The observation of a subpicosecond bottleneck state in an iron porphyrin

Volume 117. num~r 2

THE OBSERVATION OF A SUBPICOSECOND IN AN IRON PORPHYRIN RI.

7 Juae 1985

CHEMICAL PHYSICS LEJTERS

BOTTLENECK

STATE

GREENE

A T&T Bell Laborarories.

Murray Hill. NJ 07974.

USA

Received 16 January 1985

Iron(II1) tenaphenylporphme chloride (FeTPP) displayed a O-7531015 ps transient absorbance al 625 nm rollowng excilation with 0.15 ps laser pulses at eilher 625 or 312.5 nm. We suggest that this transient wmraponds to a hltherm unobserved singlet charge lransfer botllcneck state Relaxation into his slate following exciralion at either 625 or 312 5 nm occurred in less than 30 fs

I- Introduction The ultrafast electronic relaxation known to occur in iron chromophores of several biologically significant molecules has been the subject of extensive previous investigation. Light scattering (fluorescence and resonant Raman) [l--5], direct picosecond time-resolved studies [6-l l] and non-linear polarization spectroscopies [12] have alI suggested subprcosecond excrtedstate lifetimes in many such systems. In this paper, we report results of direct subpicoseccnd time-resolved experiments performed on iron(IIl) tetraphenylporphine chloride (FeTPP). We choose FeTPP as the subject of our study for two reasons As a model heme, illucidation of femtosecond timescale electronic relaxations and bottleneck states could prove valuable in the interpretation of many already existing studies on hemo~ob~, met~opo~hy~s and related molecules- gecondly, FeTPP has specifically been the subject of previous picosecond and frequency domain measurements [l l--13]_ Dynamic models that have implicatedultrashort-lived excited states in this molecule cart now be directly tested and expIored with ~30 fs time resolution_

2. Fxperimental Time-resolved

measurements

were performed

with

subpicosecond light pulses obtained from a passively mode-locked colliding pulse rmg dye laser [ 141. The experunents are of the excite and probe variety, where one pulse optically excites the sample, and a second time-delayed pulse probes (by absorption) the decay of the initially excited molecular population. Our am-

plified laser system produces 0.15 ps, 0.5 mJ pulses at 625 nm with a repetitron rate of 10 H.z. Pulses at 625 nm were divided at a beamsplitter, and sent down separate optical paths. A computercontrolled optical delay line situated in one optical arm simultaneously controlled the time delay between excitation and probing, and digitally recorded the analogue data signal. Sample excitatron was achieved by absorption of either frequency-doubled light at 312.5 run, or by laser light at the fimdarnental wavelength 625 urn. probe pulses consisted of either hght at 625 run, 460 rim, or a white light continuum pulse. Two photodiodes were utilrzed for &rgle-wavelength probing- One diode monitored probe light before the sample (IO), the other detected the probe light transmitted through the sample(X)_ The two resultant 10 Hz signals were separately integrated in a 0.5 s RC circuit, and fed into separate channels of a differential amplifier_ For small signals (changes in transmission of less than 10%) the amplifier output is roughly proportional to AOD Calibrations were achieved by placing suitable neutral density falters in the I beam. 191

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CHEh4ICA.LPHYSICS LEITERS

Volume 117, number 2

Continuum generation in water facilitated both 460 run and broadbanded probing. A spectrograph and optical multichannel analyzer were used to obtain a transient absorption spectrum between roughly 350 and 550 run. Spectral notch falters were used to obtain kinetics at 460 run. FeTPP was obtained from Strem Chemical Inc. and used without further purification_ Chloroform solutions, 5 X lo4 M for measurements probing at 625 nm, and 1 X lo4 M for broadbanded or 460 run probing, were either flowed in a 0.5 mm pathlength flow cell, or contained statically in a 1 .O mm pathlength cuvette For 4-60 nm and broadbanded probing it was necessary to flow the sample solution at a rate ensuring a new sample voIume for each laser shot. Static samples evidenced a permanent increase in absorbence at 460 nm within just a few seconds of W laser exposure_

3. Results The results of two single-wavelength pump and probe experiments are shown in figs. 1 and 2. Data in fig. 1 were obtained wrth excitation and probing pulses both at 625 run, whereas data in fig. 2 utilized 312.5 nm excitation and a 625 run probe. In both experiments, pump and probe pulses were polarized parallel with respect to each other. Both sample responses exhibit an instantaneous (mstrument-resolved) rise of a positive absorbance at 625 run, followed by the subprcosecond decay of this absorbance. We note that the sIower signal rise evident in the UV pump data (fig 2) can be attributed to differences in group velocity at 3 12.5 and 625 run. This results in temporal walkoff between pump and probe pulses in the sample, and a concomrtant degradation in instrument response [ 151. More importantly however, the transient signal at

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Fe_ L Induced absorbance in FeTPP versus delay time; 625 nm pump, 625 nm probe_ 192

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Fig. 2. Induced absorbance

nm pump, 625 nm probe.

in

20

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FeTPP versus delay tune; 3125

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CHEMICAL PHYSiCS LEITERS

625 nm induced by a 625 nm excitation (fig. 1) decays to a negative asymptotic value with a best-fit singleexponential time constant of 0 8 f 0.1 ps. When excited by W light (fig 2) the same transient signal :s observed to decay to a positive asymptotic value with a time constant of 0.7 f 0.1 ps Similar measurements to those described above were performed with a 312.5 nm pump and a 460 nm probe (not shown). A positwe transient signal was observed to rise with instrument resolution, and decay with a similar= 0.7 ps time constant to a constant positive asymptotic value. A rough determination of the decay kinetics of the asymptotic signal at 625 nm induced by 3 12-5 nm excitation yielded a value of 25 f 10 ps. A determination ofthe lifetime of the negative transient resultant from 625 nm excitation was not made due to the small

magnitude of this signal. For all single-wavelength pump and probe measurements described above, signal strength versus pump power was determined to be linear over a factor of at least 5 _This observation was made at the delay time corresponding to the peak signal. Careful attention waspaid however to ascertain whether or not the functional form of the entire temporal response was invariant to pump power over this same five-fold range. For all our measurements, both the kinetics and relative

amounts of prompt versus asymptotic signal remained constant as a function of pump power over a five-fold range. Finally, a transient absorbance spectrum taken at

7 June 1935

6 ps delay following W excitation is shown in fig. 3. This spectrum is in qualitative agreement with a previously pubhshed spectrum derived after excitation of FeTPP by 353 m-n light [ll].

4. Discussion An extensive body of research conducted largely over the last 20 years has attempted to place the optical spectroscopy of porphyrins and metal-substituted porphyrins into an orderly framework [ 161. Spectral features manifest by these compounds are often diffuze and overlapping, with the mere qualitative assignment of electronic configuration for a particular transition being the legitimate subject of both theoretical and experimental research_ This holds particularly true for the iron porphyrins, where strong metal-porphyrin interactions mix excited triplet and singlet porphyrin (X,X*) stateswith “porphyrin to metal” charge transfer transitions [ 17,181. The complex spectroscopy of these iron systems has necessarily raised intriguing questions concerning relaxation dynamics and intermolecular

energy

flow.

Such kinetic information is recognized to be germane to the detailed understanding of numerous picosecond flash polyolytic studies performed on hemoglobin? and related molecular systems [6-10,19-231. Frequency domain experiments have indicated extremely short lifetimes(2-100 fs) for the excited states comprising the visible transitions in these iron chromophores. Specifically, absolute and comparative measurements of resonance Rarnan versus fluorescence intensities, in addition to analysis of absorption linewidths have enabled workers to predict subpicosecond lifetimes for excited electronic states in several hemeproteins [ 1-S]

Fig. 3. Transientabsorbancespectrumof FeTPP 6 ps subscquent to 312.5 urn excitation

Novel non-linear optical measurements reported by Andrews and Hochstrasser [ 121 employ polarization spectroscopy to derive lifetime information for the Soret transition in FeTPP. These authors conclude that the transition is essentially homogeneously broadened, corresponding to a population lifetime of approximately 3.5 fs. Transient grating studies performed on FeTPP by these s+me authors report an energy deposition yield of 0.93 f 0.07 withm just a few nanoseconds of photoexcitation [ 131. Prior to the present work, only one direct picosecond 193

Volume 117. number2

CHEMICAL PHYSICS LmERS

time-resolved study of relaxation dynamics in FeTPP has been reporred [ 111. Continuum probing following excitation with 8 ps, 353 run pulses, revealed a shortlived 50 f 20 ps transient difference spectrum similar to that shown in fig. 3. The results were interpreted as the first evidence of a trrplet bottleneck in an iron heme Within the context of a simple three-level model, the authors speculated that an observed inability to bleach the ground-state transition implies a rapid < 100 fs ground-state recovery in FeTPP. The results of our measurements provide dramatic new information that can now be incorporated into a more complete model of this system. _ Subsequent to either UV or visible excitation, we observe transrents at 625 run having 0.7 +- O-1 and 0.8 f 0.1 ps lifetimes, respectrvely. These lifetimes are recognized to be experimentally identical, and shall be referred to jointly as 0.75 + 0.15 ps. Since it seems unlikely that two different excited states would have identical lifetimes we conclude that both excitation wavelengths result in the population of the bottleneck state labeled B in fig 4. Our experiment measures

7 June 1985

k, + k2, the rate of depopulation of state B to be 1.3 x IO= s-1. We should note that there is potential hazard in interpretmg single-wavelength pump and probe kinetic data without pnor knowledge of the excited-state spectroscopy of the system. Previous picosecond spectroscopic studies have revealed instances where exerted-state transrtions have evolved both in spectral shape and magnitude [24-261. In the present case for instance, the 0.75 ps transient measured at 625 run could be a consequence of such a spectral bandshape evolution What we are interpreting as the decay of one electronic state into another could possibly tarrespond to the thermalization of vibrational energy on a single electromc surface. We argue against this possibility however, on the grounds of the results of the 460 run probe experiment mentioned prewously. It seems unlikely that both 460 and 625 nm be wavelengths sensitive to bandshape evolution of an electronically excited state. Far more likely however, would be that the excited electromc state B have a broad and diffuse morphology, spanning a large region of the visible and ultraviolet spectrum. It would cleariy be desirable to have a full absorbance spectrum of the 0.75 ps transient. The 3 ps of chirp in our continuum pulse between 400 and 800 nm however makes it extremely tedious to construct or identify such a shortlived spectrum.

l?ig 4. Kinetic model schematically depicting 3125 and625 mn excitation vnth subsequentrelamboni FeTPP_ Electzonk levels E and B are assumed to be predominately singlet in character. The exact energy of state B is U&IIOWU, other than that it 1.5 below 16000 cm-‘. Level Y is a~~@~ed as the lowest porphyrintriplet state, having a 25 f 10 ps lifetime. Our expenmentsmeasure (kl + k& ,theRfetrme ofthe bottleneck state B, to be 0.75 f 0.15 ps.State X issuggestedto be a charge transfer of higher multiplicity. 194

After state B decays, persistent signals remain. For the case of W excitation, the transient absorbance spectrum (fig. 3) in addition to the determination of a 25 + 10 ps lifetime for the persistent 625 nm signal identifies this state as the triplet bottleneck from the previously mentioned picosecond FeTPP study [ 1 l]_ We designate this state as Y in fig. 4. State Y has an absorbance at 460 and 625 run greater than that of the ground state G. For the case of 625 excitation however, we must postulate population of a state other than Y. In this case, state X must have a mailer absorbance at 625 m-u than the ground state. It is possible that states Xand Y could be simultaneously populated to different degrees by W and/or visible excitation_ However, that W excitation results in a positive transient at 625 run and visible excitation results in a net bleaching mandates the involvement of at least three metastable levels (G, XandY). All our data and observations are consistent with the expectation of extremely short lifetimes associated

Volume 117, number 2

with the visible and presumably UV transitions in iron porphyrins Frequency domain measurements have probed ultrashort lifetimes associated with the Soret and Q transitions. By contrast, direct picosecond and femtosecond kinetic studies most effectively expose the longest-lived states (bottleneck states). However, dtiect measurements can also place some limits on lifetimes of states through which these bottlenecks must be populated. Observation of an instantaneous rise for the 0.75 ps (state B) transient subsequent to either UV or vlslble excitation indicates a rapid (-30 fs or less) relaxation through aII intermediate levels between 3 12.5 nm and the B state. This conclusion is also supported by absence of observable rnultiexponential transient behaviour with lifetvne components shorter than 0.7 ps In the context of the kinetic scheme in fig. 4, this impIies k3+k4S-kl

References [l]

F. Adar. M. Gouterman 80 (1976) 2184

and S. Aronowib,

J. Phys Chem.

[2] J-M. Friedman, [3] [4]

[Sl [6] [7]

D.L. Rousseau and F. Adar,Rot. Natl. Aad Sci. US 74 (1977) 2607. F. Adar. J. Phys. Chem. 82 (1978) 230. J M. Friedman and D.L Rousseau, Cbem Phys. Letters 55 (1978) 488. P.M. Champion and R. Lange, J Chem. Phys. 73 (1980) 5941. C.V Shank, E-P. Ippen and R. Bersohn. Science 193 (1976) 50. D. Huppert. K D. Shaub and P.M. Rentzepis, Rot. Natl. Acad. Sci. US 74 (1977) 4139.

[ 81 B.I. Greene, R.M. Hochstrasser, R B. W&man and !!'.A

Eaton. Proc. Nat1 Acad. Sci US (1978) 5255. [9] J-L Martin, A Migus, C. Poyart. Y Lecarpentien, R Asticr and A. Antonetti, Proc. NatL Acad. Sci. US 80 (1983)

173.

[lo1 L.J. Noe. in: Biologid events probed by ultrafast laser

+kz_

ed R R Alfano (Acadcnuc Press, New York, 1982) p_ 339. P-A. Cornelius, AW. Steele, D.A. Chemoff and R.M. SP=t==VY,

We turn fmally to a discussion of the identlty of states B and X. The diffuse nature of the low-energy transitions in Fe(III) porphyrin systems has been at-

tnbuted to strong interactions between porphyrin singlet and triplet states with “porphyrin to metal” charge transfer states [16-l 81. As has been proposed previously in the context of hemoglobin relaxation dynamics however, it still seems reasonable to suppose that in azeroth-order approximation, relaxation occurs

first through imtiaIly excited “singletstates” and then via states of different multiplicity 181. It is tempting therefore to hypothesize that B corresponds to the lowest-energy singlet charge transfer state, and the state X, a charge transfer state of higher multiphcity.

In conclusion,

7 June 1985

CHEMICAL PHYSICS LE-l-rERS

we have demonstrated

the existence

of a 0.75 ps lifetime bottleneck state in FeTPP. Population of this state results both from excitation of porphyrin states at 3 12.5 run and from excitation of mixed porphyrin-charge transfer states at 625 nrn In agreement with the predictions of frequency domain studies, the lifetimes of optically accessible electronic states situated between 625 and 312.5 run are directly determined to be shorter than 30 fs.

[ll]

Hochstrasser. Chem. Phys. Letters 82 (1981) 9. [121 J-R Andrews and R M. Hochstrasser. hoc. Nat1 Acad. Sci. US 77 (1980) 3110. 1131 J R. Andrews and R.M Hochm. Chem. Phys Letters 76 (1980) 207. (14) R.L. Fork, B-1 Greene and C.V Shank, Appl. Phys. Letters 38 (1981) 671. [15] M-R. Topp and G.C Chns, Opt. Commun. 13 (1975)

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Amd Sd. US 75 (1978) 573. A.H. Reynolds, S.D. Rand and P.M. Rentzepis, Rot. Natl. Aad Sci US 78 (1981) 2292. P.A. Cornelius, AW Steele. D.A Chemoff and R.M Hochstrasser. Rot. NatL Aed. Sd. US 78 (1981) 7526. D-A Chemoff, R.M. Hochsfxasser and AW. Steele, Rot. Natl. Amd. Sci US 77 (1980) 5606. J. Temer. J.D. Sixon~. T-G. Suno. M. Nmumo. hf. Nicol ani kM. El-S&d. Pro-i. NiitL A&d. Sb US 78

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1241 B.L Greene, R.M. Hochshxsser and R.B. Weisman, in. Piax-eomd phenomena, eds. C.V. Shank, E.P 1ppe11 and

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