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Photochemistry from higher excited states of aromatic molecules: The possible role of Rydberg-like states
0146-5724/91 $3.00+ 0.00 PergamonPress plc
Radiat. Phys. Chem. Vol. 37, No. 5/6, pp. 667-671, 1991 Int. J. Radiat. Appl. Instrum., Part C Printed in Great Britain
PHOTOCHEMISTRY FROM HIGHER EXCITED STATES OF AROMATIC MOLECULES: THE POSSIBLE ROLE OF RYDBERG-LIKE STATES GO H rKIED KOHLER InstitUt fiJr theoretische Chemie und Strahlenchemie, University of Vienna, W~hringerstr. 38, A-1090 Vienna, Austria Abstract--The appearance of wavelength dependent photochemistry (H-abstraction) and fluorescence in some typical polar aromatics (i.e. phenols, anilines, indoles) is correlated to the energetic position of Rydberg type excited states. This behaviour is also demonstrated for carbazole as an example of a larger aromatic molecular system. In bichromophoric molecules, interrelation between excited states of valence and Rydberg type is demonstrated. The significance of higher excited state photochemistry for primary radiation chemical processes is discussed.
INTRODUCTION Formation of excited molecules by irradiation with high energy radiation is either due to direct excitation by interaction of molecules with slow electrons or, probably more important, due to recombination processes of ion-electron or ion-ion pairs (Bednar, 1990). During charge-recombination higher excited states are most generally formed, as the coulombic energy is released to the system. In this case, the ion pair does undergo spin relaxation, singlets and triplets should result with a probability distribution of approximately 1:3 (Bednar, 1990). These states, which are formed primarily, relax in condensed phase by isoenergetic internal conversion and vibrational relaxation to the thermally equilibrated states and photochemistry is normally assumed to proceed therefrom. Nevertheless, it was shown frequently, that photochemistry can also occur from non-relaxed states, prior to relaxation (Turro et al., 1978). One class of such systems which could also be of special relevance to radiation chemistry are simple aromatics, mostly benzene derivatives, with a polar group attached to the ring, i.e. molecules like anilines, phenols and indoles. When the excitation wavelength dependence of these molecules was studied, a remarkable decrease of the fluorescence quantum yield, given by qv, was found, as the excitation was into states higher than the lowest singiet $1 (K6hler and Getoff, 1976, 1980; Zechner et al., 1981). Such wavelength effects were, however, only found in cases where hydrocarbons or water were used as solvents. For other polar environments like alcohols, ether, acetonitrile etc. such an effect appears negligibly small. This decrease in q~ was correlated to an increase of H-atom release in the case of hydrocarbon solvents, but to enhanced
solvated electron ejection for aqueous solution. The latter process was also found by consecutive two photon excitation of higher states via the lowest singlet or triplet (Grabner et al., 1977). The lowest excited states of aliphatic molecules like hydrocarbons, alcohols or water, compounds which are preferentially used as solvents in radiation- and photochemistry, are most generally of Rydberg type, i.e. states of large spatial extension which are derived from atomic orbitals of higher principal quantum number than the valence states (n t> 3 for organic compounds) (Robin, 1974). Such excited states correlate along the bond dissociation coordinate to a dissociative valence state (Robin, 1974). Homolytical bond cleavage is thus the predominant relaxation pathway of the lowest excited state of these molecules. One exception might be cyclohexane, which excited state is appreciably long lived to show remarkable fluorescence (Rothman et al., 1973). The most important exceptions are, however, tertiary amines, exhibiting high fluorescence yields (e.g. triethylamine in n-hexane at room temperature qr = 0.68; K6hler, 1986) but low radical yields (Halpern, 1981). The Rydberg orbital is centred on the nitrogen and NC-bond cleavage must overcome an activation barrier making this process dynamically improbable (Ashfold et al., 1979). Rydberg-like states could also play a dominant role in the relaxation of higher excited states of polar aromatics, as they were found for some anilines (Fuke and Nagakura, 1977) and indoles (Lami, 1977) in their gas phase absorption. They are positioned at energies near to the Sl/$2 overlap region, i.e. the wavelength range where qF decreases. A relaxation mechanism via the Rydberg manifold might be postulated to explain wavelength effects on qr and the efficient H-atom release at the higher energies. This renders a discussion on the possible importance
667
668
GO-~III~-~.DK6HLER
of such Rydberg excitations in polar aromatics worthwhile, which will be presented in this paper.
anilines) and indoles in hydrocarbon solution (K6hler and Getoff, 1976, 1980; Zechner et al., 1981). The universal pattern found is characterized by the following items: (1) qv is constant at wavelengths corresponding to excitations into the S l state. In the $2/S~ overlapping region the yield decreases and stays near a constant value again for $2 excitation. This can be interpreted by the existence of a deactivation pathway originating from the relaxed $2. The quantum yield of such a process is then given by 1 - ~ with
EXPERIMENTAL
The experimental setup and the procedure for obtaining wavelength dependence of qF has already been discussed in detail (K6hler and Getoff, 1976, 1980). The compounds used throughout this work were of best available quality, were sublimed or distilled in vacuo and kept under argon prior to use. Purity was checked by gas chromatography. N,N-dimethyl2-phenylethylamine was prepared as reported previously (Kthler and Getoff, 1980). All solvents used were of spectroscopic or analytical grade. Hydrocarbons were further purified by column chromatography (SiO2 and basic A1203, Merck, under argon athmosphere), ethanol by refluxing over molcular sieve 10 •. RESULTS
AND
=qF(A=,o)/qF(S,).
(2) This process is only found when O - H or N - H bonds are present. Methylation supresses this effect. (3) Only hydrocarbon solutions show a decrease of qE on A=~c(with the important exception of aqueous solutions which shall not be considered in this discussion). Intermolecular hydrogen bonding makes this additional $2 relaxation pathway inefficient. ( 4 ) Homolytical O - H or N - H cleavage is the main photochemical primary process competing to internal conversion. For phenols nearly quantitative agreement between its yield from the $2 state, denoted by c<, and 1 - # is found. For the anilines and indoles quantitative agreement is, however, not achieved. It is interesting to prove whether such effects are restricted to this class of molecules mentioned above or whether they are also found for larger molecular systems. For this purpose the excitation wavelength dependence of qF of carbazole and N-methylcarbazole was examined and the results are given in Fig. 1. Clearly a similar picture in comparison to the smaller molecules is found: qF of carbazole decreases in steps when the excitation energy is increased. There are, however, important differences to the smaller systems. The first step appears at higher energies than the $2 excitation and the observed decrease is relatively small ( ~ 10%). A further step is observed near the high energy flank of the third absorption band. No wavelength effects were, however, observed for methylated carbazole and for carbazole in diethylether in the
DISCUSSION
Most studies on excitation energy effects on the fluorescence yield--excluding bichromophoric systems and some special molecules (Turro et al., 1978)---have been done on benzene and its derivatives and on indole. Benzene itself will not be concerned, as wavelength effects are of different origin compared to its polar derivatives (Braun et al., 1963). For benzene this phenomenon is related to the "channel three" problem and the process concurring with relaxation to the fluorescent state is suppressed as the symmetry is lowered by substitution (Birks, 1970). This is completely different to the general pattern found for its polar derivatives. On the other hand, for typical scintilators (e.g. PPO or BBOT), dyes (e.g. rhodamine B) and condensed aromatic systems (e.g. naphthalene, anthracene, pyrene) no )~°~dependence of qF was found for )~=x~> 190 nm and such substances were thus used as quantum counters (Kthler and Getoff, 1976; Birks, 1970). Discussion is thus concentrated on that class of molecules as polar benzene derivatives (phenols, ,.",
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300 A [nm] Fig. 1. Dependence of the relative fluorescence yield/~ on the excitation wavelength A~ for carbazole in n-hexane (1, x and • for two different concentrations, 2 x 10-6 and 1 x 10-3 M) and diethylether (2). An absorption in n-hexane. Rydberg transitions calculated as given in the text are marked in this figure.
Photochemistry of aromatic molecules respective wavelength range. Excitation wavelength effects are thus restricted to :t < 260 nm (i.e. 38,500 cm-~), and are less efficient for this larger molecule. On the other hand, they are suppressed by methylation and in hydrogen bonding solvents as well. It is attempted to correlate the onset of an additional relaxation pathway occuring at higher excitation energies with the appearance of Rydberg-like states. They should be of the same type as are the lowest excited states in the corresponding aliphatic systems, alcohols and amines. The (n,3s) transitions of amines are found around 240-250 rim, and for the alcohols near 200 nm (Robin, 1974). The (n,3p) bands of the amines are located around 200 nm (Robin, 1974). The frequencies of Rydberg transitions fit to the modified hydrogen atom formula (Robin, 1974): hv = li - R / ( n - 6
)2.
(2)
Herein/~ is the ionization potential towards this series converges for large principal quantum numbers n. The second term in equation (2) containing R the Rydberg constant (R = 109, 737.1 cm -m) is called the "term value". 6 is the quantum defect and differentiates between transitions to 3s and 3p type excited orbitals. Increasing the chain length, the term value for 3s converges to approx. 27,200 cm -I, corresponding to ¢5 = 1.0 and for 3p it is typically 18,500 em -l. These values seem, however too high for aromatic molecules. For N,N-dimethylaniline a term value of 23,700 cm-t was found, when related to the vertical transition, for indole 20,100 cm -t and for N-methylindole 22,500 cm -~. The value found for indole seems most likely to be too small as the lowest transition might not be resolved. A term value around 23,000 cm-~ is thus reasonable for these aromatics and was used to calculate Rydberg transition energies. It might, however, be a littletoo small for the phenols, as in the corresponding alcohols the excited states are littlemore valence in character than for the amines. In the same way a term value of 20,000 cm- t was excepted for (n,3p) transitions.Energies of the Rydberg states obtained by this procedure and related to adiabatic ionization potentials are compiled in Table 1, together with the wavelength of onset of the observed qF decrease. A good correlation between these Rydberg energies and the appearance of excitation Table 1. Calculated (n, 3s) Rydbergtransition energies, ionization potentials li (Ran et al., 1979), and wavelength of onset of qv decrease (~.o,) Ii (~,3s) 2o~ Compound (eV) (nm) (nm) Phenol 8.37 228 235 Pyrocatechol 8.28 232 238 Resorzinol 8.30 231 234 Hydrochinon 8.20 237 245 o-Cresol 8.24 234 240 p-Cresol 8.20 236 250 Aniline 7.71 260 265 N-Methylaniline 7.35 281 275 DMPEA 7.70 260 255 lndole 7.76 257 250 Carbazole 7.60 266 265
669
energy effects is obvious. For carbazole calculated Rydberg energies are marked in Fig. 1. Crossing from the vertically excited state to the Rydberg surface is thus energetically feasible. The energy gap between the respective S,(~,~*) and the 3s Rydberg is considerably smaller than between the S, and the lower lying (Tt:r*). Therefore, the (Tt,~*)---,(Tt,3s) internal conversion process could compete effectively to that within the (~,~*)-manifold. Internal conversion to the (~,3s) must, however, be promoted by motions destroying the molecular plane of symmetry (antisymmetric 7~-orbitals) in order to couple these to states with totally symmetric 3s upper orbital. It was shown previously, that deuteration of phenol does not affect fl significantly (Dellonte e t al., 1987). O--H and C - H stretching vibrations should, therefore, be inefficient in promoting internal conversion. This is, however, to be expected for internal conversion to a lower (~,~*) state, as these vibrations possess the largest quantas of energy. This rationalizes also the proposed mechanism. A mechanism via the Rydberg surface should make N - H and O - H bond cleavage from the higher excited state very efficient.Back conversion to the S~ and especially to the So state, at the crossing point of the Rydberg and the respective valence surface, could also occur. The latterprocess would account for the discrepancy between ~ and l -fl values. A n alternative explanation for this discrepancy between the qr decrease found and the measured H2-yields might be due to inhomogcneous cage recombination. The Rydberg states of the aliphaticcompounds are centred on the heteroatom. This is not a priori the case for the aromatic molecules as the firstionization potential corresponds to the 7r-HOMO. It is, therefore, not a priori convincing that these 0t,3s) states correlate to repulsive (n,#*) valence states.Nevertheless, the H O M O can most generally be described as an antibonding combination of the heteroatom lone pair n and the respective aromatic x-orbital. Removing an electron from the ~-system increases the positive charge density on the heteroatom. The (n,3s) should thus closely resemble the (n,3s) in character. The question remains, why for methylated compounds like anisole, N,N-dimethylaniline, N-methylindole, and N-methylcarbazole processes competing to the S,--*S~ internal conversion are rather inefficient, although Rydberg states do exist as well. It is important to notice that cleavage of the respective O-CH3 and N-CH3 bonds in the corresponding aliphatic compounds is also much less efficient as O - H or N - H rupture, but the Rydberg surface is likewise repulsive. The best example, triethylamine, is highly fluorescent (qF = 0.68) and no photochemistry is observed. This stability must be dedicated to dynamical properties of the excited state surface. From this discussion it seems rather probable, that the appearance of molecular Rydberg like states makes homolytical bond cleavage from the higher
670
GOrirRlm KtHt~
PEA
C )
DMPEA
Fig. 2. 2-Pbenylethylamine (PEA) and N,N-dimethyl-2phenylethylamine (DMPEA) in their AM1 geometries (C3 sp3 and CAR aromatic carbon atom). excited (~,~*) enough efficient to compete effectively with S,(~,~*)-*S, Or,lr*) internal conversion. To look closer to the importance of Rydberg excitations in this energy region, wavelength effects were studied on bichromophoric molecular systems, one chromophore of (~,~*) and one of Rydberg type. As model compounds 2-phenylethylamine (PEA) and N,N-dimethyl-2-phenylethylamine (DMPEA) were used (Fig. 2) (Kthler et al., 1977; Kthler et al., 1981). Fluorescence of both systems were studied previously (Shizuka et al., 1979; Bryce-Smith et al., 1973). DMPEA in hydrocarbon solvents exhibits a broad emission spectrum (maximum at 306 nm), red shifted versus the fluorescence spectrum of toluene (maximum around 280 nm). Increasing solvent polarity slightly (e.g. in 1,2-dichloroethane) emission shifts significantly to lower energies, whereas in strongly polar solvents like alcohols dual fluorescence is observed (maxima at 284 and 431 nm). This phenomenon was interpreted by intramolecular charge transfer exciplex formation. Contrary, PEA shows only fluorescence quenching when compared to toluene (Shizuka et al., 1979). Absorption spectra of these systems (Figs 3 and 4) can be constructed by superimposing the spectra of the
two separated chromophores. Figure 2 shows AM1 (Dewar, 1983) geometries for these compounds. In equilibrium the amino group is placed near to the aromatic ring in both molecules. The energy of this minimum is, however, only significantly below the energy of a stretched conformation. The side chain should, therefore, remain flexible but the minimum geometry could still be favoured in liquid solutions, because of dynamical reasons. A distance of 3 A between N and the nearest C atom makes mutual perturbation of the two absorption transitions small. On the other hand, excited state interactions e.g. by charge transfer are favoured by such a geometrical conformation. Figure 3 shows the wavelength effects found for DMPEA in n-hexane and 1,2-dichloroethane, monitored at their emission maxima. No spectral changes were observed varying 2ext. This emission can be attributed to a (Ar- ... N +) intramolecular charge transfer state (Bryce-Smith et al., 1973). Thermal equilibrium is established fast but a distinct dependence of the fluorescence yield on the primarily excited state was found. In n-hexane solution qF remains near to a constant for 2~ >215rim with a slight increase near the (n,3s) absorption. Population of the c.t.-state should thus be only slightly less efficient from the (lr,~*) than from the (n,3s) state (fluorescence from both unperturbed states is very near in energy at ;. ~ 280 nm). A considerably different behaviour is observed for dichloroethane solutions. In this case population of the c.t.-state from the (n,n*) is significantly more efficient than from the (n,3s). The Rydberg state centred on the amino group is efficiently quenched by electron accepting molecules. This is most likely because of the large spatial extension of the 3s orbital. These results demonstrate that Rydberg states of aromatic molecules can be excited in the condensed phase and that it is possible to monitor their specific deactivation pathways. Only a single fluorescence spectrum around 280 nm is observed for PEA in both, n-hexane and methanol as solvents• The spectrum is indpendent of ~.~c and can be attributed to 0r,~*) emission. No fluorescence
:
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270 A [nm]
Fig. 3. Dependence of ~ on ~o for N,N-dimethyl-2-phenylethylamine in n-hexane (H) and 1,2-dichloroethane(DC1E). Absorption spectrum (A) in DCIE and for toluene (AT)and triethylamine (A^) in the same solvent.
671
Photochemistry of aromatic molecules CONCLUSION -800 F3
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Fig. 4. Dependence of fl on R~ for 2-phenylethylamine in methanol plus 0.I M HCIO4 (Fl), n-hexane (F2), and methanol (F3) and for toluene in methanol (F4).Absorption spectra given for solutions in methanol plus 0.I M HCIO4 (Ai) and n-hexane (A2).
is observed from primary and secondary aliphatic amines but N - H - b o n d cleavage is very efficient with a quantum yield near to 1 (Ashfold et al., 1979). Intramolecular quenching was described for PEA relative to toluene (Shizuka et al., 1979). Figure 4 gives qF of PEA as a function of )~xc for n-hexane and methanol as solvents. Differently to toluene, considerable solvent effects are observed for the fluorescence of PEA. They are comparable to solvent effects found for, e.g. aniline. Hydrocarbon solutions show a strong decrease of qF, but in alcoholic solvents wavelength effects are considerably smaller. In cyclohexane also Q(H2) increases at higher excitation energies (K6hler and Getoff, 1980). The amino group, although separated from the ring, induces thus a behaviour which is similar to aniline but different to toluene. In the case the amino group is protonated (Fig. 4) qF decreases in methanol also. In this case no (n,3s) state is present. To illustrate this, the effect of protonation is shown for N,N-dimethylaniline in methanol in Fig. 5. Also here, wavelength effects are observed but they are in the same order of magnitude than for toluene (see Fig. 4). Solvent-dependent wavelength effects, which are typical for polar aromatics, thus disappear and they become neady independent of the environment, as was found for benzene and toluene as well.
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01
REFERENCES
Ashford M. N. R., Macpherson M. T. and Simons J. P. (1979) Top. Carr. Chem. 86, 1. Bednar (1990) Theoretical Foundations of Radiation Chemistry. Kluwer Academic Publishers, Dordrccht. Birks J. B. (1970) Photophysics of Aromatic Molecules. Wiley-Interscience, London. Braun C. L., Kato S. and Kipski S. (1963) J. Chem. Phys. 39, 1645. Bryce-Smith D., Gilbert A. and Klunklin G. (1973) J. Chem. Soc. 330. Dcllonte S., Marconi G. and Monti S. (1987) J. Photochem. 3o, 37. Dewar M. J. S. (1983) J. Molec. Struc. 100, 42. Fuke K. and Nagakura S. (1977) J. Molec. Spectr. 64, 139. Grabner G., K6hler G., Zechner J. and Getoff N. (1977) Photochem. Photobiol. 26, 449. Halpern A. (1981) J. Phys. Chem. 85, 1682. Kthler G. (1986) J. Photochem. 35, 189. K6hler G. and C-etoff N. (1976) J. Chem. Soc., Faraday Trans. 1 72, 2101. K6hler G. and Getoff N. (1980) J. Chem. Soc., Faraday Trans 1 76, 1576. K6hler G. and C-etoff N. (1981) J. Luminesc. 24/25, 547. K6hler G., Rosicky C. and Getoff N. (1977) Excited States in organic Chemistry and Biochemistry (Edited by Pullman B. and Goldblum.). Reidel, Dordrecht. Lami H. (1977) Chem. Phys. Lett. 48, 447. Rao C. N., Basu P. K. an Hegde M. S. (1979) Appl. Spectr. m,v. 25, 1. Robin M. B. (1974) Higher Excited States of Polyatomic Molecules, Vol. I. Academic Press, New York. Rothman W., Hirayama F. and Lipsli S. (1973) J. Chem. Phys. ~ , 1300. Shizuka H., Nakamura M. and Morita T. (1979) J. Phys. Chem. g3, 2019. Turro N. J., Ramamurthy W., Cherry W. and Farneth W. (1978) Chem. Rev. 78, 125. 7_echner J., Kthler G., G-etoffN., Tatischeff I. and Klein R. (1981) Photochem. Photobiol. 34, 163.
-,00 ,
0
220 240 260 A Into] Fig, 5, Dependence of fl on 2,~ and the absorption spectrum (A) for protonatc~d N,N-dimethylaniline in methanol in the 200
presence of HCIO4. P..PC 3"11516--D
In this paper the significance of molecular excited states of Rydberg type for high energy photochemistry of some polar aromatics is discussed. Such states correlate along the bond dissociation coordinate to the dissociative valence states of N - H or O - H bonds. Their energetic position is in good accordance with the appearance of fast homolytical bond cleavage. Such a process is reflected in a decrease of the fluorescence yield. The measured yields are, however, determined by the dynamics on the Rydberg surface, especially when Rydberg/valence mixing occurs. In related bichromophodc systems, which are of n-valence and Rydberg type respectively, crossing from the valence to the Rydberg surface can be demonstrated and consecutive photochemical processes from the latter are observed. As in radiation chemistry such higher excited states are predominantly produced by the primary processes, their deactivation pathways are thus of general importance also in this respect.
Radiat. Phys. Chem. Vol. 37, No. 5/6, pp. 667-671, 1991 Int. J. Radiat. Appl. Instrum., Part C Printed in Great Britain
PHOTOCHEMISTRY FROM HIGHER EXCITED STATES OF AROMATIC MOLECULES: THE POSSIBLE ROLE OF RYDBERG-LIKE STATES GO H rKIED KOHLER InstitUt fiJr theoretische Chemie und Strahlenchemie, University of Vienna, W~hringerstr. 38, A-1090 Vienna, Austria Abstract--The appearance of wavelength dependent photochemistry (H-abstraction) and fluorescence in some typical polar aromatics (i.e. phenols, anilines, indoles) is correlated to the energetic position of Rydberg type excited states. This behaviour is also demonstrated for carbazole as an example of a larger aromatic molecular system. In bichromophoric molecules, interrelation between excited states of valence and Rydberg type is demonstrated. The significance of higher excited state photochemistry for primary radiation chemical processes is discussed.
INTRODUCTION Formation of excited molecules by irradiation with high energy radiation is either due to direct excitation by interaction of molecules with slow electrons or, probably more important, due to recombination processes of ion-electron or ion-ion pairs (Bednar, 1990). During charge-recombination higher excited states are most generally formed, as the coulombic energy is released to the system. In this case, the ion pair does undergo spin relaxation, singlets and triplets should result with a probability distribution of approximately 1:3 (Bednar, 1990). These states, which are formed primarily, relax in condensed phase by isoenergetic internal conversion and vibrational relaxation to the thermally equilibrated states and photochemistry is normally assumed to proceed therefrom. Nevertheless, it was shown frequently, that photochemistry can also occur from non-relaxed states, prior to relaxation (Turro et al., 1978). One class of such systems which could also be of special relevance to radiation chemistry are simple aromatics, mostly benzene derivatives, with a polar group attached to the ring, i.e. molecules like anilines, phenols and indoles. When the excitation wavelength dependence of these molecules was studied, a remarkable decrease of the fluorescence quantum yield, given by qv, was found, as the excitation was into states higher than the lowest singiet $1 (K6hler and Getoff, 1976, 1980; Zechner et al., 1981). Such wavelength effects were, however, only found in cases where hydrocarbons or water were used as solvents. For other polar environments like alcohols, ether, acetonitrile etc. such an effect appears negligibly small. This decrease in q~ was correlated to an increase of H-atom release in the case of hydrocarbon solvents, but to enhanced
solvated electron ejection for aqueous solution. The latter process was also found by consecutive two photon excitation of higher states via the lowest singlet or triplet (Grabner et al., 1977). The lowest excited states of aliphatic molecules like hydrocarbons, alcohols or water, compounds which are preferentially used as solvents in radiation- and photochemistry, are most generally of Rydberg type, i.e. states of large spatial extension which are derived from atomic orbitals of higher principal quantum number than the valence states (n t> 3 for organic compounds) (Robin, 1974). Such excited states correlate along the bond dissociation coordinate to a dissociative valence state (Robin, 1974). Homolytical bond cleavage is thus the predominant relaxation pathway of the lowest excited state of these molecules. One exception might be cyclohexane, which excited state is appreciably long lived to show remarkable fluorescence (Rothman et al., 1973). The most important exceptions are, however, tertiary amines, exhibiting high fluorescence yields (e.g. triethylamine in n-hexane at room temperature qr = 0.68; K6hler, 1986) but low radical yields (Halpern, 1981). The Rydberg orbital is centred on the nitrogen and NC-bond cleavage must overcome an activation barrier making this process dynamically improbable (Ashfold et al., 1979). Rydberg-like states could also play a dominant role in the relaxation of higher excited states of polar aromatics, as they were found for some anilines (Fuke and Nagakura, 1977) and indoles (Lami, 1977) in their gas phase absorption. They are positioned at energies near to the Sl/$2 overlap region, i.e. the wavelength range where qF decreases. A relaxation mechanism via the Rydberg manifold might be postulated to explain wavelength effects on qr and the efficient H-atom release at the higher energies. This renders a discussion on the possible importance
667
668
GO-~III~-~.DK6HLER
of such Rydberg excitations in polar aromatics worthwhile, which will be presented in this paper.
anilines) and indoles in hydrocarbon solution (K6hler and Getoff, 1976, 1980; Zechner et al., 1981). The universal pattern found is characterized by the following items: (1) qv is constant at wavelengths corresponding to excitations into the S l state. In the $2/S~ overlapping region the yield decreases and stays near a constant value again for $2 excitation. This can be interpreted by the existence of a deactivation pathway originating from the relaxed $2. The quantum yield of such a process is then given by 1 - ~ with
EXPERIMENTAL
The experimental setup and the procedure for obtaining wavelength dependence of qF has already been discussed in detail (K6hler and Getoff, 1976, 1980). The compounds used throughout this work were of best available quality, were sublimed or distilled in vacuo and kept under argon prior to use. Purity was checked by gas chromatography. N,N-dimethyl2-phenylethylamine was prepared as reported previously (Kthler and Getoff, 1980). All solvents used were of spectroscopic or analytical grade. Hydrocarbons were further purified by column chromatography (SiO2 and basic A1203, Merck, under argon athmosphere), ethanol by refluxing over molcular sieve 10 •. RESULTS
AND
=qF(A=,o)/qF(S,).
(2) This process is only found when O - H or N - H bonds are present. Methylation supresses this effect. (3) Only hydrocarbon solutions show a decrease of qE on A=~c(with the important exception of aqueous solutions which shall not be considered in this discussion). Intermolecular hydrogen bonding makes this additional $2 relaxation pathway inefficient. ( 4 ) Homolytical O - H or N - H cleavage is the main photochemical primary process competing to internal conversion. For phenols nearly quantitative agreement between its yield from the $2 state, denoted by c<, and 1 - # is found. For the anilines and indoles quantitative agreement is, however, not achieved. It is interesting to prove whether such effects are restricted to this class of molecules mentioned above or whether they are also found for larger molecular systems. For this purpose the excitation wavelength dependence of qF of carbazole and N-methylcarbazole was examined and the results are given in Fig. 1. Clearly a similar picture in comparison to the smaller molecules is found: qF of carbazole decreases in steps when the excitation energy is increased. There are, however, important differences to the smaller systems. The first step appears at higher energies than the $2 excitation and the observed decrease is relatively small ( ~ 10%). A further step is observed near the high energy flank of the third absorption band. No wavelength effects were, however, observed for methylated carbazole and for carbazole in diethylether in the
DISCUSSION
Most studies on excitation energy effects on the fluorescence yield--excluding bichromophoric systems and some special molecules (Turro et al., 1978)---have been done on benzene and its derivatives and on indole. Benzene itself will not be concerned, as wavelength effects are of different origin compared to its polar derivatives (Braun et al., 1963). For benzene this phenomenon is related to the "channel three" problem and the process concurring with relaxation to the fluorescent state is suppressed as the symmetry is lowered by substitution (Birks, 1970). This is completely different to the general pattern found for its polar derivatives. On the other hand, for typical scintilators (e.g. PPO or BBOT), dyes (e.g. rhodamine B) and condensed aromatic systems (e.g. naphthalene, anthracene, pyrene) no )~°~dependence of qF was found for )~=x~> 190 nm and such substances were thus used as quantum counters (Kthler and Getoff, 1976; Birks, 1970). Discussion is thus concentrated on that class of molecules as polar benzene derivatives (phenols, ,.",
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i
"~
300 A [nm] Fig. 1. Dependence of the relative fluorescence yield/~ on the excitation wavelength A~ for carbazole in n-hexane (1, x and • for two different concentrations, 2 x 10-6 and 1 x 10-3 M) and diethylether (2). An absorption in n-hexane. Rydberg transitions calculated as given in the text are marked in this figure.
Photochemistry of aromatic molecules respective wavelength range. Excitation wavelength effects are thus restricted to :t < 260 nm (i.e. 38,500 cm-~), and are less efficient for this larger molecule. On the other hand, they are suppressed by methylation and in hydrogen bonding solvents as well. It is attempted to correlate the onset of an additional relaxation pathway occuring at higher excitation energies with the appearance of Rydberg-like states. They should be of the same type as are the lowest excited states in the corresponding aliphatic systems, alcohols and amines. The (n,3s) transitions of amines are found around 240-250 rim, and for the alcohols near 200 nm (Robin, 1974). The (n,3p) bands of the amines are located around 200 nm (Robin, 1974). The frequencies of Rydberg transitions fit to the modified hydrogen atom formula (Robin, 1974): hv = li - R / ( n - 6
)2.
(2)
Herein/~ is the ionization potential towards this series converges for large principal quantum numbers n. The second term in equation (2) containing R the Rydberg constant (R = 109, 737.1 cm -m) is called the "term value". 6 is the quantum defect and differentiates between transitions to 3s and 3p type excited orbitals. Increasing the chain length, the term value for 3s converges to approx. 27,200 cm -I, corresponding to ¢5 = 1.0 and for 3p it is typically 18,500 em -l. These values seem, however too high for aromatic molecules. For N,N-dimethylaniline a term value of 23,700 cm-t was found, when related to the vertical transition, for indole 20,100 cm -t and for N-methylindole 22,500 cm -~. The value found for indole seems most likely to be too small as the lowest transition might not be resolved. A term value around 23,000 cm-~ is thus reasonable for these aromatics and was used to calculate Rydberg transition energies. It might, however, be a littletoo small for the phenols, as in the corresponding alcohols the excited states are littlemore valence in character than for the amines. In the same way a term value of 20,000 cm- t was excepted for (n,3p) transitions.Energies of the Rydberg states obtained by this procedure and related to adiabatic ionization potentials are compiled in Table 1, together with the wavelength of onset of the observed qF decrease. A good correlation between these Rydberg energies and the appearance of excitation Table 1. Calculated (n, 3s) Rydbergtransition energies, ionization potentials li (Ran et al., 1979), and wavelength of onset of qv decrease (~.o,) Ii (~,3s) 2o~ Compound (eV) (nm) (nm) Phenol 8.37 228 235 Pyrocatechol 8.28 232 238 Resorzinol 8.30 231 234 Hydrochinon 8.20 237 245 o-Cresol 8.24 234 240 p-Cresol 8.20 236 250 Aniline 7.71 260 265 N-Methylaniline 7.35 281 275 DMPEA 7.70 260 255 lndole 7.76 257 250 Carbazole 7.60 266 265
669
energy effects is obvious. For carbazole calculated Rydberg energies are marked in Fig. 1. Crossing from the vertically excited state to the Rydberg surface is thus energetically feasible. The energy gap between the respective S,(~,~*) and the 3s Rydberg is considerably smaller than between the S, and the lower lying (Tt:r*). Therefore, the (Tt,~*)---,(Tt,3s) internal conversion process could compete effectively to that within the (~,~*)-manifold. Internal conversion to the (~,3s) must, however, be promoted by motions destroying the molecular plane of symmetry (antisymmetric 7~-orbitals) in order to couple these to states with totally symmetric 3s upper orbital. It was shown previously, that deuteration of phenol does not affect fl significantly (Dellonte e t al., 1987). O--H and C - H stretching vibrations should, therefore, be inefficient in promoting internal conversion. This is, however, to be expected for internal conversion to a lower (~,~*) state, as these vibrations possess the largest quantas of energy. This rationalizes also the proposed mechanism. A mechanism via the Rydberg surface should make N - H and O - H bond cleavage from the higher excited state very efficient.Back conversion to the S~ and especially to the So state, at the crossing point of the Rydberg and the respective valence surface, could also occur. The latterprocess would account for the discrepancy between ~ and l -fl values. A n alternative explanation for this discrepancy between the qr decrease found and the measured H2-yields might be due to inhomogcneous cage recombination. The Rydberg states of the aliphaticcompounds are centred on the heteroatom. This is not a priori the case for the aromatic molecules as the firstionization potential corresponds to the 7r-HOMO. It is, therefore, not a priori convincing that these 0t,3s) states correlate to repulsive (n,#*) valence states.Nevertheless, the H O M O can most generally be described as an antibonding combination of the heteroatom lone pair n and the respective aromatic x-orbital. Removing an electron from the ~-system increases the positive charge density on the heteroatom. The (n,3s) should thus closely resemble the (n,3s) in character. The question remains, why for methylated compounds like anisole, N,N-dimethylaniline, N-methylindole, and N-methylcarbazole processes competing to the S,--*S~ internal conversion are rather inefficient, although Rydberg states do exist as well. It is important to notice that cleavage of the respective O-CH3 and N-CH3 bonds in the corresponding aliphatic compounds is also much less efficient as O - H or N - H rupture, but the Rydberg surface is likewise repulsive. The best example, triethylamine, is highly fluorescent (qF = 0.68) and no photochemistry is observed. This stability must be dedicated to dynamical properties of the excited state surface. From this discussion it seems rather probable, that the appearance of molecular Rydberg like states makes homolytical bond cleavage from the higher
670
GOrirRlm KtHt~
PEA
C )
DMPEA
Fig. 2. 2-Pbenylethylamine (PEA) and N,N-dimethyl-2phenylethylamine (DMPEA) in their AM1 geometries (C3 sp3 and CAR aromatic carbon atom). excited (~,~*) enough efficient to compete effectively with S,(~,~*)-*S, Or,lr*) internal conversion. To look closer to the importance of Rydberg excitations in this energy region, wavelength effects were studied on bichromophoric molecular systems, one chromophore of (~,~*) and one of Rydberg type. As model compounds 2-phenylethylamine (PEA) and N,N-dimethyl-2-phenylethylamine (DMPEA) were used (Fig. 2) (Kthler et al., 1977; Kthler et al., 1981). Fluorescence of both systems were studied previously (Shizuka et al., 1979; Bryce-Smith et al., 1973). DMPEA in hydrocarbon solvents exhibits a broad emission spectrum (maximum at 306 nm), red shifted versus the fluorescence spectrum of toluene (maximum around 280 nm). Increasing solvent polarity slightly (e.g. in 1,2-dichloroethane) emission shifts significantly to lower energies, whereas in strongly polar solvents like alcohols dual fluorescence is observed (maxima at 284 and 431 nm). This phenomenon was interpreted by intramolecular charge transfer exciplex formation. Contrary, PEA shows only fluorescence quenching when compared to toluene (Shizuka et al., 1979). Absorption spectra of these systems (Figs 3 and 4) can be constructed by superimposing the spectra of the
two separated chromophores. Figure 2 shows AM1 (Dewar, 1983) geometries for these compounds. In equilibrium the amino group is placed near to the aromatic ring in both molecules. The energy of this minimum is, however, only significantly below the energy of a stretched conformation. The side chain should, therefore, remain flexible but the minimum geometry could still be favoured in liquid solutions, because of dynamical reasons. A distance of 3 A between N and the nearest C atom makes mutual perturbation of the two absorption transitions small. On the other hand, excited state interactions e.g. by charge transfer are favoured by such a geometrical conformation. Figure 3 shows the wavelength effects found for DMPEA in n-hexane and 1,2-dichloroethane, monitored at their emission maxima. No spectral changes were observed varying 2ext. This emission can be attributed to a (Ar- ... N +) intramolecular charge transfer state (Bryce-Smith et al., 1973). Thermal equilibrium is established fast but a distinct dependence of the fluorescence yield on the primarily excited state was found. In n-hexane solution qF remains near to a constant for 2~ >215rim with a slight increase near the (n,3s) absorption. Population of the c.t.-state should thus be only slightly less efficient from the (lr,~*) than from the (n,3s) state (fluorescence from both unperturbed states is very near in energy at ;. ~ 280 nm). A considerably different behaviour is observed for dichloroethane solutions. In this case population of the c.t.-state from the (n,n*) is significantly more efficient than from the (n,3s). The Rydberg state centred on the amino group is efficiently quenched by electron accepting molecules. This is most likely because of the large spatial extension of the 3s orbital. These results demonstrate that Rydberg states of aromatic molecules can be excited in the condensed phase and that it is possible to monitor their specific deactivation pathways. Only a single fluorescence spectrum around 280 nm is observed for PEA in both, n-hexane and methanol as solvents• The spectrum is indpendent of ~.~c and can be attributed to 0r,~*) emission. No fluorescence
:
~.
,%
H
E
P "...'~..A
0.5 i i AT
0
i
210
A " ............... "":'" 1 :.,~...' ........ " :..~:.~%,.. i
230
i
i
250
i
-200
,
270 A [nm]
Fig. 3. Dependence of ~ on ~o for N,N-dimethyl-2-phenylethylamine in n-hexane (H) and 1,2-dichloroethane(DC1E). Absorption spectrum (A) in DCIE and for toluene (AT)and triethylamine (A^) in the same solvent.
671
Photochemistry of aromatic molecules CONCLUSION -800 F3
~
1.0-
-
|
-
I
-
-
~
-600
:00y
/ f~T,C.:.:.~
-200 ~
x~
rio
'
24o
rio'
250
' 270 Alnm]
Fig. 4. Dependence of fl on R~ for 2-phenylethylamine in methanol plus 0.I M HCIO4 (Fl), n-hexane (F2), and methanol (F3) and for toluene in methanol (F4).Absorption spectra given for solutions in methanol plus 0.I M HCIO4 (Ai) and n-hexane (A2).
is observed from primary and secondary aliphatic amines but N - H - b o n d cleavage is very efficient with a quantum yield near to 1 (Ashfold et al., 1979). Intramolecular quenching was described for PEA relative to toluene (Shizuka et al., 1979). Figure 4 gives qF of PEA as a function of )~xc for n-hexane and methanol as solvents. Differently to toluene, considerable solvent effects are observed for the fluorescence of PEA. They are comparable to solvent effects found for, e.g. aniline. Hydrocarbon solutions show a strong decrease of qF, but in alcoholic solvents wavelength effects are considerably smaller. In cyclohexane also Q(H2) increases at higher excitation energies (K6hler and Getoff, 1980). The amino group, although separated from the ring, induces thus a behaviour which is similar to aniline but different to toluene. In the case the amino group is protonated (Fig. 4) qF decreases in methanol also. In this case no (n,3s) state is present. To illustrate this, the effect of protonation is shown for N,N-dimethylaniline in methanol in Fig. 5. Also here, wavelength effects are observed but they are in the same order of magnitude than for toluene (see Fig. 4). Solvent-dependent wavelength effects, which are typical for polar aromatics, thus disappear and they become neady independent of the environment, as was found for benzene and toluene as well.
,.01
° %
01
REFERENCES
Ashford M. N. R., Macpherson M. T. and Simons J. P. (1979) Top. Carr. Chem. 86, 1. Bednar (1990) Theoretical Foundations of Radiation Chemistry. Kluwer Academic Publishers, Dordrccht. Birks J. B. (1970) Photophysics of Aromatic Molecules. Wiley-Interscience, London. Braun C. L., Kato S. and Kipski S. (1963) J. Chem. Phys. 39, 1645. Bryce-Smith D., Gilbert A. and Klunklin G. (1973) J. Chem. Soc. 330. Dcllonte S., Marconi G. and Monti S. (1987) J. Photochem. 3o, 37. Dewar M. J. S. (1983) J. Molec. Struc. 100, 42. Fuke K. and Nagakura S. (1977) J. Molec. Spectr. 64, 139. Grabner G., K6hler G., Zechner J. and Getoff N. (1977) Photochem. Photobiol. 26, 449. Halpern A. (1981) J. Phys. Chem. 85, 1682. Kthler G. (1986) J. Photochem. 35, 189. K6hler G. and C-etoff N. (1976) J. Chem. Soc., Faraday Trans. 1 72, 2101. K6hler G. and Getoff N. (1980) J. Chem. Soc., Faraday Trans 1 76, 1576. K6hler G. and C-etoff N. (1981) J. Luminesc. 24/25, 547. K6hler G., Rosicky C. and Getoff N. (1977) Excited States in organic Chemistry and Biochemistry (Edited by Pullman B. and Goldblum.). Reidel, Dordrecht. Lami H. (1977) Chem. Phys. Lett. 48, 447. Rao C. N., Basu P. K. an Hegde M. S. (1979) Appl. Spectr. m,v. 25, 1. Robin M. B. (1974) Higher Excited States of Polyatomic Molecules, Vol. I. Academic Press, New York. Rothman W., Hirayama F. and Lipsli S. (1973) J. Chem. Phys. ~ , 1300. Shizuka H., Nakamura M. and Morita T. (1979) J. Phys. Chem. g3, 2019. Turro N. J., Ramamurthy W., Cherry W. and Farneth W. (1978) Chem. Rev. 78, 125. 7_echner J., Kthler G., G-etoffN., Tatischeff I. and Klein R. (1981) Photochem. Photobiol. 34, 163.
-,00 ,
0
220 240 260 A Into] Fig, 5, Dependence of fl on 2,~ and the absorption spectrum (A) for protonatc~d N,N-dimethylaniline in methanol in the 200
presence of HCIO4. P..PC 3"11516--D
In this paper the significance of molecular excited states of Rydberg type for high energy photochemistry of some polar aromatics is discussed. Such states correlate along the bond dissociation coordinate to the dissociative valence states of N - H or O - H bonds. Their energetic position is in good accordance with the appearance of fast homolytical bond cleavage. Such a process is reflected in a decrease of the fluorescence yield. The measured yields are, however, determined by the dynamics on the Rydberg surface, especially when Rydberg/valence mixing occurs. In related bichromophodc systems, which are of n-valence and Rydberg type respectively, crossing from the valence to the Rydberg surface can be demonstrated and consecutive photochemical processes from the latter are observed. As in radiation chemistry such higher excited states are predominantly produced by the primary processes, their deactivation pathways are thus of general importance also in this respect.