Picosecond flash photolysis and spectroscopy: 3,3′-diethyloxadicarbocyanine iodide (DODCI)

Volume

27, number

1

CHEhlICAL

PICOSECOND

FLASH

PHYSICS

PHOTOLYSIS

I July 1974

LETTERS

AND

SPECTROSCOPY:

3,3'-DIETHYLOXADICARBOCYANlNEIODIDE(DODCI) Douglas MAGDE and Maurice W. WINDSOR Deparrt?lerrr of Clrerr~irfry. Ii’ashitlgrorr Stare University, A~lhfra~r. rvaslfitfgr0t~99/ 63, US.4 Received

28 hlarch

1974

We dcmonstratc transient absorption spectroscopy with picosecond time resolution and nanomctcr spectral rcsolution over the entire visible region. To resolve mu1 tiple discrepancies in the literature, we studied 3,3’-dicrhyloxadicarbocyanine iodide. We measured both cscited sinflct liFerime and ground sure deplaion and recovery. The kinetics of the two Processes are the same. The characteristic time is 1.15 f 0.15 nsec in ethanol and 560 + 70 pscc in waler. WC

report lluorcscence

yield dererminntions

and comment

1. Jntroduction Picosecond kinetic spectroscopy using mode locked lasers [l] has now evolved to the point where itjs routinely generating informalion. A recent review [2] , although not intended to be comprehensive, offers an introduction to much of the field and emphasizes current advances in one laboratory. Until very recently,

studies of picosecond transient species have been confined to observations at a few discrete wavelengths. This limits attention to systems which have been well characterized previously on slower time scales and, even then, risks misinterpretation. Clearly it is desirable to have broad band detection capability and several groups have been working toward this end. Alfano and Shapiro first suggested in print [3] that the extensive white continuum generated when an in. tense picosecond laser pulse is focused into almost any medium could be used for picosecond transient absorption studies. This phenomenon, generally called self phase modulation (SPM) even though the effective mechanism may involve several non-linear optical processes, was much studied

even able tory been time

on the nanosecond

on the controversy

over photoisomer

generation.

ing unambiguous time-resolved absorption spectra of picosecond transients in a variety of molecules [G] _ Japanese workers have reported success [7] without details. Actual spectra are exhibited in two recent reports [S, 91. Those in the latter, a study of 3,3’diethyloxadicarbocyanine iodide (DODCI) by Busch, Jones, and Rentzepis @JR), show rather clearly what could be bleaching and return of ground state absorption. Unfortunately, the data reported exhibit apparent serious discrepancies with other recent results [10-l I]. Since we plan to make strong claims for a variety of measurements using SPM.based methods as a routine tool, this controversy must be resolved in order to pro. mote confidence in the method. Furthermore, these othei results are not entirely consistent. There is animmediate practical significance because DODCJ is currently a preferred dye for mode-locking both pulsed and cw tunable rhodamine 6G lasers. In the interest of brevity, we will first describe our measurements and then combine a review of the controversy with a discussion of the significance of our data.

time scale

before their work and is still a topic of considerresearch [4]. Still the implementation of a satisfacpicosecond flash photolysis technique has not easy. We have been pursuing this goal for some [5] and have only recently succeeded in obtain-

2. Experimental

design

A schematic diagram of the picosecond flash photo. lysis apparatus is shown in fig. la. A single optical pulse of 5-8 psec duration and 1060 nm wavelength 31

CfIEhlICAL PHYSICS LETTERS

Volume 27, number 1

(a)

4-i

F P

SPM

1 July 1974

and unescited referetice volumes both above and below the excited region. The entire area is then imaged at about 1:3 demagnification onto the slit of a spectrograph and, ultimately, onto photographic film. By having a reference spectrum both above and below a purported transient spectrum, we greatly reduce the risk of misinterpreting random fluctuations in the intcnsity of the continuum as genuine transient effects. The excitaticln beam travels at a IO” angle to the probe

and, therefore, does not enter the spectrograph. There are HO optical filters between the sample and the spectrograph. The probe pulse can precede the pump in arrival time or it can be delayed by up to 4 nsec. At any particular frequency band, the probe has the same short duration as the original laser pulse; the probe pulse is, (bl

hp. 1. (a) Picosecond flash photolysis and spectroscopy appnratus: FD. frequency doubler; DM, dichroic beam splitter; SPA!, self phase modulation cell: DL, opricnl delay line; CT. cylindrical telescope; P, pump pulse path; S. probe continuum pulse path; C, sample cell; MON, excitation intensity monitor;

F, colored glass filters. Mirrors and lenses arc not labellcd. The spectrograph is a 3/J m grating instrument. (b) Enlargement of the beam geometry at rhc sample cell to illustrate how S probes both the excited volume and adjacent unexcited rcgions.

is selected from the output of a mode-locked Nd3*glass oscillator, amplified in a second glass rod, and passed through a KDP frequency doubler. A dichroic mirror sends the green 530 nm pulse of 2 or 3 mJ energy around one optical path of variable length. The residual infrared pulse of about LS mJ energy is focused into the SPM cell, which contained Ccl, in the present instance. This produces a white continuum extending from 430 nm through the visible and near infrared. The energy in the visible (passed by 5 mm of Schott KG3 glass) was about 1 pJ. After collimation and removal of the 1060 nm radiation by a fdter, the continuum pulse follows a path along a second variable length optical delay line 2nd finally passes through the sample cell in the same region as the green excitation pulse. The detail of the geometry in this region is most important and is shown enlarged in fig. lb. The key point is that the probe beam samples simultaneously an excited volume of 1 mm cross section in the center 32

however, slightly “chirped”, that is, there is a slight dispersion in the arrival time of the various colors at the sample due to the relatively long path (by picosecond standards) through dispersive media. This has been fu!ly characterized [6] ; it amounts only to 16 psec be-

tween 430 and 650 nm and is easily accounted for in interpreting results. The photographic method is all but essential for survey studies of transient species (given the present state of the art of picoseccnd lasers); it is not the most convetient method for determining precise kinetic data. Therefore, once the photographic spectra reveal the best wavelengths to monitor, we replace the camera with a slit and photoelectric detection. Although we then monitor only a single narrow band of wavelengths, by using a 500 we reruirl the reference information

channel optical multichannel analyzer or OMA (SSR Instruments) to obtain an intensity profde along the length of the slit which is exactly equivalent to a densitometer trace vertically across the photographic spectrogram. The present apparatus incorporates three innovations that account for its versatility and reliability: (1) We generate the SPM probe continuum using the “waste” infrared left over after frequency doubling. This has two beneficial results: it conserves the green for excitation purposes, and it greatly simplifies the task of separating the weak SPM continuum from the overwhelmingly intense laser line. (2) We have abandoned attempts to gather data at multiple time delays in a single laser shot using an echelon [2] . This technique, although valuable at very short time delays, places severe constraints on the spatial uni-

Volume

27. number

CHEMICAL

1

PHYSICS

1 July

LETTERS

s Bnly :’ S ;c: opaque

1974

obaorbrr

‘s t OQOCI

Ponly Hg ‘,

-20, ‘._, .+20 :. 300 .600

p+s ,,

DELAY IN

; : ,,900’

PSEC

‘. 1500 :

,Jl

579,577

.I

‘:546

;

‘. ,,:”

“..

I 436

24dO.

., ’

‘. .., Hg: Wavslrnglh : ;. ‘.

In nm

Fig. 2. Picosecond flash photolrsis of DODCI in ethano! (2 mm path at 5 X IO-’ bl): Hg lines prove rhat the specrra exrend from red (650 nm) on the left to deep violet (435 nm); resolution is 1.2 nm. The red cut-off is limited only by tilm sensitivily. Top four spectra are for calibration: S is the continuum wirh no sample. The “opaque absorber”. 3 1 mm pnper strip. mimics intense absorption over entire visible and verifies lhat sample region is sharply imaged onto camera. Wirh S only, normal Sround state absorption is seen; with P only, scattered P and red-orange fluorescence are seen. Below nppcar spectrn of excited DODCI 81 different time delays. These are the thin central portions undwiched between reference spectra. At -20 psec there is no central band because prove it develops there are no excited molecules. At +20 psec the excited s~nte spectrum is fully dcvelopcd. (Other measuremenrs

within our 5 psec time resolution.) fully discussed in the text.

It then

decays

gradually

over

formity of the continuum; furthermore each echelon provides only discontinuous jumps in time delay over a limited temporal range. Instead we have improved the reliability of the laser itself to make practicable the use of multiple shots and continuously variable delay lines. This has permitted us to work with absorption changes anywhere in the range 3 psec to 10 nsec with no change in apparatus. (3) Perhaps most important we have developed a scheme which records on each shot a reference spectrum against which to measure absorption changes. Our principal measurements were ma<; on a 5 X 10m5 M solution of DODCI in ethanol using 2 mm path length cells. The dye, from Eastman Kodak, was used as received. These conditions mimic those of previous investigators [lo, 121, which in turn were chosen to reflect the conditions actually used in mode.iocking applications. Using thin layer chromatography, moreover, we could resolve no contaminants. In water DODCI deteriorates over a period of hours to days. We did not undertake a characterization of this degradation. However, we do report results on freshly mixed solutions having optical density of 09

1 or 2 nsec.

The

several

features

of the excited

state

specttx

are

at 530 nm in order to permit direct comparison with the results of BJR. Our kinetic data are characteristic, therefore, of the DODCI-water solution, but should be interpreted with allowance for the fact that the molecular constitution of this solution may not be well defined,

3. Results Our main results are displayed in fig. 2. This shows several calibration spectra together with time resolved spectra of excited DODCI. Note that the excited state spectra appear in each display as a rather thin horizontal band sandwiched between reference spectra which show unexcited DODCI absorption. One may distinguish three phenomena in the excited DODCI spectra: In the blue and violet a dark region reveals excited singlet absorption S, f S, . In the yellow-green, just to the left of the scattered 530 nm excitation light, a bright central region amidst the ground state absorp. tion results from enhanced transmission due to ground state depopulation. Further to the red-orange, there 33

Volume 17, number 1

I July 1974

CHEMICAL PHYSICS LETTERS 5082-4203

PIN photodiode

in conjuction

with a

Tektronix 7904-7A19-7B92 oscilloscope. We measured the system response using attenuated green laser pulses. The fluorescence was observed from the rear of the cell at an angle of 30” from the direction of the excitation

beam, with the diode located about 2 cm from the sample and with only a 3 mm Schott OG 570 filter in between. Again several measurements were made and; considering them all, we estimate that r = 1.2 + 0.3,

I 4

DELAY Fig. 3. The

0

,

8

12

TIME

(iOm’o secl

of rhe probe intensity Innsmitted through the exited region of the sample IO thil~ transmitted through uncscited neighboring regions is plotlcd on a semilog graph neainsr the time delay of the probe pulse. Data were raken at 610 nm in a 3 nm bandwidth for a 7 mm. 5 X IO-’ hl solution of DODCI in ethanol. The line of best fit to tirst order decay yields 7 = I. I6 nsec for this particular run. lognrithm

OF the

is still a bright central

ratio

streak,

due now to amplificatiorz

of probe light, the gain arising because fluorescence from the relaxed excited stale occurs in a region where there is no ground state absorption. We have a four-level laser amplifier. We did verify that under the conditions of measurement only amplification occurred; there was no laser cscillation or superradiance. It is clear that all three effects have the same lifetime. There is partial decay by 1 nsec and after 2 nsec the spectrum appears almost as it does when the probe beam slightly precedes the pump. Calibration of the film response with a photographic step tablet gives a lifetime of about 1 nsec. Direct photoelectric detection using the OMA, as described above, measured r more precisely. Fig. 3 shows the data from one of several such experiments. Considering all such measurements, we assign the excited state lifetime of DODCI as T = 1.15 to.15 nsec. Electronic detection was also used to verify that the gain spectrum in the red was identical to the published fluorescence spectrum of DODCI itself [ 10, 1 l] . By using the OMA with its silicon detector, measurements could be extended to wavelengths longer than the film

cut-off. Given T = 1.15 nsec, there seemed to be an excellent chance to observe fluorescence decay directly with a fast photodiode. 34

We used a Hewlett-Packard

consistent with, but less precise than, the transient ab. sorption measurement reported above. The following control experiments were performed: One or more measurements were made on ethanol solutions of concentration lo4 M and 4 X 1OA M and with cell lengths of 1 mm and 10 mm. Excitation pulse

energy was varied by a factor of six. OMA detection was used at 460, SSO, 600,610, and 650 nm. We searched particularly for relaxations with characteristic times near 300 psec and around 10 or SO psec. We found no such components. Measurements were made with the polarization of the probe beam both parallel to and perpendicular to that of the pump; we could see no change. A sensitive differential

measurement, performed at an optimum excitation level, might resolve rotational relaxation [ 131 in addition to the dominant electronic relaxation. Water solutions of DODCI have absorption spectra virtually identical to those in ethanol, but do show somewhat different kinetic behavior. Time-resolved photographic data indicate a shorter lifetime. Electronic detection sets T(water) = 560 ? 70 psec. The spontaneous fluorescence decay measured by direct photodiode detection was clearly faster than in ethanol, but still slower than the response of the detection system to the laser pulse itself. Deconvolution yields 0.4 < 7 < 0.8 nsec, consistent with the above much more precise datum.

4. Fluorescence

quantum

yields

To round out our study of the photophysics of DODCI, we measured fluorescence quantum yields, still under conditions of negligible photoisomer concentration (see below). For the ratio R = QF(EtOH)/ &(H,O), we find R = 2.05 i 0.1, in agreement with the ratio R' = T(EtOH)/T(HzO) =Z 2.05 of the emission lifetimes. We would expect R = R’ if the transition OS-

Volume 21, number 1 cillntor

strength

CHEMICAL PHYSICS LETTERS

is the same in the two solvents_

Calcu-

lation of the oscillator strength of DODCI in water from its absorption spectrum is problematic in view of its limited solubility and the instability of that solution. We also confirmed an earlier measurement [lo] of OF for DODCI in ethanol relative to rhodamine 6G in the Same solvent, finding QF(DODCI)/$~(R~G) = 0.48 * 0.04. The above bF measurements were made using 5 X 10m5 M DODCI in ethanol and I mm path lengths in order to duplicate the conditions of the kinetic measurements. Concentrations af other solutes were adjusted to comparable optical density. Rhodamine 6G, however, is not an accepted fluorescence standard. Hence, we next determined OF for DODCI in ethanol (concentration below 5 X lo-CM) relative to sodium fluorescein in 0.1 N NaOH. The latter was assumed to have QF = 0.90. A critical evaluation of this compound as a standard has been given recently [14]. We found QF = 0.42 i 0.02.

5. The DODCI controversy 1. Dempster et al. (DMRT) [lo] first undertook a study on the microsecond time scale of DODCI in ethanol and found that the quantum yield for fluorescence was tiF = 0.49, for intersystem crossing QISc = 0.005, for internal conversion GIc = 0.43, and for photoisomerization to a metastable isomer (lifetime 1.3 msec) QPI = 0.08. Integrated absorption data gave a radiative lifetime which, with the measured OF, led to a lifetime for the Sl state T = 1.24 + 0.15 nsec. Since their QF was based on assuming the quantum yield for rhodamine 6G to be 1 .O, their QF and T may be slightly too large. Both our kinetic data and our remeasurement of & argue that they are. But much more significant is the fact that several independent methods now converge on a value near r = 1.15 nsec for DODCI under conditions where no photoisomer is present, that is, either low intensity continuous excitation or excitation by a sirlgfe picosecond pulse of arbitrary power. DMRT also deduced a lifetime for the photoisomer of not more than 300 psec and suggest that it is this species which is important for mode locking. 2. BJR [9] studied bleaching of DODCI in water after excitation by a single picosecond laser pulse. They infer that most of the normal ground state absorption

1 July 1974

returns in only 10 psec and all of it in 50 psec. They “observed no evidence for any transient absorbing species”. BJR attribute their 10 and 50 psec spectral observations to rapid recovery of the ground state population following bleaching, but offer no interpretation of this at the molecular level. We have been unable to observe such a fast component to the recovery. Furthermore, we are at a loss to account

for the non-observn-

tion by BJR of the excited state absorption shown in our fig. 2. The fact that they studied water solutions while

others

[IO--121

used ethanol

seems,

on the basis

of our measuremenrs, to be insufficient to account for the bulk of the discrepancy. 3. Arthurs et al. (AER) measured fluorescence decay [ 111 as well as ground state depopulation and recovery :12] at two different wavelengths which were assigned to the two species, DODCI itself and the isomer. Since they used long trains of picosecond pulses, there was time for the build up of photoisomer and both species should have been present. They obtzin T = 300 psec for both species and conclude that both species are important in mode locking. They did not comment on the coincidence of identical lifetimes. They did report a new and lower value for photoisomer

yield opI = 0.01.

,9BR were aware of the discrepancy between DMRT and BJR and despite intensive efforts were also unable to duplicate the kinetic result of BJR. They further express surprise that BJR did not observe the photoisomer in absorption. This latter, however, should not have been a point of contention. Regardless whether Qpl =O.Ol or as high as 0.08., one cannot generate very

much isomer with a siflgle picosecond pulse. Only with a singe long pulse or with a train of short pulses, can the population be significantly converted to photoisomer. The photographic technique of BJR would not be expected to resolve a trace component, especially when the spectrum of that compo’nent is rather similar to DODCI itself. The experiments reported here also use single pulse excitation and indeeci we do not observe the photoisomer. We do, however, observe transient absorption of the DODCI excited singlet state in contrast to BJR. ABR are not so explicit in discussing the discrepancy between their DODCi lifetime and that of DMRT. They appear to discount the earlier result, but do not say whether they question + or the absorption data. Our data, however, rule out any’possibility that T = 3.5

Volume 27, number 1

300 psec for an isolated CDDCI molecule in ethanol. If we accept that the spectral isolation used by ABR was adequate to distinguish the spectral regions, then we must conclude that under conditions of high levels of excitation, the spectral properties of the solution are not simply due to a superposition of two non-interacting species, as ABR implicitly assume. The concentrations used in previous experiments [lo-121 rule out simple dipole-dipole quenching of DODCI by its isomer. (Diffusion controlled quenching, of course, is out of the question.) One mighr invoke a loose complex, sufficient to keep the isomer within range for efficient energy transfer one to speak of the two spectrally

1 July lY74

CHEMICAL PHYSICS LETTERS

but still permitting distinct molecules.

Or one may have to reinterpret the spectral properties of the solution at high levels of excitation with a substantially revised model.

vision or, at least, extension of the current model and, most likely, additional data.

Acknowledgement

We thank Mssrs. E. Wood, S. Prince, and B. Bushaw for assistance with construction of parts of the apparatus. The fluorescence quantum yield determination relative tc fluorescein was made by Mrs. T. Cremers using = fxility made available by our colleague Professor C.

Crosby. This research was supported in part by the Office

of Naval Research.

References [l] A.J. Dehlaria, W.H. Glenn Jr., h1.J. Brienza and h1.E.

6. Conclusions

Since the multiple controversies over the photokinetics of DODCI involve distinguishing DODCI from its photoisomer, we have concentrated on clarifying first the properties of the parent species. We believe that single picosecond pulse excitation with broad band monitoring of transient species is the most direct approach for such a study. Under these conditions, we pointed out, the photoisomer should not be and was not observed. Using our technique, then, we obtained lifetimes for the DODCI lowest excited singlet state of 1 .I5 + 0.15 nsec in ethanol and 560 k 70 psec in water. The same lifetime applies to excited state absorption S,, + S,, recovery of ground state depletion, and fluorescence gain, each in a different spectral region. These values are confirmed by direct photoelectric measurement of the spontaneous fluorescence lifetime and by the consistency with fluorescence quantum yield data. Our directly measured 7 value in ethar?ol is in quite

h&k. Rot. IEEE 57 (1969) 2. [2] T.L. Netzel, W.S. Struve and P.hl. Rentzepis. Ann. Rev. Phys. Chem. 24 (1973) 473. [3] R. Alfz~-~oand S.L. Shapiro, Chem. Phys. Letters 8 (1971) 631. R.L. Carman and F. Shimizu. Phys. Rev. A8 (1973) 1486, and references therein. [51 M.W. Windsor. Inrra-Science Chemistry Reports 4 (1970) 231; J.R. Novak and M.W. Windsor, Proc. Roy. Sot. A 308 (1968) 95; M.W. Windsor and J.R. Novak, AFOSR Contract Final Report (Dec. 30, 1971), available as AD-735789 from DDC, Cameron Station, Arlington, VA 22314. [61 D. hfagde, B.A. Bushaw and h1.W. Windsor, to be pub-

141 I. Reinrjes,

lished. 171 H. Kuroda and S. Shionoya, Solid State Commun. 13 (1973) 1195. 181P.M. Rentzepis, R.P. Jones and J. Jormer, J. Chem. Phys. 59 (1973)

766.

PI G.E. Busch, R.P. Jones and P.M. Rentzepis, Chem. phys. ietters 18 (1973) 178.

[lOI D.N. Dempster, T. Morrow, R. Rankin and G.F. Thompson, I. Chem. Sot. Faraday II 68 (1972) 1479. [Ill E.G. Arthurs, D.J. Bradley and A.G. Roddie, Chem. Phys. Letters

22 (1973)

230.

good agreement with the original value deduced by DMTR [lo].

1121 E.G. Arthurs, D.J. Bradley and A.G. Roddie, Opt. Commun. 8 (1973) 118. [ 131 T.J. Chuang and K.B. Eisenthal. Chem. Fhys. Letters 11

We find no evidence for relaxation on the IO-50 psec time scale. It may be possible to reconcile our results with the 300 psec figure. To do so will require re-

[14]

36

(1971) 368. J.N. D-emas and G.A. Crosby, 991.

J. Phys. Chem.

75 (1971)