Quenching of Low-Lying Excited States in Porphyrins by Electron

20 Quenching of Low-Lying Excited States in Porphyrins by Electron Acceptors in Rigid Matrices

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: October 15, 1986 | doi: 10.1021/bk-1986-0321.ch020

Zbigniew Gasyna, William R. Browett, and Martin J. Stillman Department of Chemistry and Centre for Chemical Physics, University of Western Ontario, London, Ontario, N6A 5B7 Canada

The photochemical formation of one-electron-oxidized, porphyrin π-cation radical species, in frozen solutions containing alkyl halides, has been characterized by optical absorption, magnetic circular dichroism (MCD) spectroscopies and the measurement of fluorescence intensity decay curves. The absorption and MCD spectra were analysed using a spectral envelope deconvolution computer program based on least squares and Simplex fitting procedures. The results of the deconvolution calculation of these spectra are consistent with the A and A models of the electronic ground state for ΖnΤΤΡ and MgOEP •, respectively. The electron transfer process has been investigated by measuring fluorescence decay curves using a single-photon counting technique. The rate constant of the reaction has been determined for a number of the donor-acceptor systems, and was found to exhibit a strong dependence on exothermicity, reaching a maximum of 10 s in the most favorable reactions. These results are interpreted as indicating that there is an electron transfer reaction between the lowest excited singlet state of the porphyrin donor and the alkyl halide acceptor molecule. 2

2

zu +.

1u

+

13

-1

Electron transfer reactions of photoexcited porphyrins and related compounds, have long attracted interest as model reactions f o r the photosynthetic apparatus of chloroplasts. In previous studies ( 1 - 3 ) , we have shown that the products of photolysis i n frozen solutions of a number o f porphyrin-alkyl chloride donor-acceptor systems, a r e the one-electron o x i d i z e d r a d i c a l species of the porphyrin and the reduced r a d i c a l species o f the acceptor. We now report a spectroscopic characterization of the porphyrin ττ-cation r a d i c a l species formed, using absorption and magnetic circular dichroism (MCD) spectroscopies. In addition, we have c a r r i e d out a complete spectral envelope deconvolution calculation that uses both absorption and MCD data to obtain band energy and intensity values f o r these complicated spectra. These results a r e compared with the spectral values predicted by theoretical calculations f o r both the A and A model species ( 4 ) . 2

2

2 U

0097-6156/ 86/ 0321 -0298506.00/ 0 © 1986 American Chemical Society Gouterman et al.; Porphyrins ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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GASYNA ET AL.

Quenching of Low-Lying Excited States in Porphyrins

299

The study of the mechanism of the light-induced, intermolecular electron-transfer that occurs i n f r o z e n , r i g i d matrices between porphyrin electron donors, and quinone and alkyl halide electron acceptors, i s an attractive experimental approach. Experiments c a r r i e d out i n the s o l i d state provide a good environment i n which to test the dependence of both the separation distance and excitation energy on the reaction rate, because the effects of diffusion become relatively insignificant. In such systems, electron transfer may be considered as a process that i s competitive with the normal radiative decay of the e x c i t e d state. Accordingly, it has been shown i n a number of studies ( 5 - 7 ) , that the efficiency of the "static" quenching of the donor or acceptor luminescence intensity correlates well with the calculated energetics f o r the electron transfer reaction i t s e l f . Analyses of pulsed excitation experiments (8-11) have demonstrated the presence of the predicted, but unusual, kinetics of the fluorescence decay measured from organic molecules which were studied i n cryogenic glasses i n the presence of electron acceptors. The photochemical oxidation of porphyrins i n frozen solutions containing alkyl halides, i s shown i n this paper to involve the lowest excited singlet state of the porphyrin. We present results of deconvolution calculations c a r r i e d out on the fluorescence intensity decay curves, from which we are able to show that the rate constant for the r a d i c a l pair formation depends on the exothermicity of the electron transfer reaction. Experimental Materials and spectroscopic procedures. H T P P , ZnTPP and MgTPP were synthesized according to published procedures ( 1 2 ) . MgOEP was kindly supplied by Dr. J . F a j e r (Brookhaven National Laboratory). Reagent grade 2-chlorobutane, BuCl (BDH) was freshly d i s t i l l e d under nitrogen. C B r (Kodak) was purified by recrystallization. Spectranalyzed CC1 ( F i s h e r ) was used without further purification. A l l other solvents were distilled under nitrogen before use. The quinones, ρ-benzoquinone, BQ (Fisher), tetrachloro-p-benzoquinone, ρ -C1 Q ( Baker ), tetrachloro -o -benzoquinone, ο - Cl Q ( Aldrich ) , and 2,3-dichloro -5,6-dicyanobenzoquinone, D DQ ( Kodak ) were purified by recrystallization. The porphyrins were dissolved i n an appropriate solvent to which the acceptor was added. For the optical absorption measurements, the porphyrin solutions were placed i n an optical c e l l , which was then plunged into liquid nitrogen i n order to prepare the glass. The c e l l was quickly transferred into an Oxford Instrument CF204 cryostat, and the optical absorption spectra were recorded on a Cary 219 spectrophotometer. A 300 W tungsten-halogen Kodak projector lamp was used for the photolysis of the samples, which was c a r r i e d out i n the CF204 cryostat. The light was f i l t e r e d through appropriate Corning f i l t e r s . The MCD spectra were obtained with an Oxford Instruments SM-4 magnet which was mounted on a CD spectrometer built i n this laboratory. A l l spectral data were stored digitally as r e c o r d e d . 2

4

4

4

4

S p e c t r a l data analysis. The absorption and MCD spectra of ZnTPP and MgOEP were fitted with Gaussian lineshapes, and the band parameters were determined using the r i g i d - s h i f t , Born-Oppenheimer, and Franck-Condon assumptions ( 1 3 ) . Iterative, non-linear, least squares and SIMPLEX fitting procedures, used f o r the absorption and MCD band fitting, respectively, were developed i n this laboratory ( 1 4 ) . In order to enhance the quality of the fits calculated f o r each band envelope, these programs allowed the parameters calculated f o r the absorption spectrum to be used directly with

Gouterman et al.; Porphyrins ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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the MCD and vice v e r s a . The data presented here were calculated using the same parameters to f i t both the absorption and the MCD spectrum measured for each complex. Fluorescence and phosphorescence measurements. Steady-state fluorescence and phosphorescence measurements were made using a cylindrical Pyrex tube (4 mm o.d.) inserted into a quartz Dewar f i l l e d with liquid nitrogen. The fluorescence and phosphorescence spectra were determined on a Perkin-Elmer MPF-4 spectrofluorometer. Low-temperature fluorescence decay data were obtained using a Photochemical Research Associates (PRA, London, Ontario, Canada) Model 3000 Nanosecond Lifetime Fluorometer ( 1 5 ) , which i s based on the technique of time-correlated single-photon counting ( 1 6 ) . A pulsed hydrogen a r c lamp operating at ~30 kHz was used a s the excitation source. Fluorescence lifetimes and decay curves were obtained by application of fast F o u r i e r transform convolution and deconvolution techniques ( 1 7 ) . Suitable programs were developed i n this laboratory for use with the IBM Instruments S9001 computer (18) • Results and Discussion Photolysis of Η , Τ Ρ Ρ . Figure 1 shows a s e r i e s of optical absorption spectra measured at 79 Κ during the visible light irradiation of a glassy solution of H T P P i n BuCl which contained 0.8 mol/L of C B r . The irradiation results i n a decrease i n the intensity of the porphyrin absorption bands located at 650, 600, 550, 525, and 420 nm (Figure 1, line a ) , while new, more prominent absorption bands grow i n at about 690, 495, and 455 nm (Figure 1, lines b, c and d). Figure 1, line e , shows a spectrum which was calculated by subtracting, by computer, an estimated residual amount of the spectrum of the remaining unoxidized porphyrin (Figure 1, line a ) , from the spectrum recorded at the end of the irradiation ( Figure 1, line a ) , on the basis of an estimated 85X photolytic conversion. The spectral features of the photochemical product displayed i n Figure 1, line e , a r e characteristic of a porphyrin π-cation r a d i c a l species ( 1 , 2 ) , thus, we characterize the spectral changes shown i n Figure 1 a s arising from the formation of the Η ΤΡΡ"*"· s p e c i e s . Z

4

2

Photolysis of ZnTPP and MgOEP. A number of metalloporphyrins have been photooxidized to produce the ττ-cation r a d i c a l species at 77 Κ ( 1 - 3 ) . The high yields of the photooxidized product obtained when alkyl halide glasses have been used, has resulted i n the almost complete conversion to the π-cation r a d i c a l species i n s e v e r a l cases. Photolysis of ZnTPP and MgOEP can be c a r r i e d out at low temperatures to y i e l d nearly 100Z of the photooxidized products. The s o l i d lines i n both the absorption (upper) and MCD (lower) panels of Figures 2 and 3, show the spectra recorded after photolysis of ZnTPP (Figure 2) and MgOEP (Figure 3) at 77 K, i n the presence of C C 1 . The starred bands represent a small amount of remaining, unoxidized porphyrin. 4

Analysis of the absorption and MCD spectra of ZnTPP* · and MgOEP**" *. The optical absorption and MCD spectra of ZnTPP"*" · and MgOEP"*" ·, shown i n Figures 2 and 3, a r e representative of the A «""»d A ground states, respectively, that a r e found f o r porphyrin π-cation r a d i c a l s p e c i e s . Results from deconvolution calculations c a r r i e d out f o r both the absorption and MCD spectral envelopes of these ττ-cation r a d i c a l spectra a r e also shown i n Figures 2 and 3, plotted a s the individual component bands that a r e 2

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Figure 1. Absorption spectra measured at 79 Κ f o r ( a ) H T P P before irradiation, and ( b - d ) H T P P after increasing irradiation times, i n a B u C l - C B r (0.8 mol/L) glass, ( e ) The calculated absorption spectrum of 100X Η Τ Ρ Ρ · i n B u C L - C B r (0.8 mol/L) at 79 K. 2

2

4

+

2

4

Figure 2. Computer deconvolutions of the absorption and MCD spectra of Ζ η Τ Ρ Ρ · i n BuCl containing C C 1 at 79 K. The starred band represents part of the spectrum of unoxidized ZnTPP. +

4



Figure 3. Computer deconvolutions of the absorption and MCD spectra of MgOEP i n BuCl containg CCI* at 79 K. The starred band represents part of the spectrum of unoxidized MgOEP. +


20.

GASYN A ET AL.

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responsible for the total, observed intensity. The parameters used for each of these component bands are tabulated i n Table I. This table summarizes data from the theoretical calculations of Edwards and Zerner (4) for both ZnTPP " ·, with either a A or A ground state and MgOEP "*, with a A ground state, giving the energy ( i n units of 1000 cm" ) and oscillator strength (osc) for transitions to a number of e x c i t e d states, identified by their excited state symmetry. The experimental data listed i s for component bands used to fit the observed 77 Κ absorption and MCD spectra (•'Calculated"), with the oscillator strength ( o s c ) indicated for absorption bands and A/D values indicated for bands which were fitted with A terms i n the MCD. In addition, for each absorption spectrum, a s e r i e s of band centres were obtained ("Observed") by measuring the wavelengths of the band maxima directly from the spectra. The deconvolution calculations were constrained to using the minimum number of Gaussian bands that would provide a "best f i t " for both the absorption and MCD spectra using the same set of band centre and band width parameters. Our experience indicates that for spectra containing a large number of overlapping bands with similar intensity, fitting either the absorption or the MCD spectra separately produces ambiguous results. The predicted oscillator strengths ("osc" i n Table I) of the absorption bands i n the visible region of the theoretically-calculated spectra ( 4 ) , roughly follow the trends actually observed i n the absorption spectra, and quantified by the deconvolution calculation, Table I. S p e c i f i c a l l y , in the case of the A ZnTPP " · species, a s e r i e s of overlapping, weak bands i s observed extending from 15 000 - 23 000 cm" (667 - 440 nm), with no well resolved features. By way of contrast, a well resolved band i s seen as the lowest energy band at 15 400 cm" (649 nm) f o r the A MgOEP "* s p e c i e s , this band i s followed to higher energy by a s e r i e s of overlapping bands that are similar to those observed f o r ZnTPP " *. F o r both s p e c i e s , the deconvolution data show that i n the UV region the absorption spectrum i s dominated by several transitions, and that these transitions are a l l much more intense than the bands i n the visible region. The deconvolution results c l e a r l y indicate that these ττ-cation r a d i c a l spectra a r e f a r more complicated than the theoretical predictions have suggested ( 4 ) , and that the band energies determined by the theoretical calculations ( 4 ) , increasingly skew to higher energies as the energies of the transitions i n c r e a s e , relative to the band energies calculated directly from the spectral data. The MCD data not only help to constrain the absorption spectrum fitting calculations, but also provide information about the degeneracy of the e x c i t e d states involved, in the form of MCD A or Β terms. The theoretical calculations (4) have indicated that many of the transitions which give r i s e to intensity i n the absorption spectrum should be degenerate and this should result i n A terms being observed i n the MCD spectrum. However, few distinct A terms were observed in the spectra, Figure 2 and 3. No significant A term contribution was found in the visible region MCD spectrum of the A ZnTPP "· species, although there appears to be a distict A term at 24 300 cm" (412 nm). In contrast, a resolved A term was found i n the visible region of the A M g 0 E P * spectrum, at 15 300 cm" (653 nm), while to higher energies, the A terms appear to be much l e s s intense, i f present at a l l , than was found i n the ZnTPP "· spectrum. 4

2

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1

1

2

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1

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4

Fluorescence intensity decay curves for Η , Τ Ρ Ρ . Figure 4A shows fluorescence decay profiles recorded f o r H T P P i n BuCl and i n B u C l - C B r glassy solutions at 77 K. A single-exponential deconvolution was found to 2


4

304

PORPHYRINS: EXCITED STATES AND DYNAMICS

Table I .

Results of spectral envelope deconvolution calculations c a r r i e d out on the absorption and MCD spectra of ZnTPP*** * and MgOEP"** *.

ZriTOP"** · Ex. State *Azu Symm. E n e r g y ( o s c ) a

E

* 8

*

ZE

Z E

8

Theory

Experiment

0

15

Energy ( o s c )

15.5 ( 0 . 0 1 ) 16.9 ( 0 . 0 0 )

15.0 ( 0 . 0 4 ) 17.4 ( 0 . 0 0 )

19.8 ( 0 . 0 1 )

18.6 ( 0 . 0 1 )

Observed Calculated Energy ( o s c ) Energy 14.6 15.9 16.8 17.9 19.4 20.8 21.8 22.6 23.4 23.9 24.1 24.3 24.5 25.3 26.6 30.1 31.2

16.6 17.9 19.4


21.3 ( 0 . 0 1 ) 22.6

** ** E

E

30.0 ( 0 . 0 1 ) 32.9 ( 4 . 4 2 )

30.6 ( 0 . 0 2 ) 32.5 ( 3 . 8 5 )

24.3 25.3 26.7

*S *S E E

36.6 ( 0 . 0 0 ) 42.2 (0.21) 43.5 ( 0 . 3 9 )

MgOEP*** · Excited s t a t e symm.

37.0 ( 1 . 3 7 ) 40.5 ( 0 . 4 7 )

Theory

0

0

(0.033) (0.056) (0.035) (0.067) (0.088) (0.045) (0.134) (0.169) (0.135) (0.159) (0.008) (0.157) (0.182) (0.095) (0.163) (0.055) (0.118)

MCD A/D

0.863 1.070 -

0.453

-

Experiment Observed Energy

Energy ( o s c )

'Eg

13.9 (0.077)

E

* *

16.1 (0.013)

'Eg

17.9 (0.002)

15.4

17.1 18.5

25.2 (0.032) 'Eg

29.7 (0.502)

'Eg

30.7 (2.434)

25.3

34.2 (0.405) 38.4 (0.850) 41.2 (0.264)

26.6

0

MCD A/D

7

Calculated* Energy ( o s c ) 14.4 (0.007) 15.3 (0.027) 15.9 (0.013) 16.4 17.1 18.3 20.1 21.9 23.5

(0.010) (0.021) (0.044) (0.017) (0.077) (0.068)

25.1 25.2 25.7 26.5 27.4 29.8

(0.295) (0.136) (0.103) (0.154) (0.330) (0.468)

1

0.471

-

-

-

0.409

0.522

-

Prom r e f . ( 4 ) . Band centres a r e given i n units of 1000 cm" . Values obtained by measuring X i n the absorption spectrum only. Calculated with band shape fitting programs to provide acceptable fits i n both the absorption and the associated MCD spectrum. m

a

x


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GASYNA ET AL.

give a satisfactory fit for the fluorescence decay of H T P P i n the neat BuCl glass at 77 Κ (line a). The fluorescence p r o f i l e s f o r H T P P i n the presence of C B r were, however, found to deviate significantly from calculated, single-exponential decay curves ( F i g u r e 4A, lines b and c ) . This deviation was observed with excitation into either the Β band (424 nm) or the Q band (560 nm) regions; both excitations l e d to identical quenching effects. Figure 4B shows the results obtained f o r MgTPP, c l e a r l y quite similar to the H T P P system. In both of these systems, we attribute the fluorescence quenching by C B r to the electron transfer reaction: MTPP* + CBr -> Μ Τ Ρ Ρ · + C B r ~ * ( 1 - 3 ) . The electron transfer process can be treated by formal analogy to the triplet-triplet energy transfer of Dexter* s electron exchange mechanism ( 1 9 ) . We have tested the function derived by Inokuti and Hirayama (20) f o r the decay of donor luminescence in a diliusion i r e e , random ensemble of donor and acceptor molecules. The steady-state luminescence intensity of the donor i s predicted, under these conditions, to depend exponentially on the concentration, C, of the electron acceptor, Z

2

4

2

4

+


4

4

l / l

0

= e

-

4

^

C

/

3

(1)

where, Rq i s a " c r i t i c a l transfer distance". A test of this dependence i s shown i n Figure 5 f o r the fluorescence quenching observed i n a number of ZnTPP-acceptor solutions at 77 K. The time-dependence of the relative emission intensity, I ( t ) / I , may be given by D

I(t)/I

0

= e "I

%

o

%

+

W2)*C

(2)

«

Quenching of Low-Lying Excited States in Porphyrins by Electron

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