\‘olumc 109. nun> bcr 3 CIlE311CXL PllYSICS LETTERS 
SPECTRAL PECULlARITlES OF NH-TAUTOMERISM IN KOCYCLE-CONTMNIN\I’G PORPHYRINS 
AND THEIR COVALENTLY LINKED DINERS 
EA. ZEXKEVICIf. A.M. SHUtGA. A.V. CI-IERNOOK and G-P. GURINOVICH 
I~~sritlcte qf’Pi@cs. t3S.W A dettty of Sciettces. Xittsk 223602. USSR 
Ix has been found e~~~eri~?xe~t~ll~ tbar a whole class of isocyclc-containing porphyrins. inchiding synthetic and natural 
objects and rheir corafently linked dimcrs. SIXXYS XI&taulomerisrn xvhich manifests itscff in isotropic solutions in normal 
cfcctronic spcctm both 11 77 ii and h@her temperatures (up 10 500 IQ. 
I _ introduction 
The investigation of isocycle-containing porphyrins 
and rheir dimers has aroused interest for several rea- 
sow. First, the isocycle is 3 structure element of 
chlorophyll and its natural analogues, and, even more 
so. it participates in the formation of pigment aggrc- 
gates [ 11. Second. it has been shown recently that the 
reacrion ccntcr in photosynthetic systems contains a 
pair of strongly interacting chlorophyll molecules, the 
so-called “special pair”, which is involved in the initial 
photophysic processes of the reaction center (see, 
for csample. ref. [Z!]). So, ;I synthetic porphyrin dimer 
in which isocycles are covalently linked may be a good 
model system for studying natural chlorophyll aggre- 
gated forms and pbotopbysical processes in pigment 
aggregates. 
The investigation of such systems requires taking 
into account the possibility of the appearance of some 
specific effects characteristic of porphyrin molecules. 
First of all. it is known that in porphyrins two inner 
hydrogens which. in their stable conforll3~tion, are 
bound 10 opposite nitrogens. may jump from one pair 
of nitrogens to another [3]_ At room temperature 
these hydrogitns migrate relatively quickly [4]. But at 
77 K the above tnutomerism ceases when the porphyrin 
rr~9ccule is in the ground electronic state IS]. The 
migration of the center protons may srill be induced 
even at l~q~~id-~icli~i~~i temperature by photoexcitation 
of the porphyrin molecule. Such pltototrsnsforn~ation 
306 
between two tautomers at Iow temperatures has been 
observed in two types ofexperiments: (i) photoinduced 
reversible conversion of admixture centers into one 
another for porphyrin-free bases incorporated in an 
n-octane Sltpol’skii matrix [6,7], (ii) variations in the 
emission intensity and fluorescence depolarization of 
glassy solutions of porphyrins at 77 K during excita- 
tion by constant polarized monochromatic light [S]. 
In most cases the spectral separation between the 
individual tautomers (2X) does not exceed 100 cm- 1 
[6,7]. Therefore NH-tautomerism can be effectively 
detected at 77 and 4.2 K in the Shpol’skii matrix only. 
Accordingly, the fluorescence depolarization due to 
photoinduced proton migration may be observed on- 
ly with selective monochromatic excitation [5] _ In 
chlorins, the spectral separation between the two tau- 
toniers is large: 4E = SOO- 1500 cm- 1 [8--i 0] . But 
the second tautomer which can be produced by photo- 
excitation can be stable only at 42 K due to the fast 
reverse pfiotocbeuiical relaxation [IO] _ 
Subsequent to a preliminary study of cyclopentan- 
porpil~r~n ~H-tautol?lerisnl [I I], we present in this 
paper a more detailed consideration of some new ex- 
perimental results which show that a whole class of 
porphyrins - with rhe isocyclic ring - including nat- 
ural objects, their derivatives and covaIentIy linked 
dimers, exhibit NH-tautomerism in isotropic solutions 
both at 77 K and at higher temperatures (500 K), 
which can be detectable in normal absorption and 
fluorescence spectra. 
VOiUlllC 109, nulllber 3 CHEMICAL PHYSICS LETTERS 17 August 1981 
2. Experimental 
Synthesis and ~denti~c3t~on of cycIopen~npor- 
phyrins and their chemical dimers covalently linked 
via isocycles have been described in previous papers 
[ 12,13]. The porphyrins with the cyciopentanonc 
ring have been prepared and purified by farnil& meth- 
ods [14--161. Our csperimentaf results were obtained 
when investigating the following compounds: 
3’,S’cyclo-3’-metl~yl-~,7,8,12.13,17,1S-hcptaethyl- 
porphyrin (1); 3’ ,5’-cyclo-3’-e.uometl~ylen-2,7,S,12, 
13,17,1 S-J~eptaeti~pl-22H,2~l~-l~orphin (2); phyllo- 
erythrin methyl ester (3); deosophylloerythrin methyl 
ester (4); lo-ethosyphylloerythrin methyl ester (5): 
pheoporphyrina5 dimethyyl ester (6); 3-vinylpheopor- 
phyrin-a5 dimethyl ester (7); 4-vinylprotopheophytin 
a (8): j?~eso-tripropyl~yc~opentanporp~lyr~ (9) and 
5’,5’-cyclodimer (10) which consists of two molecules 
1 and 3: 
Cl), RI = H, R2 = CH- 3 (3), R = H 
(2): R,, R? = =CH, (5), R = OC,H, 
(6), R = COOCH, 
Me 
COOR, ‘OoMe 
(7), R, = CH=CH,, 
(S), R, = CH=CH,, 
RI! = C,H5, R, = CH, 
R? = Ck=CH, 
(g), R1 = CH2CH$H3, 
R2 = CH,CH, 
Compounds 1, 3,4, 9, 10 were dissolved in EPIP 
(diethyl ester, petroleum ester, isopropanol 5 : 5 : 2) 
at concentrations <IO-” mol 2-l whereas compounds 
3,s. f&7. 8 were dissolved in Tl-lF-DE (terrahydro- 
furan, diethyl ester 1 : 1) 31 the same concentrariona 
These solvent mi.utures can be frozen as glassy sam- 
ples at 77 K. The absorption spectra were recorded 
on a standard SF-1 0 spectrophotometer or a Beckman- 
5270. The ~~?easure~~~e~~ts of fluorescence excitation 
and emission spectra were made with the aid of an 
SLiG?SOO specrrofluorometer with automatic correc- 
tion of spectral response. 
3. Results and discussion 
When analyzing in detail the spectroscopic observa- 
tions of the invesrigated compounds and symmetrical 
porphyrins [3], some spectral pecuiiariries should be 
emphasized, which are characteristic only of isocycle- 
containing porphyrins. Figs. l-4 give some esperi- 
mental results obtained for porphyrins with different 
side and isocycle substituents. 
(af Absoq~tiort speettp [fig. 2). fn most ca.us, along 
with the four-band spectrum in the visible region. 
which is normaf for porphyrin free bases [3], an addi- 
tional real band or shoulder can be observed on the 
Iong-wave side of the main O-O absorption band at 
293 K. The relative intensity of this new band be- 
comes greater on heating the solutions to 500 K. In 
contrast, the intensity of the additional band decreases 
as the solutions are cooled to 77 K but this band be- 
comes sharper and narrower (fig. 1 b). 
[bj Fhorescence spectra [fig. 2/_ At 77 K the spec- 
tra exhibit features common to all objecis: besides the 
two bands of the main product, there is a new system 
of bands which corresponds to the additional absorb- 
ing center. The relative intensity of these new bands 
307 
Voflrrlie 109. number 3 CHEMICAL PHYSICS LETTERS 17 Augst 1984 
Fig. 1. Nxorf~tion spcctru of compounds 2 (a) and 9 (b) in 
EI’II XXI 5 (c) in l’f-If:-DE. (1) 500 K (in nlcttlylnapIlI~lalenc). 
f2f 793 K. (3) 77 ti. 
is strorlg~y dependent on tbo exciting light wavelengtir. 
It should be noted that the nuoresce~ce lifetimes of 
the 0-O bands for the two Iumincscent centers, mca- 
sured at 77 K, are rather ctose (for example, in solu- 
tion of coltI~ou;~d 2 ?f = 2 1.9 ns at X,, = 6X? nm 
and 72 = 20.3 11s at x,e, = 637 mn). At ;oom temper- 
ature the spectral pat&n is not so dear due to the 
temperature broadening and shift of the fhxescence 
bands. N~vertlle~css, in some cases ~ddit~oI~a1 bands 
can be obscrvcd even at 293 K {fig. 2b). 
(c) l5cirrzriolt s~x?crm offll1or~scc~lcc (f&. 3). The 
analysis of fluorescence excitation spectra recorded at 
different emission wavelengths shows that the system 
of ihorcscence bands ~nay be assigned on@ to two 
fluorescent centers. Comparison between different 
cscitation spectra of nuoresccnce [Fig. 3) and absorp- 
tion spcctrs of the same con~pounds (Gg_ I) permits 
us to conclude that iti ail the cases the two absorp- 
tion cerlters are tlo~i-interacting. Ueariy, at 77 K the 
existence of two centers which Itave their own ff uores- 
cence and ~b~~rptio~ spectra can be revealed for aft 
to 
0.8 
0.6 
0.4 
02 
0 
I?& 2. f~fuorescence emission spectrrt of compounds 2 (a) and 
9 (b) in EPIP and 5 (c) in Tf IF-DE ot 293 K (1) imd 77 K 
f?). Ahescft = 4 nrn, Shnlonit = :! nm. Escitation wwc- 
fen~ths are sIx.wn in prtrentl~eses. 
IO 
08 
06 
04 
02 
0 
450 500 600 h,nm 
Fig. 3. Ftuoresccnce excitation specrrcl af cornpounds 2 (a’) 
and 9 (b) in EPlP and 5 (cj in THF-DE at 77 I(. The reccrd- 
ing \~~~i?ict$$ths are !&OWR in panWfle!ie% AXescit = 2 I%%, 
Ahmonit = 4 nm. Correction of exciting fight spectrai distri- 
bution has been done up to 600 nm. 
308 
‘Jolurnc 109, number 3 CHEMICAL PHYSICS LETTERS 17 Auglsr 1964 
Maxima of absaption and fiuorescence spectra (A, nm), molar decimal extinction coefficients (E X 10e3, P mom-’ cm” i in paren- 
theses) and energy separation &E between So -S1 transitions of tautomers in porphyrins with iwcycle and their dimer 
1 293 399(185) 494(15.5) 531(4.0) 566(6.7) 62Oc7.8) 622 628 
x a) 77 399 499 532 560 61-1 614 679 
** 77 403 496 529 569 622 624 692 
2 293 407( 199) iO8(14.1) 545C13.1) i72(8.6) 627(4.2) 629 696 
* 77 407 507 544 568 621 622 689 
** 77 413 504 535 580 636 637 707 
300 
360 
5 
* 
** 
293 416(700) 519(9.5) 560(15.0) 577(12.5) 628(0.6) 632 696 
77 418 514 5.57 571 623 624 692 
77 420 507 533 561 633 639 709 
380 
370 
7 
* 
*r 
293 419 524 i66 586 639 641 710 
77 322. 523 566 590 637 638 707 
77 47-6 513 546 592 651 652 726 
3z.l 
340 
8 
* 
** 
293 42s 516 568 590 644 649 716 
77 429 530 570 i88 643 644 714 
77 431 516 549 606 653 634 730 
160 
240 
9 
* 
** 
293 414(345) 516(I;.Oj 554(15.8) 587(i. 1) 64iG.4) 646 71s 
77 412 520 55, -“7 582 638 639 709 
77 413 513 543 594 653 654 728 
360 
370 
10 293 397(306) 
293 410(291) 
* 
** 
77 400 
77 412 
77 423 
iOO(24.0) 
SO6(26.1) 
500 
509 
501 
SW(9.1) 
538(18.3) 
534 
549 
534 
566cll.O) 621C8.6) 623 690 330 
574(13.6) 628(4.7) 630 700 300 
561 613 
569 622 
580 637 
624 692 
638 709 
260 
380 
in dirners (these results xvi11 be published elsewhere). 
Increase in the porphyrin concentration by 300-700 
times (say, for compounds 1 and 2) does not cause 
additional changes in electronic spectra as against 
diluted solutions_ lf the results obtained from temper- 
ature experiments (fig. lb) and measurements of flu- 
orescence lifetimes in different bands are taken into 
account, one may conclude that the additional centers 
observed in solutions of the investigated compounds 
are not due to porphyrin molecule aggregation. Below 
we present experimental results which clearly show 
that the two absorbing and fluorescing centers are 
due to No-tautomerism. 
isocycfe-containing porphyrins (tabfe f). The spectral 
separation between the So--S, transitions of these 
centers is different as can be seen from table I _ There- 
fore_ it is difficult for some compounds to find the 
second center at 293 K, but for compounds 2 and 9, 
for example, this center can easily be detected even at 
higher temperatures (413 and 500 K) (fig. 1 b). 
It should be noted that the existence of different 
centers is also found in the covalently linked dimer 
10. Moreover, the analysis of all data obtained for 
dimers points clearly towards the efficient transfer of 
the excited singlet state energy from two centers of 
compound 1 to two acceptor centers of compound 3 
309 
VoIume 109, number 3 CHEMICAL PHYSICS LEXTERS 11 Atqust 3984 
fd] ~~iotoill~llce~ re ersibie co~wersio~~ of the ten- 
fers (pig. 4. Because of the intrinsic asy~lln~etry caused 
by the isocyclic ring, the tautonmrs in the investigated 
compounds are chenlicaily inequivalent and can ab- 
sorb at rather different frequencies (see table 1). There- 
fore. it is possible to realize transformation between 
the two tautomers, which can be achieved by irradiat- 
ing either of them iI1 the region of the priority absorp- 
tion. As can be seen front fig. 4, the fluorescence in- 
tensity of the first tautomer decreases in time upon 
excitation into its absorption band by constant (yolar- 
izcd or non-polarized) light. At the same time, the 
baud of the first tautomer in the fluorescence exci- 
tation spectra decreases while that of the other in- 
creases. The reverse tfaJ?SfOrm7atiOn can be realized 
by exciting the solution into the absorption band of 
the second tautomer. it is essential that we could ob- 
serve in such expcrintents not only a decrease in the 
fluorescence intensity of the first tauromer, but an 
increase in the fluorescence intensity of the other, 
too (fig. 43. curve 3). Consequently, this photoprocess 
is exactly Phototr~nsformation and not ~ilotodestruc- 
Fig. 4. Cftan~cs of fh:orcscence intensity and fluorescence 
cscitation specrra of compounds 2 in EPIP (3) and 5 in 
Tf II:-DE (b) wit11 irrndiation time at 77 K. ahc,cit = 4 nnt, 
lhmonit = 4 nm. (a) (I), (I’): hex = 621 nm. hmon = 690 
nn:; (21, (2’): h,. = 636 nm. h,on = 690 nm (1’ and 2’ - 
dcutcmtcd compound 2 in TIlF-DE); (3): Xes = 622 nm. 
hmOn = 707 nm: (4): poiyvinylburbyral fim as a solvent, T 
= 293 K, h,. = 627 nm, Xmon = 697 nm. {b) (I): h,, = 623 
nm, h,,,on = 692 nm; (2): hex = 637 nm. ,xrnOn = 692 nm. 
Exitation spectrx- initizf: - - - sfzes irradiation into 
the absorprion band of tbc frrst tautomer; _.. after irradIa- 
tion into the ~~)~orption band of the second ~lon~-~v~vei~o~th~ 
?3UtOlIlC~. 
tion. The rate of trans~orIllation under the same exci- 
tation conditions is greater for long-wave tautomers 
than for short-wave ones, which is due to the existence 
of a potentia1 barrier at photoinduced NH-proton jump- 
ing at 77 K. From the temperature dependence of the 
absorption spectra of compound 9 we found that the 
enthalpy of activation was A!!?N~~ =$30--570 CIII-~ in 
the ground state. 
We flave also investigated the effect of deuterium 
substitution of the inner protons in the molecule 
center. For example the addition of deuteroethanol 
to THF-DE solution of compound 2 causes a signifi- 
cant difference between the kinetic curves and. COJI- 
sequently, the quantum efficiency of photoprocesses 
as compared with a similar case where normal ethanol 
is added (fig. 4a). This fact can readily be associated 
with the ~rlllibitjon of ND-tauton~erist~? 3s against NH- 
tautomerism due to the growth of the mass of the 
jumping particles. Additional evidence of the NH- 
tautomerism process is the observation of only one 
absorbing and luminescing form in solutions of tile 
2%complex of conlpound 2 and its di~rotonated de-. 
rivative and the absence of any p~ototransforR]ati~~ 
processes under similar conditions. Finally, we have 
not observed any photoinduced transformations of 
the two tautomers of compound 2 in rigid films of 
polyvinylbuthiral at 293 K. This latter result is due to 
the quick thermal migration of inner protons, which 
inhibits the photoselection of individual tautomers, 
So. directing our attention to the experimental part 
of the present work, it should be noted that for iso- 
cycle-containing porphyrins and their dimers, the 
spectra1 and energy characteristics of individual NH- 
tautomers can be detected with assurance when using 
normal electronic spectra. These objects are fairly con- 
venient for the study of the detailed mechanism of 
tautorller pf~oto~duced tr~j~sfor~ilation- Such esper- 
iments are performed in our laboratory. 
References 
[l] J.J. Katz, J.R. Norris and L-L. Shipman, Brookllaven 
Symposia in Biology No. 28 (1976) p. 16. 
f2j V.A. Shuvalov and A.A. lirasnovsky. Biophysics 26 
(1981) 544. 
f3l G.1’. Gur~ovjch, A.N. Sevchenko and K.N. Solov’ev, 
Spcktroskopiya ~htoro~~a i rodstvenny~h soedinenii 
(Nauka i tekhnika, Minsk, 1968). 
310 
Volume 109, number 3 CHEMICAL PfiYSlCS tETTERS 17 Aqust 198-I 
f4] Ii.X SoIov’ev. V-A. hIssJzenkov, A.-F, Gradyushko. A.11 
Tttrkova and V-P. Lezina, 21~ frikf. Spekrroskopii 13 
(1970) 339. 
IS] EL Zafcsski, V.W. Kotlo, A.N. Serchcnko, K.N. Solor’cv 
and SF_ Sbkirman. D&lady Mad. Nriuk SSSR X07 
t19f2) 1314_ 
{61 0.x Korwiev nnu R.1. f’crsonov. apr. i Spcktrosktopi?‘a 
32 (1972) 400. 
171 6.X Solov’cv, I.E. Zalesski. V.N. Kotlo and S.F. 
Sbkirman. Pis’ma v ZhETPh 17 (1973) 463. 
IS] S.F. Sbkirman, S.M. Arabcy and G.D. Egorow, Zh. 
Prikl. Spektroskopii 31 f19791817. 
f9j S. Vr)ljxr and R.M. %Xfscf&ane. Mol. Cryst. Liquid 
Cryst. 50 (1979) 213. 
311