The excited state absorption kinetics of anthrone at 533 nm

15 December

CHEMICAL PHYSICS LElTERS

Volume 52, number 3

THE EXCITED STATE ABSORPTION

KINETICS OF ANTHRONE

1977

AT 533 nm *

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

of Gzlifomia.

Riverside.

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.

work

1. Introduction

There have been several studies of the picosecond

recent

The present

and Mitschele

on benzophenone

study is a preliminary

report

which in-

vestigates this alternative explanation for the previous-

kinetics of excited state absorption of aromatic ketones

of Rentzepis

[l--6] in room temperature solutions. A study by Kobayashi and Nagakura [6] on

ly observed the present

absorption kinetics in anthrone [6]. work, a broad, relatively featureless

In

anthrone than in the parent ketone, benzophenone [l-3] _ It was suggested that a more rapid intersys-

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 indicative of the intersystem crossing rates in anthrone, are reported. To differentiate between S, + S, and

tern crossing

T-T

anthrone

in benzene

for an excited tation

at 347

reported

state absorption nm. The excited

a risetime at 400

of 70 ps

nm after exci-

state absorption

at

400 nm was assigned to a T, + TI buildup, implying an intersystem crossing (ICS) rate which is slower in

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

absorption,

a triplet

quencher

is used as solvent.

2. Experimental 2. I. Materialsami apparatus

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 Corporation, the National Science Founddtion, and the Committee on Research of the University of California, Riverside.

Anthrone (Aldrich Chemical Co.) was used without further purification to provide comparison with pre-

vious work [6]. Freshly prepared anthrone solutions were used for the experiments and did not show interfering concentrations of the common impurity 9anthranol as monitored by the absorption at 400 nm[7] _The anthrone solutions used and the optical densi-

ty (0-D.)

at 355

nm in the 1 mm quartz

sample cell

431

Volume 52, number 3

Mi

CHEMICAL

PHYSICS

LETTERS

15 December

1977

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

were 0.24 M in benzene with an 0-D. of 1.40 and 0.06M in cis-1,3-pentadiene with an 0-D. of 0.40. 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 position 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. 2.2.

Results

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

oscilloscope

(7904).

averages as a function of delay time are plotted in fig. 2. 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. A(eA) 2 B(eB) 2

C(cc),

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 coeffcients of eB and eC at 533 nm, respectively. The smooth curves based on eq. (1) are given in fig. 2.

excitation

The relevant parameter

optical density changes at numerous delay settings,

caption.

and the data treatment was similar to that used in previous work [2--41. The normalized optical density 432

(1)

values are given in the

Volume 52, number 3

CHEMICAL PHYSICS LEI-I-ERS

15 Decernbkr 1977

to some lower vibrational level of S, or (2) direct

Anthrone

/Benzene

--_ ----___ -----_________ i i I

1

50

,

0

TIME

loo IPSCC)

J

1

zco

I50 -

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 dcnsities 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

other

aromatic

to UV excitation

ketones)

is as follows:

of anthrone

(or

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 competing processes can occur: (1) vibrational relaxation

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 radiationless 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 information. 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. 433

Volurnc 52, number

CHEMICAL PIIYSKS

3

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 the ISC rate for thy 355 nm.

434

tW0

d0

excitation

not

ftd!C out

differences jn 347 and

wavelengtl~s,

LE’ITERS

15

December 1977

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 anonabsorbing 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 moiecule 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 favoring 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

molecules) with higher excited state extinction coefficients. 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.

15 Dccembcr

1977

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. JoussotDubien (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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