Intramolecular triplet energy transfer in ester-linked bichromophoric

J. Phys. Chem. 1992,96,1524-1535

7524

Intramolecutar Triplet Energy Transfer in Ester-Linked Bichromophoric Azoalkanes and Naphthalenes Paul S. Engel,**+Douglas W. Horsey,+John N. Scholz? Takashi Karatsu,t and Akihide Kitamura*J Department of Chemistry, Rice University, P.O. Box 1982, Houston, Texas 77251, and Department of Chemisfry, College of Arts & Sciences, Chiba University, Yayoi-cho. Chiba 260, Japan (Received: February 21, 1992; In Final Form: May 29, 1992)

The photophysics of a series of compounds has been studied wherein a triplet sensitizer such as benzophenone (BB) or thioxanthone (TH) is linked via an ester group to an azoalkane such as 3,3,5,5-tetramethyl-1-pyrazoline (PY)or 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO). The observation of phosphorescence from the sensitizer moiety in a frozen glass and triplet lifetime measurements in solution show that intramolecular triplet energy transfer (ET) can be remarkably slow. Model compounds in which naphthalene replaces the azo group behave normally, exhibiting complete and rapid intramolecular ET. Despite the small size of the azo group, its critical radius, & for intermolecular ET in a frozen glass is comparable to that of naphthalene. When the donor and azo chromophores are closely linked by alkyl groups, ET is very rapid. No phosphorescence or T-T absorption of the azo group could be detected, but quenching experiments show that the triplet lifetime of cyclic azoalkanes is much shorter than their singlet lifetime.

Of the mysteries surrounding the deceptively simple azo chromophore,' the one that has received the least attention is the nature of its lowest triplet statee2Neither phosphorescence3" nor transient absorption' has ever been observed from an azoalkane triplet. Product studies generally'~8~9 but not alwayslOJ'show that intersystem crossing (ISC) is not an efficient processI2 while indirect evidence3J3suggests that azoalkane triplets are exceedingly short lived. The combination of poor ISC and rapid deactivation render observation of an azoalkane triplet by emission or transient absorption spectroscopy a challenging problem. On the brighter side, several groups have used a graded series of sensitizers to show that the n,a* triplet energy of azoalkanes lies in the region of 50-60 kcal So-Tl ab~orption'~J~ and electron impact studiesI9 support a triplet energy in this range. The present study employs intramolecular ET to bypass the usual inefficient ISC of azoalkanes. We reasoned that irradiation of the sensitizer moiety of a bichromophoric molecule20should rapidly populate the sensitizer triplet state which should then transfer energy to the azo chromophore. This approach should allow quenching of the azoalkane triplet by external reagents without complications from quenching of the sensitizer moiety. Furthermore, we hoped that phosphorescence or transient absorption spectroscopy might allow the direct observation of an azoalkane triplet. This study, which employs both frozen glasses and solution, is the first one, to our knowledge, that deals with intramolecular triplet ET to a chromophore as small as the azo group. Remarkably, the rate of intramolecular ET to the azo group in solution has been found to vary over a range of nearly los; however, attempts to observe the triplet state of this chromophore have been unsuccessful. Quenching experiments reveal that the lifetime of azoalkane triplets is much shorter than that of their excited singlet state.

Selection and Synthesis of Compounds To maximize the chances of observing the triplet state or of intercepting it with external quenchers, we avoided azoalkanes that have an efficient deactivation pathway. 2,3-Diazabicyclo[2.2.l]hept-2-ene (DBH), for example, would be a poor choice because its quantum yield for triplet sensitized loss of nitrogen is unity.' Likewise, trans acyclic azoalkanes have little to recommend them because, even though their triplet states generally do not undergo homolysis in solution,2l radiationlessdecay is likely to be rapid for these flexible molecules.22We therefore confined our studies mainly to the 3,3,5,5-tetramethyl-l-pyrazoline(PY) and 2,3-diazabicyclo[2.2.2]oct-2-ene(DBO) structures. Rice University.

'Chiba University.

0022-3654192/2096-1524S03.00/0

PV

DBO

We selected the ester group to link the sensitizer and azoalkane moieties for ease of synthesis. Benzophenone was chosen as the 3(n,7r)* sensitizer because its triplet energy (68.5 kcal/mol) lies above that of azoalkanes while its singlet energy is low enough to permit selective excitation. In order to shorten the effective interchromophoric distance,23thioxanthone was employed as a 3(7r,7r)* sensitizer whose triplet energy (ET = 65.5 kcal/mol) is still above that of azoalkanes. The longer lifetime of 3(17,7r)* sensitizers was also expected to make ET more competitive with radiationless decay, and the high extinction coefficient of thioxanthone allows the use of low concentrations that minimize bimolecular ET. The first azoalkane employed as an energy acceptor is a pyrazoline (PY),whose triplet sensitized N2 quantum yield (er) is 0.23.13 However, DBO should be an even better candidate than PY for detection of the triplet state because its benzophenone sensitized e, is only 0.014. Furthermore, DBO is probably a more rigid structure than PY,a feature that should diminish the rate of radiationless decay (kd).22 There were two indications that triplet DBO might survive long enough to be detectable. Firstly, radiationless decay of TI is spin forbidden and is normally expected to be slower than decay of SI.Since we had previously determined kd of DBO*' to be 1.4 X lo6 s - I , ~ ~ the fact that radiationlessdecay is the dominant fate of DBO*' suggested that the lifetime of this species might be reasonably long. Secondly, the triplet state of 1-cyclopropyl-DBOreacts with neat CC14,8bimplying a lifetime sufficient for bimolecular processes to occur. The same conclusion can be drawn from our observation that the photoreduction of DBO can be sensitized by 2-a~etonaphthone.~~ To provide a p i n t of attachment for the sensitizer moiety, the following azoalcohols were prepared: 3-(hydroxymethyl)-3,5,5trimethyl-1-pyrazoline (1),25 2-(tert-butylazo)-2-methyl-1-propanol (2),26 1-(hydroxymethy1)-DBO (5), and 1-(2-hydroxyethyl)-DBO (6). The Diels-Alder reaction of 1-(2-hydroxyethyl)-l,3-cyclohexadiene2' and 1-(hydroxymethy1)-1,3-~yclohexadiene~~ with N-methyltria~olinedione~~ afforded adducts 3 and 4 that were converted to the azoalkanes by hydrogenation and hydrolysis. In the case of 4, protection as the trityl ether improved the yield of the water-soluble azoalcohol6. The bichromophoric esters were prepared from the azoalcohols and a sensitizer acid chloride; for example, reaction of 4-benzoylbenzoyl chloride (7) with 6 afforded the ester 4BB-2-DBO. The new esters are named here by first 0 1992 American Chemical Society

The Journal of Physical Chemistry, Vol. 96, No. 19, 1992 7525

Intramolecular Triplet Energy Transfer

n

A 1. H2, PdIC 2. Ph,CBI

*

1. Hz, PcVC 2. KOH, I-PfOH

3. KOH, i-PrOH 4. BF3.EQO

3. CUClZ 4. N k W

6H 5,n=l 6,n-2

HOOk 10

Ph AP-DBO

7

dH

6

4BB-Z-DBO

abbreviatingthe sensitizer moiety, then specifying the chain length of (CH3, and finally abbreviating the azoallrane. The ester-lied bichromophoric azoalkanes prepared for this study are shown below.

4BB-l-DB0, 4BB-2-DB0,

ZBB-PV, 3BB-PV, 4BB-PV,

n=l n.2

ortho mota

qqq 0 O

I

?

P

TH-1-DBO, TH-2-DBO,

O

0

Me0 Meo&

0

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DMA

AP

ZBB.Me, 388.tn0. 4BB-Me,

orlho mota pars

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TH-PV

Two compounds with even shorter interchromophoricdistances were synthesized, one a 3,4-dimethoxyacetophenonelinked to a pyrazoline (DMA-PY) and the other an acetophenone connected to a DBO (AP-DBO). The DMA chromophore was chosen because its longer wavelength absorptionwthan that of acetophenone would allow more selective irradiation of the sensitizer moiety. Reaction of 2-propenylmagnesium bromide3' with 3,4-dimethoxybenzaldehyde followed by Jones oxidation of 8 and 1,3-dipolar ~ycloaddition~~ of 2diazopropane to the enone afforded DMA-PY.

crq

&

0

para

N

TH-ATB

1.

the triazolidine ring is bent toward the C-C double bond. Hydrogenation of 9 occurs from the less hindered exo face, leading to the endo acid As shown in the drawing of AP-DBO, the C 4 group points toward the nitrogens, the 0-N2 and 0-N3 distances being 3.59 and 2.96 A, respectively. Although the benzoyl group of AP-DBO could have been placed at the bridgehead, we feared that this radical-stabilizing substituent a to the azo group would enhance photolability (see be lo^).'^,^^ In addition to the bichromophoric azoalkanes described above, we examined two kinds of model compounds; the first contains only the sensitizer moiety while the second consists of the sensitizer bonded to a well-behaved acceptor, naphthalene. Their structures and abbreviations are shown.

*

"eo*

N=N

2. Me+Ndd

Me0

DMA-PV

AP-DBO was prepared via a multistep sequence analogous to that for DBO itself and for the carboxylic ester of its next lower homolog, DBH." Diels-Alder reaction of 3,4-dihydrobenzoic acid34with N-methyltriazolinedionegave the adduct 9 which was transformed in the usual manner27to 10. Conversion of 10 to AP-DBO was accomplished via the acid chloride using diphenyl~admium.~~ The stereochemistry of the carboxylic acid group is determined at the hydrogenation step and was shown by X-ray crystallography of AP-DBO to be end^.^^ This result is in accord with the structure37of 9 lacking a COOH group, wherein

TH-2-NA

coo

All of the compounds were purified by column chromatography, HPLC, and/or recrystallization; they were then analyzed by HPLC prior to photophysical and photochemical studies. Results Absorption Spectra. As seen in Figure 1, the UV spectrum of 4BB-PY is merely the sum of the spectra of the individual chromophores. The same is true for 2BB-PY, except that in this case, significant absorption does not extend beyond 380 nm. The spectra of both compounds allow selective excitation of the sensitizer moiety in the region 350-380 nm. Since TH-Me exhibits a strong band at 385 nm (t 7000 M-' cm-I) which dominates over the much weaker absorption of the azo moiety (e = 200-300 M-' cm-I), the spectrum of TH-PY (Figure 2) is nearly identical to that of TH-Me. This favorable ratio of extinction coefficients ensures that 350-380 nm irradiation of all TH compounds will excite only the sensitizer. The absorption of DMA extends to 360 nm, barely beyond that of 1. However, the bichromophoric molecule DMA-PY absorbs more strongly, particularly in the longer wavelength region, than the sum of its components (Figure 3), indicating that the chromophores interact. The absorption spectrum of DBO and 6 has a window at 320 nm (t 20 M-l cm-I) where acetophenoneexhibits a n,a* maximum (c 50 M-' cm-', cf. Figure 4). The azo chromophore is plainly visible in the spectrum of AP-DBO but is somewhat less intense than in DBO itself. These relatively minor changes in e coupled with the

-

--

7526 The Journal of Physical Chemistry, Vol. 96. No. 19, I992

+2"ool \

Engel et al.

i\ I

,

t0.m. t0.m

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.

.

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WAVELMOlH

Figure 1. UV spectra in benzene of 4BB-Me (0.0107 M), 4BB-PY (0.00423 M), and PY-OH (1) (0.00521 M).

"'"1

\

TABLE I: Extinction Coefficients (M-I c d ) in Benzene of Compouads Used in This Study compound 266nm 313 nm 337nm 355nm 366nm PY" 93 113 1 138 176 6 DBO 13 17 41 95 6 21 48 99 ATBb 8 10 14 15 DMA 135 4960 34 DMA-PY 582W 600 12000 136 35 AP 48 45 17 AP-DBO 116W 72 147 64 100 TH-Me 560 6600 2060 3750 4-BB-Me 114 86 128 100 3BB-Me 71 116 103 69 2-BB-Me 113 111 24 56 TH-2-NA 2360 1690 820 4100 4BB-2-NA 584 130 136 95 4BB-1-NA 679 117 150 153 TH-2-DBO 668 5900 3900 2230 TH-1-DBO 1740 5550 483 3347 TH-PY 6910 2000 455 4000 TH-ATB 6120 1200 1390 2560 4BB-2-DBO 354 170 195 220 555W 4BB-1-DBO 389 165 203 24 1 4BB-PY 490 320 156 127 192 3BB-PY 282 115 82 2BB-PY 290 277 33 73

u\

w

*0.100-

tO.400

o.woo-!

Figure 4. UV spectra in benzene of DBO (0.00482 M), AP-DBO (0.00374 M), and AP (0.0138 M).

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rbo

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,

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Figure 2. UV spectrum in benzene of TH-PY (0.00022 M).

+"oool 1\

In hexane.

I

W

0.0000

-0

300

Be4

WAVUENETH

Figure 3. UV spectra in hexane of DMA (1.56 X l0-l M) and DMA-PY (1.13 X lo4 M).

absence of noticeable spectral band shifts speak for weak interaction of the chromophores." The UV spectra of 4BB-1-NA and 4BB-2-NA correspond to the sum of the benzophenone and naphthalene spectra. Table I summarizes the extinction m f f i cients at photochemically useful wavelengths of the compounds relevant to this study. E~aisshoSpectra. The phosphorescence spectra of the esterlinked bichromophoric azoalkanes at 77 K looked like those of the respective model compounds 2BB-Me, 3BBMe, 4BBMe, and TH-Me; hence, we too have failed to detect any emission from an azoalkane triplet state.- On the other hand, it is remarkable that phosphorescence of the sensitizer moiety was seen, in view of its absence in related bichromophoric m0Wes.m~4143 Several experiments were undertaken to explain this continued donor

In CH3CN.

TABLE II: Phospborescemce hkaritiea md Lifetimes for E s t e r - W e d BichnwopLoric MokHks at 77 K in EPA comwund k.,~ nm intensitvb rm/ ms 4BB-Med 365 1.o 4.9e 4BB-PYd 365 0.006 4.8' 2BB-Med 350 1.o 4.9e 2BB-PYd 350 0.003 4.2c TH-Md 300, 370 1 .o 114 TH-PYf 370 0.06 1138 TH-2-DBU 370 0.03 1188 For relative intensity experiments. bIntensity at ,A, relative to I = 1.0 for the model methyl caters. CLifetimeswere determined in a s e p arate set of experiments using 2-mm sample t u h and an excimer laser. d2.3 X M. 'Excited at 351 nm in methylcyclohexane. f9.3 X M. gExcited at 337 nm in 2-methyltetrahydrofuran.

emission despite the presence of a nearby acceptor. Phosphorescence lifetimes of our esters were measured by pulsed laser irradiation of frozen (77 K)glagses and monitoring the fust-order emission decays with an oscilloscope. Phosphor#Kxnce intensities relative to the model compounds were determined quantitatively in 8-mm sample tubes using a chopper. As seen in Table 11, the bichromophoric azualkana emit much less strongly than the model esters but with the same lifetime. No meaningful phosphorescence was detected from DMA-PY and AP-DBO.

Intramolecular Triplet Energy Transfer

The Journal of Physical Chemistry, Vol. 96, No. 19, 1992 7527

TABLE I11 Quenching of Benzophenow and Thioxanthone Phosphorescence in Ether-Alcohol at 77 C donor acceptor A,, nm A,, nm pointsb slopec Ro,d A 4 6.73 13.9 365 414, 443 BB' NA 4 8.6 15.1 443 DBO 300 BB' 365 414, 444 5 6.06 13.4 BB' PY 370 460 3 6.8 13.9 TH' NA 4 8.4 14.9 300 460 TH' DBO 370 460 4 4.1 11.8 PY TH'

TABLE V Lowest Triplet- and Singlet-State Energies of AzoaUunes (kcal/mol)

"Maximum quencher concentration was 0.1 M. bNumber of quencher concentrations run. CSlope of In (Zo/I) versus quencher concentration. dCritical radius for triplet ET, calculated as radius of a M. '9.3 X sphere whose volume was slope/6.023 X lozo. c2.3 X IO-' M.

" Singlet-triplet splitting based on band origins which are difficult to locate in the broad absorption spectra of acyclic azoalkanes. *Onset. CReference 19. Band maximum. Reference 18. 'Reference 47. gAll isomers. hReference 14. 'Reference 48. /This work. k2,3-Diazabicyclo[2.2. I ] hept-2-ene. 'Reference 6. This value is higher than the previously reported4 60 kcal/mol.

TABLE I V Quenching of Sensitizer Triplets by Azoalkanes" azoalkane sensitizer ETb IO-*&,, M-' DBO BB 68.5 38' TH 65.5 40 chrysene 56.6 17 biacetyl 54.9 5.5d 53.3 0.18 9-fluorenone PY p-MeO- A P 71.5 30 65.5 6.9 TH phenanthrene 61.8 0.01

s-I

" By flash photolysis in benzene at room temperature. Triplet energy of sensitizer, kcal/mol. From ref 3. d By phosphorescence lifetime quenching. 'p-Methoxyacetophenone. In order to evaluate the possible role of the ester linkage in slowing intramolecular ET, phosphorescence spectra of the model compounds 4BB-1-NA and 4BB-2-NA in a frozen glass were obtained using the fluorescence mode so as not to exclude any short-lived BB emission. The first samples studied exhibited sizeable BB-type phosphorescence but extensive purification reduced that emission to less than 2% of the NA phosphorescence in the case of 4BB-2-NA and to less than 0.5% in 4BB-1-NA. It should be noted that BB phosphoresces about 25 times more i n t e n ~ e l than 9 ~ ~NA, so even a small amount of a BB-containing impurity would cause an observable emission. The phosphorescence spectrum of TH-2-NA consisted only of NA emission. We conclude that intramolecular ET is complete in these model compounds, as found earlier for alkyl-linked BB and NA bichromophoric m0lecules.4~~~ Since the model compounds 4BB-n-NA and TH-ZNA behaved normally, it seemed unlikely that the ester group of our bichromophoric azoalkanes was responsible for inhibiting intramolecular ET and allowing the sensitizer moiety to phosphoresce (cf. Table 11). We next considered the possibility that the particular azoalkanes chosen were actually poor quenchers of BB and TH phosphorescence. Using the classic technique of Ermolaev,44we measured the phosphorescence intensity of BB and TH in frozen 1:1 ether-alcohol (EA) containing increasing concentrations of azoalkane acceptors (Table 111). Plots of In (IOIO were linear and our critical radius for ET (Ro)came out 13.9 A for the BB-NA pair, which compares nicely with the 13 A reported by Ermolae~.~ Moreover, & was similar for all donoracceptor pairs studied. Thus the triplet quenchers PY and DBO appear to be as effective intermolecularly as the well-studied naphthalene. Semitizer and Azorlkane Triplet Energies. Despite the normal intermolecular ET behavior found in the above donor-azoalkane pairs,the ester group of the bichromophoric azoalkanes could lower the donor triplet energy level enough to slow intramolecular ET. In order to determine the triplet energy ( E T ) of the sensitizer moiety, the phosphorescence spectra of the corresponding methyl esters were examined. The results were (compound, ET in kcal/mol) as follows: PhZCO,69.4 (lit.4668.5); 2BB-Me, 72.8; 3BB-Me, 69.4; 4BB-Me, 67.3; thioxanthone, 65.5;& TH-Me, 64.3. Thus ET of TH-Me was quite similar to that of thioxanthone, but the BB esters exhibited ET'S ranging over 5.5 kcal/mol. Even though ET of DBO had already been reported3to be 54.5 kcal mol-', we measured the quenching rate constants (k,) in

azoalkane azomethane azobutane PY DBHk DBO

ES 67,bsCWd 70,bJ 84.6dJ 70b 8 2bJ 84' 76'

ET 55.5.63' 64"' 53:s' 62.7c.d 53h.i 64i 62'9"' 53'

Es - ET' 14 17 18 22 23

TABLE VI: Stem-Volmer Slopes for Quenching Nitrogen Formation

compound 4-BB-PY 2-BB-PY DMA-PY DMA-PY' AP-DBO AP-DBOI AP-DBO'

concn,b M 0.02 0.09 0.09 0.076 0.021 0.022 0.025

slopec 108 6.5 0.058 C0.7 0.055 9.2 33.5

lT,d ns 22 1.3 0.012 C0.14 0,011 >2 6.7

irradiation wavelength, nm 366 366 366 366 313 313 313

"Benzene solvent. The quencher is piperylene unless otherwise indicated. bConcentration of bichromophoric azoalkane. Stern-Volmer slope k T . dTriplet lifetime of donor assuming k, = 5 X lo9 M-' s-l. With %i-terr-butylnitroxide. 'With 1,3-cyclohexadiene.

solution for DBO with four triplet sensitizers (Table IV). From the usual plot of log k, versus sensitizer ET, we deduced a DBO triplet energy of 53 kcal mo1-l (cf. Table V). It is therefore clear that DBO should be an efficient quencher of all of the sensitizers in this work. A similar set of experiments with 3,3,5,5-tetramethyl-1-pyrazoline (PY) gave an estimated ET of 64 kcal mol-I showing that the singlet-triplet energy gap for the cyclic azoalkanes is about 20 kcal mol-l. The most questionable case for exothermic energy transfer is TH-PY, but even here, PY quenched thioxanthone triplets with k, = 6.3 X lo8 M-' s-I. Thus the sensitizer and quencher triplet energies are arranged such that ET is exothermic in every bichromophoric compound studied here. Solution Wpse Lifetime Studies. It was important to determine whether triplet quenchers would diminish the quantum yield of nitrogen loss from the bichromophoric azoalkanes. The results could reveal not only the rate of intramolecular ET but also the lifetime of the azoalkane triplet state. Since dienes are well-known triplet quenchers, we prepared Stem-Volmer plots using piperylene at various concentrations. The results (Table VI) include two experiments with di-tert-butyl nitroxide (DTBN), an effective quencher of even low energy triplet and one experiment with 1,3-cyclohexadiene. The latter compound was tried at high concentration (6.7 M) with AP-DBO but formation of a yellow color and polymer during the 5-h irradiation forced us to employ a much lower quencher concentration (0.025 M). Although no photoreaction of DTBN occurred during irradiation, judging by the constancy of the absorbance at 450 nm, DTBN has an extinction coefficient of 5.3 M-' cm-' at the most favorable irradiation wavelength (313 nm); hence, its concentration had to be limited to avoid competitive light absorption. The data in Table VI show that the bichromophoric azoalkanes have triplet lifetimes ( T T ) very much shorter than the usual microsecond range characteristic of aromatic carbonyl compounds. Direct measurement of triplet lifetimes by kinetic spectroscopy is greatly superior to the Stern-Volmer approach because the quenching rate constant is measured rather than assumed. We studied our bichromophoric compounds by transient spectroscopy over a period of years on instruments of increasing sophistication. In order to avoid light absorption by the azo moiety, the excitation wavelength was selected as 308 nm (XeCl excimer laser), 355 nm,

7528 The Journal of Physical Chemistry, Vol. 96, No. 19, 1992 TABLE VII: Rate Constants for Intermolecular (kb)and Intramolecular (PET)Energy TransfeP compound A,, nm kb, M-I s-I kETI S-I 1.6 x 104 3.0 x 109 TH-2-DBO 308 TH-1-DBO 308 4.0 x 109 5.1 x 104 TH-PY 355 8.6 X lo6 TH-ATB 355 1.5 x 109 7.1 x 104 4BB-2-DBOb*' 266 4.3 x 108 4BB-1-DBOb'd 266 1.4 X lo8' 4.5 x 107 3 x 109 355 4BB-PY 5.0 x 107 355 3BB-PY 4.2 X lo8 355 2BB-PYd 6.8 X lo8 355 TH-2-NAd 1.3 x 109 355 4BB-2-NAb 1.2 x 109 355 4BB- 1-NAd "In degassed benzene at 25 OC using nanosecond laser excitation and continuous wave analyzing light. bHexane solvent. CUsingpicosecond laser excitation and continuous wave analyzing light. dUsing picosecond laser pulse-probe technique. '4BB-1-DBO exhibited two other decay components of lifetime 88 ns and 7 p .

or 266 nm (YAG laser harmonics). Because BB absorbs less strongly than TH, the intense, long-lived fluorescence of DBO interfered with the measurements on the two 4BB-DBO's unless 266-nm excitation was used. The wavelength 355 nm was convenient for the three BB-PY isomers and for the naphthalene compounds because that wavelength falls to the red of the acceptor absorption. Each compound was studied at several concentrations, generally in benzene, using degassed solutions in sealed tubes. No new transient absorption clearly attributable to an azoalkane triplet was ever seen. In most cases, decay was a single exponential and the rate of intramolecular ET ( k E T ) was calculated from eq 1. 1 / 7 = k b [ S ] + kd + ET (1) Here k b represents the rate of bimolecular (intermolecular) ET, [SIis the substrate concentration, and kd is the decay rate of the sensitizer moiety in the absence of ET. For those cases where T of bichromophoric azoalkanes increased at lower concentration, a plot of I / T versus [SI provided values of kb and kET4- kd (cf. Table VII). For the model compounds 4BB-Me and TH-Me, kd was determined by kinetic spectroscopy as 7.7 X lo5 s-' and 1.4 X lo4 s-l, respectively. The kETvalues in Table VI1 were corrected for the contribution of kd to the intercept of eq 1. When kETwas rapid, 7 did not depend on concentration and no value of kb could be obtained. The TH-containing azoalkanes were well-behaved except for TH-PY, which showed dual exponential decay in degassed benzene. The 2-20-w~component, whose lifetime increased irregularly with dilution, is discussed below under Ester-Linked Sensitizer Azoalkanes. In N2-purged samples, this component was minor, presumably because it was more easily quenched by adventitious oxygen. Since the shorter-lived decay exhibited essentially the same lifetime (T = 116 f 9 ns) in N2-purged samples over the concentration range 1.1 1 X lo4 to 6.68 X M, we took kETto be the reciprocal of 116 ns (cf. Table VII). The 116-ns component seems not to be due to TH*3-TH*3annihilation because its intensity relative to the long-lived component did not decrease at lower laser intensity and because its lifetime increased at lower temperatures. The observed temperature dependence was T ("C), T (ns) as follows: 10, 157; 23, 116; 30, 91; 50, 73, leading to a linear Arrhenius plot with E, = 9.1 kcal/mol and log A (s-I) = 9.86. 4BB-1-DBO also gave multiexponential decay, the fastest component (T = 7 ns) corresponding to kET = 1.4 X lo8 s-l. It is conceivable that one component of the dual exponential decay corresponds to T-T absorption of an azoalkane triplet. However, one would expect to see the same transient with the same lifetime independent of sensitizer structure. Rapid ET was clearly shown for 4BB-2-NA, where the 0.78-11s decay time of BB*' at 575 nm matched the rise time of NA*3 at 433 nm. Because we considered AP-DBO the compound most likely to show azoalkane T-T absorption, a hexane solution was excited at 266 nm and a careful search was made for transient absorption.

Engel et al. TABLE VIII: Decomposition Quantum Yields for Ester-Linked Bichromopboric Molecules in Benzene at 20 O C energy transfer," compound concn, M 0, % inter/intrac TH-2-DBOd 8.9 X 0.0061 85 0.0072 17 TH-PY' 1.8 X lo4 0.14 99 0.14 0.013 4BB-PY' 4.8 X 0.076 98 0.078 0.32 3BB-PYC 7.8 X 0.11 98 0.11 0.47 "This ratio is (kET+ kb[S])/(k,T + kb[S]+ kd + &,[SI)where S is the bichromophoric azoalkane and k, is self-quenchings3 of the sensitizer moiety. Calculated decomposition quantum yield of excited triplet azo moiety. Ratio of intermolecular to intramolecular energy transfer, kb[S]/k,T, under the experimental conditions. 3 13-nmirradiation. 366-nm irradiation.

TABLE I X Triplet Sensitized Nitrogen Quantum Yields in Benzene at 25 OC compound concn, M sensitizer A,, nm 0r PY 0.05 benzophenone 366 0.23' 0.18 4BB-PY 0.025 internalc 366 0.19 2BB-PY 0.105 internalC 366 0.74 DMA-PY 0.075 internal 366 0.014b DBO benzophenone 3 13 0.02 0.013 DBO acetophenone 3 13 0.042 0.01 1 AP-DBO 0.022 internal 313 aReference 13. bReference 24. c A t these concentrations, the extent of bimolecular sensitization is comparable to the intramolecular component.

Despite the short delay (30 ps), no transients were observed from 370 to 700 nm. In contrast, DMA-PY exhibited a transient absorption at 380-400 nm that rose within 30 ps and decayed with T = 770 ns. In view of the radical delocalizing substituent on the pyrazoline a-carbon, we suspect that the carrier of this spectrum is a species formed by loss of N2.52 The most interesting result from the kinetic spectroscopy work is that intramolecular ET to the NA acceptors occurs within a nanosecond but ET to the azo group can be nearly lo5 slower. Decompdtion Quantum Y i Decomposition quantum yields (ar)were determined for several bichromophoric azoalkanes (cf. Table VIII). Although the ratio of intramolecular to bimolecular ET varied with azoester concentration, the arvalues were always slightly smaller than for DBO and PY themselves (cf. Table IX). Whether the azo moiety receives its triplet energy from an internal or external sensitizer, the fraction of azo triplet that loses N2 (ah in Table VIII) is 0.1-0.2 for pyrazolines and about 0.01 for DBO derivatives. Thus these values depend only on the decomposition versus deactivation rate of the triplet azo moiety itself. The large increase in @I found for incorporation of a triplet sensitizer into a p e r ~ x i d edoes ~ ~ .not ~ ~carry over to bichromophoric azoalkanes. However, placing a radical-stabilizingketo group on the a-carbon, as in DMA-PY, causes @I to jump to over 0.7 (cf. Table IX). Discussion Ester-Linked Benzophenone-Napbt~lenes. 4BB-n-NA and TH-2-NA exhibited phosphorescence from only the acceptor moiety, similar to previously studied analogs 11,43 12,42.56-57 13,4l 14,59and 15.60 The ester-linked bichromophoric molecule 16 phosphoresced from both the BB and triphenylene moieties at 82 K,62but in this case the acceptor triplet energy was high enough that reverse ET could repopulate the BB triplet state. When the interchromophoric distance of 11 was increased to about 15 A using a steroid linking group, ET was only 35% e f f i ~ i e n t ~and ~v~ emission from the BB moiety was observed. Recently, the phosphorescence yield of the BB donor in 17 was found to decrease while that of the dibenz[bflazepine increased as the chain length was made shorter.65 The complete ET seen in our ester-linked NA derivatives is as expected from conformational considerations and the effect of distance on ET. It is well-known that esters prefer the cis (extended) conformation 20 over the trans conformation 21.For

The Journal of Physical Chemistry, Vol. 96, No. 19, 1992 7529

Intramolecular Triplet Energy Transfer

-C-(CH,),-0

coo

8

1s

17

0

R

0

KO,.. KO R

20

21

I

R’

example, in ethyl formate (R = H, R’ = Et), 20 is more stable than 21 by 1.7 kcal/mol. Since the free energy difference will be even greater for R = Ar, our ester-linked bichromophoric compounds remain frozen in the extended conformation 20 during phosphorescence studies at 77 K. The effect of distance on ET efficiency is conveniently viewed in terms of the “sphere of quenching” mode169*70 which proposes that ET will take place immediately if an acceptor molecule lies within a sphere of radius Rofrom an excited donor. Acceptors lying outside this critical radius do not receive energy. According to molecular models, the maximum interchromophoric distance (from C = O to the center of NA with the ester in conformation 20) is about 9.8 A in 4BB-1-NA and 10.7 A in 4BB-2-NA. Since these distances are less than the Ro = 13 A for intramolecular ET from BB to NA (cf. ref 44 and Table 111), complete ET is expected and observed in 4BB-n-NA. Turning to the solution phase results (cf. Table VII), we note that both 4BB-n-NA compounds exhibit kETvalues near 1.2 X lo9 s-’. The rate constant for Th-2-NA would probably be similar except that it was measured with an apparatus whose response time was comparable to the observed decay time of TH*3. For example, k E T of 4BB-1-NA came out considerably higher using the picosecond pulse-probe technique than with the transient digitizer. Relevant comparisons with 4BB-n-NA are found in compounds 1871and 1972in which kETis 10” s-I and 5.0 X 1Olo s-I, respectively. Since the interchromophoric distance in 18 and -

0

18 a-naphthyl 19 p n a p h l h y l

19 is shorter than in 4BB-n-NA, the 40-80-fold faster k E T for 18 and 19 is not surprising. In an elegant study, Closs and cow o r k e r ~determined ~~ k E T for a series of cyclohexane and decalin-linked BBNA compounds. When the number of linking atoms was three, &ET was in the region of 5 X lo9; however, with four atoms, &ET depended on conformation, ranging from 4.0 X lo7 to 1.3 X lo9s-l, The larger values are similar to the kET we found for 4BB-n-NA, showing that ester and alkyl linkages behave similarly. Conformational equilibration of hydrocarbon chains requires tens of nanoseconds;73hence, at the moment of excitation, several conformers of 4BB-n-NA are populated, each characterized by some value of kEP The observed value is an average over all Conformations. Interestingly, 4BB2-NA does not exhibit a slower kETthan 4BB-l-NA, despite its greater chain length. Because of chain folding, more intervening atoms do not necessarily mean a greater interchromophoric distance. Ester-IinkedSensitizer A”. As shown in Table 11, we measured both phosphorescence lifetimes (T,,) and relative intensities in frozen glasses for several bichromophoric azoalkanes. While T~ is very similar to that of the sensitizer moiety alone, the

intensities are 16 to 330 times lower. It is not likely that the residual sensitizer emission in 4BB-PY, 2BB-PY, etc. is due to impurities, because completely different samples were studied by different workers with similar results. Partial triplet63and sing1et74-76ET have been seen in a few cases and the question of intensity versus lifetime quenching has been considered theoreti ~ a l l y . ~The ~ , present ~ ~ * ~results ~ are in accord with the “sphere of quenching” model mentioned above. Despite the small size of the azo group, its critical radius is surprisingly similar to that of the much larger NA chromophore (cf. Table 111). Only in the case TH + PY do we see a somewhat smaller R,,, most likely because ET is close to thermoneutral (cf. Table V). We suggest that most of the bichromophoric azoester molecules freeze in a conformation favorable to ET, but a small fraction continue to emit with a normal lifetime. Many conformations not involving (20-0 bond rotation are obviously accessible (see below), ET from BB is known to have an optimum ge~metry,’~.~~ and geometric factors can greatly affect ET rate^.'^*'-^^ Whatever conformation disfavors ET in the ester-linked azoalkanes is either unpopulated in the analogous naphthalenes or does not inhibit ET to the larger chromophore. The fraction of conformations that emit is lower for 2BB-PY than for 4BB-PY presumably because the azo group is closer to the carbonyl in 2BB-PY. Changing the sensitizer from a 3(n,r*) type (BB) to a 3(r,r*) type (TH) decreases the degree of intensity quenching but does not affect the constancy of fp(cf. Table 11). These observations suggest that the conformation unfavorable for ET is more populated in the TH compounds. Even more than in the frozen glass, changing the nature of the sensitizer moiety in solution greatly affects kET. As seen in Table VII, ET from TH to the azo group is always slower than from BB, sometimes nearly lo5 slower. Changes in ET rate with the nature of the donor at constant interchromophoric separation have been seen before; for example, replacing the BB moiety by carbazole in a steroid-linked naphthalene lowers k E T lOWf0ld.6~In contrast, quenching of acetone n,r* triplets by phenones is slower than diffusion control, but changing the configuration of the phenone lowest triplet from a,** to n,r* slows kq only slightly.84 The excited-state configuration of sensitizers is known to influence their rate of intramolecular charge transfer quenching but the effect is opposite to the present results. Thus 3(r,r*) sensitizers are quenched more rapidly by charge-transfer interaction^^^ but 3(n,r*) sensitizers are quenched faster by azoalkanes. The slow k E T for TH-azoalkanes is particularly noteworthy because bimolecular transfer proceeds at the diffusion-controlled rate. Thus intramolecular ET in TH-2-DBO is so slow that at a concentration of lo4 M, 95% of the energy transfer takes place from the TH*3 of one molecule to the DBO of another. Since the critical radius experiments (Table 111) show that the azo group is not too small for efficient ET at these interchromophoricdistances, the ester group must hold the donor and acceptor in an orientation not conducive to ET. This unfavorable conformation may be populated in only the TH compounds but if populated in the BB cases, it must not inhibit ET from the 3(n,r)* state. An earlier case of an ester linkage slowing interaction between chromophores is found in singlet excited phenanthrene derivatives and substituted styrene^:^ In order to examine the conformations of a typical bichromophoric azualkane, we carried out molecular mechanics calculations on TH-PY.86 Rotation is possible about four single bonds, but

.

N=N

.

TH-PV

as expected for the ester group,6668the dihedral angle 2,3,5,6 was close to 180° and deviations from this value encountered large

7530 The Journal of Physical Chemistry, Vol. 96, No. 19, 1992

barriers. The thioxanthone moiety was somewhat folded and the carbonyl oxygen was 57O above the convex face (L1,2,3,4 = 57’). The other low energy rotamer about the 2,3 bond, which placed the carbonyl oxygen above the concave face (L1,2,3,4 = - 5 9 O ) , had a higher AHf by 0.3 kcal/mol and the minimized barrier between these rotamers was 5.9 kcal/mol. As a result of these restrictions, the PY moiety tended to lie near a line passing through sulfur and C=O of the TH moiety. It is possible that ET is slow in these geometries because they fail to meet some orientation requirement. Rotation about the 5,6 bond encountered no barriers greater than 2 kcal/mol over a 120° range, but bringing the ester C = O and PY moieties into proximity raised the energy enormously, as expected. Finally, there were three minima about the 6,7 bond lying within 0.7 kcal/mol but separated by a -7 kcal/mol barrier. Since interconversion over a barrier of 7 kcal/mol with an A factor of 1013s-I requires about 13 ns, several conformations of TH*3-PY are accessible during its lifetime. It is remarkable that none of these allow rapid ET. TH-PY exhibited a dual decay curve, which might be attributed to excitation of two conformers (A and B) that are separated by a large barrier.87 ET in the lowest energy conformer (A) could be slow, causing the long-lived component, while ET in B is faster. Since the lifetime of the faster component decreases at higher temperature, we might say that ET is an activated process (E, = 9 kcal/mol for k l ) ,or perhaps conformer B goes with rate constant k2 to yet a third conformer (C) wherein ET is fast. In the latter case, the observed E, = 9 kcal/mol corresponds to conversion of B to C. (TH-PY)A

tl (TH-PY)s

hv

(TH’3.PY)~

$1 (TH”.PY)B

I

(TH-PY”)A

laster ki

(TH.PY”)B

kz

%(TH-PY’3)c When there is but a single exponential decay, one conformer probably dominates and the decay lifetime corresponds to intramolecular ET in that conformer. However, it is also possible that the decay curve measures conversion of the excited dominant conformer to one in which ET is very rapid. It would be most helpful to know the actual geometry and the potential energy surface of these bichromophoric azoalkanes. This missing information points out the disadvantage of studying bichromophoric molecules linked by a single chain. However, the synthetic effort needed to prepare rigidly linked compounds is only justified by such curious results as those found in Table VII. Cldy-Lhked BichromophoricAzonlkmes. In DMA-PY and AP-DBO, the interchromophoric distance is no more than 5 A. As seen in Table VI, deazatation of these compounds was hardly quenched by piperylene, showing that the sensitizer moiety has a lifetime of about 12 ps. This lifetime is too short to allow bimolecular ET and can only be explained by an increase in kd or in kET. Since kd is unlikely to be changed by the presence of a nearby acceptor, we propose that kET must be nearly 10” s-l. Despite this rapid ET rate, neither DMA-PY nor AP-DBO exhibited any phosphorescence nor any azo group T-T absorption. rTof the azo group can be deduced from quenching experiments, but it is first necessary to consider the triplet energy of the quencher relative to those of the donor and acceptor moieties. The triplet energy of piperylene (59 kcal/mol-’) is below that of both DMA (71 kcal/mol-’) and PY (64 kcal/mol-l); hence, the 1 2 9 lifetime in Table VI is an upper limit on both chromophores. One can understand the short T~ of the DMA moiety on the basis of rapid intramolecular ET but the pyrazoline triplet might be expected to survive longer. As suspected for some y e a r ~ , ~the ,*~ azoalkane triplet state seems to possess an extraordinarily efficient radiationless decay channel. Quenching of DMA-PY was also carried out with di-tert-butyl nitroxide, which is known to be highly effective for a variety of triplet Although its absorption at 313 nm (e = 5.3 M-’ cm-’) prevented the use of high quencher (TH‘3.PY)~

Engel et al. concentrations, we can place a less restrictive upper limit of 0.14 ns on TT of DMA-PY by assuming that di-tert-butyl nitroxide quenches DMA*3 and PY*3 at the diffusion controlled rate. If TT of PY*3 is below 0.14 ns, the observed quenching of 4BB-PY and 2BB-PY (Table VI) must be an effect on the BB moiety. In fact plugging the kb and km values for 4BBPY and 2BBPY from Table VI1 into eq 1 leads to predicted T ~ ’ of S 9 and 1.3 ns, close to the lifetimes deduced from piperylene quenching (Table VI) (we assume that kb and kd are the same for 2BB-PY as for 4BB-PY). Similar experiments were carried out with AP-DBO where the triplet energies of donor and acceptor are 73.6 and 53 kcal/mol, respectively. Since piperylene lies between these values, the 11-ps lifetime shown in Table VI must be that of the acetophenone moiety, showing that ET is exceedingly fast in AP-DBO. Further progress required a quencher whose energy was low enough to intercept DBO*’. Partial success was achieved with 1,3-cyclohexadiene, even though ET is essentially thermoneutral in this case. Because of interfering side reactions, the diene concentration had to be limited to 0.022 M, leading to TT > 2 ns. In fact, the mode of interaction between AP-DBO*3and the diene should be chemical reaction.8b Di-rert-butyl nitroxide gave an interesting result, for the Stern-Volmer plot was linear up to 0.01 M quencher and it had a slope of 33.5. Assuming diffusion controlled quenching, one calculates T = 6.7 ns for the DBO triplet state. Under this assumption, the azo moiety confined to a bicyclic structure undergoes radiationless decay so rapidly that the triplet lifetime is nearly a hundred times shorter than the singlet lifetime.24 Since the triplet energy of DBO corresponds to emission at 540 nm, our failure to observe phosphorescence from AP-DBO must arise not from its inaccessibily long wavelength buthstead from the rapid rate of radiationless decay compared to radiative decay of the DBO triplet state. On the other hand, the time resolution of the transient absorption experiment was far below 6.7 ns; hence, the absence of T-T absorption implies that it is either weak or that it lies at a wavelength outside the range of the apparatus. Calculated potential surfaces for azomethane4’ and diimides8 show a crossing of the triplet- and ground-state surfaces upon twisting about the N=N bond. The suggestion that the triplet requires much less twisting than the singlet before it crosses to the ground state may account for the large difference in lifetimes. Judging from molecular models, one finds that even DBO has a surprisingly flexible skeleton, especially if the excited states are considered to possess only a partial 7r bond. In summary, we have found that intramolecular triplet energy transfer to the azo group of ester-linkedbichromophoric molecules can be remarkably slow both in a frozen matrix and in solution. When the ester linkage is removed and the acceptor is placed within 5 A of the donor, the ET rate increases to nearly 10” s-I, allowing Stern-Volmer quenching of the azo triplet state. The triplet lifetime of 1-pyrazolinesis on the order of 12 ps while that of 2,3-diazabicyclo[2.2.2]oct-2-ene(DBO) is 6.7 ns.

Experimental Section cewrrli ’ ” L Melting points were taken on a Mel-Temp apparatus and are uncorrected. NMR spectra were recorded in CDC13on a Hitachi R-600, a Jeol GSXSOO, or an IBM AF-300 spectrometer. UV spectra were obtained on a Hitachi 220 or a HP8452A diode array spectrometer interfaced to a PC. Phosphorescence spectra were recorded using EA or EPA solvent on a Hitachi F4010 fluorescence spectrometerwith a phosphorescence attachment. The laser flash photolysis experiments were carried out on two nanosecond and two picosecond systems. One nanosecond system, which belongs to Professor Katsumi Tokumaru at Tsukuba University in Japan, used an XeCl excimer laser (Lambda Physik EMG-101,308 nm, 10 ns fwhm) to pump a dye laser (Lambda Physik FL-3002) for sample excitation, and a pulsed xenon arc for probing. The monitoring light from a monochromator was detected by a photomultiplier and the signals were stored in an oscilloscope and were transferred to a personal computer. The other systems, located at the CFKR in Austin, TX (see Acknowledgment), were based on Nd:YAG lasers.89

Intramolecular Triplet Energy Transfer

These were operated in either the Q-switched mode to give 10-ns pulses or were mode locked to produce 30-ps pulses. The low nanosecond experiments used the picosecond laser for excitation and a transient digitizer to record the decay profile. The picosecond experiments were done with the pulseprobe technique, wherein part of the 1064-nm laser light was converted to the third or fourth harmonic for sample excitation and part was converted to white light for probing. Comj”is. 1,3-Pentadiene, 1,3-~yclohexadiene,and di-ferfbutyl nitroxide were distilled prior to use as quenchers. The bichromophoric esters were synthesized from the appropriate alcohols and acid chlorides, which were produced by reaction of the acids with S0Cl2. The three isomeric benzoylbenzoic acids were purchased and thioxanthone-4-carboxylicacid was synthesized by minor alteration of the literature The alcohols were synthesized as described below. 3 . ( H y ~ o x y m e t b y l ) - 3 , ~ ~ ~ ~ t b y l - l - p(1) y rwas p Zprepared according to the literature procedure.25 l-(Hydroxymetbyl)-2,3-dlazcrbicyclo[2.2.2~t-2-e11e (5) was synthesized from 2,3-dihydrobenzylalcohoP and Cmethylurazole in the same way as DB0.91 l - ( % H y ~ x y e t b y l ) - S ~ ~ b i c y c l o [ 2 . 2 . 2 l o c t(6). - % ~The known urazole, 1-(2-hydroxyethy1)-4-methyl-2,4,6-triazatricy~10[5.2.2.0~~~]undecane-3,5-dione (dihydro-4) was prepared according to the literature proced~re.~’The urazole (1.80 g, 7.55 mmol), triphenylmethyl bromide (2.68 g, 8.30 mmol), 4-(N,Ndimethy1amino)pyridine (0.060 g, 0.5 mmol), and 2 mL of Et3N were dissolved in 35 mL of CH2C12 The mixture was stirred under nitrogen for 3 days at room temperature then poured over ice. The aqueous layer was extracted 3 times with CH2C12. The combined extracts were washed twice with saturated NH4Cl. After removal of solvent by rotary evaporation,the product was recrystallized from cyclohexane yield- the trityl ether as a white solid (3.03 g, 6.30 mmol, 83% yield). NMR: 7.2-7.5 (15 H, m), 4.25 (1 H, br s), 3.33 (2 H, t, J = 6 Hz), 3.01 (3 H, s), 2.42 (2 H, t, J = 6 Hz), 1.79 (8 H, AB quartet). The above trityl ether (2.44 g, 5.1 mmol), 150 mL of 2-propan01, and 11.5 g of KOH were placed in a round-bottom flask fitted with a reflux condenser, nitrogen inlet, and magnetic stirring bar. This mixture was heated to reflux for 16 h and cooled to room temperature. The solids were filtered off and washed with CH2C12 Rotary evaporation of the filtrates gave a semisolid slurry which was dissolved in 150 mL of water and neutralized with 1 N HC1. Some precipitation of the product occurred. Three extractions with CH2C12followed by rotary evaporation and recrystallization from hexane afforded white crystals of 1-(2-(triphenylmethoxy)ethyl)-2,3diazabicyclo[2.2.2]oct-2-ene(the trityl ether of 6), mp 114-115 OC (1.58 g, 4.0 mmol,74% yield). NMR: 7.20-7.55 (15 H, m), 5.06 (1 H, br s), 3.48 (2 H, t, J = 6 Hz), 2.44 (2 H, t, J = 6 Hz), 1.1-1.7 (8 H, m). The DBO trityl ether (3.5 g, 8.8 mmol) was dissolved in 120 mL of dry CH2C12in a three-neck flask equipped with a magnetic stirring bar, nitrogen inlet, and serum stopper. Boron trifluoride etherate (2.3 mL, 2 equiv) was added by syringe and the mixture was stirred for 20 h at room temperature. The very water soluble DBO alcohol 6 was extracted by repeatedly washing the organic layer with small portions of water. The combined aqueous washings were continuously extracted for 3 days with CH2C12. Rotary evaporation of the solvent followed by vacuum distillation in a microstill afforded a light brown oil (1.14 g, 7.4 mmol,84% yield). UV A,, 380 nm, c 194. NMR: 5.17 (1 H, br s), 4.06 (2 H, t, J = 6 Hz), 3.67 (1 H, br s), 2.22 (2 H, t, J = 6 Hz), 1.1-1.8 (8 H, m). Anal. Calcd for C8HI2N20: (M+) m/e 154.1106. Found: 154.1103. Synthesis of fiters. Three methods were used for synthesis of the esters: reaction of an acid chloride and alcohol in pyridine solution (method A), reaction of acid chloride, alcohol, and pyridine in chloroform solution (method B), and reaction via the alkoxide (method C). One example of each method is described in detail below. Metbod A. 2-(2,1Ma~bicyclo[2.2.2~t-2-em-l-yl)ethyl4Bewoylbenzorte (4BB-ZDBO). In a flask equipped with a

The Journal of Physical Chemistry, Vol. 96, No. 19, 1992 7531 magnetic stirring bar and a condenser with nitrogen inlet tube was placed 720 mg (2.9 mmol) of 4-benzoylbenzoyl chloride and 10 mL of pyridine. A 320-mg (2.1-”01) portion of DBO alcohol 6 was added directly to-the flask The reaction mixture was heated in an oil bath maintained at 60 OC and allowed to stir for 2 h, then stirred for 12 h at room temperaure. The reaction solution was poured into water and extracted 3 times with ether. The combined extracts were washed with saturated CuS04, and then with water. After drying over Na2S04,the solution was concentrated by rotary evaporation. The red oil was purified by silica gel column chromatography, and the product was recrystallized from EtOH to yield white crystals, mp 86.5-87.5 OC (120 mg, 0.33 mmol, 15% yield). NMR: 7.4-8.2 (9 H, m), 5.17 (1 H, br s), 4.81 (2 H, t, J = 7 Hz), 2.59 (2 H, t, J = 7 Hz), 1.2-1.7 (8 H, m). Anal. Calcd for CzzHz2N2O3: C, 72.91; H, 6.12; N, 7.73. Found: C, 72.77; H, 6.12; N, 7.84. ( ~ D i a Z a b i c y ~ 2 2 2 ~ t - ~ l - y l ) mebenzoylbemolte etby (4BB-1-DBO) was synthesized by method A using 1.8 g (7.4 mmol) of 4-benzoylbenzoyl chloride and 1 g (7.1 mmol) of 1(hydroxymethyl)-2,3-diazabicyclo[2.2.2]oct-2-ene(5). This ester was obtained as white crystals, mp 150.5-152 OC (1.23 g, 3.5 mmol, 49% yield) (recrystallized from EtOH). NMR: 7.4-8.3 (9 H, m), 5.24 (1 H, br s), 4.93 (2 H, s) 1.1-1.9 (8 H, m). Anal. Calcd for C21H20N203:C, 72.38; H, 5.80; N, 8.04. Found: C, 72.29; H, 5.86; N, 8.05. 2-(2,3-Dinzabicycl0[2.2.2~t-Zenel-yl)etbylQthioxanthowcarboxylate (TH-2-DBO) was synthesized by method A from 1 g (3.6 mmol) of 4-thioxanthonecarbonyl chloride and 414 mg (3 mmol) of 1-(2-hydroxyethyl)-2,3dmzabicyclo[2.2.2]oct-2-ene(6). The yellow, crystalline product had mp 161-163.5 OC (200 mg, 0.51 mmol, 17% yield) (recrystallized from EtOH). NMR: 7.5-8.9 (7 H, m), 5.18 (1 H, br s), 4.85 (2 H, t, J = 7 Hz), 2.62 (2 H, t, J = 7 Hz), 1.2-1.7 (8 H, m). Anal. Calcd for C22H20N203S: C, 67.32; H, 5.15; N, 7.14. Found: C, 67.20; H, 5.15; N, 7.05. 2 4 1-Napbtby1)ethylQbenzoylbenzoate (4BB-2-NA) was prepared by method A starting with 1.6 g (6.6 mmol) of 4benzoylbenzoyl chloride and 1.13 g (6.6 mmol) of 241naphthy1)ethanol. The ester was a clear oil (0.6 g, 1.6 m o l , 24% yield). NMR: 7.3-8.2 (16 H, m), 4.72 (2 H, t, J = 7 Hz), 3.57 (2 H, t, J = 7 Hz). 1-Napbtbylmetbyl 4-benzoylbenzoate (4BB-1-NA) was synthesized by method A from 560 mg (2.3 mmol) of 4-benzoylbenzoyl chloride and 230 mg (1.5 mmol) of 1-naphthylmethanol. The white, crystalline product (82 mg, 0.22 mmol) was obtained in 15% yield, mp 124-125 OC. NMR: 5.85 (2 H, s); 7.20-8.30 (16 H, m). Z(l-N.pMhyl)ethyl Qthioxmthowcruboxylate (TH-ZNA) was synthesized by method A using 1 g (3.6 mmol) of 4-thioxanthonecarbonyl chloride and 512 mg (3 mmol) of 241naphthy1)ethanol. After recrystallization from EtOH, the ester was obtained as yellow crystals, mp 138.5-140.5 OC (440 mg, 1.1 mmol, 37% yield). NMR: 7.4-8.9 (14 H, m), 4.77 (2 H, t, J = 7 Hz), 3.62 (2 H, t, J = 7 Hz). Anal. Calcd for CZ6Hl8O3S: C, 76.07; H, 4.43. Found: C, 75.86; H, 4.53. l-(%Methyl-%(tart-butylazo)ppyl) 4-thio-x~late (TH-ATB) was prepared by first treating 0.38 g (1.48 mmol) of thioxanthone-4-carboxylic acid with 10 mL of S0Cl2. After refluxing under N2 for 2 h, the excess SOCl, was removed under vacuum. In a separate threeneck flask equipped with a N2 inlet, reflux condenser, and addition funnel, a 0.245-g (1.55-mmol) portion of azoalcohol 226 was dissolved in 3 mL of CH2C!,. Pyridine (0.18 mL, 2.2 mmol) was added followed by the acid chloride in 13 mL of dry CH2C12 The yellow solution was refluxed for 1.5 h but the acid chloride did not dissolve fully. After cooling to room temperature, the solution was diluted to 100 mL with CH2C12and washed with dilute HCl and then with dilute aqueous NaHCO,. The insoluble material was removed by filtration and the organic layer was dried over K2C03. After rotary evaporation of the solvent, the product was purified by preparative HPLC on silica gel using 30% EtOAc in hexane. A second pass through the column yielded 46.3 mg (7.9%) of the desired ester. NMR:

1532 The Journal of Physical Chemistry, Vol. 96, No. 19, 1992

Engel et al.

H, m). Anal. Calcd for C21H22N203: C, 71.98; H, 6.33; N, 7.99. 8.1-9.2 (m, H); 7.3-7.7 (m, H); 4.59 (s, 2 H); 1.30 (s, 6 H); 1.16 Found: C, 72.12; H, 6.33; N, 7.81. 6,9 H). Method B. (3,5,5-Trimethyl-l-pyrazolin-3-yl)methyl 4Synthesis of 3-(3,4-Dimethoxybenzoyl)-3,5,5-trimethyl-lBenzoylbenzoate (4BB-PY). A 0.353-g (1.56-"01) portion of pyrazohe (DMA-PY). 1-(3,4-Dimethoxyphenyl)-2-methyl-2Cbenzoylbenzoic acid was suspended in 7 mL of benzene in a flask propen-1-01 (8). 2-Propenylmagnesium bromide was prepared from 2-bromopropene (3.10 g, 0.0256 mol) and active Mg31 in equipped with a magnetic stirring bar, reflux condenser, and nitrogen inlet. SOCl2 (2 mL) was added and the solution was 200 mL of dry THF. 3,CDimethoxybenzaldehyde(4.13 g, 0.0248 refluxed under nitrogen for 1 h. The excess SOCl2and benzene mol) in 200 mL of dry THF was added over 30 min to the were removed by distillation and the resulting yellow solid was Grignard reagent, the reaction mixture was refluxed for 30 min, evacuated at 0.1 mmHg for 30 min. 3-(Hydroxymethyl)-3,5,5and it was then cooled to 0 OC. Saturated NH4Cl (75 mL) was trimethylpyrazoline(1)25(0.215 g, 1.51 mmol) was dissolved in added slowly to the mixture. The organic layer was decanted off 3 mL of dry pyridine in a three-neck round bottom flask fitted and the aqueous layer acidified with 1 N HCl. The aqueous layer with a magnetic stirring bar, reflux condenser, nitrogen inlet, and was e x t r a d with 3 X 50 mL ether. The combined organic layers dropping funnel. The crude acid chloride was dissolved in 3 mL were washed twice with saturated NaCl and dried over K2C03. of dry CHC13 and slowly added to the alcohol while cooling in Rotary evaporation of the solvent resulted in a brown oil which an ice water bath. The reaction mixture was allowed to warm was purified by column chromatography (CH2CI2on alumina) yielding 3.91 g (0.019 mol, 76%), clear oil. NMR: 6.82-6.92 to room temperature and was then heated to reflux for 30 min. After cooling to room temperature, 20 mL of CHC13was added (3 H, m), 5.16 (1 H, br s), 5.03 (1 H, br s), 4.92 (1 H, br s), 3.84 and the mixture was washed twice with 1 N HCl, twice with (6 H, s), 2.32 (1 H, br s), 1.57 (3 H, s). saturated NaHC03,and twice with water. The organic layer was (3,4-DimethoxyphenyI)-2-propenylKetow. The above alcohol dried over K2C03and filtered through a small amount of active (8) (2.09 g, 0.010 mol) in 50 mL of acetone was cooled in an ice alumina. Following rotary evaporation of the solvent, the flask bath and Jones' reagent92was added until the solution was slightly was evacuated at -0.1 " H g . The resulting oil crystallized (0.23 brown. The solution was stirred at mom temperature for 2 h. The g, 0.65 mmol,44% yield). Two recrystallizations from 95% EtOH upper layer was decanted off of the chromium salts, which were yielded white crystals with mp 93-94 "C. NMR: 7.3-8.2 (9 H, rinsed with petroleum ether. Addition of the petroleum ether m), 4.61 (2 H, AB quartet), 1.40-1.52 (1 1 H, 3 singlets and an extracts to the upper layer resulted in separation into two layers. AB quartet). Anal. Calcd for CzlHz2N203:(M+) m / e 350.1630. The lower layer was added to the chromium salts and was washed Found: 350.1627. twice with pentane. The combined organic layers were washed (3,5,5-Trimethyl-l-pyrazolin-3-yl)methyl 3-benzoylbenzonte twice with saturated NaCl and twice with saturated NaHC03 (3BB-PY) was synthesized by method B from 4.9 g (22 mmol) and dried over K2C03. The solvents were removed by rotary of Cbenzoylbenzoic acid and 1.16 g (8.2 mmol) of 1. The resulting evaporation. Low temperature recrystallization from pentane yellow oil was purified by preparative HPLC to yield white yielded white crystals (1.18 g, 5.7 mmol, 57%), mp 35-36 OC. crystals, mp 77-79 OC (580 mg, 1.7 mmol, 21% yield) (recrysThe product was stable to storage for several months in a freezer tallized from n-hexane). NMR: 7.3-8.4 (9 H, m), 4.61 (2 H, (--lo "C). NMR: 7.40-7.52 (2 H, m), 6.84-6.94 (1 H, m), AB quartet), 1.4-1.6 (1 1 H, m). Anal. Calcd for C21H22NZ03: 5.82 (1 H, br s), 5.56 (1 H, br s), 3.97 (3 H, s), 3.95 (3 H, s), C, 71.98; H, 6.33; N, 7.99. Found: C, 71.88; H, 6.33; N, 8.04. 2.08 (3 H, s). Anal. Calcd for CI2Hl4O3:(M') m / e 206.0943. Found: 206.0947. (3,5,5-Trimethyl-1-pyrazolin-3-y1)methyl4-thioxantbone carboxylate (TH-PY) was synthesized by method B using 2.6 g (9.5 3- (3,4-Dimethoxybenzoyl) -3,5,5- trimethyl-1-pyrazoline m o l ) of 4-thioxanthonecarbonyl chloride and 1.35 g (9.5 mmol) (DMA-PY). An excess of diazoisopropaneg3in ether was added of 1. The yellow, oily product was purified by preparative HPLC dropwise to a stirred solution of the above enone (2.13 g, 0.0103 to afford yellow crystals, mp 155.5-156.5 OC (830 mg, 2.2 mmol, mol) in 20 mL of ether at room temperature. Addition was 23% yield) (recrystallized from EtOH). NMR: 7.5-8.9 (7 H, continued until a slight pink color persisted. The solvent was m), 4.68 (2 H, AB quartet), 1.4-1.6 (11 H, m). Anal. Calcd removed by rotary evaporation and the product was purified by for C21H20N203S: C, 66.30; H, 5.30; N, 7.36. Found: C, 66.17; column chromatography (70% EtOAc in hexane on silica), followed by preparative HPLC (30% EtOAc in hexane on silica) H, 5.32; N, 7.37. yielding white crystals (1.91 g, 6.92 mmol, 67% yield). Re(2,3-Diazabicyclo[2.2.2]oct-2-ene-l-yl)methyl4-thiowntbooecrystallization from petroleum ether gave crystals melting at 45-46 carboxylate (TH-1-DBO) was synthesized by method B. A 2.56-g "C. NMR: 8.32 (1 H, d of d, J = 3.6, 8.4 Hz), 7.90 (1 H, d, (10-mmol) portion of 4-thioxanthonecarboxylic acid and 0.93 g J = 3.6 Hz) 6.94 (1 H, d, J = 8.4 Hz) 3.96 (3 H, s), 3.94 (3 H, (6.6 mmol) of 1-(hydroxymethyl)-2,3-diazabicyclo[2.2.2]cct-2-ene s), 2.64 (1 H, d, J = 12 Hz), 1.80 (3 H, s), 1.53 (3 H, s), 1.28 (5) were used. The yellow solid was purified by flash chroma(1 H, d, J = 12 Hz), 1.20 (3 H, s). Anal. Calcd for CI5H2,,N2O3: tography to afford yellow crystals, mp 165-167 OC (365 mg, 0.97 (M') m / e 276.1474. Found: 276.1468. mmol, 15% yield) (recrystallized from EtOH). NMR: 7.4-9.1 (7 H, m), 5.26 (1 H, br s), 4.99 (2 H, s), 1.2-1.9 (8 H, m). Anal. Synthesis of 5-Beazoyl-2,3-diPzabicyclo[2.2.2]oct-2-e~ (APCalcd for C21H18N203S: C, 66.64; H, 4.80; N, 7.40. Found: C, DBO). 8 - c a r b o ~ - ~ ~ t h y l - ~ 4 , 6 ~ ~ ~ ~ y ~ l o [ 5 . 2 . 2 66.36; H, 4.79; N, 7.34. 8-ene-3,5-dione. Birch reduction of benzoic acid (30.0 g, 0.242 mol) yielded 1,Mhydrobenzoic acidH in 95% yield (29.4 g, 0.237 Method C. (3,5,5-Trimethyl- l-pyrazolin-3-y1)methyl 2mol) after distillation. This material was isomerized to 3,4-diEenzoylbenzoate (2BBPY). 2-Benzoylbenzoicacid (0.75 g, 3.32 hydrobenzoic acid using aqueous NaOH according to the literature mmol) was converted to the acid chloride. The crude product was p r ~ e d u r e . ' ~The conjugated acid (22.2 g, 0.179 mol) was imdissolved in 5 mL of dry THF, and the solution was placed in a mediately dissolved in 300 mL of EtOAc and a solution of Ndropping funnel. The alkoxide of PyOH was formed by addition methyltriazolinedioneB in EtOAc was added until the persistance of 2.2 mL of 1.49 M n-butyllithium (3.28 mmol) to 1 (0.424 g, of a pink color indicated that the Diels-Alder reaction was com2.99 mmol) in 8 mL of dry THF contained in a three-neck flask plete. The adduct began to precipitate about halfway through fitted with a reflux condenser, nitrogen inlet, magnetic stirring the reaction. The solvent was removed by rotary evaporation, bar, and dropping funnel. After stirring the reaction mixture for yielding white solid which was recrystallized from 75 mL of 20 min at room temperature, the acid chloride was added over EtOAc, mp 214-215 OC (31.6 g, 0.133 mol, 74.5% yield). N M R a 10-min period. The mixture was refluxed for 45 min, cooled variable 6-10 (1 H, br s), 7.34 (1 H, m), 5.39 (1 H, br s), in an ice bath, and extracted with 3 X 15 mL ether. The ether 4.98-5.10 (1 H, m), 3.00 (3 H, s), 2.1-2.4 (2 H, m), 1.48-1.72 solution was dried over MgS04 and the solvent was removed by (2 H, m). Anal. Calcd for CIOHIIN304: (M+) m / e 237.0749. rotary evaporation, leaving 1.08 g of yellow oil. The product was Found: 237.0744. purified by column chromatography and recrystallized from EtOH, yielding white crystals, mp 82.5-84 OC (563 mg, 1.6 "01, 8-Cnrboxy-4-methyl-2,4,6triaza~cyclo[5.2.2.Oz~6~d~an~ 5456yield). NMR: 7.3-8.1 (9H,m),4.30(2H,s),1.2-1.5(11 3,5-dione. The above Diels-Alder adduct (10.3 g, 0.0434 mol)

Intramolecular Triplet Energy Transfer in 400 mL of absolute EtOH was placed into a round-bottom flask with a magnetic stirring bar. Approximately 100 mg of 105%Pd/C was added, and the flask was attached to an atmospheric-pressure hydrogenation apparatus. The contents were stirred vigorously until hydrogen absorption ceased (1 130 mL, 6 h). The catalyst was removed by filtration through Celite. Rotary evaporation of the solvent and evacuation of the flask resulted in a white solid, mp 145-151 OC (8.2 g, 0.034 mol, 78% yield). NMR: variable (1 H, br s), 4.76 (1 H, br s), 4.45 (1 H, br s), 3.08 (3 H, s), 2.8-3.0 (1 H, m), 1.5-2.7 (6 H, m). Anal. Calcd for C1&II3N3O4:(M') m/e 239.0906. Found: 239.0909. 5-Carboxy-2,[email protected]]oct-2-ene(10). The above hydrogenated adduct (8.2 g, 0.034 mol), solid 85% KOH (9.8 g, 0.15 mol), and 80 mL of i-PrOH were refluxed under N2 for 16 h. After cooling to room temperature, the salts were filtered off and washed with MeOH. The alcohol solvents were removed by rotary evaporation. The remaining sludge was dissolved in 150 mL of water and acidified with concentratd HCl. A 30-mL portion of 1 M CuC12was added and the brick red capper chloride complex slowly precipitated out. After filtration of the complex, 5 mL more CuC1, was added, resulting in more precipitate. The procedure was repeated until no more precipitate formed. The combined crops of copper complex (6.2 g) were dissolved in 50 mL of concentrated NH40H then acidified with concentrated HCl to pH = 3 in an ice bath. The aqueous solution was extracted continuously with CH,C12 for 3 days. The combined extracts were rotary evaporated yielding a brown solid, mp 133-136 OC (3.18 g, 0.0207 mol, 60.8% yield). NMR: 10.65 (1 H, s), 5.57 (1 H, br s), 5.28 (1 H, br s), 2.80 (1 H, t, J = 8 Hz), 1.1-1.9 (6 H, m). Anal. Calcd for C7H,0N202:(M+) m/e 154.0742. Found: 154.0739. 5-Benzoyl-23-diazabicyclo[2.2.2]oct-2-ene(AP-DBO). The was converted to the sodium DBO acid (10) (1.36 g, 8.83 "01) salt by dissolution in enough 0.1 N NaOH to keep the solution basic. The water was removed by rotary evaporation and the sodium salt was evacuated at 120 OC for 2 h. The salt was suspended in 20 mL of dry benzene in a flask equipped with a reflux condenser and nitrogen inlet. Approximately 4 mL of oxalyl chloride (50 mmol) was added, resulting in gas evolution. After the mixture was stirred under nitrogen for 1 h, the benzene and excess Oxalyl chloride were removed under reduced pressure. An additional 10 mL of dry benzene was added to the flask and pumped off to remove any trapped oxalyl chloride. The resulting crude acid chloride was dissolved in 15 mL of dry benzene. Simultaneously the diphenylcadmium reagent was prepared.35 PhMgBr was prepared from Mg turnings (1.09 g, 45.4 mmol) and PhBr (6.78 g, 43.2 "01) in 15 mL of dry ether. CdC12(4.35 g, 23.7 mmol) (previously dried in an oven for 2 days at 120 OC and ground to a fine powder) was added and the suspension was stirred at room temperature for 30 min. The reaction mixture turned very dark at this point. A 20-mL portion of dry benzene was added, and the ether was distilled off. An additional 30 mL of benzene was added and approximately 20 mL was distilled off. Finally the reaction mixture was refluxed for 30 min, resulting in a homogeneous gray suspension. After cooling to 0 OC, the acid chloride was added from a dropping funnel, and the reaction mixture was stirred for 18 h at room temperature. The suspension was poured into 100 mL of saturated NH4Cl and 50 g of ice. After the mixture was shaken vigorously in a separatory funnel, the aqueous layer was separated and washed with two 50-mL portions of CH2C12. The combined extracts were dried over K2C03,and the solvent was removed by rotary evaporation. The resulting oil was chromatographed on silica. The major impurity, biphenyl, was eluted first with CH2Cl2. After the biphenyl was removed, EtOAc was used as eluent and yielded the desired AP-DBO (0.93 g, 4.34 mmol, 49% yield). The AP-DBO was further purified by preparative HPLC (60% EtOAc in hexane on silica gel). Recrystallization from hexane afforded white crystals, mp 99-101 OC,NMR: 7.40-7.95 (5 H, m), 5.50 (1 H, br s), 5.26 (1 H, br s), 3.70 (1 H, t, J = 9 Hz), 1.2-2.0 (6 H, m). Anal. Calcd for C13H,4N20:(M+) m/e 214.1106. Found: 214.1105.

The Journal of Physical Chemistry, Vol. 96, No. 19, 1992 7533

TABLE X Stern-Volmer Quenching of N2Evolution in Benzene at 25 O c a ~~

~

quencher compound concn, M 9, 4BB-PY 0.000 0.178 0.005 0.111

2BB-PY

0.0 10 0.050 0.100 0.00

0.082 0.029

0.05 0.10

0.141

0.50

0.044

1.00

0.026

qutnchcr compound concn, M 9, DMA-PY 0.0 0.760 0.720 0.005 1 .o

0.014 0.192

AP-DB@

0.1 19

2.0 5.0 10.0

0.00

0.708 0.674 0.564

0.478

0.01 10 0.00281 0.0096 0.00552 0.0089 0.00861 0.0086 0.00951 0.0082

1,3-Pentadienequencher and 366-nm irradiation except for APDBO. Di-rert-butyl nitroxide quencher and 3 13-nm irradiation. PhosphorescenceExperiments. The triplet energies of model sensitizers were determined from their phosphorescence spectra in EPA at 77 K. For the quenching experiments in the solid phase, EPA or EA solutions of sensitizers and azoalkanes at various concentrations were frozen in 8-mm tubes in the phosphorimeter. The complete spectnun was s~annedfor each tube and the intensity at the emission maximum was noted. In the following listing of the data, the sensitizer and quencher are given fmt, then quencher concentration (M) and Zo/Z. (BB + NA) 0.05, 1.41; 0.075, 1.66; 0.10, 1.96; 0.15,2.74. (BB DBO) 0.025, 1.20; 0.50, 1.49; 0.075, 1.91; 0.10,2.45. (BB + PY) 0.018, 1.09; 0.026, 1.14; 0.029, 1.21; 0.041, 1.28; 0.071, 1.55. (TH NA) 0.025, 1.15; 0.05, 1.36; 0.075, 1.70. (TH DBO) 0.025, 1.21; 0.05, 1.52; 0.075, 1.85; 0.10,2.36. (TH PY) 0.016, 1.08; 0.028, 1.11; 0.048, 1.21;0.071, 1.34. For (BB + NA) and (BB PY), the values of Zo/Z are the average of those obtained using 414- and 443-nm emission. Plots of In P/Z versus quencher concentration were linear with the s l o p shown in Table 111. Transient Absorption Studies. Spectral grade benzene and hexane were used as solvents, and all samples were freeze-thaw degassed. For each compound, tubes with four or five different concentrations were prepared, keeping their optical density between 0.3 and 1.5. Lifetimes of thioxanthone derivatives were determined by monitoring T-T absorption at 700 nm, while 560 nm was used for benzophenone derivatives. The observed decay curves were first order except for TH-PY and 4BB-1-DBO, which are described under Solution Phase Lifetime Studies. The data for TH-2-DBO are given here as an example (concentration (M), lifetime (ns)): 1.79 X lo-", 1.6; 1.07 X lo-", 2.6; 7.13 X lo", 3.5; 3.56 X lW5, 7.2. When plotted according to eq 1, these data gave a straight line and provided the kb and kETvalues shown in Table VII. Decomposition quantum yields of the ester linked bichromophoric molecules were measured in 2.0 mL of benzene. The concentrations (cf. Table VIII) were selected to give an initial absorbance of 0.522 at the irradiation wavelength. The TH-PY, 4BB-PY, 3BB-PY, and TH-1-DBO solutions were irradiated at 366 or 313 nm with a 100-W high-pressure mercury lamp through a UV36B glass filter or a K2Cr04+ Na2C03solution filter, for 20 s, 2.5 min, 4 min, and 6 min, respectively. The extent of decomposition, which was under 20% in all cases, was measured by HPLC. The photon dose was calculated from the amount of N, evolved from decomposition of an actinometer consisting of benzophenone (0.063 M, a = 0.522 at 366 nm, or 8.56 X M, u = 0.522 at 313 nm) and 2,3-diazabicyclo[2.2.l]hept-2-ene (DBH) (0.05 M) in 2 mL of benzene. The nitrogen quantum yield was taken as 0.97 at both 366 and 313 nme4 Stern-Volmer Quenching. The nitrogen quantum yield of azoalkane solutions was determined in degassed, sealed tubes in a merry-go-round using a 450-W medium-pressure mercury lamp with appropriate filters to isolate the 313-nm or 366-nm mercury line. The initial azoalkane concentrations were chosen to carrespond to an absorbance of 2-3 at the irradiation wavelength. Samples containing quenchers at various concentrations were

+

+

+

+

+

7534 The Journal of Physical Chemistry, Vol. 96, No. 19, I 992

irradiated to low conversions ( 3 4 ) . (37) Agmon, I.; Kaftory, M.; Nelsen, S. F.; Blackstock, S. C. J. Am. Chem. Soc. 1986, 108, 4477. (38) Deuterogenation of 9 lacking the COOH group occurs 68% from the exo face, placing D anti to the azo group. Cf. Edmunds, A. J. F.; Samuel, C. J. J. Chem. SOC.,Chem. Commun. 1987, 1179. (39) Engel, P. S.; Nalepa, C. J. Pure Appl. Chem. 1980, 52, 2621. (40) Lamola, A. A. J. Am. Chem. Soc. 1969, 91, 4786. (41) Zimmerman, H. E.; McKelvey, R. D. J. Am. Chem. SOC.1971,93, 3638. (42) Maki, A. H.; Weers, J. G.; Hilinski, E. F.; Milton, S. V.; Rentzepis, P. M. J . Chem. Phys. 1984, 80, 2288. (43) Lamola, A. A.; Leermakers, P. A.; Byers, G. W.; Hammond, G. S. J. Am. Chem. Soc. 1965,87, 2322. (44) Ermolaev, V. L. Sou. Phys.-Usp. 1963,80, 333. (45) Kellogg, R. E.; Bennett, R. G. J . Chem. Phys. 1964,41, 3042. (46) Herkstroeter, W. G.; Lamola, A. A.; Hammond, G. S. J. Am. Chem. Soc. 1964, 86, 4537. (47) Camp, R. N.; Epstein, I. R.; Steel, C. J . Am. Chem. Soc. 1977,99, 2453. (48) Collier, S. S.;Slater, D. H.; Calvert, J. G. Phorochem. Phorobiol. 1968, 7, 737. (49) Watkins, A. R. Chem. Phys. Leu. 1980, 70, 262. Gijzeman, 0. L. J.; Kautman, F.; Porter, G. J. Chem. SOC.,Faraday Trans. 2 1973,69,727. (50) Schwerzel, R. E.; Caldwell, R. A. J . Am. Chem. Soc. 1973,95,1382. (51) See also Green, S. A.; Simpon, D. J.; Zhou, G.; Ho, P. S.;Blough, N. V. J. Am. Chem.Soc. 1990, 112, 7337. (52) Caldwell, R. A. In Kinetics and Spectroscopy of Biradicals and Carbenes; Platz, M. S., Ed.; Plenum: New York, 1990. (53) Yip, R. W.; Szabo, A. G.; Tolg, P. K. J . Am. Chem. SOC.1973,95, 447 1. (54) Leffler, J. E.; Miley, J. W. J. Am. Chem. SOC.1971, 93, 7005. (55) Thijs, L.; Gupta, S. N.; Neckers, D. C. J. Org. Chem. 1979,44,4123. (56) Keller, R. A. J . Am. Chem. SOC.1968, 90,1940. (57) Although other workerss8observed some anthrone phosphorescence from 12, Maki et aL4* support the original conclusions6 that ET is 100% efficient. (58) Hudson, J. A.; Hedges, R. M. In Molecular Luminescence; Lim, E. C., Ed.; W. A. Benjamin: New York, 1969; p 667. (59) Breen, D. E.; Keller, R. A. J . Am. Chem. Soc. 1968, 90, 1935. (60) Schaffner, K.; Amrein, W.; Larsson, I. M. Isr. J. Chem. 1975, 14,48. (61) Amrein, W.; Schaffner, K. Helu. Chim. Acra 1975, 58, 397. (62) Lamola, A. A. J . Am. Chem. Soc. 1970, 92, 5045. (63) Keller, R. A.; Dolby, L. J. J . Am. Chem. Soc. 1969, 91, 1293. (64) No triplet energy transfer was found between naphthoate and anthroate over a 20-A distance. Rauh, R. D.; Evans, T. R.; Leermakers, P. A. J. Am. Chem. SOC.1968, 90, 6897. (65) Katayama, H.; Maruyama, S.;Ito, S.; Tsujii, Y.; Tsuchida, A.; Yamamoto, M. J. Phys. Chem. 1991, 95, 3480. (66) Grindley, T. B. Tetrahedron Lerr. 1982, 23, 1757. (67) Jones, G. I. L.; Owen, N. L. J . Mol. Srrucr. 1973, 18, 1. (68) Allinger, N. L.; Chang, S. H. M. Tetrahedron 1977, 33, 1561. (69) Perrin, F. C. R . Hebd. Seances Acad. Sci. 1924, 178, 1978. (70) Smaller, B.; Avery, E. C.; Remko, J. R. J. Chem. Phys. 1965,43,922. (71) Anderson, R. W.; Hochstrasser, R. M.; Lutz, H.; Scott, G. W. Chem. Phvs. Leu. 1975. 32. 204. i72) Closs, G.'L.;hotrowiak, P.; MacInnis, J. M.; Fleming, G. R. J. Am. Chem. Soc. 1988, 110, 2652. ( 7 3 ) Winnik, M.A. Acc. Chem. Res. 1977.10. 173; Chem. Rev.1981.81, 491. (74) Speiser, S.; Katriel, J. Chem. Phys. Lerr. 1983, 102, 88. (75) Zimmerman, H. E.; Goldman, T. D.; Hirzel, T. K.; Schmidt, S. P. J . Org. Chem. 1980,45, 3933. (76) Getz, D.; Ron, A.; Speiser, S . J . Mol. Srrucr. 1980, 61, 61. (77) Inokuti, M.; Hirayama, F. J . Chem. Phys. 1965,43, 1978. ( 7 8 ) De-, J. N. Excired Stare Liferime Measurements; Academic Press: New York, 1983; p 53. (79) Roy, J. K.; El-Sayed, M. A. J. Chem. Phys. 1964, 40, 3442. (80) Eisenthal. K. B. J . Chem. Phvs. 1969. 50, 3120. (81) Okada, K.; Sakai, H.; Oda, M. J . Am. Chem. Soc. 1987,109.5534. ET from triplet phenanthrene to fluorene, which is 5.7 kcal/mol endothermic, was found to depend strongly on conformation. (82) Speiser, S.; Hassoon, S.; Rubin, M. B. J. Phys. Chem. 1986,90.5085. (83) Exchange singlet ET may depend on the nature of the group linking the chromophores. Kroon, J.; Oliver, A. M.; Paddon-Row, M. N.; Verhoeven, J. W . J . Am. Chem. SOC.1990, 112, 4868.

J. Phys. Chem. 1992, 96,7535-7546 (84) Mirbach, M. F.; Ramamurthy, V.; Mirbach, M. J.; Turro, N. J.; Wagner, P. J. Nouv. J . Chim. 1980, 4. 471. (85) Sakuragi, H.; Tokumaru, K.; Itoh, H.; Terakawa, K.; Kikuchi, K.; Caldwell, R. A.; Hsu, C . 4 . J . Am. Chem. Soc. 1982, 104,6796. (86) The program was PCMODEL from Serena Software, Bloomington, IN. (87) Wagner, P. J. Acc. Chem. Res. 1983, 16, 461. (88) Baird, N. C.; Swenson, J. R. Can. J. Chem. 1973, 51, 3097.

7535

(89) Foyt, D. C. J . Comput. Chem. 1981, 5,49. (90) Gilman, H.; Esmay, D. L. J. Am. Chem. Soc. 1953, 75, 278. (91) Cohen. S. G.; Zand, R. J . Am. Chem. Soc. 1%2,84, 586. (92) Meinwald. J.; Crandall, J.; Hymans, W. E. Org. Synth. 1966,45,77. (93) Andrews, S. D.; Day, A. C.; Raymond, P.; Whiting, M. C. Org. Synth. 1970, 50, 27. (94) Kuehne, M. E.; Lambert, B. F. Organic Synthesis, Collective Volume V; Wiley: New York, 1973; p 400.

Use of Microciusters To Simulate Cage, Trapping, and Chaperon Effects in Association Reactions Xiche Hu and William L.Hase* Department of Chemistry, Wayne State University, Detroit, Michigan 48202 (Received: March 16, 1992)

Association reactions of HAr, (n = 4, 12, 13) microclusters with CH3 to form CH4 are used to study microscopic solvation dynamics. Potential energy surfaces for the CH, + HAr, systems are constructed from a previously fitted H + CH! ab initio potential and Ar-Ar, Ar-C, and Ar-H Lennard-Jones interactions. Classical trajectory and reaction path calculations for the CH, + HAr, systems illustrate the relationship between the structure of a microcluster and its chemical reactivity. Solvating the H atom with Ar, is found to have the following three important effects on the association dynamics: (1) caging, a steric effect responsible for attenuating the association reactivity; (2) trapping, the Ar, shell creates a van der Waals well in which low relative translational energy collisions are trapped for long times; and (3) chaperon, a vibrationally/rotationally hot CH4is formed as H associates with CH, and Ar, acts as an energy sink to stabilize the excited CH4. For CH3reaction with a Ar6(H)Ar6cluster, which has a H atom in the interior of an icosahedral ATl2shell, a van der Waals well exists on the reaction path in which CH, is physisorbed on the surface of the ArI2shell. Both reaction path and classical trajectory calculations show that relaxation of this solvation shell structure is necessary for CH4 formation to occur. Trapping in the van der Waals well enhances CH4 formation by providing a sufficiently long interaction time for relaxation of the Arlzshell structure. If the H atom is sitting on the surface of the Ar, moiety this relaxation is not necessary, since reaction can occur by both direct and hopping mechanisms.

I. Introduction Although many reactions of interest occur in solution, the understanding of reactions in the liquid phase is much less complete than in the gas With the advent of experimental techniques such as picosecond s p e c t r ~ s c o p y and ~ ~ ~the ~ ~ac~-~~ cwibility of parallel processing s~percomputers~~-~~ for theoretical studies, more definitive studies of microscopic solvation dynamics are possible. To understand the role of solvation, one is concerned with the manner in which intermolecular forces and motions of the solvent perturb the reacting system. Molecular clusters provide a bridge between isolated gas molecules and the condensed phase.3e38 By systematically varying the size of the cluster, one can observe the gradual transition from the behavior characteristic of binary collisions of isolated gas molecules to the characteristics of condensed-phase dynamics. Molecular beam techniques in general and free jet expansions in particular have provided the capability to attach one or more solvent molecules to a reactant species and prepare a “guest” molecule embedded in a well-characterized local solvent configu r a t i ~ n . ’ ~ In - ~a~ slightly different approach, gas-metal surface interactions can be modeled by collisions between molecules and transition-metal cluster^.^^-^^ In the past decade considerable attention has been paid to the study of chemical reactions within van der Waals clusters (Le., intracluster reactions). An extensive research effort has been concerned with photoinduced molecular reactions within a well-characterized cluster. Lineberger and c o - ~ o r k e r shave ~~,~~ studied the photodissociation and recombination of IC in massselected cluster ions to explore the solvent “cage” Vaida and co-workers- have reported that intermolecular forces can stabilize the potential energy surface involved in predissociation, leading to changes in the dissociation dynamics. A spectroscopic technique has been developed by Scoles and cow o r k e r to ~ ~detect ~ ~ and measure the structural and dynamical properties of rare gas clusters containing active infrared molecules. This approach has been used to study dimer formation on the

surface of medium-large argon clusters.41 Intercluster reactions involve reactions between clusters or between a cluster and an atom or molecule.67 In some of the earliest work of this type crossed molecular beams were used to study the reaction of (CH31), clusters with alkali^.^^-^^ In later work the reaction between O(3P)and (NO),, clusters to form NOz was s t ~ d i e d . ~In~ very , ~ ~ recent work,’l molecular beam and picosecond laser techniques have been combined to study the AH + S! A- S,H+ proton-transfer reaction in real time. Here AH 1s a-naphthol and S, a (NH,), cluster. Of particular relevance to the work we report here are the molecular beam experiments by Naaman and c o - w o r k e r ~in~ ~which , ~ ~ the intercluster recombination reaction 0 COR,,, C02+ mR is investigated. They found that the spectator rare gas atoms R, change the reactivity, which varies with the number of rare gas atoms. However, they were unable to directly determine reactive cross sections as function of number of rare gas atoms. Computer simulations are expected to play an important role in understanding both intra- and intercluster reactions.73 For example, without simulations, there is often so much flexibility in interpreting experiments that qualitatively distinct mechanisms can be used to explain the same data. The power of simulations is that they can lead to a greater understanding of dynamical processes by calculating details which cannot be observed experimentally. For example, molecular dynamics simulations have played a decisive role in understanding mechanisms of Iz photodissociation and recombination in the condensed phase.3997c76Not surprisingly, one of the first type of cluster systems investigated by computer simulation is the photoinduced dissociation of molecules in the interior of a c l ~ s t e r . “ ~In- ~related ~ work, LeRoy and have used simulations to study dynamical and structural properties of SF6-(rare gas), van der Waals clusters. Although experiments have considered the dependence of intercluster reactions on cluster size,67*72,80,81 little theoretical work has been done on this In the work reported here the effect of solvating an atomic reactant is studied by simulating the

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0022-3654/92/2096-7535%03.00/00 1992 American Chemical Society