The photodynamics of 2-(2′-hydroxy-5′-methylphenyl)-benzotriazole in low-temperature organic glasses

91. numkr

\‘calrne

CHEMICAL

5

THE PHOTODYNAMICS

4 February

1983

OF Z-(2’-HYDROXY-5’-hlETHYLPHENYL)-BENZOTRIAZOLE

IN LOW-TEMPERATURE S.R.

PHYSICS LETTERS

ORGANIC

GLASSES

BARBARA

FLOhl and PI.

Lkpaarrrn~t~ t of Clwnrisr~r. L’nircrsiry of hlinnesora, hlinneapolis. hlinnesota 5.i4.5.5. USA Ilrccivcd 7 Ocrobcr

1981

The non-tillic-resolved and time-resolved emission spectroscopy of 2 -( 2’-hydroxy-5’-mcthylphmyl)-benzotriazole in lowtcmpcraturc organic $ISSCS has been investigted. The observed data are consistent Gth ;Lkinetic -model that has been disCIIMXI prcviuusly

by several rcsearchrrs. Our results sugest that the estrernely efficient electronic radiationless decay of this c-trnlpound is due to an esceptionally rapid decay rate for the proton-transferred form of the fist electronically excited

singlet st3te.

2. Experimental

1. Introduction Tltc special photophysical otgwii

niolcctrlcs

properties

3s ;I result

imparted

of intranioieculsr

to

hydro-

gen btmcling

have been under investigation for over _X ?-cars [ I --_:I _ in recent years. primarily as a result of nc\x’ cspcrinlrntrrl techniques (especially picosecond

sp~‘c~roscopy).

considerable

(iw nwchanistic &II‘the ccntra1

progress

understanding

has been nxtde on

of this field

issues in this regard

has been

[S].

One

the mech-

HMPB was generously supplied at no charge by Ciba-Geigy Corporation and purified by recrystallization tbrce times from absolute ethanol. Spectroscopic solvents were reagent grade and purified by treatntent with activated

alumina (methylcyclohexane and isopentane) or by distillation from calcium hydride (Smethylpentane). Sample solutions were =104

mole/liter

anistis tlet;iils of how hydrogen bonding can enhance electronic txkttitmless Jcczty in ;i variety of molesules. e.g. SW rcfs. 14-6 J . In tilis paper we con1 inue our research 17-9 ] in

quartz

this xez

study.

by stttd\ting the low-telnper~ture

static

and

dynamic tluorcsccnie sprctroscopy of 2-(2’~hydrosy5‘-~1~sti~~ipl1~~~\.1)-b~~~z0triazole (HXlPB). HMPB. 3s iornprtred

to closely

drosy-phenyl gen bonds.

related

substi~uents

has xi esccptionally

rxliatiwiless

dwrty

prrprr the

spccrroscopy

identify

pmnd.

4s

with

efficient

nwchariisrnjs).

the rclasation

responsible

con~pwtids

and intramolecular

hydro-

electronic

We esaniine

of IlhlPB in an attempt process

for the rscrptional

or processes

properties

2-hy-

in this

to

that are

of this com-

in HAlPB and contained

optical

in sealed 1 mm

cells.

Our apparatus for variable-temperature, time- and wavelength-resolved fluorescence measurements has been described

molecular

in detail elsewhere [9]. In the present excitation is at 355 nm and emission

is monitored at variable z-1 5 mn wide regions. The experimental digital time-dependent fluorescence traces (points in the figures) were fit (solid lines in the figures) by an iterative procedure with a convolution

of the experimentally

deterntined

instrument

response function (=-5O ps full width half height) and an assumed kinetic model, see section 3. Employing this procedure we routinely achieve ~10 ps effective time resolution. see elsewhere for further details [9]. Static emission spectra (4 mn resolution) were determined with 3.55 nm. 30 ps excitation and detection by a spectrograph constructed from an HR320 (Instruments SA) polychromator with a 150 groove/ 0 009-2614/83/0000-0000/S

03.00 0 1983 North-Holland

CHEMICAL PHYSICS LETTERS

Volume 94, number 5

mm holographid grating and a 1420 (Princeton Applied Research) photodiode array, PDA, with a 520 photocathode. The spectra we report have been corrected for the channel-dependent sensitivity of the PDA but not for the wavelength-dependent sensitivity of our apparatus_

3. Results and discussion 3. I. Basic photophysical

mechanism

S,z t121 se2 + hv --f s,a_

(1)

This species rapidly rearranges

by excited-state

S--II

molecular proton transfer, ESIPT, to yield a lowerenergy excited species, SJ3

intra-

(2)

512+%3-

Fig. 1 allows for the following radiative and non-radiative decay processes s, 2 + so2 + hv,

(3)

s,,

(4)

+ so3 f hv,

S,, -

The spectroscopy of HMPB and derivatives [lo--161 have been investigated by several groups. The generally accepted photophysical mechanism is closely analogous to that believed to be operating in related molecules, e.g. see refs. [2,4,S17,17]. In non-polar solvents HMPB exists predominantly in an intramolecularly hydrogen-bound form, 2, which when excited in the ultraviolet yields a Fran&-Condon excited singlet state,

4 February 1983

4

x-6 S13-

So2 + heat,

(5)

So3 + heat,

(6)

So3 + So? -f heat _

(7)

The identification of which process or combination of processes [eqs. (3)-(7)] is responsible for the exceptionally efficient electronic radiationless decay in non-polar solvents is a central issue in this study of HMPB, see below_ In polar, hydrogen-bonding solvents HMPB has been shown to exist (at least partially) in an intermolecularly bonded form. 1 [ 121. This type of spe-

4

E c

I

z-a

cn t

0

cu 0

15

S-01

Fig_ 1_ A schematic representation of the photodynamics of HMPB in the form of a Jablonski diagam.

489

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94. number

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CHEMICAL

4 February 1983

PHYSICS LETTERS

tics. which does not exhibit ESIPT [ 121. apparently also exists in non-polar solvent when small amounts of polar impurities are present_ see below. The pertinent kinetic steps associated with 1 are as fl~llows:

+iIU

s,

1

-

%l*

@I

s,, + hu.

(9)

S, I - S,,

+ herlt.

(10)

The emission spectra we observe for HMPB in lowtemperature organic glasses are displayed in fig. 2. Absorption spectroscopy of I-IhlPB in similar environments have been investigated [ 121 and reveals that 111s lowest-energy absorption band is at ~350 ml [eq_ ( 1 )] and 110 detectable absorption is present %I00 mn. The significantly Stokes-shifted 630 nm emission in ligs. 23 and 3 has been previously assigned to fluorescence Ieq_ i4)] from St, which is produced by ESIPT [cq. (‘)I. The emission we observe at =I00 nm is more diit?su11 to interpret. It would be tempting to assign this band IO llu~,resccnce from S, z [eq. (3)]_ But. a variety of
b

Wavelength

(nm)

IQ 2. Emission spectra of HMPB at low temperature with 355 nm excitation in the followin:: solvents: WX/IP (a). 3UP (b) and 3hlP with added moisture (c). Fig. 2s is truncated at 4 10 nm because of interference from a colored glass 400 nm “long pass” filter used in the collection optics for this spectrum only. A 375 nm “long pass” filter was employed in the collection of figs_ 2b and 2c.

Will.

Ttlc evidcncc just mentioned, as weI1 as the similarity of the <500 JIII~ emission spectrum (fig. 3b) to that
3.3. Time-resolved

specrroscopy

The time- and wavelength-resolved emission data are in complete agreement with fig. 1 and the arguments just made about the non-time-resolved spectroscopy. In both 3methylpentane (3MP) and a 1 : 1 mixture of methylcyclohexane and isopentane (MCH/IP), the 400 nm kinetics at <90 K appears in 2 ns. The red fluorescence band (monitored at 615 2 15 nm) also appears in
CHEhliCAL

Volume 94. number 5

3UP

I

T=SOK

.

Time

(psec)

Fig. 3. Time-resolved fluorescence of HMPB at low temperature in organic glasses. See the figure and texf for further details. The lifetime (~1) or lifetimes (~1 and ~~ biexponential decay) for the emission in each panelas determinedby a fitting procedure (see text) are as follows: (a) 7 = 2000 2 300 ps;(b)~=345~20ps:(c)~,=10+5psandr~=220~20ps; and(d)~1=10f5psand-rz=245i20ps.

PHYSICS LETTERS

4 February

1983

emission is considerably less intense than the 6 15 nm band. The non-exponential decay kinetics we observe at 615 nm may be a manifestation of time-resolvable vibrational relaxation which has been observed in many other low-temperature systems [4,5,7,18,19] _We have not investigated this aspect of the kinetics in detail, and will not discuss it further. The simplest interpretation of the fluorescence kinetics is that the species responsible for 400 mn and 615 nm fluorescence are kinetically unrelated and are due to excitation (355 nm) of two different groundstate species - presumably 1 and 2 as described above. This is supported by the observation that. while the relative non-time-resolved intensity of the 613 and 100 nm bands are dependent on sample preparation (see section 3_2), the kinetics of each of the bands are not_ Our data are in contradiction with the recent proposal by Huston et al. [lo] that S12 and St; are in rapid picosecond equilibrium, since we see no evidence of the onset of equilibrium in the 615 mn kinetics nor do we observe identical lifetimes at 615 and 400 nm - which their mechanism implies_ Based on these arguments, the 615 nm kinetics can now be analyzed to determine a lower limit for the rate constant for ESIPT, X-, in eq. (2)_ If it is assumed. as will be justified below, that k2 > X-, it can be shown simply that the rate of appearance of the 615 nm emission is a measure of X-2. Since we observe that emission at this wavelength is formed in (10 ps, even at 16 K, the rate constant for ESIPT in HMPB must be greater than 1011 s-l_ ESIPT rates as large as lOi s-l have indeed been observed for every compound structurally related to HMPB that has been studied by picosecond fluorescence spectroscopy [4,5,7,17,20]. and appears to be a general rule for this type of compound_ It has been suggested. incidentally, that the ESIPT in these compounds may be so rapid because ESIPT, in this case, is not impeded by a significant potential energy barrier and as such represents a special class of vibrational relaxation [?,2 1] _ 3.4. EIecrronic mdiationlessdecay mechanism It has been proposed that the dominant mechanism for electronic radiationless decay of HMPB [12.1~,15]

4 February 1983

CHEMICAL PHYSICS LETTERS

and relared compounds IIU -s,2

s,,

ES1P-f -s

[I ] is as follows: 13 - So3 + heat

- StI1 + heat.

(11)

The kinetics we observe at 615 mn (ij are consiswith this III~L-h311is1r1. (ii) suggest that the estrcmcly efficient decay mechanism of HMPB is due to 311 exceptionally large non-radiative decay rate (k6) of S13 as coInpared to related compounds. arid are consistcur \vith the assumption that k2 >k,. X-, can. of course. be Cslinlated sccurately from the inverse of trn~

the lifetime of the 615 nm kinetics. see figs. 3 and 4. II should 1x2mentioned. however. that low-temperature tr3w.icnt grourtd-state recovery me;lsurements will bc required to rigorously establish the validity of ccl_ ( I 1 ). \\‘c are prcscutly continuinp research on

llhll’l3 ahwg tlwse lines. The tcmpersrure and solvent dependence of the non-mdiative decay rate of S13 offer some insight on f hc IIIectIarIisIn of the decay process [es_ (G)] . see fig. 3. Seal- rhe $ss-forming Icrnperature of the solvenls [ :2 J. ;= 120 K. rhc drrrl?; rates increase rapidly with incrcasiug tcmpcrature. This is similar to related compw~ds. c-g_ 2-l ~-lI\~dros~plIen?:l)-beIIzotlIiazole (f 1131‘) [ 5 I_ and is evidcrlce tflat the decay process involvc’s. rit !cast p2rIly. a “large-amplitude internal molions'. Tltis type of iIncma1 motion. e.g. rotation sbou~ a single bond. which is available to “flexible” IIIC&XU~~S. and lends to be diIninished in the highly

.P.~C”,IP 3 M P

I 1

viscous environment of a glass solvent, can promote non-radiative decay. Incidentally, the use of large solvent viscosity effects on non-radiative decay rates as evidence for large-amplitude excited-state motion has been clearly established and quite popular [19,X---261. In the temperature range 16-90 K k6 is only slightly temperature dependent, even though the available thermal energy and solvent hardness [22] must be varying greatfy. Related compounds in the same envirdmnents exhibit radiationless decay rates that are orders of magnitude slower [4,5.7X2], because the large-amplitude motion decay mechanism of these LoI.Ipounds is essentially “turned off” in “rigid” environments. it seems reasonable then to suspect thaI the I?~ decay process has two components: (i) a process similar to other ESIPT molecules that involves large-amplitude motion and is dominant above the glass-forming temperature of the solvent and (ii) a process which is still rapid at high viscosity and extremely low temperature. The latter process seems analogous to the rapid radiationless decay observed in some “rigid” molecules [27]_ It is likely that the “rigid” component of the decay mechanism is, in fact, responsible for the exceptional photochemical stability of HMPB in “hard” environments, e.g. organic polymers [ 16]_ It is important to note. however. that the phenomenological separation of “large-amplitude” and “‘small-amplitude” decay components by viscosity effects is at best approximate in nature and only recently has been evaluated carefully by comparison to theoretical prediction [23] _The observation of weak viscosity effects on the radiative properties of obviously “rigid” molecules [3S] _ furthermore, suggest that weak viscosity effects must be interpreted with caution. The differences we observe between the decay kinetics of HMPB at -230 K in 3MP and MCH/IP may be a further example of a relatively weak “hardness” or viscosity effects on a “small-amplitude” motion controlled non-radiative decay process.

f 4. Conclusions

and summary

The non-time-resolved emission spectroscopy 2-(2’~hydroxy-5’-methylphenyl)-benzotriazole in temperature organic glasses has been investigated_ observed data are consistent with a kinetic model

of lowThe [eq.

CtiE,MICAL

PHYSI-

(1 l)] that has been discussed previously by several researchers_ Our results suggest that the extremely efficient electronic radiationless decay of this cqmpound is due to an exceptionally rapid decay rate for the proton-transferred

form (S13) of the electronically

ex-

cited singlet state.

Acknowledgement

Acknowledgement

is made for partial support

of

this research to the following agencies: the donors of the Petroleum Research Fund (administered by the American Chemical Society), the Research Corporation, and the Graduate School of the University of Minnesota. SRF would like to thank the Allied Corporation for partial support during the period of thii

research_

LETTERS

1983

[9] A. Strandjord, S. Courtney, DM Friedrich and P.F. Barbara, Excited-State Dynamics of S-Hydrosyflavone, J. Phys. Chem., submitted for publication. [lo] A. Huston, G-W. Scott and A. Gupta, J_ Chem. Phvs.. 76 (1982) 4978. Ill1 A-A. E&nov and VS. Sivokhin, Dokl. Akad. Nauk SSR 250 (1980) 387. 1121 T. Werner, J. Phys. Chem. 83 (1979) 320. 1131 VS. Sivokhin and A-A. Eftmov. Zb. Prikl. Spectroskopiys 31 (1979) 813. 1141 A. Gupta, G-W. Scott and D. Kliger, ACS Symposium Series 151 (ACS, Washington, 1960) ch. 3. 1151 J-A. Otterstedt. J. Chem. Phys. 58 (1973) 5716. [=I HJ_ He&r and H.R. Bhttmann, Pure AppL Chem 36 (1973) 141_ 1171 K.K. Smith and K J. Kaufman, J. Phys. Chem. 82 (1978) 2286. WI P.F. Barbara, P.M. Rentzepis and L.E. Brus. J. Chem. Phys. 72 (1980) 6802; B-P. Boczar and M-R_ Topp. Vibrational. Excitonic and !dolecular Site Relasation of Perylene in Solid Solution at Low Temperature. presented at the 182nd ACS National hlerting, New York, 1961.

[I91

P-F_ Barbara. S-D. Rand and P-XI. Renrrepis, J. Am.

PI

Chem. Sot. 103 (1961) 2156. GJ. Woolfe and PJ. Thisticrh~~aite, 1. Am. Chem. Sot.

References 111 W_ Kliinffer. Advan. Photochrm. 10 (1977) 311_ i2] D. Huppert~bl. Gutman and K.J. Klaufma&, Advan. Chem. Phvs. 47 (1981) 643. [3] J-F_ Ireland and P.A.H. Wyatt. Advan. Phys. Org. Chem. 12 (1976) 131. 141 PI_ Barbara. PM. Rentzepis and L.E. Brus, J. Am_ Chem. Sot. 102 (1980) 2786. 151 P-F. Barbara, LE. Brus and PM. Rentzepis, J. Am. Chem. Sot. 102 (1980) 5631. [6] H. Inoue. hl. Hida, N. Nakashiia and K. Yoshihara, J. Phys. Chem. 86 (1982) 3184. [7] P.F. Barbara, S.H. Courtney and K. Ding. Picosecond Emission Spectroscopy with Intensified Photodiode Arrays, presented at the 184th National Meeting of the ACS, Kansas City, September 1982; K. Ding. S.H. Courtney, AJ_ Strandjord, S. Flom. D. Friedrich and P-F. Barbara, Excited-State Intramolecular Proton Transfer and Vibrational Relaxation in 2-(2Hydroxyphenyl)-Benzothiazole. J. Phys. Chem., submitted for publication. [S] P-F. Barbara, A. Strandjord and S. Courtney, ExcitedState Dynamics of 3-Hydroxyllavone, presented at the 16th Great Lakes Regional ACS hleeting, Normal, Illinois, June 1982.

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D. Ford, P-J. Thistlethwite and G-J. Woolfe. Chem. Phys. Letters 69 (1980) 246. D-L. Williams and A. Heller. J. Phys. Chem. 74 (1970) 4473. G. Fischer and E_ Fischer, !%lol. Photochrm. 8 (1977) 279. S.P. Velsko and G.R. Fleming. J. Chem. Phys. 76 (1982) 3553. J-R. Taylor, h1.C. Adams and W. Sibbett, 1. Photochem. 12 (1980) 127. Th. Forster and G. Hoffman. Z. Physik Chem. (Leipzig) 75 (1971) 63. D_ Gegiou, K-A. hluszkst and E. Fischer. J_ Am. Chem. Sot. 90 (1966) 12. G.W. Robinson, Excited States 1 (1974) l_ T.G. Politis and H-G. Drickamer, J_ Chem. Phys. 75 (1981) 3203; E. Blatt, E. Treloar and K-P- Ghkino, J. Phys. Chem. 85 (1981) 2810.

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