007. (5) Holze, R. Z. Phys. Chem. 1993, 185, 1. (6) Cotton, F. A.; Czuchajowska, J. Polyhedron 1990, 9, 2553. (7) Rosa, A.; Ricciardi, G.; Lelj, F.; Y. Chizhov, Y. Chem. Phys. 1992, 161, 127. (8) Casarin, M.; Ciliberto, E.; Di Bella, S.; Gulino, A.; Fragala, I.; Marks, T. J. Inorg. Chem. 1992, 31, 2835. (9) Lukes, P. J.; McGregor, A. C.; Crayston, J. A. Inorg. Chem. 1992, 31, 4697, and references therein. (10) Van der Putten, A.; Elzing, A.; Visscher, W.; Barendrecht, E. J. Electroanal. Chem. 1987, 221, 95. (11) Moskovits, M. ReV. Mod. Phys. 1985, 157, 783. (12) Pemberton, J. E. In Electrochemical Interfaces: Modern Techniques for In-situ Interface Characterization; Abruna, H. D., Ed.; VCH: New York, 1991; p 193. (13) Hiller, H.; Dimroth, P.; Pfitzner, H. Liebigs Ann. Chem. 1968, 717, 137. (14) Chipperfield, J. R.; Woodward, S. J. Chem. Educ. 1994, 71, 75. (15) Sister, E.; Gottfried, V.; Kapon, M.; Kaftory, M.; Dori, Z. Inorg. Chem. 1988, 27, 600. (16) Azuma, N.; Tani, H.; Ozawa, T.; Niida, H.; Tajima, K.; Sakata, K. J. Chem. Soc., Perkin Trans. 2 1995, 343. (17) Goedken, V. L.; Pluth, J. J.; Peng, S.-M.; Bursten, B. J. Am. Chem. Soc. 1976, 98, 8014. (18) Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, W. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. Gaussian 92, Revision A; Gaussian Inc.: Pittsburgh, PA, 1992. (19) Fogarasi, G.; Pulay, P. In Vibrational Spectra and Structure; Durig, J. R., Ed.; Elsevier: Amsterdam, 1985; Vol. 14. (20) Bell, S.; Guirgis, G. A.; Lin, J.; Durig, J. R. J. Mol. Struct. 1990, 238, 183.
5260 J. Phys. Chem., Vol. 100, No. 13, 1996 (21) Schachtschneider, J. A. Vibrational Analysis of Polyatomic Molecules, Parts V and VI; Technical Report Nos. 231 and 57; Shell Development Co.: Houston, TX, 1964 and 1965. (22) Zhao, W. Personal communication, 1986. (23) Varsa´nyi, G. Vibrational Spectra of Benzene DeriVatiVes; Academic Press: New York, 1969. (24) Dollish, F. R.; Fateley, W. G.; Bentley, F. F. Characteristic Raman Frequencies of Organic Compounds; Wiley-Interscience: New York, 1974. (25) Gouterman, M. J. Mol. Spectrosc. 1961, 6, 138. (26) Brown, D. W.; Floyd, A. J.; Sainsbury, M. Organic Spectroscopy; Wiley: Chichester, 1988. (27) Rousseau, D. L.; Friedman, J. M.; Williams, P. F. Top. Curr. Phys. 1979, 11, 203. (28) Clark, R. J. H.; Dines, T. J. Angew. Chem., Int. Ed. Engl. 1986, 98, 131.
Bell et al. (29) Clark, R. J. H.; Dines, T. J. Chem. Phys. Lett. 1979, 79, 321. (30) Stacy, A. M.; van Duyne, R. P. Chem. Phys. Lett. 1983, 102, 365. (31) Virdee, H. R.; Hester, R. E. J. Phys. Chem. 1984, 88, 451. (32) Dines, T. J.; Peacock, R. D. J. Chem. Soc., Faraday Trans. 1 1988, 84, 3445. (33) Itoh, K.; Sugii, T.; Kim, M. J. Phys. Chem. 1988, 92, 1568. (34) Chen, J.; Hu, J. M.; Xu, Z. S.; Sheng, R. S. Appl. Spectrosc. 1993, 47, 292. (35) Tai, Z. H.; Zhang, J. F.; Gao, J. S.; Xue, G. J. Mater. Chem. 1993, 3, 417. (36) Creighton, J. A. Surf. Sci. 1983, 124, 209. (37) Creighton, J. A. In Spectroscopy of Surfaces; Clark, R. J. H., Hester, R. E., Eds.; Wiley: Chichester, 1988.
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