J. Phys. Chem. 1981, 85, 665-670
interesting to note that such an effect is not present in the methyl-substituted guanidine, which is to be expected. Table I1 displays the total energies of the planar structures. Conclusion Based on the discussed results we conclude the following: (1) Y delocalization is present in guanidine but to a lesser
66s
extent than in guanidinium ions. (2) Electronegative substituents increase the T electron localization on the carbon imino nitrogen bond but do not have much effect on the amino nitrogen bond. (3) The intramolecular hydrogen bonds and other electrostatic forces of attraction are of significance in rotational barrier energies.
The Role of Hydrogen Vibrations in the Radlatlonless Deactivation of Chromlum(II1)-Alkylamine Complexes In Their Lowest Doublet State K. Kuhn,' F. Wasgestian,' and H. Kupka Instnut fur Anorganische Chemie der Universnat zu Koin, D-5000 Koln 4 7, Federal Republic of Germny, and the Instltut fur Theoretlsche Chemie der Universltat, D-4000 DiisseMorf, Federal Republic of &many (Received: June 24, 1980; In Final Form: October 9, 1980)
A series of chromium(II1)-alkylamine complexes with CrNe skeletons have been synthesized. These complexes have similar electronic properties but different numbers of N-H bonds, ranging from 8 to 18. Measurements of absorption spectra, relative phosphorescence quantum yields, and phosphorescence decay times, obtained at liquid nitrogen temperature, showed that there is a good correlation between the radiationless transition rate and the number of active hydrogen atoms attached to the donor atoms. For the dependence of the electronic relaxation rate on the number of the maximum frequency N-H vibrations a theoretical expression is derived, which includes both the effects of configurational changes and of frequency changes in the first coordination layer. This expression adequately predicts the dependence of the nonradiative rate from the vibrational degeneracy and the displacement of the maximum frequency modes. Intraduction According to early theoretical work2+ hydrogen stretching vibrations play the dominant role as the accepting modes in the radiationless deactivation of excited electronic states. Experimentally it was found that on deuteration the phosphorescence decay of Cr(II1)-amine complexes is slowed by a factor of 50-100 at low tempera t u r e ~ . In ~ ~order ~ to study the influence of hydrogen vibrations, we prepared a series of alkylaminechromium(111) complexes with common electronic structures and varying numbers of hydrogen atoms attached to the coordinating atoms (active hydrogen atoms). In substituting the hydrogen of [Cr(NH3)6]3+by alkyl groups, the CrN6 skeleton is conserved and the ligand field changes only slightly. Also the spin-orbit coupling between the lowlying ligand field states remains constant, because it depends on the internal heavy atom effect of the central ion. Table I specifies the ligands and their abbreviations used in this study. The 2E, 4Azg(Oh)phosphorescence of chromium(II1) complexes has been widely ~ t u d i e d especially ,~ that of [Cr(NH&13+ and [Cren3]3f.12*13Hence its transition
-
(1) Department of Laboratory Medicine, University of Connecticut, Farmington, CN 06032. (2)G. W. Robinson and R. P. Frosch, J.Chem. Phys., 37,1962(1962); 38,1187 (1963). (3)11. Englman and J. Jortner, Mol. Phys., 18, 145 (1970). (4) I). J. Robbins and A. J. Thomson, Mol. Phys., 26, 1103 (1973). (6)I). R. Henry and W. Siebrand, Chem. Phys. Lett., 3,327 (1969). (6)A. Heller, J . Am. Chem. SOC.,88, 2058 (1966). (7)I. B. Neporent, E. B. Sveshnikova, and A. P. Serov, Zzv. Akad. NAUK SSSR,Ser. Fiz., 39, 1959 (1975). (8)C. D. Flint and A. P. Matthews, Chem. Commun., 954 (1971). (9)P.D.Fleischauer and P. Fleischauer, Chem. Reu., 70, 199 (1970). (10)K.K.Chatterjee and L. S. Forster, Spectrochirn. Acta, 20,1603 (19641. - -,(11) C.D.Flint and P. Greenough, J. Chem. S O ~Faraday ., Trens. 2,
TABLE I: Me,SO c y clam
Abbreviations Used
dien
dimethyl sulfoxide 1,4,8,11 -tetraazacy clote tradecane bis(2-aminoethy1)amine
ditn
bis( 3-aminopropy1)amine
en etam meam Pn
1,2-diaminoethane ethylamine methylamine 1,2-diaminopropane
tn
1,3-diaminopropane
(CH3)2S0
[ 14]anN,
H,N( CH,),"(CH2 )2 " 2 H,N( CH,),"(CH,),", H, NCH,CH, NH, CZH,", CH,NH, H NCH, CH(CH,)NH,
-
H2N(CH2)3"2
mechanism, vibronic structure, and decay kinetics are fairly well understood. In the present paper we are concerned with the problem of the nonradiative decay of the 2Estate of alkylaminechromium(II1) complexes by considering the decay times and the phosphorescence quantum yields at low temperatures. It shall be shown that the nonradiative rate increases significantly as the number of active hydrogen atoms increases. The observations are rationalized by means of the well-known three-level diagram for phosphorescence in d3 systems (Figure 1). Assuming that no photochemical reactions occur at liquid nitrogen temperature, the phosphorescence quantum yield +p and the decay time rp are14 @p = K57p*'ISC (1) 1 / =~k5 ~ + k6
+ k l ( k Z+ k 3 ) / k 4
where ki are phenomenological rate constants and
(2)
aIScis
\--
68,897 (1972). (12)C. D.Flint and A. P. Matthews, J . Chem. Soc., Faraday Trans. 2, 72,579 (1976). 0022-365418112085-0665$0 1.2510
(13)C. Conti, F. Castelli, and L. S. Forster, Znorg. Chin. Acta, 33, L171 (1979). (14)G. B.Porter in A. W. Adamson and P. D. Fleischauer, "Concepta of Inorganic Photochemistry", Wiley, New York, 1975,p 37.
0 1981 American Chemical Society
666
The Journal of Physical Chemistry, Vol. 85, No.
Kuhn et ai.
6, 198 1
(0.032 mol) of en and 15 mL (0.16 mol) of pn, the mixture was stirred for 1 h a t 60 OC. After allowing the mixture to cool slowly, ethanol was added drop by drop. The yellow precipitate was further treated as [Cretar&j(C104)3:yield, 5 g. Anal. (C8C13CrH28N6012) C, H, N, Cr. [Crcyclarnen](C104)3. cis-[CrcyclamC12]C1(3 g) was suspended in 25 mL of MezSO. After having added 5 mL of en, the mixture was stirred for 1h at 60 "C. Unreacted Flgure 1. Energy level diagram and rate processes for a d3 chromium starting material was filtered off. The filtrate was mixed system. Straight arrows indicate radiative transkions and wavy arrows with an equal volume of ethanol. Addition of ether prononradiative ones. duced a fine crystalline yellow precipitate, which was isolated and washed with ether. The further treatment the quantum yield of intersystem crossing. was as described for [Cretam6](C104)3:yield, 1.2 g. Anal. At low temperatures and for systems with an effective ( C I Z C ~ ~ C ~ H ~C,~ H, N ~N, O Cr. ~Z) intersystem crossing, we can take kq/ > k2 [Cretam6NH3I3+ and [Cr(N-N)&J3+ (N-N = en, pn, tn; k3, so that A = NH3, meam) were prepared from [Cretam&]Bq and truns-[Cr(N-N)Br,]Br, respectively, by reaction with liquid 1 / T p = k5 k6 (3) ammonia or methylamine for 4 h. After evaporation of the ammonia or methylamine, the crude products were discan be used throughout the entire analysis. solved in a small amount of cold water (NH3 complexes) A previous treatment of a similar problem concerning or ethanol (meam complexes). Pentammine species were the decay of the 2Estate in a series of Cr(II1) compounds removed by precipitation with a few drops of cancentrated has been made by Robbins and Thomson.4 Adopting the HBr. The final products were precipitated by concenmathematical framework of Englman and Jortner? these trated HC104,ethanol, and/or ether. [Crpnzmeam2]3twas authors predict the nonradiative decay rate to be proisolated as the nitrate only, by adding concentrated HN03 portional to d", where d is the degeneracy of the highest and ether to the alcoholic solution and crystallization a t frequency, which is identical with the number of active H -18 OC. Although trans-dibromo complexes were the atoms, and n is the number of vibrational quanta required starting materials, trans-cis isomerization may have taken to bridge the energy gap. Using literature values, Robbins place.24 We do not define the configuration of our comand Thomson observed an approximately linear relationplexes, because the small differences in ligand field ship. strength of the amines prevent spectroscopic characterization. Experimental Section Spectrophotometric Measurements. Absorption spectra Materials. The following compounds were prepared were recorded on a Cary 14. For the doublet absorptions, according to literature: CrBr3, [ Crmeam5Br]Br2;15cis0.01 M solutions were measured in 10-cm cells on a 10-fold [Cr~yclamCl~]Cl;'~ as well as the perchlorates of [Crscale expansion. (NH3)6]3+;17[ Crmeam6]3+;'8 [ Cren3]3+, [ Crpn3I3+, The equipment used to measure phosphorescence con[Crtn3I3+;l9[Cren2pn]3+;20[Cren2tnI3+, [ C ~ - e n t n ~ ] ~ + ;sisted ~ ~ of a high-pressure mercury lamp (Osram HBO 100 [Crdien2]3+, [Crditn2]3+;22 [Crmeam5NH3I3+.l5 W/2) in connection with interference filters (Schott) for [Cretam5Br]Br2was obtained by the same method as excitation and a monochromator (GCA/McPherson, EU[Crmearn5Br]Brzl5by using ethylamine instead of me700 2) with Peltier cooled photomultiplier (RCA C31034) thylamine. tr~ns-[Cr(N-N)~Br~]Br complexes (N-N = en, for detection of the luminescence light by a picoammeter pn, tn) were prepared by treating ca. 1 g of the corre(Keithley 410). The total light emission was obtained by sponding trans-difluoro compoundsB with 5 mL of freshly electronic integration (Kipp and Zonen BC 1). Excitation distilled concentrated HBr and allowing the mixture to conditions were standardized with [CrcyclamenI3+, the stand overnight. intensity of which was determined before each run. By [Cret~rn~](ClO Anhydrous ~)~ CrBr3 (2 g, 0.007 mol) was careful1 adjustment, a reproducibility of 10% could be added to 50 mL of dry ethylamine in small portions. The achieved. yellow-brown suspension was stirred for 2 h under reflux Phosphorescence decay times were obtained by excitaand the excess ethylamine was evaporated off. The yellow tion with a pulsed 10-kV nitrogen laser (Lambda Physics) residue was dissolved in a small amount of cold water and of 5-ns pulse width and signal averaging with a boxcar cold concentrated HCIOl was added drop by drop. The averager (Princeton Applied Research, Model 162/ 164, precipitate was washed with ethanol and ether and dryed gated integrator). in air: yield, 1 g. Anal. (C12C13CrH42N6012) C, H, N, Cr. Me2S0 and glycerol in a 1:l mixture were used as the [Crer~pn~](CZO~)~ Anhydrous CrC13 (5 g, 0.032 mol) was solvent for all measurements, because it forms a good glass suspended in 46 mL of Me2S0. After addition of 2.4 mL a t liquid nitrogen temperature.
+
+
(15)K. Kuhn and F. Wasgestian, Inorg. Nucl. C h e n . Lett., 12,803 (1976). (16)J. Glerup, J. Josephsen, K. Michelsen, E. Pedersen, and C. E. Schaffer, Acta Chem. Scand., 24, 247 (1970). (17)A. L.Oppegard and J. C. Bailar, Jr., Inorg. Synth., 3,153(1950). (18)M.Parris and N. F. Feiner, Inorg. Nucl. Chem. Lett., 3 , 337 (1967). (19)E.Pedersen, Acta Chem. Scand., 24,3362 (1970). (20)P.Pfeiffer, T. Gassmann, and H. Pietsch, 2.Anorg. Allg. C h e n . 58, 297 (1908). (21)M.Rancke-Madsen and F. Woldbye, Acta Chem. Scand., 26,3405 (1972). (22)0 . Kling and H. L. Schlafer, 2. Anorg. Allg. Chem., 313, 187 (1961). (23)J. Ferguson and M. L. Tobe, Inorg. Chim. Acta, 4, 109 (1970).
Results Absorption Spectra. The band maxima of the spinallowed transitions are presented in Table 11. They demonstrate that all complexes have very similar electronic structures. The differences in ligand field strength due to the various amines are small but significant. Band positions of the mixed complexes are calculable by linear interpolation between the ligand field stren-gths of the participating amines. Therefore we can propose the fol~~~~
~
(24)C.F.C.Wong and A. D. Kirk, Inorg. Chem., 17,1972(1978);Can. J. Chem., 53,3388 (1975).
Radiationless Deactivation of Chromium(II1)-Alkylamine Complexes
TABLE 11: Absorption Maxima of the Spin-Allowed Bands of Alkylamine-Chromium( 111) Perchloratesa
The Journal of Physical Chemistry. Vol. 85,No. 6, 1981 667 3000
1500
x 100
[wNH,), 1 3 + [ c r e n , ( ~ ~ , 1)3,, [Crpn2("3)2 13+ [ c r t n , ( ~ ~ ,1)3 +, [Crmeam,NH, 13+ [Cretam,", 13+ [ Crmeam, 13+ [ Cretam, 3 3 + [Cren,l3+ [c r p n , [Crtn3 1 [C r e n , p n I 3 + [ c r e n p n , i33++ [ Cren, t n ] [ Crentn, ] [Cren,meam, ]'+ [Crpn,meam, 13+ [Crtn,rneam2l3+ [Crdien,13+ [Crditn, 13+ [ Crcyclamen] 3 +
yt+
+
21.64 (44) 21.64 (67.7) 21.69 (74.6) 21.66 (51.6) 21.1 (61.7) 21.01 (65.2) 20.96 ( 7 1 . 8 ) 20.91 (71.5) 21.88 (85.9) 21.83 (82.6) 21.57 (66.1) 21.83 (93.5) 21.79 (84.4) 21.74 (77.5) 21.6 (65) 21.52 (84.1) 21.46 (75.6) 21.32 (55.9) 21.7 (146.4) 21.1 (70.7) 21.5 (148)
28.94 (37.2) 28.41 (56.8) 28.41 (63.1) 28.41 (46.6) 27.78 (52.9) 27.70 (56.0) 27.54 (62.5) 27.55 (61.5) 28.49 (69.9) 28.50 (73.4) 28.19 (57.2) 28.43 ( 7 7 ) 28.41 (68.9) 28.41 (70.2) 28.29 (59.7) 28.25 (71.6) 28.19 (64.7) 27.93 (48.3) 27.85 (78.2) 27.70 (66.0) 28.17 (107)
12
0 [cm-' 1
x 10
13.5
1L
1L.5
15 [kKI
at 77 K.
where n, is the refraction index of the medium (n,2= 2.2 was used for our solvent mixture), i j the wave number of the vibronic origin, and f the oscillator strength calculated from the integrated intensity? f = 4.32 X 1 0 - 9 1 c ( ~dij )
(5)
where E ( V ) is the molar extinction coefficient at wave number 8. For the complex ions of Figure 2, [Crdien2I3+, [Crditn2I3+,[Cren2pnI3+,and [CrcyclamenI3+ radiative lifetimes of 4.1,4.8,4.8, and 2.1 ms, respectively, were obtained. The remaining complexes also yielded values of this order of magnitude. Phosphorescence. All complexes showed a structured phosphorescence emission after excitation of the 4A2 "r, transition. As a typical example, the spectrum of [Crtn3](C104)3 is displayed in Figure 3. The corresponding spectrum of a glassy solution, reported by Porter and S ~ h l i i f e ris , ~similar ~ but less resolved. The 0-0 transition has been assigned to the most intense band;27the smaller components below the electronic origin are due to internal vibrations of the complex. Figure 3 further shows that at 3000-3200 cm-l below the 0-0 line an emission band is found, which may be assigned to N-H stretching vibrations, because on deuteration it is shifted to about 2500 cm-*, as was demonstrated in [Cr(NH3)6]3f.11Its existence proves the vibronic coupling of the N-H vibrations. Phosphorescence intensities and decay times of glassy solutions were measured a t liquid nitrogen temperature. Air-saturated solutions were used. [CrcyclamenI3+,the complex having the longest doublet lifetime, demonstrated that the phosphorescence intensity was not affected by degassing. Tests showed the phosphorescence intensity to be proportional to the optical density a t excitation wavelength (0.3 IOD I1).28 Therefore the intensities of different complexes could be normalized to equal amounts of excitation. Figure 2 shows that self-absorption of emitted radiation is negligible at wave numbers below 15000 cm-*. Figure 3 demonstrates that the major part of the phosphorescence is confined to a narrow range, where the change in quantum energy with wavelength can be neglected. Hence the integrated intensities of the total emission region are proportional to the phosphorescence quantum yield. For [Cren3I3+and [Cr(NH3),I3+,Chatterjee and Forster'O reported absolute quantum yields of 0.009 and 0.0033, re-
-
7-
i
I \
Flgure 2. Absorption spectra of alkylamine-chromium(II1) complexes in the region of the spin-forbidden band., 0.01 M solution in 1:l Me,SO/glycerol at room temperature.
-
500
Figure 3. Phosphorescence spectrum of [Crtn,](CIO,),
a 0.01 M solutions in Me,SO/glycerol (1: 1)at r o o m temperature.
5-
1000
-
--
lowing spectrochemical series of amine ligands: en pn dien > NH3 tn > cyclam > ditn > meam etam. Ligands forming five-membered chelate rings (en, pn, dien) produce higher field strengths than unidentate ones or those forming six-membered rings. Spin forbidden bands of all the amine complexes are located on the long wavelength tail of the first spin-allowed band. Some typical examples, where the absorption of the quartet bands has been subtracted, are depicted in Figure 2. The radiative lifetime of the doublet state may be estimated from25 (4)
(25) L. S. Forster, Transition Metal Chem., 5, 1 (1969).
(26) v. Balzani and v. Carassiti, "Photochemistry of Coordination Compounds", Academic Press, London, 1970, p 16. (27) G . B. Porter and H. L. Schlafer, 2.Phys. Chern., 38,227 (1963). (28) One of the reviewers doubted our observation of a linear relation between the phosphorescence intensity and the optical density, because the fraction of absorbed light is not linear with the optical density. A simple calculation shows that in the 0.3-1 OD region a linear approximation of the absorptivity curve produces a mean error of 3.7%, which is well within our experimental error limits.
668
The Journal of Physical Chemistry, Vol. 85, No. 6, 1981
TABLE 111: Phosphorescence Data of Chromium- Alkvlamine Comolexes'"
Kuhn et ai.
E t
vi br onic origin
fCr("3)6
i3+
2 [Cren2(NH3),'j3+ 3 [Crpn,(",),I'+ 4 [Crtn,(NH,),l3+ 5 CCrmeam.NH, 1'' 6 iCretam,NH, 7 [ Crmeam, 13+ 8 [Cretam6I3+ 9 [Cren313+ 10 [Crpn3I3+ 11 [Crtn313+ 12 [Cren2pnI3+
Is+
1 3 [ Crenpn,
13+
14 [Cren,tnI3+ 15 [Crentn,13+ 16 [ Cren, meam, ] 3 + 17 [ Crpn,meam, 13+ 18 [ Crtn,meam, 13+ 19 [ Crdien, 13+ 20 [ Crditn, 13+ 21 [CrcyclamenI3+
15.23 15.03 15.00 15.10 15.13 15.11 15.12 14.96 14.95 15.02 14.96 14.97 14.94 15.01 15.00 14.98 15.08 14.65 14.98 14.79
0.0031 0.0104 0.0075 0.0027 0.0057 0.0078 0.0055 0.0124 0.009 0.0132 0.0065 0.0112 0.0115 0.0105 0.0129 0.0136 0.0076 0.0112 0.0134 0.0146 0.0167
a Liquid nitrogen temperature, glassy solutions. tive to [ C r e n 3 I 3 +@ , p = 0.009.10
70 94 96 110 125 135 138 142 120 111 133 118 119 111 122 112 117 134 97 203 136 Rela-
spectively. Therefore we were able to use one of those as standard for our measurements. The phosphorescence decay after laser excitation was exponential in all cases. The vibronic origins, phosphorescence quantum yields, and phosphorescence decay times are presented in Table 111. The observed decay times are at least one order of magnitude shorter than the radiative lifetime calculated from the doublet absorption. So that we have k5
