Diphenylbutadiene in supersonic jets: Spectroscopy and picosecond dynamics

Volume

103, number

CHEhllCAL

1

DIPHENYLBUTADIENE

IN SUPERSONIC

16

PHYSICS LEITEKS

JETS: SPECTROSCOPY

AND PICOSECOND

December 1983

DYNAMICS

J.F. SHEPANSKI, B.W. KEELAN and A.H. ZEWAIL * Arthur Amos Noyes Laboratory of Chemwal Physics **. Cahfornia hrstrtute of Technology, Pasadena. Calrfornia 91125. USA Received

8 September

1983, in final form 26

October 1983

The picosecond dynamics and one- and two-photon ewitation of Jetc,ooled dlphenylbutadlene are reported. An Ai state IS found to lie below the dipole allowed Bi state lsomenzation and/or other non-ratitive processes are unportant above a threshold of 1050 cm-’ cwxss energy. Results are compared with those previously obtamed for stilbene

l_ Introduction

Recently, the spectra and trans-cis photoisomenzation dynamics of stilbene have been reported by Syage et al. [I] using a picosecond-jet technique. The stilbene molecules were cooled by supersonic Jet expansion and excited with picosecond laser pulses. l-he lowfrequency modes and barrier height for lsomerization (4200 cm-l) were obtained in this study. Here, we wish to extend this work to the next higher member of the all-transa,o-diphenylpolyenes (C6H5-(CzH2)n-C6H5), diphenylbutadiene (hereafter DPB). Fully conjugated linear polyerles have been of considerable spectroscopic interest as they provide simple, yet rigorous tests of molecular bonding theories and have important derivatives that are critical in such processes as vision. In particular, a great deal of effort has been focused on aromatically substituted species, which fluoresce strongly and so may be studied readily by a variety of spectroscopic techniques_ The simplest spectroscopic properties of these molecules were elucidated decades ago [2]; more recently, attempts have been made to explain anomalies in terms of the nature of the excited state [3]. These molecules possess an A, ground state (assuming C2h symmetry), and low-lying Bz and Af excited states of roughly comparable energy. In the dipole ap* Camille and Henry Dreyfus Teacher--Scholar. ** Contribution No. 6703.

pro,.imalion these states are one- and two-photon allowed, respectively. -l-he energy and oscillator strength of the Bz + A, transitlon decreases and Increases, respectively, witn mcreasing chain length. -l-he relative energy of the B: state is very sensitive to the molecular environment, whereas that of the Ai is less so. hence the orderirg and separation of the excned states IS strongly dependent on solvent. This has profound effects on the spectroscopy of these species, as the -4: state may be involved in the processes of fluorescence and photoisomerization to cis forms [3,4] _The ordering of the excited states has been reliably determined for stilbene and some of the longer chain species but there 1s stdl controversy over the ordering in DPB, which may arise from solvent effects maskmg the actual ordering in the isolated molecule_ In longer cham species the Ai state is more stabihzed relative to rhe B; state, with near degeneracy occurring for DPB in EPA glass at 77 K [ 51. In this paper, we investigate the ordering, separation, and interaction of the Al and B; states of isolated, jet-cooled DPB, ad explore the effects of these properties on the decay processes which occur following picosecond laser excitation_ We compare the results obtained for DPB with those of stilbene in order to elucidate the mechanisms of isomerization and other non-radiative decays. We also compare our findings with those of Heimbrook et al- (6 1, who very recently reported a vibronic analysis of the one-photon excltation and fluorescence spectra of DPB in a molecular 9

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CHEMICAL PHYSICS LETTERS

a preliminary two-photon excitation spectrum is presented.

beam. Finally,

2. Experimental

16 December 1983

tron beam 4 mm from the nozzle (X/D x 40). Ambient chamber pressure was 100 FTorr. The dispersed emission was measured by a fast photomultiplier and recorded on an MCA. Phrorescence decays were measured using time-correlated single-photon counting at the peak emission wavelength

DPB (Aldrich, 98%) was recrystalhzed from methanol; the purity was confiimed by melting point and thin layer chromatography. The gas-phase absorption was obtained using a pulsed molecular beam apparatus [7], while dispersed and picosecond time-resolver’ fluorescence were measured on a continuous-flow molecular beam system [8] _ The pulsed beam system consisted of a scanning dye laser pumped by a frequency-doubled YAG at 10 Hz, a KD? doubling crystal equipped with a UV-tuning f’eedback circuit, a pulsed supersonic jet, rd a fluorescence-detecting photomultiplier. The output of the dye laser (6 ns pulsewrdth, 03 cm-l bandwidth) ranged from 7 to 60 mW average power over the wavelength range of DCM and L.DS 698 dyes. The beam was frequency doubled and passed perpendicularly through the molecular jet 1 cm from the nozzle (X/D = 30). A feedback circuit continually adjusted the angular Lt of the KDP crystal to maximize UV intensity as wavelength was scanned; In the pulsed beam the sample was expanded through a 0.3 mm aperture at a nozzle temperature of 140°C. Carrier gas pressure and composition were varied to determine conditions for optimal cooling; 50 psi He was used in all studies reported herein. Ambient chamber pressure was 100 rrTorr when the nozzle was off and 300 PTorr during pulsed operation_ Fluorescence was momtored along the axis orthogonal to both beams and filtered to pass broadband DPB emission while blocking visible and scattered laser radiation. No fluorescence saturation effects were detected. The PMT signal was directed to a boxcar integrator where it was normalized with respect to a second PMT signal which monitored pulse-to-pulse laser power. The r3xcitation source for the continuous-flow molecular beam consisted of a mode-locked argon ion laser pumping a cavity-dumped dye laser. The output was frequency doubled using fiI03,

providing W

pulses with 15 ps and 3 cm-’ temporal and spectral pulsewidths, respectively. The sample was expanded through a 0.1 mm aperture at 140°C with a backing pressure of 30 psi He, and passed through the excita10

(380

nm). Data were

transferred to a laboratory computer for storage and analysis. Frequencies reported were calibrated optogalvanically using an Fe-Ne lamp.

3. Results and discussion 3.1. One-photon

excitation

spectra

In fig. 1, the excitation spectrum of DPB in a supersonic jet is presented. There dre several notable features of this spectrum. First, the spectrum is atypical in its intensity distribution. Second, both the density of bands and their apparent widths increase with increasing excess energy, leading to a somewhat r,ongested spectrum above 30600 cm-l. Finally, there is a rapid dropoff in intensity of the excitation spectrum above 30700 cm-l. The low-energy portion of the excitation spectrum is shown in fig. 2. The lowest-energy band is located at 2965i cm-I. If this is taken to be the energy reference point (though it is not necessarily the O-O tran-l-

I-

I

I1

1

3clOOQ

*

I

I1

I

I

I

ENEFGY

’

I

I.

’

31000

30500 EXCITRTION

*

I

(d>

Fig 1. Ore-photon excitition spectrum of I)PB in a supersoruciei. normalized for laser intennty. Inset shows gisphase absorption at 80°C; note that the emission in the jet becomes undetectable at energies where the gas-phase absorption is still suong.

Volume 103. number 1

CHLblIC4L

PHYSICS

16 December

LETTERS

1983

TWO-PHOTON

ONE-PHOTON

29500

29700

29900

30100

“1

30300

ENERGY tCM-l)

Fig. 2. Comparison of one-photon and two-photon excitation signals (see text). The lower trace is a hgh-resolution onexan of the lowest-energy bands observed photon excitatlc lows the two-photon evcltation sIgnal The upper W rctlon of twice the evcitatron energy. Note that plotted _ the two .o-photon bands are displaced 34 cm-’ to the red

of the two lowest-energy bands observed In the one-photon spectrun1. sition, as discussed later), then the prominent bands m the uncongested portion of the spectrum are at excess energies of 134,238,372 (238 + 134), 397,460,504 (238 + 2 X 134), 529 (397 + 134), and 582 cm-t _ The bands at 134 and 238 cm-t will be assigned as fundamentals below. The dropoff in intensity of the excitation spectrum above 30700 cm-* can be caused by either a decrease in the absorption cross sectlon or to the appearance of an efficient non-radiative decay channel. The gas phase absorption at 8O”C, depicted in the inset of fig. 1, clearly shows that the absorption is increasing in this region. This mdlcates that non-radiative decay, which includes isomerization, becomes competitrve with emission at these energies. This is further confumed by the picosecond experiments detailed below. 3.2. Decay rates versus excess energy We have-measured the excited state decay rate as a function of energy by exciting modes between 0 and 1823 cm-t excess energy. These rates are plotted as a function of excess energy in fig. 3. There are several interesting aspects to this plot. First, lifetunes up to 1050 cm-l are much longer than anticipated for an allowed electric dipole transition, as discussed below. The lifetimes decrease from 63 ns at 0 cm-l to 3 1 ns

: :

c

STILBENE

A

A

4

AAAAA

-

;

I

CJL

1000

0

EXCESS

2000

ENERGY

KM-1)

Fig 3. Excited-state decay rates of DPB (circles) as a funcUon of excess vibranonal energy, measured relrti\e xo the 1oHestener.q band in the one-photon excnanon spectrum (see text). Compared to stibene (trhn_eles), DPB has longer llfetlmes at IOH excess enerm, and the m-xc rapid mcrmse of decay rate at high excess energy. The inset shows the

common lo,grithms of the decay rates versus evess ener_g, which accentuates the presence of a 1050 cm-’ threshold. inrhcated by an arrow

at 1039 cm-t_ Second, the hfetunes at very low excess rnergrrs fluctuate considerably and so appear IO be mode-selective. Third, above 1050 cm-t, the hfetimes decrease rapidly; at 1833 cm-l the lifetime IS only 426 ps. Finally, the decay rate of DPB at high ehcess energies increases more rapldly with increasing escess energy than does that of stdbene. Considering that (1) allowed electric dipole transitions typlcally have associated lifetimes of a few nanoseconds; (2) stilbene has a lifetime of 2.7 ns at the orlgm in the Jet [I] ; and (3) the hfetime of DPB in cyclohexanc at room temperature

is 1.8 ns [9]. the lifetime

of DPB would be expected to be a few nanoseconds at low excess energy if the s&ate involved were sunilarly electric dipole allowed. As the lifetime is actually Lens of nanoseccnds, it appears that the radiative decay 11

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103. number

1

CHEMICAL

PHYSICS LETTERS

from low-energy states is only partially allowed. This would result if a dipole forbidden Ai state were coupled to a dipole allowed state, e.g. the nearby B; state.

3.3. Two-photon

excitation

and the Ai o&in

In order to test for the presence of a low-lying A: state, we attempted to detect fluorescence resulting from two-photon (red) excitation of DPB in order to contrast it with the one-photon (UV) excrtation. Two bands were observed at 148 11.5 and 14878.5 cm-l, corresponding to one-photon excitation at 29623 and 29757 Pm-L. Due to the low intensity of these bands, we could not determine their power dependence. Null scans proved that the signal arose from the DPB sample and not from the system or external contaminants. The possibility of a trace impurity in the DPB sample cannot be completely ruled out; however, the separation between the bands (134 cm-l) corresponds exactly to the most ubiquitous interval in the one-photon excitation spectrum. These bands are shifted 34 cm-l from the lowest-energy pair of bandsin the one-photon spectrum. 3.4. Vibronic analysis The prediction of an ag fundamental at 158 cm-l [lo] is consistent with the assignment of the 134 cm -1 mode as an ag fundamental. A low frequency torsional mode of a, symmetry is predicted at 33 cm-l [IO], the value of the shift between the onephoton and two-photon bands. Vibromc coupling of the Ai state to a dipole allowed Ai state would produce a partially allowed one-photon vibronic state. Such Herzberg-Teller vibronic coupling is to be expected in a molecule with nearby electronic states [ 11 lThere is an A; state in butadrene only =5000 cm-l above the B: state [ 121 in DPB it is expecte_d that this Rydberg state will be of even lower energy. The fist intense band in the spectrum is at 238 cm-l excess energy, and it has an intense coalntem-rt at 238 + 134=372 cm-l. These bandsdiffer from the bands at 0 and I34 cm-L -m several important respects. First, the 238 and 372 cm-l bands are appro&ately three times as intense as the 0 and 134 cm-1 bands. Second, the lifetimes of the 238 and 372 cm-l bands are 53 and 42 ns, whereas those of the 0 and 134 cm-1 bands 13

16 December 1983

63 and 61 ns. respectively. Finally, the dispersed fluorescence spectra obtained by exciting at 0 and 238 cm-l are significantly different. Since the B; state is only slightly higher in energy than the A: state, a relatively intense Herzberg-Teller false origin is expected due to coupling between these two electronic states by a b, vibration. The above observations are therefore consistent with the 0 and 238 cm-l bands being false origins of the A: state caused by vibronic coupling through a, (34 cm -1) and b, (238 + 34 = 272 cm-l) vibrations, respectively *_ This conclusion differs from that of Heimbrook et al. [6]. We assign two important bands in the one-photon excitation spectrum as Herzberg-Teller false origins_ Although Heimbrook et al. agree on the assignment of the second false origin, they mterpret the fist false origin as the true A; origin and ascrrbe its appearance to static or rotationally induced distortion. As discussed above, our two-photon origin is clearly lower in energy than thz first one-photon origin, indicating the false nature of the latter. Hence distortion IS not required to explain the data. Finally, note that the complexity of the excrtation spectrum at higher excess energies indicates that accurate mode assignment in this region would require a full understanding of the vibronic interactrons present. are

3.5. Isomerization

and other non-radiative

decay

channels The ordering of the states is crucial in determining lifetime behaviors of stilbene . gd DPB. In stilbene, BG lies below A;; SO, SL, and S, have minima, minima, and mzima, respectively, at 6 = 0”. As the ethylenic bond is twisted (as would occur when the molecule undergoes torsional excitation), there is an avoided crossing of the S: 37d S2 potential curve (depicted

* A b, mode predicted at 351 cm-t [ 101 likely corresponds to the 272 cm-’ mode obserbed experimentally. This difference, like that betzleen the predicted 157 cm-l and observed 134 cm-‘, is probably die to the near degeneracy of the lowest lying states which. in general, reduces the frequ~ncy of the active modes of the lowest state. We consdered the possibility that the b,, mode predicted at 53 cmSt [lo] is responsible for the 0 cm-* excess energy f&e origin. However, the magnitude of the frequency shift (36%) is much greater than that of the 272 cm-’ b,, mode (23%) or the 134 cm-’ mode (17%).

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CHEMICAL PHYSICS LETTERS

1

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16 December

1983

the Ai barrier, isomerization and/or other non-radiatlve processes should become more Important, as larger torslonal angles may be accessed_ If this picture IS correct, the isomerization threshold would be %I050 cm-l , which is close to that found for stllbene In the let (==1200 cm-l) [ 11. The value for DPB in sollltion IS 1650 2 150 cm-l [ 13]_ (However, it should be noted that the ordering and separation of the Ai and Bi states is different in these cases.) Clearly this places a lower bound of 1050 cm-l on the separation of the A; and B: states in isolated DPB molecules. Finally, it is reasonable to conclude that the more rapid increase of decay rate with increasmg energy in DPB reflects its larger sze and hence more rapldly increasmg density of states. We are currently esamming this point m greater detad.

b

4. Conclusions J,

frcans

r),

N

so-

Cl-

TwsL

Angle

I6w

(IJ)

Fig. 4. An tiustrative potential energy diagram for TPB. The avolded crossing of the A; and B: curves results m a barrier to rotation (of height A) about either of the ethylenic bonds in the A; state. schematically in fig. 3, ref. [l]). This creates a potential barrier which prevents molecules of low excess ep ergy from accessing large torsional angles. Molecule= with excess energy greater than the barrier height can access large torsional angles, where a non-radiative decay to an SO maxunum is possible. Consequently, the barrier height represents an isomerization threshold

illSmce the Ai state m DPE is below the Bi state, a potential energy diagram similar to that of the longer diphenylpolyenes [4] is the best choice for illustrative purpoeses (see fig. 4), although it may not be a perfect description of DPB. Again a barrier results from an avoided curve crossing, but the situation differs from that in stllbene in that the allowed state is higher in energy, and the lower state is accessable only through vibronic coupling. Hence distinct behaviors might be predicted for different energy regions: (1) below the A*g barrier, there should be relatively weak fluorescence, due to vibronic coupling, and slow nonradiative decay, in part via barrier tunneling; (2) above

Our principal results for Jet-COOled dlphenplbtnadlene are as follows: (1) The AL state is below the B: state as determmed by two-photon band positions, one-photon excitation spectrum intensltles, and fluorescence lifetlmcs. The A; state is Herzberg-Teller vibronically coupled to nearby electronic states, resultmg in two false ongins in the one-photon excitation spectrum. Thrs is deduced from comparison of the one-photon and two-photon excitation spectra and correlation of mode-dependent lifetimes and dispersed fluorescence spectra. (2) Measurement of decay rate versus escess energy indicates the presence of a 1050 cm-l threshold for lsomerlzation and/or other non-radlatlve processes. close to that of stilbene in the jet. Above Ihis threshold, the decay rate increases more rapidly in DPB than in stdbene, reflecting the larger size and more rapldly increasmg densrty of states of DPB.

Acknowledgement The authors grstefully acknowledge Ihe supporr of this work by the National Science Foundation under Grant No. CHE-8211356. We thank Peter hf. Felker and Lutfur R. Khundkar for their assistance in the Iifetime measurements, and Robert R. Birge and Wm. A. Goddard 111for helpful and stimulating discussions. 13

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CHEMICAL PHYSICS LEM-ERS

[ 6J L k. He~brook, B.E. Kohler and T.A. SpigIanm, Proc. Natl. Acad. Sci. US 80 0.983) 4580.

Refemms 11J J.A. Syage, Wm. R. Lambert, P.F. Feiker,-A.H. ZewaS andR.M. Hoch-trasser, Chem. Phys. Letters 88 (1982) 266. f 2J 3. Dale, A&a Chem. &and. f 1!1957) 971. [ 3 j R&Hudson and BE. Kohfer. J. Cbem, Phys. 59 (1973) : W.A. Yee, J.S Horowitz, R.A. Goldbeck, C.M. Emertz

and D-S. K&e& J. Phys, Chem. 87 (1983) 380, a& referencw therein. 141 Y.B. B&s, G.N.R. Tripathi and h1.D. Lumb, Chem. ‘Phys. 33 (1978) 18.5. [S J ;A. Bennett and RR.

4%34.

Birge. J_ Chem. Phys 73 (1980)

f7’f J.A. Syage,P.M.Fe&erand

A-H. ZewaD, J.

Chcm. Phys.,

submitted for publication [a] Wm. R. fambert, PM. FeIker and A&l Zewail; J. Chem. Phys. 75 (1981) 5958; submitted for pubtieation I91 J.B. Barks and D J, Dyson, Proc. Roy. $oc. A275 (1963) 135. i 101 B-M. Pierce and R R Bhge, Y. Phys. Chum. 86 (1982) 2651. Ill] R.M. Hoch~rasser and C. Marzzacco, in: Molecular I~~~e~~~ce, ed. EC. Lim (3enJam~. New York, f 969) p. 631. IIt] h1.A C. Nasclmento and WmA. Goddard III, Chem. Phys 53 (1980) 251. 1131 S P Velsko and G R. ~l~rn~n~,J. Chem. Phys 76 (1982) 3553.

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16 December 1983