The excited state absorption kinetics of anthrone at 533 nm

Volume 52, number 3 CHEMICAL PHYSICS LElTERS 15 December 1977

THE EXCITED STATE ABSORPTION KINETICS OF ANTHRONE AT 533 nm *

Gary W. SCOTT and L.aq D. TALLEY Department of O~emistry. University of Gzlifomia. Riverside. Riverside. California 92521. USA

Received 13 July 1977

A rapid buildup (<20 ps) of excited state absorption of anthrone at 533 nm is observed following excitation at 355 nm in benzene solution at room temperature. Under the same conditions, weak excited state absorption of anthrone is observed in the solvent cis-1,3pentadiene, a triplet quencher. These data suggest the possibility of rapid intersystem crossing in anthrone followed by slow internal conversion and/or slow vibrational relaxation in the triplet manifold. The use of a tripIet quencher as solvent should be a reasonable test to distinguish between S,, - St and T-T absorption in favorable situations.

1. Introduction

There have been several studies of the picosecond kinetics of excited state absorption of aromatic ketones [l--6] in room temperature solutions. A recent study by Kobayashi and Nagakura [6] on

anthrone in benzene reported a risetime of 70 ps

for an excited state absorption at 400 nm after exci-

tation at 347 nm. The excited state absorption at

400 nm was assigned to a T, + TI buildup, implying an intersystem crossing (ICS) rate which is slower in anthrone than in the parent ketone, benzophenone [l-3] _ It was suggested that a more rapid intersys- tern crossing in benzophenone is due to larger vi-

bronic coupling between rm* and 7~n* states in the singlet and triplet manifolds of the benzophenone molecule due to its nonplanarity [6].

Another possible explanation for the “long” buildup time of absorption at 400 nm in anthrone is that the T-T absorption peak may blue-shift with

time. In this model, vibrational relaxation and/or internal conversion within the triplet manifold could be rate limiting_ Long vibrational relaxation times in such molecules were suggested in the early

* This research was supported in part by the Research Corpora- tion, the National Science Founddtion, and the Committee on Research of the University of California, Riverside.

work of Rentzepis and Mitschele on benzophenone

The present study is a preliminary report which in-

vestigates this alternative explanation for the previously observed absorption kinetics in anthrone [6]. In the present work, a broad, relatively featureless

region of weak T-T absorption in the visible (533 . nm) was studied. Although the weakness of the absorption in this region introduces some experimental difficulties, the results should be unaffected by spectral shifts. Buildup kinetics, perhaps more indica- tive of the intersystem crossing rates in anthrone, are reported. To differentiate between S, + S, and T-T absorption, a triplet quencher is used as solvent.

2. Experimental

2. I. Materials ami apparatus

Anthrone (Aldrich Chemical Co.) was used without further purification to provide comparison with previous work [6]. Freshly prepared anthrone solutions were used for the experiments and did not show interfering concentrations of the common impurity 9- anthranol as monitored by the absorption at 400 nm- [7] _ The anthrone solutions used and the optical density (0-D.) at 355 nm in the 1 mm quartz sample cell

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Volume 52, number 3 CHEMICAL PHYSICS LETTERS 15 December 1977

Mi SA

PC

Fig. 1. Experimental arrangement: Mode-locked Nd3? glass laser cavity consisting of a 99.9% reflector (Ml), saturable absorber dye cell (SA), Nd3’? glass rod (NDI) and a 50% reflector (M2); single pulse extractor consisting of polarizers (Pl and PZ), Pockeis cell (PC), and laser triggered spark gap (LTSG); amplifier (ND2); second (SHG) and third (THG) harmonic generating crystals; dichroic mirrors (DM); v&able delay line (VD) for the 533 nm pulse: sample cell (S) of 1 mm length: photodiodes (PD); beam splitters (BS); diffuser (D); lenses (L); filters (F); and Tektronix oscilloscope (7904).

were 0.24 M in benzene with an 0-D. of 1.40 and averages as a function of delay time are plotted in

0.06 M in cis-1,3-pentadiene with an 0-D. of 0.40. fig. 2.

The experimental apparatus shown in fig. 1 and the procedure used were simiIar to previous work 12-41, except that the 355 nm pulse (excitation) and 533 nm pulse (probe) traverse a collinear path through the sample in the same direction. The single pulse energy at 355 nm was *OS--l.0 mJ. The posi- tion oft = 0 (the delay time giving maximum overlap of the excitation and probe pulses at the sample) was determined in a separate experiment as previously reported [4J _ The pulse duration was established in that experiment as 10 ps for pulses taken from near the beginning of the pulse train.

In order to distinguish amongst possible kinetics schemes in the three solvents, a model based on the following assumptions was used:

(1) Excitation and probe optical pulses were taken to be gaussian.

(2) The following general kinetics scheme, with required parameters, was compared to the experimental results.

2.2. Results

The kinetics of absorption at 533 nm foLIowing excitation at 355 nm were determined by measuring

optical density changes at numerous delay settings, and the data treatment was similar to that used in previous work [2--41. The normalized optical density

A(eA) 2 B(eB) 2 C(cc), (1)

where A is a state formed by the excitation pulse with an extinction coefficient at 533 nm of eA, B and C are sequentially formed states with extinction coeff- cients of eB and eC at 533 nm, respectively. The smooth curves based on eq. (1) are given in fig. 2. The relevant parameter values are given in the caption.

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Volume 52, number 3 CHEMICAL PHYSICS LEI-I-ERS 15 Decernbkr 1977

Anthrone /Benzene

--_ ----___ -----_________

i i

1 I 1 J , 0 50 loo I50 zco

TIME IPSCC) -

Fig. 2. Optical densities at’533 nm due to excited state absorption of anthsonc in (a) benzene and (b) cis-1,3-pentadiene resulting from singlet state excitation at 355 nm. The i are the normalized experimental optical densities. The error bars at each point give the standard deviation in the average. The smooth curves are the calculated optical dcnsi- ties for different kinetics schemes. Parameter values of eq.

=0 8) and - (kI = 1.4 x 10” ‘‘, k2 = I.; ?;;‘I s-i, EA/‘B = 0). In (b) EA and EB values were obtained from analogous curves in (a), and the calculated O.D. was reduced to account for incomplete absorption of the photolysis pulse.

3. Discussion

A simplified model for the processes which can occur subsequent to UV excitation of anthrone (or

other aromatic ketones) is as follows: In nonpolar

solvents, anthrone, excited at 355 nm, is prepared in a vibronic state that is = 1600 cm-’ above the zero point of the lowest singlet (Inn*) state. Then two com- peting processes can occur: (1) vibrational relaxation

to some lower vibrational level of S, or (2) direct intersystem crossing to an excited vibrational level of either T t (3nn*) with a3200 cm-’ excess energy

of T2 C3nn*). The exact location of the lowest ‘XT* state (Tz) in anthrone is not known, but it is reasonable to assume that it is near the zero point level of S 1 (e.g., the 3,~* (T,) state of benzophenone has been inferred to be = 100 cm-’ above S, [8,9]). Thus the rate of formation of a thermally relaxed T, state

may be determined by vibrational relaxation and/or internal conversion in the triplet manifold if these processes are slower than intersystcm crossing.

To determine the rates of all competitive radiation- less processes would require a number of different measurements of excited state absorption kinetics at various excitation and probe wavelengths in a low temperature sample. In lieu of such detailed informa- tion. measurements at fixed excitation and probe wavelengths can be useful in distinguishing between the possiiole decay routes. In particular, the present experiments were designed to distinguish between a slow S, + T1 intersystem crossing step and a rapid S 1 + T* intersystem crossing step followed by slow interns1

conversion and/or vibrational relaxation in the triplet manifold.

Some selected excited state absorption kinetics parameters from the literature for the parent aromatic ketone, benzophenone, and anthrone may be compared with the present results on anthrone. For benzophenone excited at 347 nm in benzene, a decay time of 15-20 ps for a transient absorption at 694 and 976 nm was reported in the early work of Rentzepis and Mitschele [l] _ These workers reported more rapid decay following excitation at 385 nm. Their interpretaticn WAS that the ISC rate was faster than vibrational relaxation from the upper vibronic level, and that ISC from the zero point level of S1 was still faster by a factor of three. The relative magnitudes of the intersystem crossing rate constants are consistent with a theoretical model developed [lo] for a closely-spaced, two electronic

level system (S, and Tt) in the intermediate coupling case. (However, low temperature linewidth measurements in benzophenone [ 111, indicate that one higher vibronic state has a shorter lifetime than the zero point level of S, .) The observed decay time following excitation at 347 nm is consistent with a report [3 j of the absorption buildup kinetics at 533 nm following excitation at 355 nm.

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Volurnc 52, number 3 CHEMICAL PIIYSKS LE’ITERS 15 December 1977

J For anthrone in benzene, excitation at 347 nm yields an exponential buildup time for absorption at 400 nm of 50 ps. (The value of 50 ps was caicuhned from the published experimental data of Kobayashi and Nagakura lb] assuming gaussian excitation and probe pulses of 20 ps (fwhrn). They reported 70 ps for this value.) In the present work, an exponential buildup time of less than 20 ps for excitation at 355 nm and probe at 533 nm was determined. It is reasonable to assume that absorption due to either the initially excited state (ST) or an intermediate state (S,, Ts, or TT) may be contributing to the short time absorp tion at 533 nm. If S, C- St absorption occurs at 533 nm with an extinction coefficient approximately equal to the T, + T 1 absorption at 533 nm, then it would be difficult to determine ISC rate constants from the observed kinetics,

An alternative, but not necessarily exclusive, model which accounts for the rapid buildup of absorption at 533 nm in anthrone is to assign the short time absorption to a T, +- Ts or T,, + TT transition. In this ease, the ISC rate constant in anthrone could be similar to the rate constant for benzophenone. In what follows, there is no distinction between absorption due to TT (3nn*) or Tt (3mr*) or a mixed character state. Using the nomenclature of eq. (l), Ts,would be state A, and T, would be state B. Three cases are given in fig. 2a; (1) If eA = eB then k, can have any value. (2) If fA = 0.8 eR and ki' = 50 ps then a reasonable, somewhat better fit to the experimental data is obtained. (3) A simple buildup of absorption at 533 nm with a kr’ = 50 ps does not reproduce the ex;;r:rimental data.

The above model provides an explanation of the relatively long buildup of absorption at 4-00 nm ‘.

A buildup time of 50 ps could be due to (1) slow internal conversion from Tq or (2) slow vibrational relaxation within the tripIet manifoId from Tf. In the first case the T,, 4 TT absorption spectrum might be expected to be different from T, + T, absorption, requiring simpty that the extinction coefficient at 400 nm be lower for the T, + T? absorption but roughly the sdme at 533 nm as the T, + T, absorption. In the second case, a simple blue-shift of the spectrum

’ Of CouISe, the PVZSeIlt ESUltS d0 not ftd!C out differences jn the ISC rate for thy tW0 excitation wavelengtl~s, 347 and 355 nm.

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during vibrational relaxation would give the long buildup at 400 nm, providing the Franck-Condon maximum for the relaxed state absorption is to the red of the O-O of the T, + T, absorption.

To distinguish between S, + St and T-T absorption at 533 nm in the short time frame, a triplet quencher - cis-1 ,Zpentadiene - was used as solvent _ If the short time absorption is rapidly quenched (510 ps) in this solvent, then it is reasonable to assume that ‘I’$ (or Tf) is rapidly formed. If, however, quenching takes Ionger (&SO ps) then it is reasonable to assume that the interfering absorption is due to S, + S 1 (or ST). Of course, it must be assumed that the kinetics and spectra of anthrone in the &s-l ,3-pentadiene solvent are not drastically changed from those in benzene.

The experimental results are shown in fig. 2b, and the parameters for the two curves were chosen as follows: (1) Curve - - - represents the closest fit to the experimental points assuming that the initially formed state Sr is state A, that it decays to state T, (B) with kl' = SO ps and that T, decays to anon- absorbing state C with kr* = 10 ps (a quenching rate comparable to that observed for benzophenone in cis- 1,3-pen tadiene 141). The peak optical density was determined by using the asymptotic optical density observed in the nonquenching benzene solvent, but by reducing the excited state concentration to account for lower absorption at 355 nm due to the lower solubihty in cis-l,3-pentadiene (see section 2.1). This curve does not provide a good fit to the experimental data. (2) Curve - provides a more reasonable fit to the experimental data. It is based on the model of T$ (or TT) formation in 7 ps and decay in 9 ps, using benzophenone kinetic parameters

141. Unfortunately, the experiment does not provide

clear cut evidence for rapid intersystem crossing (Le., comparable to benzophenone) in the anthrone moie- cule due to the resulting low peak optical density. The experimental uncertainty of ~0.02 optical density units is slightly Iess than the differences in the two curves. Further, a negative systematic error in the data may be biasing the experiment toward the model favor- ing more rapid intersystem crossing_ To be definitive for this molecule, a higher excited state concentration would be required. However, the technique reported here should prove quite useful for wavelengths (or

Volume 52. number 3 CHEMICAL PHYSICS LfXTERS 15 Dccembcr 1977

molecules) with higher excited state extinction coeffi- cients. Rapid intersystem crossing in anthrone, consistent with other work 161, requires that vibrational relaxation or internal conversion in the triplet manifold be slow.

In summary, this preliminary study reports experimental evidence that intersystem crossing of anthrone in benzene and cis- 1 ,Zpentadiene may be more rapid than previously reported [6] and may occur at a similar rate to that previously determined for benzophenone [l-3 J. A model which accounts for the known experimental data on anthrone requires a spectral change in the T-T absorption of this molecule after intersystem crossing. If this change is simply a blue-shift of the spectrum with time, then the kinetics of excited state absorption just to the red of a strong T-T absorption region (say at 440 nm) might show an initial fast buildup followed by a decay to a steady state absorbance. Experiments along these lines are in progress.

Acknowledgement

We thank Mr. Charles D. Merritt for writing the computer programs used in the kinetics analysis.

References

P.M. Rentzcpis, Scicncc 169 (1970) 239; P.M. Rentzepis and C.J. Mitxhele, Anal. <‘hem. 42 No. I4 (1970) 20A. R.M. Hochstrasscr. H. Lutz and G.W. Scott, Chem. Pbys. Letters 24 (1974) 162. R.W. Anderson Jr., R.M. Hochstrasscr, H. 1 utz and G.W. Scott, Chcm. Phys. Letters 28 (1974) !53; Bull. Am. Phys. Sot. 20 (1975) Abstract D13, p. 46. R-W. Anderson Jr., R.M. Hochstrasscr, II. Lutz end G.W. Scott, J. Chcm. Phys. 61 (1974) 2500. R.M. Hochstrasscr and A.C. Nelson, in: Lasers in physical chemistry and biophysics, cd. J. Joussot- Dubien (Elsevier, Amsterdam, 1975) p# 305; Opt. Commun. 18 (1976) 361. T. Kobayashi and S. Nagakura, Chem. Phys. Letters 43 (1976) 429. M. Pope, N. Geacintov and S. Michelson, Mol. Cryst. 1 (1966) 125. J.M. Morris and D.F. Williams, Chem. Phys. Letters 25 (1974) 312. M. Batley and D.R. Kearns, Chcm. Phys. Letters 2 (1966) 423. A. Nitzan, J. Jortner and P.M. Rcntzcpis, Chem. Phys. Letters 8 (1971) 445.

[ 111 S. Dym and R.M. Hochstrasser, !. Chcm. Phys. 5 1 (1969) 2458.

[ 121 G. Porter and M.R. Topp. Proc. Roy. Sot. A31 5 (1970) 163.

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The excited state absorption kinetics of anthrone at 533 nm