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Energy levels of doublet-doublet transitions for some diphenylmethyl radicals
SpectroohimicaActa.Vol.26A,~~.2827
to 2232. PerwunonPreaa1970.PdntedinNorthernIreland
Energy levels of doublet-doublet transitions for some diphenylmethyl radicals G. FAVAEtO Institute of Physical Chemistry, University of Padova, hdOV8 Laboratory of Photochemistry end High Energy Radiations of the Consiglio Nezionale delle Ricerche, Bologna (Receiwd 9 April 1969) Al&a&-Absorption and fluorescencespectroscopy in rigid gkses et 77% haa been used to study the fhst two electroniotransitions of photoohemicallyproduced diphenylmethyl radicals. The long wave fluorescenceemissionof diphenylmethylredid, firstly describedin a previous paper, is now illustrated in more defail end a vibrational analysis attempted. The study hi been extended to some pra-substituted derivatives and the substituent effects on the yield of photolysis and on the energy levels ww reported.
IN the course of & study [lj on the primary photolysis products of arylalkyl thiocyanates, isothiooyanates snd disulfldes in a rigid matrix at 77”K, we have identified benzyl, diphenylmethyl and triphenylmethyl radicals through their U.V. absorption and visible fluorescence spectra. Particularly interesting was the detection of the long wavelength emission for diphenylmethyl (benzhydryl) radicsl, until now unobserved but expected for such an odd cllternant radical, where both excitations of electrons into and out of the singly occupied orbital are possible [2]. Benzyl [3] and triphenylmethyl [4] radicals show two absorption systems, in the near U.V. and in the visible region respectively, the latter being weaker and observable only with a long absorption p&h; the oorresponding fluorescence emission, on the contrary, is rather strong and experimentally observable even when the concentration of radicals is too small for the detection of the U.V. absorption [5]. Both the long wave and the short wave transitions lie in the spectral region theoretically predicted [2], and the relative intensities are in the expected order. The diphenylmethyl radical trapped in a rigid matrix of EPA, displays similar characteristics : its absorption system at 336 nm is well known [6]; the feeble emission, observed in the 620-680 nm region [l], constitutes good proof of the existence of a low lying excited state for this radical. We now report the absorption and emission spectra of the photolysis products of some symmetrically and asymmetrically substituted diphenylmethyl thiocyanates and/or carbinols in a rigid matrix at 77°K. The photolysed compounds are benzhydryl thiocyanate and its 4,4’-dimethyl, 4-phenyl, 4-chlorine and 4-n&0 derivatives; benzhydryl carbinol and its 4-phenyl and 4-chlorine derivatives. [l] [Z] [S] [4]
G. FAVARO and U. MAZZUCATO, Photochern.PhotobioZ.6,689 (1967). H. C. LONUU~T-HIININSand J. A. POPLE,Proc. Phys. Sot. AM, 591 (1965). G. PO~TYER and E. STBACXXAN, &~~lrocochi~. Acta U, 299 (1968). (a) G. N. LEWIS, D. LIPKIN snd T. T. htAGEL, J. Am. Chtm. Sot. 66, 1679 (1944). (b) T. L. C!E,Uand S. I. WEISSWAN, J. Chem. Phya. 22, 21 (1954). [6] 8. LEACH,~?~&~wTMx? of organic and Inorganic i+fat&&, p. 176. John Wiley (1962). [tl] G. PORTER and E. STRA~~AN,Trane. Fmzd.ug Sot. 64,1696 (1968). 2327
2328
Cl. FAVARO
The observed spectra sre reasonably assigned to the co~~~n~g diphenylmethyl radicals, produced by homolytic eleav&ge at the C-OH or C-SCN bonds, the identity of spectra obtained from both types of compounds giving a good support to this tentative assignment. In every case, the recorded spectrragive evidence of the existence of two excited states, whose relstive positions are sensibly influenced by substitution. The ~ornpo~~ had been prepared by standard procedures for previous kinetic work [7]. The solvent w&s E.P.A. and the solute concentrations were of the order of lo-4 M. The transparent silica cell and cryostttc used for this work were described in previous papers [l, S]. Irradiations of the undegassed solutions were performed by an untiltered Gallois low pressure mercury lamp, emitting mainly the 2537 A line; times of photolysis were generally of one hour: in some cases, longer &r&i&ion times were required to produoe the trapped radicds. A hydrogen discharge lamp was used for measuring the absorption speotra, while the mercury lamp was utilized 8s the exciting source for luminescence speotra; for this purpose it was filtered by a Chance OX7 filter, which outs out visible radiation. Times of exposure for the luminescenoe spectra ranged from twenty minutes to one hour with & slit of about 200 fi. The photographic plates were Kodak 103-D and 103-F; sn Hilger Medium quartz s~c~o~ph (~persion 15 ~~rnrn at 3000 A) was used for recording the rtbsorption spectra and an Hilger D187 (dispersion = 40 A/mm at 6000 A) for the emission spectra. RESULTS
The near U.V. absorption msxima and the visible fluoresoence maxima of the diphenylmethyl rtiuals, with the exception of the absorption for the 4-nitroderiv&ive, we reported in Table 1. Fluoresoenoe spectra are shown in Fig. 1. The ws,ve numbers and relative intensities of the ~u~esce~~ bands of ~substitu~d diphenylmethyl are reported in Table 2, along with sn attempted vibrational analysis. Relatively shorter irradiation times were necessary to photolyze the carbinoles than the corresponding thiocysnates. However, the structure of the aryl alkyl psrtner seemed to be the factor which mainly influences the yield of homolytic dissouiation. A relative rough comparison of these yields could be made either from the appearance of the absorption of the radicals s,nd/or of their luminescence, if one admits that cabsorptionooefficients and luminescence quantum yields are of the same order for the radioels under study, although this gumption is not without question. For instance, while absorption as well as fluorescence of the photolytic product of 4,4-dimethyl derivative both appear after a short irradiation time, diphenylmethyl red&al sbndits 4-chlorine derivative are more easily detectable from their absorption than from fluorescence, the latter requiring a higher ~ncentr~tion of radicals to be [7]A. JLXJETO, A. PAVA, U. WA~ZUCATOand 0. ROSSIWTO,J. Am. Chem. Sot. S&2729 (1961). [8]S.LEACH, E. MIUIRDICYANand L. GRAJOAR,J. Ckiwa. Phg~ 58, 749 (IQ61).
A ,mm Fig.
1. Fluorescence spectra, of diphenylmethyl radical (1) antd its 4,4’-dimethyl (2), 4-phenyl (3), 4-chloro (4) and 4-nitro derivativ ‘BS.
2329
Energy levels for doublet-doublet transitions Table 1. Ultraviolet absorption and visible fluorescencedate (A,, radical and some of its derivatives para-subatituenta Absorption
Fluoreaoenoe
in nm) for diphenylmethyl
H
4,4’-dimethyl
Cphenyl
4ohlorine
4nitm
335.5 330 323 312 523 532 641 560 659 669 689 606 630 656 680
342 323 310
370 362 332
318 315 297
not observed
486 480 466 494 602 611 618 636 557 671 585 613 648 686
464 476 486 499 607 639 649 507 613 649 676
636 667 608 630 660 680
637 668 586 612 660 680
Table 2. Vibrational analysis of the long wave fluorescencespectrum of diphenylmethyl radical Wave numbem
Relative intensities
19,120 18,800 18,480 18,180 17,890
8 6 7 4 6
17,670 16,980
10 4
16,600
7
16,870
6
16,270 14,710
2 1
AnelyL3if.J 0.0 0.0-320 0.0~640 O@-940 0.0-1230 0.0-Z x 640 0.0-1660 0.0-940-2 x 640 o*O-1660-640 0~0-1550-940 0+1660-1230 O*O-1660-940-640 0.0-Z x 1650 0~0-1660-940-2 x 640 0+1660-940-3 x 640
Assignment CC bend CC bend CC stretch +CH stretch C-C
stretch
On the contrary, the absorption of the 4-nitro derivative was not detected even though its fluorescence was easily measurable. From a comparison of the radioal spectra, it can be pointed out that there are three main substituent effects: (a) on the sharpness of the vibrational bands, whioh are somewhat reduced; (b) on the position of the absorption bands, which are hypsoohromioally displaced for the 4-uhlorine derivative and bathochromically displaced for the dimethyl and phenyl substituted radicals ; (c) on the position of the fluorescence bands, measured on the vibrationless transition, which are shifted towards shorter wavelengths for the dimethyl and phenyl derivatives and towards longer wavelengths for the chlorine and nitro diphenylmethyl radicals. observed.
2330
G. FAVARO
Some of the frequency differences between the O-O band and successive bands in every fluorescence system are similar for different radicals (this is particularly evident when the vibrational structure is well developed, namely for diphenylmethyl and its dimethyl and phenyl derivatives), indicating that substitution mainly affects the energy of the emitting electronic state, but not the vibrational levels of the ground state of the radicals. Such a behaviour was found by LEWIS et al. for radicals obtained from the triphenylmethyl derivatives [a]. No vibrational information is obtainable from the frequency differences between the absorption bands in the U.V. transitions, due to the scarce development of the progressions. It would certainly be interesting to investigate the feature of the long wave absorption, corresponding to the fluorescence emissions. With this aim we have a longer path quartz cryostat under construction, permitting the irradiation and recording of absorption spectra along a 20 cm path. DISCUSSION
The photoproduced radicals can be regarded as derived from benzyl type radicals by substitution of an hydrogen atom of the methylene group. The prominent frequencies, constituting the main progressions in the fluorescence spectra (see Table 2), are similar to those of benzyl [9]. Somewhat similar frequencies are also active in the toluene molecule [lo]. In view of these considerations, although the low resolution and the great broadness of the fluorescence bands would probably permit a possible error of flO0 cm- l, the attempted analysis reported in Table 2 for diphenylmethyl does not appear so misleading. An analogous analysis could be proposed for the dimethyl and phenyl derivatives, while it would be very difficult in the case of the nitro and chlorine derivatives, due to the poorness of bands detected in their emission spectra. The relative readiness for photolysis can be interpreted in terms of the relative stability of radicals produced. The high intensity of emission of 4,4’-dimethyl diphenylmethyl radical and its appearance after few minutes of irradiation could be justified by the relatively high stability of this radical, due to a hyperconjugation effect by the para methyl groups. As to the 4-phenyl derivative, conjugation between the two directly linked benzene rings may be assumed to be quite small, as it is for instance in the case of biphenyl [ll], where the rings are supposed to be slightly twisted relative to each other. So, only a small enhancement of stabilization is to be expected by 4-phenyl substitution in the diphenylmethyl radical. A 4-nitro or a 4-chlorine group seems to destabilize the corresponding radicals if we consider the cage recombination as being mainly responsible for the weakness of the radical spectra, and also after long irradiation, but the situation is not very clear because of the expected great radical stabilizing capacity of the n&o-group. As to the relative level position of the two excited states involved in the near U.V. absorptions and visible emissions, it can be considered that the substituents affect [9] L. GRAJCAR and S. LEACH, J. CAirn. Phye. 61, 1523 (1904). [lo] Y. KANDA tma H. SPONER, J. chewa. p&8. 28, 798 (1968). [ll] me e.g.: Y. KANDA, R. Smsm A and Y. SAW, fi’pectrochim. Acta 17, 1 (1961).
Energy levels for doublet-doublet trauaitions
2331
the optioal properties by changing the distribution of the unpaired eleotron in the n-system. The perturbation introduced in the unsubstituted odd alternant hydrocarbon might shift the energy levels with respect to the degenerate one in such a way that the long wave and the short wave transitions are deplaoed in opposite directions, as found. The electron-donating methyl-group shifts the O-O visible luminescence band to higher frequencies while the electron-aocepting substituents Cl and NO, shift the bands in the opposite directions. The behaviour of the phenyl diphenylmethyl
4,4 dimcthyl
4
phony1
4 chloro
4
nitro
30,000
-I ‘E :
1 1
‘5 26,000
-
1
Fig. 2. Scheme of the short wave (absorption) and long wave (fluorescence) electronic trausitionaof diphenyhnethyl radical and some derivatives.
group approximates that of two methyl groups. As to the more energetic absorption transition, it seems to follow an opposite trend, though the lack of the nitro derivative spectrum makes the correlation very uncertain. The separation between the two excited vibrationless levels is much smaller for 4,P’dimethyl (Av = 5450 cm-l) and C-phenyl derivatives and higher for 4-chlorine derivative (12,700 cm-l) with respect to the case of the unsubstituted radical, where the configuration interaction between the degenerated doublet states, corresponding to excitation of electron into and out of the singly occupied orbital, leads to a separation of 10,700 cm-l (see sketch in Fig. 2). The Hammett equation has been frequently applied to the correlation between optical spectra (mainly absorption frequencies) and structure of substituted aromatic systems. Its applicability to the properties of open shell systems is not so clear. WALTER [12] gives a criterion to predict the nature of the substituent effects, distinguishing two classes of stable aromatic radicals which exhibit an Hammett-like or non-Hammett behaviour. The Author concludes that triphenylmethyl radicals [I21 R. I. WALTER, J. Am. Ohem.Sot. 88, 1923 (1966).
2332
G. FAVARO
fall into the clrtssfor which both donor and acoeptor substituents affect their properties in the same dire&ion. In our case, on the contrary, an Hammett-type behaviour, if very poor, is followed, but the random distribution of the few data at disposal fails to be interpreted on the basis of a general theory. AcknowledgmentThe and discussions.
author wishes to thank Prof. U. MAZZUCIATO for his helpful suggestions
to 2232. PerwunonPreaa1970.PdntedinNorthernIreland
Energy levels of doublet-doublet transitions for some diphenylmethyl radicals G. FAVAEtO Institute of Physical Chemistry, University of Padova, hdOV8 Laboratory of Photochemistry end High Energy Radiations of the Consiglio Nezionale delle Ricerche, Bologna (Receiwd 9 April 1969) Al&a&-Absorption and fluorescencespectroscopy in rigid gkses et 77% haa been used to study the fhst two electroniotransitions of photoohemicallyproduced diphenylmethyl radicals. The long wave fluorescenceemissionof diphenylmethylredid, firstly describedin a previous paper, is now illustrated in more defail end a vibrational analysis attempted. The study hi been extended to some pra-substituted derivatives and the substituent effects on the yield of photolysis and on the energy levels ww reported.
IN the course of & study [lj on the primary photolysis products of arylalkyl thiocyanates, isothiooyanates snd disulfldes in a rigid matrix at 77”K, we have identified benzyl, diphenylmethyl and triphenylmethyl radicals through their U.V. absorption and visible fluorescence spectra. Particularly interesting was the detection of the long wavelength emission for diphenylmethyl (benzhydryl) radicsl, until now unobserved but expected for such an odd cllternant radical, where both excitations of electrons into and out of the singly occupied orbital are possible [2]. Benzyl [3] and triphenylmethyl [4] radicals show two absorption systems, in the near U.V. and in the visible region respectively, the latter being weaker and observable only with a long absorption p&h; the oorresponding fluorescence emission, on the contrary, is rather strong and experimentally observable even when the concentration of radicals is too small for the detection of the U.V. absorption [5]. Both the long wave and the short wave transitions lie in the spectral region theoretically predicted [2], and the relative intensities are in the expected order. The diphenylmethyl radical trapped in a rigid matrix of EPA, displays similar characteristics : its absorption system at 336 nm is well known [6]; the feeble emission, observed in the 620-680 nm region [l], constitutes good proof of the existence of a low lying excited state for this radical. We now report the absorption and emission spectra of the photolysis products of some symmetrically and asymmetrically substituted diphenylmethyl thiocyanates and/or carbinols in a rigid matrix at 77°K. The photolysed compounds are benzhydryl thiocyanate and its 4,4’-dimethyl, 4-phenyl, 4-chlorine and 4-n&0 derivatives; benzhydryl carbinol and its 4-phenyl and 4-chlorine derivatives. [l] [Z] [S] [4]
G. FAVARO and U. MAZZUCATO, Photochern.PhotobioZ.6,689 (1967). H. C. LONUU~T-HIININSand J. A. POPLE,Proc. Phys. Sot. AM, 591 (1965). G. PO~TYER and E. STBACXXAN, &~~lrocochi~. Acta U, 299 (1968). (a) G. N. LEWIS, D. LIPKIN snd T. T. htAGEL, J. Am. Chtm. Sot. 66, 1679 (1944). (b) T. L. C!E,Uand S. I. WEISSWAN, J. Chem. Phya. 22, 21 (1954). [6] 8. LEACH,~?~&~wTMx? of organic and Inorganic i+fat&&, p. 176. John Wiley (1962). [tl] G. PORTER and E. STRA~~AN,Trane. Fmzd.ug Sot. 64,1696 (1968). 2327
2328
Cl. FAVARO
The observed spectra sre reasonably assigned to the co~~~n~g diphenylmethyl radicals, produced by homolytic eleav&ge at the C-OH or C-SCN bonds, the identity of spectra obtained from both types of compounds giving a good support to this tentative assignment. In every case, the recorded spectrragive evidence of the existence of two excited states, whose relstive positions are sensibly influenced by substitution. The ~ornpo~~ had been prepared by standard procedures for previous kinetic work [7]. The solvent w&s E.P.A. and the solute concentrations were of the order of lo-4 M. The transparent silica cell and cryostttc used for this work were described in previous papers [l, S]. Irradiations of the undegassed solutions were performed by an untiltered Gallois low pressure mercury lamp, emitting mainly the 2537 A line; times of photolysis were generally of one hour: in some cases, longer &r&i&ion times were required to produoe the trapped radicds. A hydrogen discharge lamp was used for measuring the absorption speotra, while the mercury lamp was utilized 8s the exciting source for luminescence speotra; for this purpose it was filtered by a Chance OX7 filter, which outs out visible radiation. Times of exposure for the luminescenoe spectra ranged from twenty minutes to one hour with & slit of about 200 fi. The photographic plates were Kodak 103-D and 103-F; sn Hilger Medium quartz s~c~o~ph (~persion 15 ~~rnrn at 3000 A) was used for recording the rtbsorption spectra and an Hilger D187 (dispersion = 40 A/mm at 6000 A) for the emission spectra. RESULTS
The near U.V. absorption msxima and the visible fluoresoence maxima of the diphenylmethyl rtiuals, with the exception of the absorption for the 4-nitroderiv&ive, we reported in Table 1. Fluoresoenoe spectra are shown in Fig. 1. The ws,ve numbers and relative intensities of the ~u~esce~~ bands of ~substitu~d diphenylmethyl are reported in Table 2, along with sn attempted vibrational analysis. Relatively shorter irradiation times were necessary to photolyze the carbinoles than the corresponding thiocysnates. However, the structure of the aryl alkyl psrtner seemed to be the factor which mainly influences the yield of homolytic dissouiation. A relative rough comparison of these yields could be made either from the appearance of the absorption of the radicals s,nd/or of their luminescence, if one admits that cabsorptionooefficients and luminescence quantum yields are of the same order for the radioels under study, although this gumption is not without question. For instance, while absorption as well as fluorescence of the photolytic product of 4,4-dimethyl derivative both appear after a short irradiation time, diphenylmethyl red&al sbndits 4-chlorine derivative are more easily detectable from their absorption than from fluorescence, the latter requiring a higher ~ncentr~tion of radicals to be [7]A. JLXJETO, A. PAVA, U. WA~ZUCATOand 0. ROSSIWTO,J. Am. Chem. Sot. S&2729 (1961). [8]S.LEACH, E. MIUIRDICYANand L. GRAJOAR,J. Ckiwa. Phg~ 58, 749 (IQ61).
A ,mm Fig.
1. Fluorescence spectra, of diphenylmethyl radical (1) antd its 4,4’-dimethyl (2), 4-phenyl (3), 4-chloro (4) and 4-nitro derivativ ‘BS.
2329
Energy levels for doublet-doublet transitions Table 1. Ultraviolet absorption and visible fluorescencedate (A,, radical and some of its derivatives para-subatituenta Absorption
Fluoreaoenoe
in nm) for diphenylmethyl
H
4,4’-dimethyl
Cphenyl
4ohlorine
4nitm
335.5 330 323 312 523 532 641 560 659 669 689 606 630 656 680
342 323 310
370 362 332
318 315 297
not observed
486 480 466 494 602 611 618 636 557 671 585 613 648 686
464 476 486 499 607 639 649 507 613 649 676
636 667 608 630 660 680
637 668 586 612 660 680
Table 2. Vibrational analysis of the long wave fluorescencespectrum of diphenylmethyl radical Wave numbem
Relative intensities
19,120 18,800 18,480 18,180 17,890
8 6 7 4 6
17,670 16,980
10 4
16,600
7
16,870
6
16,270 14,710
2 1
AnelyL3if.J 0.0 0.0-320 0.0~640 O@-940 0.0-1230 0.0-Z x 640 0.0-1660 0.0-940-2 x 640 o*O-1660-640 0~0-1550-940 0+1660-1230 O*O-1660-940-640 0.0-Z x 1650 0~0-1660-940-2 x 640 0+1660-940-3 x 640
Assignment CC bend CC bend CC stretch +CH stretch C-C
stretch
On the contrary, the absorption of the 4-nitro derivative was not detected even though its fluorescence was easily measurable. From a comparison of the radioal spectra, it can be pointed out that there are three main substituent effects: (a) on the sharpness of the vibrational bands, whioh are somewhat reduced; (b) on the position of the absorption bands, which are hypsoohromioally displaced for the 4-uhlorine derivative and bathochromically displaced for the dimethyl and phenyl substituted radicals ; (c) on the position of the fluorescence bands, measured on the vibrationless transition, which are shifted towards shorter wavelengths for the dimethyl and phenyl derivatives and towards longer wavelengths for the chlorine and nitro diphenylmethyl radicals. observed.
2330
G. FAVARO
Some of the frequency differences between the O-O band and successive bands in every fluorescence system are similar for different radicals (this is particularly evident when the vibrational structure is well developed, namely for diphenylmethyl and its dimethyl and phenyl derivatives), indicating that substitution mainly affects the energy of the emitting electronic state, but not the vibrational levels of the ground state of the radicals. Such a behaviour was found by LEWIS et al. for radicals obtained from the triphenylmethyl derivatives [a]. No vibrational information is obtainable from the frequency differences between the absorption bands in the U.V. transitions, due to the scarce development of the progressions. It would certainly be interesting to investigate the feature of the long wave absorption, corresponding to the fluorescence emissions. With this aim we have a longer path quartz cryostat under construction, permitting the irradiation and recording of absorption spectra along a 20 cm path. DISCUSSION
The photoproduced radicals can be regarded as derived from benzyl type radicals by substitution of an hydrogen atom of the methylene group. The prominent frequencies, constituting the main progressions in the fluorescence spectra (see Table 2), are similar to those of benzyl [9]. Somewhat similar frequencies are also active in the toluene molecule [lo]. In view of these considerations, although the low resolution and the great broadness of the fluorescence bands would probably permit a possible error of flO0 cm- l, the attempted analysis reported in Table 2 for diphenylmethyl does not appear so misleading. An analogous analysis could be proposed for the dimethyl and phenyl derivatives, while it would be very difficult in the case of the nitro and chlorine derivatives, due to the poorness of bands detected in their emission spectra. The relative readiness for photolysis can be interpreted in terms of the relative stability of radicals produced. The high intensity of emission of 4,4’-dimethyl diphenylmethyl radical and its appearance after few minutes of irradiation could be justified by the relatively high stability of this radical, due to a hyperconjugation effect by the para methyl groups. As to the 4-phenyl derivative, conjugation between the two directly linked benzene rings may be assumed to be quite small, as it is for instance in the case of biphenyl [ll], where the rings are supposed to be slightly twisted relative to each other. So, only a small enhancement of stabilization is to be expected by 4-phenyl substitution in the diphenylmethyl radical. A 4-nitro or a 4-chlorine group seems to destabilize the corresponding radicals if we consider the cage recombination as being mainly responsible for the weakness of the radical spectra, and also after long irradiation, but the situation is not very clear because of the expected great radical stabilizing capacity of the n&o-group. As to the relative level position of the two excited states involved in the near U.V. absorptions and visible emissions, it can be considered that the substituents affect [9] L. GRAJCAR and S. LEACH, J. CAirn. Phye. 61, 1523 (1904). [lo] Y. KANDA tma H. SPONER, J. chewa. p&8. 28, 798 (1968). [ll] me e.g.: Y. KANDA, R. Smsm A and Y. SAW, fi’pectrochim. Acta 17, 1 (1961).
Energy levels for doublet-doublet trauaitions
2331
the optioal properties by changing the distribution of the unpaired eleotron in the n-system. The perturbation introduced in the unsubstituted odd alternant hydrocarbon might shift the energy levels with respect to the degenerate one in such a way that the long wave and the short wave transitions are deplaoed in opposite directions, as found. The electron-donating methyl-group shifts the O-O visible luminescence band to higher frequencies while the electron-aocepting substituents Cl and NO, shift the bands in the opposite directions. The behaviour of the phenyl diphenylmethyl
4,4 dimcthyl
4
phony1
4 chloro
4
nitro
30,000
-I ‘E :
1 1
‘5 26,000
-
1
Fig. 2. Scheme of the short wave (absorption) and long wave (fluorescence) electronic trausitionaof diphenyhnethyl radical and some derivatives.
group approximates that of two methyl groups. As to the more energetic absorption transition, it seems to follow an opposite trend, though the lack of the nitro derivative spectrum makes the correlation very uncertain. The separation between the two excited vibrationless levels is much smaller for 4,P’dimethyl (Av = 5450 cm-l) and C-phenyl derivatives and higher for 4-chlorine derivative (12,700 cm-l) with respect to the case of the unsubstituted radical, where the configuration interaction between the degenerated doublet states, corresponding to excitation of electron into and out of the singly occupied orbital, leads to a separation of 10,700 cm-l (see sketch in Fig. 2). The Hammett equation has been frequently applied to the correlation between optical spectra (mainly absorption frequencies) and structure of substituted aromatic systems. Its applicability to the properties of open shell systems is not so clear. WALTER [12] gives a criterion to predict the nature of the substituent effects, distinguishing two classes of stable aromatic radicals which exhibit an Hammett-like or non-Hammett behaviour. The Author concludes that triphenylmethyl radicals [I21 R. I. WALTER, J. Am. Ohem.Sot. 88, 1923 (1966).
2332
G. FAVARO
fall into the clrtssfor which both donor and acoeptor substituents affect their properties in the same dire&ion. In our case, on the contrary, an Hammett-type behaviour, if very poor, is followed, but the random distribution of the few data at disposal fails to be interpreted on the basis of a general theory. AcknowledgmentThe and discussions.
author wishes to thank Prof. U. MAZZUCIATO for his helpful suggestions