2298
J . Phys. Chem. 1987, 91, 2298-2303
with phenylaminyl and with triplet phenylnitrene lends support to our previous proposal that the additional weak bands of the latter in the range of 400 to 500 nm are due to n ?r* and/or ?r n transitions.’ Second, the relationship is expected to hold only as long as the ground-state geometries of two isoelectronic species do not differ markedly (cf. the case of diphenylmethyl and diphenylaminyl discussed above).
-
-
Acknowledgment. This work is part of Project 2.21 3-0.84 of the Swiss National Science Foundation. Financial support was
received from Ciba-Geigy SA,Hoffmann-La Roche SA, Sandoz SA, and the Ciba-Stiftung. M.S.P. is indebted to the International Division of the US NSF for a travel grant. Registry No. H2, 1333-74-0; tert-butoxy, 3141-58-0;phenylaminyl, 2348-49-4; 1-naphthylaminyl, 93439-96-4; 2-naphthylaminyl, 9343997-5; diphenylaminyl, 2143-67-1; carbazolyl, 83045-62-9; indolyl, 50614-06-7; 1-pyrenylaminyl, 107099-16-1; aniline, 62-53-3; 1naphthylamine, 134-32-7; 2-naphthylamine, 91-59-8; diphenylamine, 122-39-4;carbazole, 86-74-8; indole, 120-72-9; 1-pyrenylamine,160667-3.
Charactedzatlon of the Exclted States of Photoreactive Dlhydrophenazine Doped in Molecular Crystals B. Prass, C. von Borczyskowski,* P. Steidl, and D. Stehlik Fachbereich Physik, Freie Universitat Berlin, D-1000 Berlin 33, West Germany (Received: October 10, 1986)
Chemically unstable dihydrophenazine can be stabilized by substitution into molecular crystals. The structure of ground and excited states of dihydrophenazine doped in fluorene crystals has been investigated by optical and EPR spectroscopy. The lowest excited singlet and triplet states reveal a planar structure whereas the ground state is nonplanar.
1. Introduction A substantial amount of literature is available concerning the physical and photochemical properties of various 5,lO-dihydrophenazine (PH2) derivatives in solution and glass matrices.’” They are known for the ease with which they transfer an electron and have been used as electron donors for the studies of electron-transfer mechanisms: Particularly the photoinduced ionization found to be the most active photochemical channel has been widely investigated. Moreover, the oxidation-reduction characteristics of PH2 derivatives are of biological importance, as they play a role in biological electron transport systems.’-I0 Ideally, the investigations should start with the simplest molecule in the series of PHI derivatives, i.e., PH2itself. However, PH2 has been found very difficult to handle due to its extreme instability in air, where it oxidizes very rapidly to phenazine. Consequently, only very few data exist on the photochemical properties and even less on the spectroscopic properties of PH, itself. To our knowledge, only Bailey et al.” and Wheaton et a1.12 have published UV absorption and IR spectra of PH2. In some solid-state matrices, however, we found that PH2 is effectively stabilized in the ground state. This offers the possibility to investigate PHI and elementary photochemical reactions of PH2, which presumably start with an electron transfer as the primary reaction steps6 Moreover, due to the fixed geometry of reactants (1) Rubasowska, W.; Grabowski, Z. R. J. Chem. SOC.,Perkin Trans. 2 1975,417.
(2) BriIblmann, U.; Huber, J. R. J . Phys. Chem. 1977,82, 386. ( 3 ) Ruaseger, P.;Moms, J. V.; Huber, J. R. Chem. Phys. 1980, 46, 1. (4) Nelson, R. F.; Leedy, D. W.; Seo, E.T.; Adams, R. N. Fresenius’ Z . Anal. Chem. 1966, 224, 184. ( 5 ) Chalvet, 0.;Jaffe, H.H.; Rayez, J. C. Photochem. Phorobioi. 1977, 26, 353. (6) Keller, H. J.; Soos, 2.G. In Topics in Current Chemistry; SpringerVerlag: Wcat Berlin, 1985; Vol. 127, p 169. (7) Halaka, F.G.; Barnes, Z. K.; Babcock, G. T.; Dye, J. L.Biochemistry 1%4, 23, 2005. (8) Low, A.; Vallin, H.; Vallin, I. Biochem. Blophys. Acto 1%3,69, 361. (9) Michaelis, K.; Schubert,L.; Schubert, M.P. Chem.Rev.1938,22,253. (10) Zaugg, W. S . J. Biol. Chem. 1964, 239, 3964. (11) Bailey, D. N.; Roe, D. K.; Hercules, D. M. Appl. Spectrosc. 1968, 22, 185. (12) Wheaton, G. A.; Stoel, L. J.; Stevens, N. B.; Frank, C. W. Appi. Spectrosc. 1970, 24, 339.
and products, PH2-doped molecular crystals can be regarded as ideal model systems for such reactions. The primary intention of this publication is to give the first detailed report on the optical excitation and emission spectra together with the excited-state lifetimes as well as EPR results for the excited triplet state of PHI doped in fluorene single crystals. Although not in the same detail, spectroscopic data of PH2 in the host crystals biphenyl, dibenzofuran, carbazole, and dibenzothiophene will be presented here. One question of major importance is the geometry of PH2 in the ground and excited states since it is expected to have a strong effect on the photochemical and spectroscopic properties of PH2. If PH2 is planar, the nitrogen “lone pair” orbitals would still participate in the conjugation of the whole ring system. On the other hand, if PH2 is bent along the N-N axis, the conjugation is interrupted a t the N positions and the molecule would be diphenylamine-like.’3 Photoinduced chemical reactions originating from PH2 could be observed in all host crystals which we have investigated. Depending on the host, these reactions can probably be described as a sequence of electron-, proton-, and/or hydrogen-transfer processes. The corresponding data will be published in detail separately.I4J5 2. Materials The structure of PH2 as well as the molecular axes notation used is shown in Figure 1. For easier identification the short notation for phenazine (P) is supplemented with the substituents hydrogen (H) or deuterium (D) in capital letters for the 5- and/or 10-positions of PH2, while the outer ring substituents are given in small letters if required. Selectively deuteriated fluorene host material was prepared as described earlier.I6 Single crystals were grown from the melt, which was doped with 2000 ppm of the guest molecule. PH2 was (13) Adams, J. E.;Mantulin, W. W.; Huber,J. R. J. Am. Chem. Soc. 1973, 95, 5417. (14) Prass, B.;von Borczyskowski, C.; Stehlik, D., to be published. (1 5 ) Steidl, P.; von Borczyskowski, C.; Fujara, F.; Prass, B.; Stehlik, D., submitted for publication in J . Chem. Phys. (16) Furrer, R.; Heinrich, M.;Stehlik, D.; Zimmermann, H.Chem. Phys. 1979, 36, 27.
0022-3654/87/2091-2298$01.50/00 1987 American Chemical Society
Dihydrophenazine Doped in Molecular Crystals
Y
+ o / I
The Journal of Physical Chemistry, Vol. 91, No. 9, 1987 2299 LOO00
30000
25000
I
I
1
20000
E Icm-’
1
15M)O I
T=77K
/
/
Figure 1. Molecular axe9 notation of 5,lO-dihydrophenazine (PH,). The dotted line indicates the two aniline moieties.
synthesized as described in ref 12. Care was taken to prevent any contamination with oxygen during the synthesis and the doping procedure. Some of the experiments reported here were, however, done with crystals grown from the melt and doped with phenazine as the only guest. By analysis of optical data as well as EPR data, it was realized and confirmed that PH2 was inevitably formed as a result of an intrinsic reaction in fluorene during the crystal preparation from the melt and was, besides P, incorporated into the growing crystal as a second substitutional guest. P is known to be photoreduced to PH2 in hydrogen-donating solvents; see ref 17. As the fluorene methylene group provides rather acidic hydrogens, this reaction is likely to occur also in the melt of fluorene in the ground state when doped with a guest containing a hydrogen-attracting group like nitrogen in azaaromatics. The involved hydrogen abstraction from the fluorene methylene group should produce the same fluorenyl radicals as for example shown for the photochemical reaction reported earlier.16 However, no trace of such radicals has been found in the final crystal. Presumably the radicals deriving from fluorene undergo secondary reactions like neutralization at defects and surfaces or dimerization, the resulting larger molecular structure of which may not be incorporated efficiently into the fluorene lattice. In any case, except for PH2 and P, no other molecules of significant concentration could be observed by optical spectroscopy or EPR in freshly grown crystals. In the reduction of P to PH, just described, the hydrogen atoms in the 5,lO-pitions of PHI are expected to come specifically from the fluorene methylene group. Consequently, in the melt of the available host materials fluorene-h8-H2 and fluorene-d8-H2, dihydrophenazine (PHI) will be formed. For the same arguments PD2 will be formed in the hosts fluorened8-Dz and fluoreneh8-D2. Hyperfine data together with the isotopically sensitive lifetime data of the dihydrophenazine T1 state of synthetically prepared PH, and P-d8H2doped into the fluorene matrix support the above statements. Independently, fluorene single crystals doped with synthesized PH2 have been grown. Spectroscopic results are the same, except for the fact that now only PH2 is the dominant guest molecule. Moreover, when doping PH2, it is possible to obtain isotopic guestrhost combinations like PH2 in fluorene-h8-D2.
3. Experimental Section The excitation and emission spectra were taken with a commercial Shimadzu fluorescence spectrometer (Model R F 540). The phosphorescence spectra were obtained at 77 K with the sample in a glass bath cryostat which can be equipped with a cylindrical rotor, whose window geometry allowed only long-living luminescence to reach the detection monochromator. Phosphorescence lifetimes were measured with a pulsed nitrogen laser as excitation source and signal averaging via single photon counting in a multichannel analyzer. In this case temperature variation was achieved in a He flow cryostat. A platinum resistor in close proximity to the sample was used as temperature indicator. (17) Beak, P.; Messer, W. R. In Photochemistry of Heteroaromatic Nitrogen Compounds;Chapman, 0.L., Ed.; Marcel Dekker: New York, 1969; VOl. 2.
Figure 2. Survey of the excitation and total emission spectra together with the phosphorescence spectrum of P-h8H2in fluorene-h8-D2taken at 77 K. A complete listing of the SI So and T1 So vibrational transitions is given in Table I.
-
-
TABLE I: Fluorescence and Phosphorescence Vibrational Energies and Transitions A, nm E, cm-l E - Em, cm-’ intensity” Fluorescence 433.3 23 079 0 vw 438.0 447.7 453.0 460.0 463.0 474.0 481.0 498.0
22831 22 336 22 075 21 739 21 598 21 097 20 790 20080
510.6 5 15.0 520.3 526.1 530.7 536.0 547.5 553.0 559.0 566.0 575.0 580.0 602.0
9 585 9417 9 220 9 008 8 843 8 657 8 265 8 083 7 889 17668 17 391 17 241 16611
248 742b 1003c 1340 1480b 1982c 2289b 2998b
W
vs W
vw W
m S
vw
Phosphorescence
0 168 365 577 742b 928 1320 1502b 1696 1917 21936 2344b 2973b
vw W W W
vs W
W W W W S
S
vw
‘v = very, w = weak, m = medium, s = strong. bNote the alternating intensities in the progression. Isotope-dependent vibrations which are no longer observed upon deuteriation of the aromatic PH2 protons.
The fluorescence lifetimes were measured at the BerlinerElektronspeicherring fur Synchrotronstrahlung (BESSY) using the radiation of the synchrotron in the single bunch mode for excitation. The repetition rate was 4.8 M H z with a n excitation pulse width less than 0.6 ns. Time correlation spectroscopy18was used. A nitrogen flow cryostat served for temperature variation. The EPR spectra were taken with a commercial Varian X-band spectrometer E-109 using a 100-W Hg lamp in combination with a Schott SFK 5 filter as excitation source. For temperature variation we used a He flow cryostat. Details of this setup are described elsewhere.15 4. Results Photochemistry of PH2 upon UV excitation could be observed in all matrices. Consequently, for spectroscopic investigations of PHI, to which we will restrict ourselves in this publication, only (18) Lopez-Delgado, R.; Tramer, A.; Munro, I. H. Chem. Phys. 1974,5, 12.
2300 The Journal of Physical Chemistry, Vol. 91, No. 9, 198 7
Prass et al. 2 50' .
101
I
I
I
180
200
I
I
1
I
I
220
2LO
260
I
1
280 T I K 300
F i e 4. Temperaturedependence of the fluorescence lifetime of P-d8H2 in fluorene-d8-H, ( 0 ) .The dotted and solid lines correspond to the fits of eq 1 and eq 2, respectively, to the data (see text). The S, lifetime of P-d8D, in fluorene-dlo-D2(X) is also given for two temperatures.
Figure 3. Total emission (left side) and phosphorescence spectra (right side) of PH2 in dibenzothiophene(a), carbazole (b), dibenzofuran (c), and biphenyl (d) host crystals taken at 77 K with an excitation wavelength of 350 nm. The dotted lines indicate the wavelength of maximum emission of PH, fluorescence and phosphorescence in the fluorene matrix (cf. Figure 2).
moderate excitation intensities have been applied in order to minimize these effects. 4.1. Excitation and Emission Spectra. Figure 2 shows the excitation and emission spectra of fully protonated PH2 in fully protonated fluorene at 77 K. The emission spectrum is essentially due to PH2as checked with an undoped crystal. At wavelengths longer than 510 nm the emission spectrum is composed of both fluorescence and phosphorescence. The phosphorescence is also shown separately in Figure 2. Phosphorescence and fluorescence spectra show a very similar vibrational structure and Franck-Condon envelope. The vibrational energies are given in Table I. In both cases the zero vibronic transition is strongly suppressed and the emission intensity rises slowly, reaching a maximum at 447 and 530.7 nm for fluorescence and phosphorescence, respectively. These maxima are part of a predominant vibronic progression with an energy of about 750 cm-l, which is identified immediately in both spectra (cf. Table I). This is in good agreement with the two very strong bands at 735 and 745 cm-' found in the IR spectrum of PH2.12 Thus, the origins of the emission spectra can be calculated to give a wavelength of about 433 nm for the fluorescence and 5 1 1 nm for the phosphorescence. Most of the remaining vibrational transitions that cannot be associated with the 750-cm-I progression appear in both fluorescence and phosphorescence spectra. The SI So transition is found to be polarized parallel to the c axis of the fluorene host crystal, Le., the long molecular axis, whereas the T, So transition is polarized perpendicular to the c axis. Fluorescence spectra were also taken for various isotopically substituted PH2guest molecules. While deuteriation of the PHI aza protons had little or no effect, spectral changes could be observed upon deuteriation of the aromatic ring protons, the predominant of which is the disappearance of the line at 474 nm. The corresponding vibrational energy of about 1982 cm-' cannot be directly interpreted by a C-H vibration, but it could be associated with the second harmonic of the weak vibrational band at 453 nm (1003 cm-') which shows a similar isotope effect and can be attributed to C-H out-of-plane vibrations.12 N o notable line narrowing could be observed for both the excitation and emission spectra when going to lower temperatures. The large line width is probably due to the existence of various PH2sites in the fluorene host crystal. Direct indications for such a site distribution were found in recent measurements where drastic line narrowing and spectral changes could be observed at 1.5 K in the PH, fluorescence upon using narrow-band excitation into
-
-
the long-wavelength absorption edge with an excimer pumped dye laser. However, these measurements are still at an early stage and we have not yet taken extensive luminescence spectra. The excitation spectrum in Figure 2 shows almost no structure. The excitation origin is suppressed with respect to higher vibronic lines. This is accompanied by a Stokes shift of about 1000 cm-' (see section 5). In Figure 3a the total emission spectra of PH2 at 77 K are collected for the host matrices dibenzofuran, carbazole, dibenzothiophene, and biphenyl. In contrast to fluorene, the PH2 fluorescence has practically no structure in these hosts and the emission maxima show a red shift, which is strongest in the biphenyl host, where the maximum of emission is found at about 500 nm. A similar red shift can also be observed in the excitation spectra. As in the fluorene host, again large absorption-emission energy gaps can be observed. Note the strong increase in phosphorescence intensity relative to the fluorescence in the dibenzothiophene host. This is the result of an increased spin-orbit coupling due to the external heavy-atom effect of the host sulfur atom. In Figure 3b the phosphorescence spectra are shown separately. While the emission maxima in carbazole and dibenzofuran are at nearly the same wavelength as in fluorene (dotted line), a red shift of the maximum to 542 nm can be observed in the biphenyl host. In the latter the spectrum is best resolved. On the other hand, in the dibenzothiophene host the phosphorescence maximum is shifted to higher energies by about 330 cm-I as compared to the case of fluorene. 4.2. Fluorescence Lifetime. The SIlifetime of P-d8H2in fluorene-d8-H2 was measured between 170 and 300 K. In all cases the S, decay was found to be exponential over at least 2 orders of magnitude in the signal intensity. With a signal-to-noise ratio better than 40:1, the experimental error in the lifetime was less than 3%. The results are shown in Figure 4. The lifetime decreases from 40 ns at 180 K by about a factor of 2 on going to room temperature. For the fully deuteriated system P-d8D2in fluorene-d8-D2 the S1 lifetime has been measured at 180 and 300 K. These data are also shown in Figure 4. One question of importance is whether the observed photochemistry has any major effect on the temperature dependence of the SI lifetime. In this case the responsible reaction must essentially be reversible since the rate constant for the observed PH2decomposition is on the order of m i n ~ t e s . ' ~No J ~ short-living photoproducts have, however, been observed so far. Furthermore, the lack of a marked isotope effect upon deuteriation of the photochemical active central protons (cf. Figure 4) excludes any reaction involving nuclear tunneling that is strongly dependent on nuclear masses as has been observed for H-transfer reactions in acridine and phenazine-doped fluorene crystal^.'^^^ Therefore, ~~
(1 9) Tietje, M.; von Borczyskowski, C.; Prass, B.; Stehlik, D. Chem. Phys. Lett. 1986, 127, 415.
The Journal of Physical Chemistry, Vol. 91 No. 9, 1987 2301
Dihydrophenazine Doped in Molecular Crystals
~
TABLE 11: Triplet Lifetimes (in ms) of Dihydrophenazine in Various Matrices' 4.2 K I1 K 300 K P-hSH2 in F-h& 490 340 20 510 30 P-dsH2 in F-h& P-d&2 in F-d& 830 530 P-d& in F-dsD2 1850 P-d8D2 in F-h& 3350 1700 70 P-h& in DBT 460 70 " F = fluorene, DBT = dibenzothiophene.
since electron-transfer reactions are contrary to H-transfer reactions not much affected by temperature,21we believe that the temperature dependence of the SI decay is not of photochemical nature. The temperature dependence of the fluorescence lifetime of PHI derivatives in various solutions has been investigated by Huber et aL3J3 The decay was found to be of intramolecular nature, and it was proposed that intersystem crossing (ISC) is the dominant SI decay route thus also determining the temperature dependence, while on the other hand internal conversion from SI to So is of negligible importance. In all cases the temperature dependence of the decay could be described by
k ( T ) = ko + kle-Ea/kT
(1)
and preexponentials in the range of 2 X lo8 s-l together with activation energies between 360 and 820 cm-I were found. In our case the fit of eq 1 to the SI decay rate of P-dsHz in fluorene-d8-Hz (broken line in Figure 4), however, shows deviations from the data which are rather more of systematic nature than statistical. A substantially better fit (see Figure 4) could be achieved assuming an additional temperature-dependent decay channel. The SIdecay rate is then described by
k(T) = ko
+ k,e-El/kT+ kZe-El/kT
(2)
The results obtained for the parameters from the fit of eq 2 to the experimental data are ko = 2.3 X lo7 s-l, k , = 1.6 X lo8 s-l, El = 570 cm-', k2 = 3.6 X lOI4 s-l, and E z = 3400 cm-'. The values for the parameters kl and E, lie well in the range of those derived from the temperature dependence of the intramolecular SI decay rate obtained for the PHz derivatives in ref 13. In accordance with the interpretation in ref 3, they presumably describe the temperature dependence of the ISC process for which preexponentials in the range of lo8 s-I are quite typical. Depletion of the SI state via an energetically close Sz state seems a likely process responsible for the additional high-temperature decay channel. This provides independent evidence for a closelying Sz state about 3400 cm-I above SI (see section 5.1). Since the SI S2energy difference is expected to be strongly solvent dependent, this could explain why the high-temperature decay channel is not observed in the temperature dependence of the SI decay of the PHI derivative^.^ An increase of not more than 500 cm-I in the SI Sz energy gap would be sufficient to suppress the high-temperature decay channel nearly completely up to room temperature. 4.3. Phosphorescence Lifetime. In contrast to the fluorescence lifetime, evaluation of the TI triplet lifetime of PHz turned out to be rather difficult. Over the whole temperature range between 4.2 and 300 K the TI decay was found to be nonexponential. A biexponential fit gave little more satisfying results. From the latter it could be concluded that in all cases the deviation from an exponential decay is less than 6% in the amplitude. The most severe problem, however, was that the decay curves were not reproducible after a temperature cycle. The time constants for successive exponential fits varied up to 15%. The nonexponential TI decay is presumably a convolution of various contributions from PHz sites, and the irreproducibility after warming up the sample can result from a change of either this convolution or the pho-
-
-
(20) Colpa, J. P.; Prass, B.; Stehlik, D. Chem. Phys. Lett. 1984, 107, 469. (21) Marcus, R. A.; Sutin, N. Eiochim. Biophys. Acta 1985, 811, 265.
TABLE III: Experimental Fine Structure Parameters mixed crystal
P-d& in F-d& P-d& in F-h& P-dsD2 in F-h& aniline-d2 in p-xylene phenazine in F-hgD2
PI,
y4
cm-
1157 ( 5 ) 1145 (5) 1190 (2) 1229 (5) 734 ( I )
PI,
cm-'
90 (5) 83 (5) 156 (2) 352 (5) 107 (3)
L(z~,N, deg 54 (1) 53 (1) 57 (1)
ref EPR EPR ODMR 25 24
tochemistry between PH, and fluorene. Despite a quite large error, crude TI lifetime data obtained from freshly cleaved crystals during the first temperature cycle and a single-exponential fit of the whole decay curve are listed in Table 11. Two major results should be noted here. The first is the strong temperature dependence of the lifetime observed in all host crystals. For example, the TI lifetime of fully deuteriated in fluorene-h8-Dz decreases from about 3.4 s at 4.2 K to 70 ms at room temperature. This is more than a factor of 30. Whether a photochemical TI decay channel is responsible for this strong temperature dependence is a subtle question and will not be discussed in this context. The second result to be pointed out is the strong isotope effect observed upon deuteriation of the 5,lOpositions of PH2 which cannot be attributed to a different nonexponentiality. The TI lifetime increases by more than a factor of 3. This is an unusually large deuteriation effect considering that only two hydrogen positions have been isotopically substituted. On the other hand, deuteriation of the remaining aromatic ring protons leads only to a comparably small increase in the TI lifetime despite the increased number of substituted positions. As will be discussed in detail in the following sections, there are strong indications for a nonplanar So ground state of PH2, the molecule being bent along the N-N axis. Consequently, the hydrogen atoms at the 5,lO-positions attain nonaromatic character in the ground state. It is, however, known that the radiationless conversion of electronic energy into vibronic energy of nonaromatic protons can be considerably more effective than for aromatic protons.22 Hence, we believe that the extreme sensitivity of the TI decay upon deuteriation of the aza protons as compared to the aromatic ring protons can be well understood in terms of an intramolecular mechanism. 4.4. Electron Paramagnetic Resonance. Both P-d8Hz-doped flUOrene-d8-Hz and P-d8Hz-dopedfluorene-h8-Dz single crystals were investigated. Due to the molecular and crystal symmetry, measurements with the magnetic field oriented within the crystalline a,b plane were sufficient to evaluate the fine structure parameters and the angles between the principal axes and the crystalline b axis from the angular dependence of the EPR sign a l ~ The . ~ ~results are summarized in Table 111. For comparison, the corresponding data for phenazine24 and are also given in Table 111. The hyperfine structure due to the I4Nnuclei could be partially resolved. The line pattern is very similar to that observed for the phenazine triplet state.23 From that we conclude (i) the predominant hyperfine splitting is due to two equivalent I4N nuclei and (ii) the anisotropy between the largest h y p e r k e coupling A,, along the out-of-plane axis z and Ayyalong the short in-plane axis is at least A,,/A, > 5 . Assuming two equivalent nuclei, the EPR spectra could be simulated quite well. The resulting tensor elements of the I4N hyperfine interaction are shown in Table IV together with the data for phenazine.z3 By optically detected magnetic resonance (ODMR) of P-dgDz in fluorene-h8-Dz in zero magnetic field at 1.5 K, it has been possible to observe the ID(+ (El and ID1 - IEl transitions at 3.304 0 1 (2) and 3.801 (2) GHz, respectively. The resulting values for 1 and IEl (see Table 111) slightly differ from the EPR results, which (22) Yang, N. C.; Muror, S . L.; Shieh T.-C. Chem. Phys. Left. 1968, 3, 6. (23) Furrer, R.; Petersen, J.; Stehlik, D. Chem. Phys. 1979, 44, 1. (24) Furrer, R.; Gromer, J.; Kacher, A.; Schwoerer, M.; Wolf, H. C. Chem. Phys. 1975, 9, 445. (25) van Noort, H. M.; Vergragt, Ph. J.; Herbich, J.; van der Waals, J. H. Chem. Phys. Lett. 1980, 71, 5.
2302
The Journal ofphysical Chemistry, Vol. 91, No. 9, 1987
TABLE IV: I4N Hyperfine Tensor Elements Determined for the Triplet States of Phenazine,O Phenazinyl," and Dihydrophenazine in Fluorene L(ZHmb),
guest molecule
aisx
lAJ, MHz
IAyyl,MHz
deg
28.4 (5)
> A,,,,, A x 2 3 )has been observed unambiguously in the PH2 triplet state (see section 4.4). A point of interest is the 14N hyperfine coupling constant A,, of PHz, which is about a factor of 2 smaller as compared to that of phenazine. This is presumably the result of a reduced r-spin density at the N positions since the Q factors relating the hyperfine coupling constant to the *-spin density32should be nearly identical in phenazine and PH2. In fact, a reduction of the *-spin density can be rationalized within a simple M O consideration. In phenazine the highest bonding and lowest antibonding orbitals which are singly occupied in the T, state both contribute almost equally to the *-spin density at the N positions, as can be concluded comparing the *-spin densities of the phenazine triplet state23and the corresponding phenazine anion radical.33 In the case of planarity of the TI state the *-electron system of PHz differs from phenazine in that additionally two more electrons are accommodated in the *-system, occupying the two lowest antibonding orbitals. Since the second antibonding orbital is of symmetry A, within the DZhpoint group and thus has a nodal plane containing the S,lO-positions, it cannot contribute to spin density at the N positions. Thus, only the unpaired electron in the lowest antibonding orbital can give rise to spin density at the N position in the central ring, and consequently only half the amount of spin density is observed. Since these symmetry arguments require a planar TI state of PHI, this can be regarded as further evidence +
-
(31) Burns, D. M.; Iball, J. Proc. R. SOC.London, A 1955, 227, 200. (32) Camngton, A.; McLachlan, A. Introduction to Magnetic Resonance; Harper International Edition: London, 1969. (33) Carrington, A.; dos Santos-Veiga, J. Mol. Phys. 1962, 5 , 21. (34) Stehlik, D.; Furrer, R.; Macho, V. J . Phys. Chem. 1979, 83, 3440.
for the planarity of the excited T, state. It is interesting to note that for the phenazinyl radical the *-spin density is also about a factor of 2 smaller at the N position of the N-H fragment as compared to the opposing aza position (see Table IV). As a consequence of the low spin density in the central ring, the D value of PHz should differ substantially from phenazine. Indeed, as can be seen in Table 111, the D value comes very close to that of aniline. Hence, the unpaired electron spins are located simultaneously in either the left or the right side of the dotted line intersecting the PHz molecule in Figure 1. In this sense PH2 can be regarded as a uminiexcitonn consisting of two interacting aniline moieties, as has been suggested by Huber et al.30 6. Conclusions
The major results can be summarized as follows. Photochemically active PH2can be stabilized in molecular crystals and, hence, for the f m t time allows a detailed characterization of the structure and dynamics of PHz. The conclusions from the optical as well as the EPR data were found to agree well with those previously reported for 5,lO-substituted PH2 derivatives in s o l ~ t i o n s . ~ ~ ~ ~ The optical spectra as well as the S1 lifetime data of PH2 suggest a symmetry-forbidden So S1 transition borrowing intensity via Herzberg-Teller coupling from an allowed S2state lying closely above S,. Most likely the nontotally symmetric skeletal vibration observed in both phosphorescence and fluorescence spectra is effective in this coupling. Furthermore, depletion of S1 via a low-lying Sz state can explain the additional high-temperature S1decay channel observed in the temperature dependence of the PHI fluorescence lifetime. From the optical spectra it can be concluded that the geometry of the excited SI and TI state cannot differ substantially. The EPR data of the lowest T1 state confirm the planarity of the PH2 central ring, which has been predicted independently by theoretical calc~lations.~ Consequently, the PHz geometry in S1 should also be essentially planar. Furthermore, it could be concluded from the different behaviors of the absorption and emission spectra that a marked change in the molecular configuration takes place upon excitation. There are indications that the geometry change involves a change of the hybrid character of the nitrogen orbitals, leading to a butterfly-like structure in the ground state, the molecule being bent along the N-N axis. Such a geometry is, however, unfavorable in the rigid host matrices and requires to some extent a redistribution of the neighboring lattice molecules. As a consequence, a site distribution of PHI molecules is not unlikely. The broad fluorescence and phosphorescence line widths, the nonexponential TI decay, together with the sensitivity of the PH2 emission spectra on the host matrices strongly indicate such a distribution. In view of the photochemistry observed between PHz and the neighboring fluorene host molecules,15a site distribution allowing for different reaction pathways can be of considerable importance. The change of the binding character of the nitrogen orbitals upon excitation is presumably a major cause for the strong photochemical activity of PHI at the central ring positions. Removement of an electron or hydrogen atom from the 5,lOpositions could stabilize the planar structure even in the ground state. At least the latter reaction is clearly observed in the fluorene host crystal^.'^ Moreover, there are also indications that electron transfer is an intermediate step to the above hydrogen abstraction.I4 -+
Acknowledgment. This work was supported by the DFG (Sfb 16 1). Crystals were prepared by R. Brunn. Dihydrophenazine
was synthesized by M. Schulz. Isotopic modifications of fluorene were synthesized by H. Zimmermann (Heidelberg).
