Inter- and intramolecular quenching of the singlet excited state of

J. Phys. Chem. 1993, 97, 2596-2601

2596

Inter- and Intramolecular Quenching of the Singlet Excited State of Porphyrins by Ferrocene Richard Ciasson,’ Eric J. Lee, Xiaohong Zbao, and Mark S. Wrighton’ Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received: July 27, 1992; In Final Form: November 30, 1992

Inter- and intramolecular quenching of the lowest singlet excited state of three porphyrins by ferrocene derivatives is reported. 5,15-Bis(4-tolyl)-2,3,7,8,12,13,17,18-octamethylporphyrin (1) and two derivatives of 1 where one of the tolyl methyl groups was replaced by a ferrocenylvinyl group, 2, or by a ferrocenylethyl group, 3, were prepared. Porphyrin 2 was isolated as a mixture of cis (73%) and trans (27%) isomers. Singlet excited state properties were studied by steady-state emission spectroscopy and by emission lifetime measurements. The relative quantum yields of fluorescence for 2 and 3 compared to 1 are 0.38 and 0.84, respectively. Fluorescence decay lifetimes of 1and 3 a r e 15 and 14 ns, respectively. Fluorescence of 2 is revealed to be due to the emission of two species (cis and trans isomers) with lifetimes of 4 and 13 ns. The shorter fluorescence lifetimes and smaller fluorescence quantum yields for 2 and 3 compared to 1 are attributed to quenching of the singlet excited state of the porphyrin by the ferrocenyl centers. However, the fraction of quenching by electron transfer and energy transfer could not be quantitatively measured. The rate constant for quenching is no more than lo8 s-I, consistent with electron-transfer quenching. Intermolecular quenching rate constants for the quenching of the porphyrin singlet excited state by ferrocene derivatives were also found to be consistent with an electron transfer quenching mechanism.

In recent years, considerable effort has been devoted to studies of photoinduced intramolecular electron transfer in multicomponent As part of a program on optical energy con~ersion,~ our laboratory has been involved in the preparation and the study of such molecules.8 Recent work has focused on the synthesis and the characterization of a series of porphyrins 1-3, twoof them having linked ferrocenyl centers. Theporphyrin/ ferrocene system is attractive because the porphyrin chromophore absorbs over most of the solar spectrum: its singlet excited state can be reduced by ferrocene according to thermodynamic arguments, and the ferrocenyl centers can be reversibly oxidized. The lowest excited singlet state of ferrocene is higher in energy than the lowest excited singlet state of the porphyrin, ruling out efficient singlet energy transfer from the excited porphyrin to the ferrocene. There have been several reports of ferrocene-substituted porphyrins in theliterature.lSl3 The preparation of a porphyrinferrocene-quinone assembly was described by Beer and Kurem.Io Intramolecular quenching of porphyrin fluorescence was observed, but the quenching was attributed to rapid intramolecular electron transfer from the excited porphyrin chromophore to the quinone acceptor. It is likely that the ferrocene center employed was incapable, thermodynamically, of reducing the singlet excited stateof the porphyrin chromophore. Maiya, Barbe, and Kadish” observed that ferrocene quenches the triplet excited state of a metalloporphyrin. Although the authors did not rule out quenching by an electron-transfer mechanism, the data were interpreted in terms of an energy-transfer mechanism. Quenching of the triplet excited state of porphyrin by an energy-transfer mechanism is likely since ferrocene has a low-lying triplet excited stzte. In this paper, we present results consistent with the conclusion that ferrocenyl centers can reduce the porphyrin singlet excited state intermolecularly and intramolecularly. Others have claimed ferrocene to reduce the lowest excited state of UO2*+.I4 It has been recently shown that ferrocene can serve as an electron donor to the lowest excited state of Cr(2,2’-bi~yridine)3~+ I s and can quench excited Ru( 11) polypyridyls by both electron donation and energy transfer.16 * Address correspondence to this author.

’ Current address: Dtpartement deChimie (204), Universittde Montreal.

C.P. 6128, Succ. A, Montrtal, Qutbec, Canada, H3C 357.

0022-3654/93/2097-2596$04.00/0

Results Synthesis of 1, 2, and 3. Porphyrin 1 was synthesized in two steps, Scheme I, using known methodology.17-18Condensation of 4-met hylbenzaldeh yde (4) with 3,3’,4,4’-tetramethyldipyrrylmethane (5)I7gaveporphyrinogen 6in 92%yield. Subsequent oxidation of 6 with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone, DDQ, afforded porphyrin 1 in 96% yield. The synthetic precursor 7 of porphyrins 2 and 3 was prepared similarly, Scheme I, using this time 1 molar equiv of 4-methylbenzaldehyde, 4, and 1 molar equiv of 4-(hydroxymethy1)benzaldehyde, 8, with 2 molar equivs of 3,3’,4,4’-tetramethyldipynylmethane, 5. Since the resulting mixture of porphyrinogens, 6 , 9 , and 10 is unstable, the mixture was oxidized with,DDQ to the corresponding mixture of porphyrins, 1, 7, and 11 prior to separation. The desired porphyrin 7 was then isolated from the mixture by flash chromatography (CHC13/MeOH 99: 1) and was isolated in 22% yield (or 44% of the theoretical yield assuming statistical formation of the porphyrins 1, 7, and 11). Oxidation of compound 7, Scheme 11, with pyridinium chlorochromate, PCC,19 gave aldehyde 12 in 80% yield. The ylide 13, obtained by deprotonation of (ferrocenylmethy1)triphenylphosphonium iodide20 with n-BuLi in THF, was added to the aldehyde 12 to give porphyrin 2 in 82% yield as a mixture of cis (73%) and trans (27%) isomers. HPLC analysis showed evidence of two isomers, but the resolution was inadequate to effect separation on a preparative scale. Finally, catalytic hydrogenation of the unsaturated porphyrin 2 in acetic acid gave porphyrin 3 in 82% yield. Optical Absorption and Emission Properties of 1, 2, and 3. Absorption spectra of 2 and 3 are nearly identical to that of 1, Figure 1, as is expected due to the low absorptivity of the ferrocene chromophore (A,, = 440 nm, c = 90 M-1 cm-1)21compared to porphyrin. No additional bands due to charge transfer or perturbation of the porphyrin chromophore are observed. This indicates relatively little ground-state electronic interaction between the electron donor (ferrocene) and electron acceptor (porphyrin) subunits. Emission spectra of dilute, deoxygenated solutions of 1,2, and 3 in 2-methyltetrahydrofuran, 2-MTHF, were recorded at room temperature. All three species show two emission bands centered at 630 and 698 nm. Relative quantum yields of fluorescence of 0 1993 American Chemical Society

The Journal of Physical Chemistry, Vol. 97, No. I I, 1993 2597

Inter- and Intramolecular Quenching by Ferrocene

SCHEME I: Synthesis of 1 and Precursor 7

li

I

4

4: R a w 0: R a CH&H

0: R a R ' a C ) I , 9: R 8 c y ; R' a chon IO: R a R ' a C y O H

SCHEME 11: Conversion of 7 to 2 and 3

7

12

2

2 and 3 compared to 1 are 0.38 and 0.84, respectively, Figure 1. The decrease in fluorescence intensity in 2 and 3 is attributed to intramolecular quenching of the porphyrin singlet s W e by the ferrocenyl centers. Intermolecular quenching of fluorescence of porphyrins 1, 2, and 3 by added ferrocene was observed in 2-MTHF at room temperature. Stern-Volmer analysis22 yields linear plots with kqr values of 48.3 M-1, 23.1 M-1, and 45.1 M-1 for 1, 2, and 3, respectively. Assuming that the porphyrins are quenched at the same rate by ferrocene, the ratio of the Stern-Volmer constants, k , ~ is, equal to the ratio of the lifetimes. The value of T * / T ~

obtained by this method is 0.48, and that of q / r l is 0.93. These are in agreement, within experimental error, with the relative quantum yields of fluorescence. Oxygen was alsoshown toquench the fluorescenceof porphyrins 1, 2, and 3 by comparing the intensity of emission of solutions saturatedwitheitherAr(O%02),air (20%02),orpure02 (100% 0 2 ) . Thevalue of 72/71 obtained from the ratioof Stern-Volmer constants for 0 2 quenching is 0.48, and that of 7 3 / 7 1 is 0.86. These values are in agreement with the values obtained for the relative quantum yields of fluorescence and for the intermolecular quenching with ferrocene.

2598 The Journal of Physical Chemistry, Vol. 97, No. 11, 1993

Giasson et al.

TABLE I: One-Electron Redox Potentials for Porphyrins 1-3a

A aacilolion 504nm 2 -mslhyl THF 298 K

~~

WAVELENGTH ( n m l I-

-

4

I

I

550

600

650

I

1

700 750 WAVELENGTH (nm)

~~

compd

E I /(redn) ~

E112(oxidn)

1 2 3

-1.38 -1.32 -1.36

+0.85 +0.58, +0.91 +0.50, +0.86

Redox potentials (V vs Ag) were measured a t low concentrations, 10 b-10-5 M, by cyclic voltammetry at a 25 pm Pt disk electrode. The E112values are the potentials a t which one-half of the plateau current is obtained at the microelectrode. The measurements were performed in CH2Cl2 containing 0.1 M [n-Bu4N]CIO4.

I

800

Figure 1. Relative fluorescence intensityof 1 (-), 2 (- - -),and 3 (- .) in deoxygenated 2-methyltetrahydrofuran at room temperature (A,,, = 504 nm). All three samples were measured under identical conditions, therefore the relative intensities represent relativeemission quantum yields. The inset shows the absorption spectrum of 1 in T H F .

Thefluorescencelifetimesof 1,2,and3 weredirectly measured by the time-correlated single-photon-counting technique in 2-MTHF at room temperature. Fluorescence of 1 and 3 showed single exponential decays with lifetimes of 15 and 14 ns, respectively. Fluorescence of 2 showed a double exponential decay consistent with a short-lived species having a lifetime of 4 ns and a longer-lived species with a lifetime of 13 ns. This result is consistent with the presence of cis and trans isomers in solution, but definitive assignment of a lifetime to a given species depends on isolation of the pure isomers. Transient Absorption Spectroscopy. The transient absorption spectra following a IO-ns pulsed laser excitation of 1, 2, and 3 were monitored with an optical multichannel analyzer with a IO-ns gate width. The transient spectrum of 1 was of the triplet state absorption band23between 430 and 480 nm which decayed at a single exponential rate of 9.5 X lo4 s-]. Transient spectra of 2 and 3 showed a similar broad absorption band between 430 and 480 nm with single exponential decay rates of 4.7 X lo5s-I and 2.1 X lo5 s-I, respectively. The spectrum of reduced porphyrin23overlaps with that of excited-state porphyrin, and so we are unable to unambiguously assign the transient absorption spectra of 2 and 3. If we assume the transient absorptions are due to triplet states and that triplet state lifetimes for all porphyrins excluding ferrocene quenching are the same, we can calculate intramolecular triplet statequenching rate constants of 3.8 X lo5 s-I for 2 and 1.2 X lo5 s-] for 3. Ferrocene has a low-lying triplet excited state and is known to be an effective triplet q~encher.2~ An important nonradiative process in excited-state porphyrin is intersystem crossing. A possible quenching mechanism of the singlet excited states of 2 and 3 is enhanced intersystem crossing relative to 1 due to the pendant ferrocene. The fluorescence quantum yield of porphyrin~2~ is 0.1-0.2, less than the intersystem crossing quantum yield of 0.6-0.8, so any additional intersystem crossing which significantly affects singlet emission intensity should have a small but measurable effect on triplet state population. The transient absorption spectra show that the a b s o l u t e concentration of triplet 2 and 3 following excitation is no greater than for 1, indicating that little or no enhancement of intersystem crossing is taking place. Cyclic Voltammetry of 1,2, and 3. The cyclic voltammograms of porphyrins 1-3 were recorded in CHzClt using 0.1 M [n-Bu4N]clod as electrolyte, and the half-wave potentials are listed in Table I. The cyclic voltammogram of porphyrin 1 shows two wavesat-1.38and+0.85V (vsAg)correspondingto thereduction and oxidation of the porphyrin macrocycle, respectively. In addition to similar waves for the reduction and oxidation of the porphyrin macrocycle, cyclic voltammograms of 2 and 3 show an additional wave corresponding to the oxidation of the ferrocene groupat +OS8 and +0.50 V, respectively. Thecyclic voltammetry of 2 does not show different waves for cis and trans isomers.

0

0.2

0.6 0.8 Oxygen Fraction

0.4

1.0

1.2

Concentration of Ferrocene (MI Figure 2. (a) Stern-Volmer plot of 02 quenching of porphyrin 1 with variousconcentrationsofferrocenein solution. (b) Ratioofstern-Volmer

constant versus ferrocene concentrations gives a Stern-Volmer plot for quenching of porphyrin 1 by ferrocene (see text).

IntermolecularQuenchingof 1 by Ferrocene Derivatives. SternVolmer quenching constants were obtained for a series of ferrocene derivatives with a wide range of oxidation potentials. Some ferrocene derivatives such as diacetylferrocene absorb significant amounts of excitation light at theconcentrations needed toquench the porphyrin chromophore, a problem which requires cumbersome correction techniques with traditional Stern-Volmer methods. We report here a new technique which provides for a simple, direct measure of the Stern-Volmer constant even under conditions in which the quencher attenuates the intensity of the excitation light. A stock solution of porphyrin in THF is made. An aliquot is removed and mixed with a known amount of ferrocenederivative. This solution is divided into three samples, and the fluorescence intensity in 0 2 , air, and Ar saturated solvent is measured. A Stern-Volmer constant, k,s, is generated for the quenching of that solution’s fluorescence by 0 2 . This is repeated for aliquots of the porphyrin solution withdifferent concentrationsof ferrocene derivative, including a solution with no ferrocene derivative, Figure 2a. Comparing thestern-Volmer constants of the solutions with a given concentration offerrocenederivative to thesolution without ferrocene derivative allows the ratios of the fluorescence lifetimes to be determined: S V ( o x y ) O/SV(~,,)[ F c l = kq(oxy)fO/kq(oxy)f[FcJ = r o / f ( F c ] . Plotting f 0 / f for various concentrations of ferrocene

The Journal of Physical Chemistry, Vol. 97, No. 11, 1993 2599

Inter- and Intramolecular Quenching by Ferrocene octomethylferrocene ferrocene phenylferrocene

ocetylferrocene

d i o c e t y l ferrocene

1081

1

1

I

I

I

I

0.0

I

0.5

Eox(V

vS.

I

I

e

I

iI

I. 0

SCE)

Figure 3. Quenching constant for the quenching of porphyrin 1 fluorescence as a function of the oxidation potential of the ferrocene derivatives (vs SCE in CHlCN containing 0.1 M [n-BurN]ClOa).

derivative provides the Stern-Volmer plot for quenching of porphyrin fluorescence by that ferrocene derivative, Figure 2b. The singlet excited state of porphyrin 1 was quenched by ferrocene derivatives with oxidation potentials between 0.00 and +0.85 V vs SCE. Using the measured lifetime of 15 ns for porphyrin 1, intermolecular quenching rate constants were calculated for five ferrocene derivatives, Figure 3.

Discussion Steady-state emission spectroscopy and Stern-Volmer analyses were used to probe the quenching of the porphyrin singlet excited state by the ferrocenyl moieties in porphyrins 2 and 3. The ratio of lifetimes of porphyrin 3 to 1,73/71, obtained by these methods, ranges from 0.84 to 0.93. The ratio of the lifetimes obtained by direct measurement (73/71 = 14/15 ns = 0.93) is in good agreement with the results from relative fluorescence and intermolecular quenching. Using the relationship k3 = l / q 1/ T , and fluorescence lifetimes of 15 ns for porphyrin 1 and 14 ns for porphyrin 3,a rate constant of 4.8 X lo6s-I can be obtained for the quenching of the porphyrin excited singlet state by the attached ferrocenyl center. The ratio of lifetimes of porphyrin 2 to 1, T ~ / T obtained ~ , by steady-state techniques, ranged from 0.38 to 0.48. Time-resolved fluorescence decay shows two species present in solution with lifetimes of 4 ns (TZ/TI = 0.26) and 13 ns ( T ~ / T = I 0.87). Using a steady-state 7 2 / 7 1 average of 0.43, the relative contribution to total quenching by the fast and slow components can be calculated tobe72:28. Theratioofcis:transisomers insolutionasdetermined by N M R is 73:27. Assuming the two quenching rates are due to the two isomers, the fluorescence lifetimes correspond to quenching rate constants of 1.8 X 108 s-I for the cis isomer and 1.0 X lo7 SKIfor the trans isomer. Intermolecular quenching of porphyrin 1 with a series of ferrocene derivatives shows that the quenching rate constant increases as the ferrocene oxidation potential becomes more negative. This pattern follows that expected for electron transfer as the rate of electron-transfer quenching increases as the driving force for electron transfer increases.16 However, the quenching pattern is not in complete accord with an electron-transfer mechanism, becauseconsiderable quenching still takes place when there is no driving force for electron transfer. The quenching rate pattern of the five ferrocenederivatives is also not compatible with an exclusively energy transfer quenching mechanism, because the energy levels of the singlet excited state of the best ferrocene quenchers are higher or equal to that of the worst quencher. It should be noted that acetylferrocene (A,, = 455 nm, t = 420 M-1 cm-1)26 and diacetylferrocene (A,, = 470 nm, t = 440 M-l cm-1)26have significantly lower energy singlet excited states that octamethylferrocene and ferrocene (A,, = 440 nm, t = 90 M-I cm-1).21-26 Thus, energy transfer may dominate the quenching

mechanism for these quenchers, since electron transfer is not thermodynamically favorable. Several arguments support charge transfer as the dominant quenching mechanism of the porphyrin singlet excited state by the ferrocenyl centers in 2 and 3. First, thermodynamic arguments support the assignment of the quenching mechanism to electron transfer from the ferrocenyl moiety to the excited porphyrin. In high dielectric constant solvents, the sum of the redox potentials for oxidation of a donor and the reduction of an acceptor is a good estimate of the energy level of the radical-ion pair of a chargeseparated species.27 Thus, for compounds 2 and 3, the energy level of the charge-separated state is, respectively, 1.90 and 1.86 eV above the ground state. Since the lowest singlet excited state of these porphyrins is 1.97 eV above the ground state, the photoinduced intramolecular electron-transfer reaction is exothermic by 0.07 eV (1.6 kcal mol-’) and 0.1 1 eV (2.5 kcal mol-’), respectively, for 2 and 3. Second, the singlet excited state of ferrocene (2.46 eV)Z1is above that of the porphyrin chromophore (1.97 eV), thus quenching by energy transfer would be endothermic (0.49 eV or 11.3 kcal mol-’). Third, triplet yields of 2 and 3 are not larger than for 1, ruling out quenching due to enhanced intersystem crossing. Therefore, we conclude that the ferrocene centers in 2 and 3 quench the singlet excited state of the porphyrins by an electron-transfer mechanism. All three linked molecules cis-2, trans-2, and 3 have a donor separated by the same number of atoms from the chromophore. Quenching in the ethyl linked assembly 3 is half as fast as the vinyl trans linked trans-2, despite the somewhat greater driving force of 3. The quencher held trans to the chromophore, trans-2, quenches an order of magnitude slower than that in the cis geometry, cis-2. At these low driving forces for electron transfer, geometric arrangement of the porphyrin and ferrocene and nature of the linking group greatly alter the quenching rate.

Experimental Section General. 3,3’,4,4’-tetramethyldipyrrylmethane (5) was prepared asdescribed by Gunter and Mander.17 (Ferrocenylmethy1)triphenylphosphonium iodide was prepared as described by Pauson and Watts.zo 4-Methylbenzaldehyde (Fluka) was distilled prior to use. Toluene-4-sulfonic acid monohydrate (Fluka), 2.3dichloro-5,ddicyano- 1,4-benzoquinone (Aldrich), and pyridinium chlorochromate (Aldrich) were used as received. THF was distilled over LiAlH4, and CH2C12was distilled over P205. Flash chromatography was performed as described by Still et al.28 using 230-400-mesh silica gel (Aldrich). IH N M R spectra were recorded on a Bruker WP-250 instrument, and chemical shifts are reported as ppm downfield from Si(CH3)4. UV-visible spectra were recorded on a Hewlett-Packard 8452A diode array spectrophotometer using a 1-cm quartz cell. 5,15-Bis(4-tolyl)-2,3,7,8,12,13,17,18-octametbylporpbyrinogen (6). A solution of 4-methylbenzaldehyde (4)(0.30 g, 2.50 mmol), 3,3’,4,4’-tetramethyldipyrrylmethane(5) (0.50 g, 2.47 mmol), and toluene-4-sulfonic acid monohydrate (0.12 g, 0.62 mmol) in 10 mL of methanol was stirred 45 min at 0 OC and under argon. The light-pink solid that formed (0.69 g, 92%) was recovered by filtration and washed with cold methanol. The product is unstable and was used in the next step without further purification: NMR (CDC13)6 7.12-6.94 (m, 8 H), 5.35 (s, 2 H), 3.66 (s, 4 H), 2.31 (s, 6 H), 1.90 (s, 12 H), 1.74 (s, 12 H). 5,15-Bis(4-tolyl)-2,3,7,8,12,13,17,18-octamethylporphyrin( 1). A solution of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) (0.70 g, 3.1 mmol) in 10 mL of T H F was added to a solution of porphyrinogen 6 (0.50 g, 0.82 mmol) in 20 mL of THF, and the resulting mixture was stirred for 2 h a t room temperature and under argon. The solvent was evaporated, and the residue was treated with NaOH solution (10% aqueous, 15 mL) to dissolve the hydroquinone. The insoluble porphyrin 1 is collected by filtration, washed with water, and dried. Recrystallization in

2600 The Journal of Physical Chemistry, Vol. 97, No. I I, 1993

Giasson et al.

pmol of Pd) in 10 mL of acetic acid was stirred for 2 h at room CHCl3 gave 0.48 g (96%) of purple crystalline solid: NMR temperature under an atmosphere of hydrogen. The solvent was (CDCI,/S% trifluoroacetic acid) 6 10.22 (s, 2 H), 8.1 1 (d, J = evaporated under reduced pressure, and the product was isolated 7.9 H z , 4 H), 7.72 ( d , J = 7.9 H z , 4 H), 3.23 (s, 12 H), 2.77 (s, 6 H), 2.25 (s, 12 H), -2.51 (s, 4 H); HRMS calcd for C4*H4>N4 by flash chromatography (CHCIj/MeOH 99.5:0.5). Thedesired porphyrin 3 was obtained as a purple solid (O.O082g, 82%): NMR 602.3409, obsd 602.3404. 5 4Hydroxymetbyl-4pbenyI)-15-( 4-tolyI)-2,3,7,8,12,13,17,18- (CDC13) 6 10.21 (s, 2 H), 7.92-7.89 (m, 4 H), 7.55-7.46 (m, 4 H), 4.24 (s, 5 H), 4.22 (m, 2 H), 4.17 (m, 2 H), 3.53 (s, 12 H), octametbylporpbyrin(7). Toluene-4-sulfonic acid monohydrate 3.17 (m, 2 H), 2.94 (m, 2 H), 2.72 (s, 3 H), 2.49 (s, 12 H), -2.42 (0.167 g, 0.88 mol) was added to a cold (0 "C) solution of (br s, 2 H); HRMS calcd for C53Hj2N4Fe800.3541, obsd 4-methylbenzaldehyde (4) (0.106 g, 0.88 mmol), 4-(hydroxym800.3539. ethy1)benzaldehyde (8) (0.120 g, 0.88 mmol), and 3,3',4,4'Fluorescence Spectra and Stern-Volmer Experiments. Fluotetramethyldipyrrylmethane (5) (0.356 g, 1.76 mmol) in 2.5 mL rescence spectra were recorded on a Perkin-Elmer MPF-44 of methanol. The reaction mixture was then stirred 15 min at spectrophotofluorimeter using a 1-cm optical glass cell. Solutions room temperature. The precipitate containing the expected with the same absorbance (A = 0.040 at 504 nm) were prepared mixture of porphyrinogens (6,9,and 10) was collected and washed for porphyrins 1,2,and 3 in 2-MTHF. The spectra were recorded withcold MeOH. Becauseoftheinstabilityoftheporphyrinogens, after the solutions were deoxygenated with 2-MTHF-saturated no attempt was made to separate the components of the mixture Ar for 30 min. Additional spectra were recorded after bubbling at this stage. The porphyrinogens were dissolved immediately in the solutions with 2-MTHF-saturated air and 2-MTHF-saturated 25 mL of T H F and treated with DDQ (0.391 g, 1.72 mmol). The oxygen for 30 min. These additional data were used in the Sternreaction mixture was stirred at room temperature for 80 min. Volmer analysis for the quenching of porphyrins 1, 2, and 3 The solvent was evaporated, and the residue was treated with a fluorescence by oxygen. solution of NaOH (10% aqueous, 25 mL). The mixture of porphyrins (1,7,and 11) (0.266 g) was collected by filtration and The Stern-Volmer experiments using ferrocene as quencher washed with water. The desired porphyrin 7 was isolated (0.122 were performed in 2-MTHF. Concentration of porphyrins 1,2, g, 23% yield) from the mixture by flash chromatography using and 3 is constant within a set of solutions (A = 0.050 at 504 nm) CHC13/MeOH 99:l aseluant: NMR (CDC13/5% trifluoroacetic and concentration of ferrocene varies from 0 to 2 X M. acid) 6 10.20 (s, 2 H), 8.26 (d, J = 7.9 Hz, 2 H), 8.13 (d, J = Samples were subjected to three freezepumpthaw cycles and 7.9 Hz, 2 H), 7.89 (d, J = 7.9 Hz, 2 H), 7.72 (d, J = 7.9 Hz, sealed under vacuum in 1-cm Pyrex tubes. 2 H ) , 5 . 1 3 ( ~ , 2 H ) , 3 . 2 1( ~ , 1 2 H ) , 2 . 7 6 ( ~ , 3 H ) , 2 . 2 4 ( ~ , 6 H ) , The Stern-Volmer experiments using both oxygen and a 2.21 (s, 6 H), -2.12 (br s, 2 H),-2.14 (br s, 2 H); HRMS calcd ferrocene derivative as quenchers were performed in THF. for C42H42N40618.3359, obsd 618.3354. Solutions containing the same concentration of porphyrin 1 (A 5-(Formyl-4-phenyl)-15-(4-tolyl)-2,3,7,8,12,13,17,18-0cta- = 0.040 at 628 nm) were prepared. One of the solutions contained methylporpbyrin (12). A solution of porphyrin 7 (0.039 g, 62.5 no ferrocene derivative. The others contained known concenpmol) and pyridinium chlorochromate (PCC) (0.020 g, 93.8 pmol) trations of one of the following ferrocene derivatives: octamein 5 mL of CH2C12was stirred for 1 h at room temperature and thylferrocene, ferrocene, phenylferrocene, acetylferrocene, or under Ar. After addition of a few drops of Et3N, the reaction diacetylferrocene. A 1-cm optical glass cell was used. Spectra mixture was filtered through a pad of silica. Evaporation of the were recorded for each solution after purging 30 min with THFsolvent gave porphyrin 12as a purple solid (0.03 1 g, 80%): NMR saturated Ar, after purging 30 min with THF-saturated air, and (CDCl3/5% trifluoroaceticacid) 6 10.43 (s, 1 H), 10.25 (s, 2 H), after purging 30 min with THF-saturated oxygen. 8.49 (m, 4 H), 8.12 (d, J = 7.6 Hz, 2 H), 7.74 (d, J = 7.6 Hz, Fluorescence Lifetime Measurements. Time-resolved fluores2 H), 3.24 (s, 6 H), 3.22 (s, 6 H), 2.77 (s, 3 H), 2.24 (s, 6 H), cence measurements were performed at the MIT Laser Research 2.23 (s, 6 H), -2.27 (br s, 2 H), -2.35 (br s, 2 H), HRMS calcd Center which is a National Science Foundation Regional for C42H40N40 616.3202, obsd 616.3206. Instrumentation Facility. Excitation light of 588 nm was produced Synthesis of Porphyrin 2. A solution of n-BuLi (2.2 M in by a Coherent 590 cavity-extended dye laser synchronously n-hexane) was added to a suspension of (ferrocenylmethy1)pumped by the second harmonic of a Coherent Antares 7 6 4 triphenylphosphonium iodide (0.057 g, 97 pmol) in 2.5 mL of mode-locked Nd:YAG laser (70 ps fwhm, 76 MHz). The T H F at 0 OC and under Ar. n-BuLi was added until the reaction repetition rate of the dye laser was reduced to 1 MHz using a mixture became a red homogeneous solution. The ylide 13solution cavity dumper. The output of the dye laser has a 5-ps pulse was then transferred over a cold (0 "C) suspension of porphyrin width. Fluorescence was collected at right angles to theexcitation 12 (0.010 g, 16.2 pmol) in 2.5 mL of THF. The reaction mixture beam and sent through a magic angle polarizing filter and was stirred 1 h at room temperature and then quenched by the monochromator to select out the 700-nm emission. A microaddition of 25 pL of water. The solvent was evaporated, and the channel plate PMT (Hamamatsu R1564-U-07,9O-p transit time residue was purified by flash chromatography (CHC13/MeOH spread) detected the fluorescence, and the time difference between 99.5:O.j). The isolated product is a mixture of cis and trans this signal and a reference signal from a photodiode monitoring isomers (&/trans = 73:27): HRMS calcd for Cj3HsoN4Fe the incident beam was converted into a voltage via a time-to798.3385, obsd 798.3379; Cis isomer NMR (CDC13) 6 10.21 (s, amplitude converter. This voltage was sent to a multichannel 2 H), 7.96 (d, J = 8.0 Hz, 2 H), 7.90 (d, J = 7.8 Hz, 2 H), 7.71 analyzer and stored in a microcomputer. The overall temporal (d, J = 8.0 Hz, 2 H), 7.53 (d, J = 7.8 Hz, 2 H), 6.80 (d, J = response of the TCPC system was 150 ps. 12.0 Hz, 1 H), 6.57 (d, J = 12.0 Hz, 1 H), 4.49 (t, J = 1.8 Hz, Transient Absorption Experiments. Transient absorption 2 H), 4.31 (t, J = 1.8 Hz, 2 H), 4.26 (s, 5 H), 3.53 (s, 12 H), experiments were performed at the MIT Laser Research Center 2.72 (s, 3 H), 2.49 (s, 12 H),-2.4 (br s, 2 H); Trans isomer NMR which is a National Science Foundation Regional Instrumentation (CDC13) 6 10.21 (s, 2 H), 7.98 (d, J = 7.8 Hz, 2 H), 7.90 (d, J Facility. Transient absorption signals were acquired following = 7.8 Hz, 2 H), 7.79 (d, J = 7.8 Hz, 2 H), 7.53 (d, J = 7.8 Hz, a laser excitation pulse of 416 nm ( 1 mJ/pulse) from a Quanta2 H), 7.20 (d, J = 16.3 Hz, 1 H), 7.03 (d, J = 16.3 Hz, 1 H), Ray RS-l Raman shift laser pumped by a Quanta-Ray DCR-3 4.61 (t, J = 1.6 Hz, 2 H), 4.38 (t, J = 1.6 Hz, 2 H), 4.29 (s, 5 Nd:YAG laser (third harmonic, 3 ns fwhm). A 150-W highH), 3.55 (s, 12 H), 2.72 (s, 3 H), 2.59 (s, 12 H), -2.4 (br s, 2 pressure Xenon arc lamp provided the broad band probe light. H). Transient spectra were obtained by an EG & G 1421 OMAdiode Synthesis of Porphyrin 3. A mixture of porphyrin 2 (0.010 g, array detector gated through a EG & G Model 1302 fast pulser. 12.5 pmol) and Pd catalyst (0.010 g of 10% Pd on carbon, 9.4 The delay time from laser pulse leading edge to sampling pulse

Inter- and Intramolecular Quenching by Ferrocene falling edge wascontrolled by a SRS Model DG535 digital delay/ pulse generator. Electrochemical Measurements. Cyclic voltammograms were performedata 25 pmPtdiskelectrodeinCH2Cl2/0.lM [n-BurN]C104and under argon. A PAR model 175 universal programmer, a BAS PA-1 preamplifier, and a Kipp & Zonen BD91 XYrecorder were used. Reported potentials are relative to a Ag reference electrode.

Acknowledgment. Part of this work was performed at the MIT Laser Research Center which is a National Science Foundation Regional Instrumentation Facility. We express our sincere thanks to Dr. Young Park for his assistance with the time-resolved emission measurements. We thank the United States Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences for support of this research. In addition, R.G. is grateful to the Natural Sciences and Engineering Research Council of Canada for support as a postdoctoral fellow. References and Notes ( I ) (a) Hofstra, U.;Schaafsma,T. J.;Sanders,G. M.; Van Dijk, M.; Van der Plas, H. C.; Johnson, D. G.; Wasielewski, M. R. Chem. Phys. Lett. 1988, I51 (l-2), 169. (b) Wasielewski, M. R. Photochem. Photobiol. 1988,47(6), 923. (c) Wasielewski, M. R.; Niemczyk, M. P.; Svec, W. A,; Pewitt, E. B. J . Am. Chem. SOC.1985,107 (4). 1080. (d) Wasielewski, M. R.; Niemczyk, M. P.; Svec, W. A.; Pewitt, E. B. J . Am. Chem. SOC.1985, 107 (19), 5562. (e) Wasielewski, M. R.; Niemczyk, M. P. J . Am. Chem. SOC.1984,106 (17), 5043. (2) (a) Moore, T. A.; et al. Isr. J . Chem. 1988, 28 (2-3), 87. (b) Gust, D.; Moore, T. A.; Moore, A. L.; Makings, L. R.; Seely, G . R.; Ma, X.; Trier, T. T.; Gao, F. J . A m . Chem. SOC.1988, I10 (22), 7567. (c) Gust, D.; et al. J . Am. Chem.Soc. 1988,110 ( I ) , 321. (d) Land, E. J.; Lexa, D.; Bensasson, R. V.; Gust, D.; Moore, T. A.; Moore, A. L.; Liddell, P. A,; Nemeth, G . A. J . Phys. Chem. 1987, 91 (18), 4831. (e) Gust, D.; Moore, T. A,; Makings, L. R.;Liddell, P. A.; Nemeth, G . A.; Moore, A. L. J . A m . Chem. SOC.1986, 108 (25), 8028. (3) (a)Schmidt,J. A.; Liu,J. Y.; Bolton, J. R.;Archer, M. D.;Gadzekpo, V. P. Y. J . Chem. SOC.,Faraday Trans. I 1989,85 ( S ) , 1027. (b) Schmidt, J. A.; McIntosh, A. R.; Weedon, A. C.; Bolton, J . R.; Connolly, J. S.;Hurley, J. K.;Wasielewski, M. R. J . Am.Chem.Soc. 1988,110(6), 1733. (c)Archer, M. D.; Gadzekpo, V. P. Y . ;Bolton, J. R. Croat. Chem. Acta 1987, 60 (3), 577. (d) Archer, M. D.; Gadzekpo, V. P. Y.; Bolton, J. R.; Schmidt, J. A.; Weedon, A. C. J . Chem. Soc., Faraday Trans. 2 1986, 82 (12). 2305. (e)

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