4530
J. Phys. Chem. 1994,98, 45304535
Distance Dependence of Excited-State Double Proton Transfer in Porphycenes Studied by Fluorescence Polarization Jacek Waluk' Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44, 01 -224 Warsaw, Poland Emanuel Vogel Institut f i r Organische Chemie der Uniuersitiit Kiiln, Greinstrasse 4, 0-50939 Cologne, Germany Received: October 27, 1993; In Final Form: February 14, 1994'
Polarization of fluorescence excitation has been measured both for free-base porphycene and three of its alkyl derivatives which differ by the length of their intramolecular hydrogen bonds. Examination of the measured anisotropy values reveals a rapid photoinduced tautomerization occurring in the lowest excited singlet state. The reaction rate is crucially dependent on the distance between the hydrogen-bonded nitrogen atoms. The process becomes too slow to be observed on a nanosecond time scale in the derivative with the largest Ne-N separation. A more detailed analysis of the anisotropy data makes it possible to determine the directions of the electronic transition moments.
Among the many systems that undergo tautomerization, of particular interest are those in which the reactants and products are formally the same. Representative examples include porphyrin, malonaldehyde, tropolone, and dimers of carboxylicacids. The ground-state behavior of such structures has been studied by numerous experimentaland theoretical methods.'-% By contrast, relatively little is currently known about the excited-statereactivity of such "narcissistic" systems. This is because the detection and monitoring of the relevant excited-state processes often requires the use of such less-routine techniques as matrix isolation and supersonicjet spectroscopy. Nonetheless, high spectral resolution sometimes makes it possible to detect tunneling splittings: they have been found for hydroxyphenalenonesS1*s2 and tropolone.s3.s4 Also, site splittings between the two tautomeric forms have been observed in the electronic and IR spectra of porphyrin20.sSs66and phthal~cyanine~~ in rigid low-temperature matrices. Such splittings reflect the fact that the formally similar tautomerization reactants and products interact differently with the matrix. Similarly, a shift of a proton in a molecule immobilized in a rigid matrix can serve to effect a change of the position of the whole molecule with respect to the microenvironment. As a result, when the tautomerization is induced with polarized light, the resulting pseudorotation can lead to a partial alignment of the sample, provided that the back proton transfer in the ground state is frozen out. Such alignment phenomena have been observed for porphyrin in low-temperature rare gas matrices.s5v56s68 Indeed, linear dichroism studies of partially aligned porphyrin have enabled the determination of the transition moment directions in both the IRSs and visibles6 regions. In this work, we use fluorescence polarization spectroscopy as a tool for the study of excited-statetautomerization in a series of free-base porphycenes, namely the parent porphycene ( l ) , 2,7,12,17-tetra-n-propylporphycene (2), 9,10,19,20-tetra-n-propylporphycene(3), and 2,3,6,7,12,13,16,17-octaethylporphycene ( 4 ) (Chart 1). The porphycenes are newly-reported isomers of porphyrins that are becoming the subject of increasing attention of both chemical and spectroscopicresearch gro~ps.'6J0-~8As compared to the porphyrins, the porphycenes exhibit both similarities and differences. Some of the latter are revealed by investigations of the internal N H tautomerization chemistry. In prior work, for instance, it has been found by *SNCPMAS-NMR that the rate of ground-state tautomerization in porphycene 1 is much faster
* Abstract published in Aduance ACS Abstracts, March 15, 1994. 0022-3654f 94f 2098-4530%04.50 f0
CHART 1
-
2 R
1
'MR R
R &R
H
n-C3H7 R
H
than in porphyrin and, in fact, cannot be frozen out on the NMR time scale at 107 K.16 Similarly, fluorescence polarization spectroscopic studies'' indicated that upon excitationporphycenes 1-3 undergo rapid tautomerization, a process that occurs on a time scale faster than the decay time of the first excited singlet state (ca. 10 ns). Again, this behavior is contrary to that of porphyrin, where the phototautomerization process is slow and probably only occurs in the triplet manif0ld.~~~~~.62 Left undetermined, however, by these earlier studies is whether the unique tautomerization behavior of the porphycenes, relative to the porphyrins, reflects an intrinsic property of the porphycene skeleton or is due, in particular, to the smaller N-N distances found in the porphyrcene series of tetrapyrrolicmacrocycles. Thus, what was needed was to carry out a study by, e.g., fluorescence polarization spectroscopy, of the tautomerization behavior of a series of porphycenes in which the internal N-.N distance could be varied at will. Such a study, which constitutes a subject of the present report, was made possible by the availability of a series of porphycenes in which the N-.N distance had been varied in a systematic way. Recently it was found that the substitution of appropriately positioned peripheral hydrogen atoms in 1 by alkyl groups leads 0 1994 American Chemical Society
Double Proton Transfer in Porphycenes
The Journal of Physical Chemistry, Yol. 98. No. 17, 1994 4531
B.
toa modulation of theinner cavity dimensions with regard to the parent system." In fact, depending on the number, size and position of the alkyl substituents, it is possible to decrease the distance between the hydrogen-bonded nitrogen atoms, as in, for instance, compound 3, increase it, as in 4, or keep it unchanged, as in 2. Now, from analysis of the fluorescence polarization data for these four compounds (Le. 1-4) we find that the rate of phototautomerizationin porphycenes is highly dependent on the magnitude of this critical distance. Indeed, as discussed bclow, excited-statedouble proton transfer, rapid in compounds 1-3, is not observed at all in compound 4, where in the relevant N.-N separation is increased hy only 0.14.2 A. Experimental Section The syntheses of porphycenes were previously describcd.6+'1 The compounds were purified by recrystallization as follows: Compound 1from dichloromethaneln-hexane, 2 from n-hexane, 3 from cyclohexane, and 4 from dichloromethane/methanol. Solvents propanol, 2-methyltetrahydrofuran (Merck), and 3methylpcntane (Fluka) were purified hy chromatography or distillation and checked for residual luminescence prior to use. Fluorescence, fluorescence excitation,and polarization spectra were recorded on a Jasny ~pectrofluorimeter.7~ controlled by a PC. The spectral resolution was better than 90 cm-I in the Soret region and better than 40 cm-I in the Q and fluorescence hand regions, respectively. Specialcarewas taken toensurethat thevaluesoffluorescence polarization were not distorted by reabsorption, energy transfer, or formation of aggregates. The presence of the latter is manifested by the appearance of a new fluorescence hand that is red-shiftedwith respect to that observed for normal monomer emission. Such red-shifted emission bands are observed at low temperatures in both 3-methylpentane and propanol even at concentrations as low as 5 X IW M. Thus, the sample concentrations were always kept below 3 X 1W M. Under these conditions, the shape and intensity of the fluorescence band were found to be unchanged as function of excitation wavelength. Moreover, the fluorescence decay profiles were found to be monoexponential." indicating that no aggregates were present. The 0-0 hands of absorption and emission were found to overlapstrongly. This forced us toshift the region of observation to one of lower energy when the SI0-0 band was being excited. Unfortunately,the polarizationof fluorescence was found tovary slightly across the fluorescence band, decreasing in the lower energy portion. In order toget reliablevaluesfor the fluorescence anisotropy in the 0-0 region of the emission corresponding to theexcitationintothe W b a n d o f t h e f i r s t excitedsinglet state, we used the following procedure: The wavelength of monitoring the fluorescence was shifted away from the 0-0 band and the polarization of the fluorescence excitation spectrum recorded in the region of the SI and S2absorptions. This was done starting from the highest possible excitation wavelength that did not produce artifacts in the emission (i.e., scattering errors due to spectraloverlapbetweenthe excitation and emission bands). This process was repeated until the whole 0-0 band corresponding to the SI - S O absorption had been scanned. Superposition of the polarization curves obtained at different emission wavelengths revealed that they overlapped by 50.02 anisotropy units. Thus, this defines the relevant margin of error. The anisotropy values of the 0-0 fluorescence band for the 0-0 SI excitation may be higher than theonesobtainedforslightly lower emissionenergies by only this small amount. For each porphycene, theobserved anisotropy values were found to he quantitatively the same independent of solvent. ThelNDO/Smethodsl was used for thecalculationof excitedstateenergiesand transition moment directions. About ZOOlowest singly excited configurations were taken into account in the CI
Figure 1. Calculated directions of transition moments for the low-lying (T.T*) states of porphycene.
E l "LzJ m c
Q
- 2 14 -.2 14
16 16
18 18
20 20
26
28
30xlO'cm-'
Fiyre 2. (top) Fluorescence (dashed line) and fluorescence excitation spectra (solid line); (bottom) anisatropy of fluorescence excitation of 1 in propanol at 113 K. Fluorescence was detected at the emission maximum, except for the region below 16300 cm-I, where the p r d u r e
described in the Experimental Section was used. Q and B symbols mark the positions of different electronic bands, taken from MCD work." procedure. Thegeometryof 1used for thesecalculations was the result of minimization by molecular mechanics using the MMX method?2 ReSults
Figure 1 presents the results of INDO/S calculations of the transition moment directions for compound 1. The moments of the first two transitions (QI and Q2) are predicted to be nearly orthogonal. On the other hand, those correspondingto the Soret bands (BIand B2)are predicted to bisect approximately the angle formed by theQl andQ,transitionmoments. Verysimilarresults werealsoobtained when thesame typeofcalculations werecarried out using the CNDOIS and PPP methods." Thus, one is led to expect strong negative emission anisotropy in the region of the second electronictransition, Q2,positive anisotropy for the Soret bands and naturally, highly positive values for the transition to the first excited singlet state, QI. The experimental results, shown in Figures 2-5, reveal that the above prediction is confirmed only for 4, but not for the other three molecules. In particular, the absolute values of the anisotropy in 1-3 are much lower than expected. This is most striking in the region ofthe first transition, where theexperimental values are much below the theoretical limit of 0.4. It is also true in the region of the second electronic band, where thevalues close tozeroareobserved,insteadof thestrongly negativeanisotropies predicted by calculations. We have previously interpreted the low anisotropy values in 1-3 as being the manifestation of a rapid excited-state tautomerization proecss. If such a proccss occurs on a time scale shorter than the SI lifetime, then the transition moment corre-
Waluk and Vogel
4532 The Journal of Physical Chemistry, Vol. 98, No. 17, 1994 I
I
I
cx C 1
-C C
0 .ffl
.-
E
W
16
1
18
20
24
26
28
30
14
32 I L
c
p ;2
‘C Q
16
18
24
20
26
28
Figure 3. (top) Fluorescenceand fluorescenceexcitation spectra;(bottom) anisotropy of fluorescence excitation of 2 in propanol at 113 K. See caption to Figure 2 for details. 7
14
14
16
16
18
18
16
18
23
24
26
28
30
~
24
26
28
30xlO’cm-’
Figure 5. (top) Fluorescence and fluorescenceexcitation spectra;(bottom) anisotropy of fluorescence excitation of 4 in propanol at 113 K. See caption to Figure 2 for details.
-
24
24
26
26
28
28
-
Figure 6. Changes of transition moment directionsupon tautomerization of porphycene. ct is the angle between SI SOtransition moments in the initial and final form,3!, denotes the angle between the moment of some higher transition and the horizontal molecular axis.
30
30x1O3cm-’
Figure 4. (top) Fluorescence and fluorescenceexcitation spectra;(bottom) anisotropy of fluorescence excitation of 3 in propanol at 113 K. See caption to Figure 2 for details. sponding to the emission should oscillate between two directions: That of the initially excited form and that of its tautomer (Figure 6). Let us denote by a/2and 0 the angles formed by the moments of the emitting and absorbing transitions with the horizontal molecular axis. The fluorescence anisotropy is given by
rta,P) = ’ / f ( P - 4 2 ) + l/2r(P + 4 2 ) = {3[cos2(P- a/2) cos2(P a/2)]- 21/10 (1)
+
+
The factor 1 / 2 results from the assumption of equal fluorescence intensities in both tautomeric forms. For the parallel orientation of the transition moments of the absorbing and emitting states (e.g. for absorption to S I )/3 = a/2 and eq 1 is reduced to
r ( a , a / 2 ) = 1 / 2 r (+ ~ )1 / 2 r ( a=) [3 cos2(a)
+ 11/10
(2)
It is clear from the above expression that for all values of a different from zero the resulting anisotropy will be lower than 0.4. Moreover, measuring the anisotropy when exciting into S1 may yield the value of a and thus the direction of the S1 So transition moment. From the polarization spectra shown in Figures 2-4 we obtain the following values of a/2: 30 f 3’ (l), 31 3’ (Z), and 40 f 5’ (3). They arevery similar in the three molecules and agree well with the INDO/S theoretical prediction of 42O (Figure 1). Using the above values of a and the experimental anisotropy values makes it possible to find the values of 0 from eq 1, and
-
*
20
-14
30xlO’cm-’
18
0
-.2
14
16
thus to determine the direction of transition moments of the higher excited states. The results, however, will be accurate only when the electronic bands are well separated. This is obviously not the case in the Soret region of the porphycene spectra, when the B1 and Bz bands strongly overlap. Also, another ( T , T * ) transition of medium intensity has been calculated to lie about 2000 cm-1 below B1. This transition may be responsible for the relatively high anisotropy values observed in the region preceding the Soret bands in 1, 2, and 4, since the calculated transition moment direction is nearly parallel (within 2’) to that of the emitting state. The fact that the polarization pattern of 1-3 is completely different from the “normal” behavior of 4 has profound chemical consequences. It implies that the excited-state tautomerization process, rapid in 1-3, is too slow to be observed in the S1 state of 4. Another explanation of the anisotropy pattern of 4,which retains the idea of fast tautomerization in this molecule, would require that the transition moment directions in 4 are tilted by about 4 5 O with respect to those of 1-3. This is very unlikely in view of both experimental and theoretical results. Such a hypothesis can therefore safely be rejected. Thus, we conclude that compound 4 qualifies as a “normal” chromophore, with its transition moments firmly fixed within the molecular frame. To the extent the above conclusion is correct, Perrin’s classic formula84 may be used to determine the angle 0 between the transition moments for absorption and emission:
r(e) = (3 cos2@) - 1)/5
-
(3)
-
From the anisotropy values in the region of Sz (Qz),we obtain 79O f 5’ for the angle between S 1 SOand Sz SOtransition moments, in excellent agreement with the theoretical value of 85’.
In the Soret region of the absorption spectrum of 4,we observe varying degrees of anisotropy. This is caused by the strong overlap
Double Proton Transfer in Porphycenes
The Journal of Physical Chemistry, Vol. 98, No. 17, 1994 4533
TABLE 1: Distances (A) between Nitrogen Atoms along with 1H NMR Shifts 169 270
371 487
2.63 2.62 2.53 2.80
2.83 2.83 2.90 2.13
3.15 3.04 6.82 0.6Y
Vertical distance in the formula. Horizontal distance. Reference 83.
Figure 7. Trans (left) and cis (right) tautomeric forms of 1.
of at least three separate transitions. Thus, only limiting values for the angles 0 may be estimated. We obtain 0 < 33O for the absorption region around 25 000 cm-l, 0 > 45’ for the peak at 26 600 cm-l, and 0 > 50’ for the 28 000-29 000-cm-1 region. The finding that the rateof phototautomerization in 4isreduced in comparison to that observed in 1-3 can be explained in terms of the differences in the ground state N.-N distances observed by X-ray diffraction methods and in terms of the differences in the measured lH NMR chemical shifts for the N H protons (Table 1). In fact, from an analysis of these separate but interrelated phenomena, it is evident that compound 4 not only has the largest Ne-N separation but also the weakest NH to N hydrogen bonds. This is perhaps a reflection of the fact that in compound 4 the ”horizontal” N.-N distance is smaller than the “vertical” one and that, as compared to 1-3, different pairs of nitrogen atoms are actually involved in the hydrogen bonding. In any case, it is interesting to note that a relatively small increase in the N.-N distance leads to a significant decrease in the rate of photoinduced tautomerization. Indeed, since the lifetime of S1 in 4 is about 6 ns,gO km for this compound must be < 1 O S s-1, whereas for compounds 1-3 km must be much greater than los s-I. Another consequence of the anisotropy results for compounds 1-4 is the presence of one or more barriers in the excited-state potential energy surface associated with the tautomericmovement of the internal protons. In thecaseof a single-minimum potential, the transition moments can only be oriented along the horizontal or vertical axes, as a result of symmetry considerations. In such an instance, the observed anisotropy should be as high as that of the Q1 band in the region of one of the Soret bands and as low as that of the QZband in the region of the other. Experimental results, however, reveal the highest anisotropy values for the Q1 band. This is consistent with the predicted double-minimum character for the excited state and not for a single-minimum surface. In view of the above, it is of interest to note that the doubleminimum character has also been assigned to the ground state of crystalline porphycene 1.16 In this instance, however, the double-minimum character could not be taken for granted. This is because the N-N distances in 1 are very short compared to those in porphyrin and, in fact, come close to those seen in socalled proton sponges, molecules in which very strong hydrogen bonds are 0bserved.*5-*~However, it may be that the doubleminimum character in the ground state is related to the fact that the hydrogen bonds in porphycenes are not linear. Our MMX calculations for 1 give the value of 138’ for the N-H-N angle. An important consideration that has yet to be addressed with respect to the structure of the porphycene tautomers concerns the possible existence of more than two equivalent species. So far, we have based our discussion on the assumption that there are only two trans tautomeric forms and it is these two species that interconvert between one another. In principle, however, one should also take into consideration the possible existence of cis tautomers (Figure 7). This is particularly true since the N M R studies16 revealed the presence of both cis and trans tautomers in the crystalline parent porphycene, 1. Assuming that (i) excited-state tautomerization involves four different forms, (ii) the interconversion rate is rapid with respect to the lowest singlet excited-state lifetime, and (iii) all the species have the same fluorescence quantum yield, one arrives a t the
following expressions for the emission anisotropies:
Here, y and b are the angles between the horizontal axis x and the moments of the emitting and absorbing transitions in the cis form (due to symmetry, they can only assume values of 0’ or 90°), gis the ground-state fraction of cis tautomers being excited (in general, it may be wavelength-dependent), e denotes the fraction of the cis tautomers in the excited state and r(a,@)and r(0) are given by eqs 1 and 3, respectively. In previous we have simulated the anisotropy values for g and e varied from 0 to 1, using the calculated transition moment directions of the Q and B states. The conclusion was that a certain amount of cis form in theground state is acceptable, as long as this form is not dominant in the excited state. The present results are consistent with this suggestion. In particular, they do not provide clear spectral evidence for the presence of cis tautomers. For instance, the fluorescence intensity of porphycenes 1-4 is observed to decay in a monoexponential manner while the shape of the emission is found to be independent of excitation wavelength. However, this does not preclude the possibility of both forms being present, especially if a rapid equilibrium between cis and trans tautomers is established in the excited state. A somewhat stronger argument against the presence of the cis form comes as a result of calculations. The dipole moment values computed by INDO/S for the cis tautomer are 3.7 D for the ground state (3.3 D by MMX), 6.7 D for the SIstate, and 0.4 D for the S2 state. Such large changes in the calculated dipole moments should be reflected in terms of observed solvatochromic shifts in both the absorption and emission bands. However, both the absorption and emission spectra of porphycenes are but weakly solvent dependent. This points to the dominance of trans tautomers which, of course, have a zero dipole moment as a result of their symmetry. As further support for the above conclusion, we have checked that the anisotropy data may be interpreted without involving a substantial contribution from the cis form. As stated above, the a/2 angles obtained for 1-3, made under the assumption that only the trans tautomers are present, were not only found to be similar in all three compounds, but also to be in good agreement with the results of calculation. The results for the angle @ for the Sz state, derived under the same assumption, are also reasonable. For compound 1, for instance, we obtain @ = 112 3’. This is to be compared with the calculated vlaue of 127’. Further, and perhaps of greater interest, is a comparison of the angle between the S1and Sz transition moments found in 1 and 2 with those found in 4 ( the value for compound 4 is obtained from the experimental data by the “ordinary” formula of eq 3). We get 88 f 5’ for 1, 79 f 4’ for 2, and 79 f 5’ for 4. In compound 3, the SIand Sz bands overlap too strongly to allow an accurate determination of @.The calculations for compound 1 give 85O, in excellent agreement with experiment.
*
4534
The Journal of Physical Chemistry, Vol. 98, No. 17, 1994
Conclusions
In summary, the present work serves to illustrate that fluorescence polarization analysis can be a very valuable tool for studying fast excited-state tautomeric interconversion processes between formally identical porphycene structures. The methods and analyses presented here should not, however, be limited to the study of porphycenes. Rather, they could prove informative in analyzing phototautomerization processes in a wide range of compounds. Further, these methods could find application in other areas such as, for instance, the study of photoinduced isomerization reactions or in the analysis of fast electron-transfer processes. In terms of the porphycenes per se, the main finding of this work is the discovery that the rate of tautomerization is an extremely sensitive function of the distance between the atoms linked by the internal NH-H hydrogen bonds. In this context, it is to be recalled that changes in the internal N-N separation can be realized in a formally simple manner, namely by steric modulation of the porphycene system by alkyl substituents in proper positions. In order to assign numerical values to the rate of excited-state tautomerization, time-resolved studies in the pico- or possibly, subpicosecond range will be required. Such studies, which would allow changes in transition moment direction to be observed as a function of time, would provide a direct measure of the speed of proton transfer. Such studies, therefore, are currently under consideration. Finally, there remains the intriguing question concerning the importance of cis tautomers. If present, these structures could probably be spectrally separated from their trans counterparts by using high-resolution methods, such as matrix isolation spectroscopy. Such studies are now in progress.
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