Stabilization of excited states by association

30 December 1994

ELSEVIER

CHEMICAL PHYSICS LETTERS

Chemical Physics Letters 231 (1994) 541-546

Stabilization of excited states by association. Emissive and reactive exciplexes from a non-emissive precursor Yuan L. Chow, Carl I. Johansson Department of Chemistry, Simon Fraser University, Burnaby, British Columbia, Canada V5A IS6

Received 1 I June 1994; in final form 1 November 1994

Abstract Acetylacetonatoboron difluoride and benzene derivatives photolytically add to form aromatic ketone products, presumably through 2 + 2 photocycloaddition. In dioxane, acetylacetonatoboron difluoride does not show detectable emission but fluoresces in the presence of added toluene, xylenes or mesitylene within a narrow excitation window of 300-320 nm. These broad emissions (A,, of 4 15-435 nm) were assigned to an exciplex that had lifetimes of 1.1-4.2 ns and fluorescence quantum yields of (3-7) X 10m3.These exciplexes were found to be precursors of the cycloaddition products through competitive quenching experiments. From kinetic evaluations, *AABF* was identified to be the reactive state with a lifetime of zz 1 ps.

1. Introduction The [ 2 + 21 photocycloadditions of acetylacetonatoboron difluoride (AABF*) and other alkyl-substituted 1,3-diketone BFI complexes with benzene and its derivatives have been concluded to occur from their singlet excited state [ 11. Similar photolysis with naphthalene and phenanthrene remains uncertain as to which species ( 1,3-diketone BF2 complex or the aromatic) excited state is responsible for the observed photocycloaddition owing to significant absorption overlap of the substrates in the 300-360 nm region [ 1,2 1. Recently, it has been established that the exciplex from singlet excited phenanthrene with AABF2 is the precursor to the products: the absorption overlap did not allow an unambiguous proof that the excitation of AABFl also leads to the observed emissive exciplex and/or adduct [2]. While the analogous dibenzoylmethanatoboron difluoride (DBMBF2) fluoresces with a reasonable quantum yield (&=0.08-O. 10) [ 3,4], AABFz is non-fluorescent in methylene chloride, acetonitrile, dioxane, cy-

clohexane and other non-aromatic solvents. In view of the propensity of DBMBFz to form highly emissive and non-reactive exciplexes with methyl benzenes [ 3-5 1, we have searched for the intermediates in the photoreaction of AABFz with these benzene derivatives [ 21; we wish to describe the formation of fluorescent exciplexes that are indeed the precursors to the adducts in their photocycloadditions. Owing to the unresolved absorption bands and the lack of AABF2 emission, well-defined conditions are required to allow the observation of such an exciplex fluorescence.

2. Experimental AABF2 was prepared as previously described [ 11, and benzene samples were distilled before use. Fluorescence and absorption spectroscopy were carried out on the same apparatus reported before [ 3,6]. The exciplex fluorescence quantum yield (c&“) was determined by the optically dilute method on a Photon

OOO9-2614/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSD10009-2614(94)01304-7

Y.L. Chow, C.I. Johansson /Chemical Physics Letters 231(1994) 541-546

542

Technology International LS- 100 spectrofluorimeter using anthracene (&=0.24) in the same aromatic solvent as the fluorescence standard [ 7 ] where e is related to a standard by [ 8 ] @ ? = (&iFunk/&nkFstd)

(1)

@F

The ‘Unk’ subscript refers to the unknown and ‘Std’ to the anthracene standard, and the symbols have the following meaning: A is the absorption at the excitation wavelength and F is the integrated fluorescence emission area. Both the unknown and the standard showed a molar absorptivity < 0.0 1 at the excitation wavelength of 3 13 nm.

3. Results and discussion In contrast to the photocycloaddition of AABF, to benzene and toluene [ 11, irradiation of AABFz and m-xylene or o-xylene (m-X or o-X) in dioxane proceeded slower and gave complex mixtures that became more complex on over-irradiation. With o-X, products 2 and 3 were isolated, among others, and presumably derived from adduct 1. Similar irradiation with m-X gave products 4, 5 and 6 after extensive chromatography. These structures were determined by their spectroscopic data. The aromatic proton coupling patterns indicated the substitution patterns for 3-5. The diketone 2 showed the cis-oriented ring protons at 3.35 and 5.55 ppm (J= 1.5 Hz), each of which was coupled to the neighboring protons. Enone 6 showed an IR absorption at 1670 cm-’ and a proton singlet at 3.37 ppm (2 H) which is proven to be located structurally near the proton at

2.08 (methyl), 6.97 (aromatic) and 6.07 (olelin) ppm by NOE experiments; 6 is the most likely structure among other possible ones. The physical data for these products will be reported in the thesis in preparation. Owing to the complex product patterns arising from secondary thermal and photolytic reactions, the preparative work was discontinued. Xylenes showed absorption maxima at 250-265 nm in dioxane with molar absorptivities of 230 M-’ of 1 M the optical density cm- ‘; at a concentration above 300 nm is negligible. AABF2 has a strong absorption at 289 nm (E= 17500 M-’ cm-‘) which extends above 310 nm at moderate concentrations of 1Op3- 1O-* M. Fig. 1 shows that AABF2 and m-X (or other xylenes) do not form ground state complexes in dioxane; NMR spectroscopic monitoring also indicates the lack of chemical shift changes. The previously reported fluorescence of AABFz [ 9 ] is now conclusively shown to arise from impurities; as stated above, it is non-fluorescent in many organic solvents, even in a rigid glassy matrix (methylcyclohexane glass at 77 K). In dioxane on excitation at 245 nm (where AABFz absorbs weakly), m-X showed a strong fluorescence peaking at 302 nm. On the addition of AABF2 at concentrations as low as 1O-5-lO-4 M (OD < 0.1 at 245 nm), the fluorescence peak intensity was drastically reduced to 30% and its shape is distorted on the low wavelength side due to the internal filter from AABF2. No new fluorescent peak above 370 nm was detected even at 10e3 M. This over-effi-

37--T---

Wavelength

4

5

Scheme 1.

6

(nm)

Fig. 1. Absorption spectra in dioxane; (1) [m-X] =O.OOM M; (2)-(4) [AABF2] =0.045 M and (3) and (4) also contain [mX] ~0.052 and 0.25 M, respectively.

543

Y.L. Chow,C.I. Johansson/Chemical PhysicsLetters231 (1994) 541-546

cient reduction could not be entirely due to dynamic quenching, since the estimated Stern-Volmer constant Ksv of 23000 M-’ requires the singlet excited m-X to have a lifetime of 2.3 us (the actual lifetime in nonpolar solvents is 3 1 ns [ lo] ) assuming that the quenching rate constant is diffusion controlled ( 10” M- ’ s- ’ ). If singlet excited m-X forms an exciplex at all, it is not fluorescent. In dioxane or methylcyclohexane glass (77 K), AABF2 ( 10-5-10-2 M, excitation wavelength at 300, 310 or 320 nm) does not show any detectable fluorescence. However, in dioxane ( [AABF2] = ( l4) x lop3 M, excitation wavelength at 310 nm) the addition of increasing amounts of freshly distilled methylbenzenes (but not benzene itself) resulted in the emergence of a broad featureless fluorescence (Fig. 2) where the A,,,,, was sensitive to the ionization potential of the methylbenzene (Table 1); this feature is characteristic of an exciplex [ l-5 1. To support that the broad fluorescence band originates from the interaction of singlet excited AABF2 with methylbenzenes, a dioxane solution of m-X (0.26 M) was excited in the presence of increasing amounts of AABF2 which resulted in a more intense fluorescence of the 424 nm band (Fig. 3). On the basis that the excited state [ *AABF2] is proportional to the ground state [ AABF2], this parallel between the increase in

I

1

I _.

430 Wavelength

(nm)

Fig. 2. Typical example of the emergence of exciplex fluorescence from [AABF2]=4.0x 10-l M and mesitylene in dioxane (A,,=310 nm) under air; Raman emission at 345 nm. [Mesitylene]=O, 0.072, 0.144,0.288,0.431 and 0.575 M for (l)+(6). The weak broad emission underneath the exciplex emission is from an unidentified impurity in dioxane. The inset shows the evaluation of KSvaccording to Eq. (2)

the 424 nm band fluorescence and [AABF2] supports the position that singlet excited AABF2 interacts with methylbenzenes to form an exciplex. Furthermore, using methylbenzenes as a solvent, the addition of AABF2 exhibited the same, but more intense and hypsochromically shifted, broad fluorescence band. These observations support that singlet excited AABFz interacts with methylbenzenes to form an exciplex. The lifetimes of AABFJmethylbenzene exciplexes (Table 1) were probed by oxygen quenching of fluorescence [ 12, 13 ] using the Stern-Volmer equation (vide supra), It/Z,= 1 +kd’rr,[O2]. In dioxane, kdiff[02] was adapted from the previous study of AABF,+*phenanthrene to be 1.98 x lo7 and 9.47 x 1O7s-’ under air and pure oxygen respectively [2]. In neat aromatic solvents, kdiff[O*] was calculated from reported concentrations of O2 [ 141 and k,i,,wasassumedtobe2X10’0M-‘s-‘. The quantum yields of exciplex fluorescence (@p) in these aromatic solvents were determined using anthracene as a fluorescence standard [ 8 ] where it was assumed that *AABF2 was completely quenched. As shown in Table 1, both the fluorescent quantum yield ( @r) and lifetime (r,) increase as the methylbenzene’s oxidation potential decreases; this parallels the recently reported DBMBFJmethylbenzene system [ 4 1. It is likely that benzene also forms an exciplex with *AABF2 which may fluoresce but scarcely beyond the detection capability ( Qp< 10P4). The parallel between T, and @F in neat aromatic solvent can be expected since e is a function of the lifetime (e=k,,z,). The charge transfer character of the exciplexes *AABFJm-X and *AABF2/p-X was qualitatively assessed by the solvatochromic shifts of the exciplex fluorescence maxima (v,,,); for both exciplexes the 2500 cm-’ (between shift in vmax is approximately cyclohexane and CH,Cl,), which indicates that these exciplexes have a substantial charge transfer character (Z 10 D) as referenced to the N,N-dimethylaniline/anthracene exciplex [ 15 1. While the quenching efficiency could not be determined directly, it can be indirectly evaluated from the exciplex emission according to [ 12 ] I,’

= (IF)-‘+(KsvZ~[D])-’

and demonstrated

,

(2)

by the inset of Fig. 2.1, and I? are

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Y.L. Chow, C.I. Johansson /Chemical Physics Letters 231(1994) 541-546

Table 1 Properties of exciplexes of lAABF2 and benzene derivatives Substrate (E,. versus SCE)

K a (G-1)

dioxane benzene (2.62 V) = toluene (2.25 V) = c-xylene (2.14V) = m-xylene (2.13V) = pxylene (2.06 V) ’ mesitylene (2.02 V) e

4y (xlOX) d neat

~~(ns) ’

A,, (nm)’ neat

dioxane

neat

1.8

2.9

_ 0.87 (0.88) a

415

405

1.1

1.2

425

422

_

0.69 (0.70) =

424

415

2.8

4.6

3.1

1.2 (1.0) a

436

420

2.6

5.1

4.2

0.64

430

420

4.2

7.0

7.6

a The figures in parentheses are those measured under argon; the error margin is k 30%. b In dioxane the solution was excited at 310 nm and in neat solutions at 320 nm. ’ Determined by oxygen quenching as in the text: the error margin is k 20%. d Anthracene in the respective aromatic solvent was used as a fluorescence standard where it was assumed that 4$=0.24 in benzene [ 71 is the same in all aromatic solvents. Errors f 30%. ’ Oxidation values were obtained from Ref [ II].

Iba PI

hu ‘A

A=

-w

kPx *AD

-

1

Zkta ka

I

A+D zklx

Scheme 2.

440 Wavelength

(nm)

Fig. 3. The exciplex formation of AABF* (at concentrations 0.033, 0.066and0.132Mfor(1),(2)and(3))with[m-X]=0.26M in dioxane (A,,=320 nm) under air. The rather good isoemissive point is fortuitous.

the relative exciplex fluorescence intensities and the exciplex fluorescence intensity as [D] -co, i.e. the exciplex fluorescence intensity obtained if all the *AABF2 interacts with methylbenzenes to form an exciplex. Ksv is the Stem-Volmer constant, which is equal to k,,r,, and TV= 1/C kl, (Scheme 2) is the

lifetime of singlet excited AABFz (directly inaccessible due to the lack of fluorescence). The KS, ( =slope/intercept) values determined according to Eq. (2) are in Table 1. Since double reciprocal plots generally generate significant errors, compounded by the change in the solvent nature at aromatic concentrations > 1 M, the results in Table 1 provide only approximate values. These Ksv values (Table 1) show small differences and are apparently constant. This lack of any trend in KS, can be attributed to either that the quenching of *AABF* by methylbenzenes is diffusion-controlled or experimental error. The lack of fluorescence from singlet excited AABF2 is speculated to arise from a short lifetime as suggested by the relation @r=kfa/C kla=kara. From the molar absorptivity of AABFl of e= 17500 M- ’ cm-’ at 289 nm, kf, is estimated to be 10’ SK’ for a spin-

Y.L. Chow, C.I. Johansson /Chemical Physics Letters 231(1994) M-546

allowed transition [ 161; this calculation assumes that the fluorescence and absorption states are connected, and that AABFz does not undergo relaxation to a twisted state. If OF is taken as < 10m4 (detection limit of the fluorimeter), ra is calculated to have the limiting value of one picosecond. This is supported by the invariance of KS, under either air or argon as in Table 1 [ 12, 13 1. This r, may be just about the right value since for Ksv z 1 M-’ (seeTable 1) ak,,= 10” M-i s-l is calculated. Although this value is two orders of magnitude greater than the diffusion limit of 10” M- ’ s- ‘, diffusion-controlled reactions are known to have remarkably large transient effects in the picosecond range [ 171 which is manifested by a larger kdir than that generally expected. The comparatively long lifetimes of the exciplexes ( > 1 ns) in Table 1 require that the rate constant of dissociation of the exciplexes must be about lo9 s-i or less; that is, the exciplex formation is irreversible [ 18 1. It remains to establish that the fluorescent exciplex is indeed the precursor to the products. Taking advantage of a short lifetime of *AABF2, which is not likely to be significantly quenched by anisole at low concentrations, and that exciplex formation is believed to be irreversible, the interaction of the *AABF2/m-X exciplex with anisole was examined. In dioxane this exciplex fluorescence, generated from [m-X] =0.82 M and [AABF2] =4.0x 10e3 M, was decreased by anisole (0.015-0.092 M). The SternVolmer analysis gave KS, = 8 k 2 M- ‘; the quenching rate constant by the addition of anisole is calculated to be ( 3 ? 1) X 1O9M- ’ s- ’ by using the exciplex lifetime (~~5,) of 2.8 + 0.6 ns (Table 1). As the photocycloaddition of AABF2 to xylenes is complicated by secondary reactions, the quantum yields of products have limited use for quantitative kinetic analysis. From the corresponding photocycloaddition of [AABF2] =0.05 M and [m-X] =0.5 M, the addition of [anisole] =0.092 M decreased the yields of the three products 4, 5, and 6 by 40%-60% under comparable conditions. Using KS, = 8 M- ’ for fluorescence quenching of the *AABF,/m-X exciplex by anisole, the reduction across the three products is calculated to be 40°/6-50%. Qualitatively it is concluded that the exciplex is the precursor to the products. This in turn indicates that the excitation of AABF2 initiates the photocycloaddition to benzene derivatives via their exciplexes.

545

The window available for excitation to observe the 424 nm AABF2/m-X exciplex fluorescence is limited to the narrow range 300-320 nm. Below 300 nm the absorption by m-X (and other homologues) starts to become significant as the concentration becomes higher than 1 M. In theory, singlet excited m-X should also interact with AABFI, either directly or indirectly through energy transfer, to generate an exciplex. The failure to observe the same 424 nm fluorescence under these conditions could be simply interpreted that this is a different exciplex from that observed from *AABF2 shown in Scheme 2 and Fig. 2. Alternatively, the xylene fluorescence could be quenched by AABFl at distances longer than the encounter distance ( z 7 A); in this case, *AABF2 should decay faster than a bimolecular encounter to form an exciplex. While such an exciplex is analogous to that from singlet excited phenanthrene and AABF2 [ 21, it is not possible to prove that this exciplex also gives the same products 4-6 due to overlapping absorption. Recently, we have demonstrated that singlet excited DBMBF, (which has 7,=0.30 ns [ 191 and ~&=0.08-O. 10 in acetonitrile [ 3,4,5,19] ) forms exciplexes with methylbenzenes in their neat solvents, showing higher fluorescence quantum yields (e = 0.4-0.7) andlongerlifetimes (7,=2-13 ns) [4]. The *AABF2 aromatic exciplexes show a similar trend with an enormous increase in the lifetime from the picosecond to the nanosecond time domain. Singlet excited AABF2 (and other alkyl-substituted BF2 complexes) must be stabilized by associating with the rcelectron systems of aromatic compounds, most likely to slow down the internal conversion and intersystern crossing (non-radiative) processes; it is speculated that these two are slowed down more than the radiative process. The observed photocycloaddition with aromatic compounds [ 1 ] is thus facilitated by the intermediary of long-lived exciplexes that allow bond reorganization to proceed. The failure of these alkyl BF2 complexes to react with oletins is, therefore, most likely due to kinetic factors; short lifetimes in dioxane or do not provide enough time for bond formation to occur.

Acknowledgement We are indebted to S.P. Wu (Simon Fraser University) for some initial laboratory work and the re-

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Y.L. Chow, C.I. Johansson /Chemical Physics Letters 231 (1994) 541-546

viewer who made a number of valuable suggestions. The authors are grateful to the Natural Sciences and Engineering Research Council of Canada for generous financial support of this project.

References [ I] Y.L. Chow and X.X. Ouyang, Can. J. Chem. 69 ( 1991) 423; X.X. Ouyang, M.Sc. Thesis, Simon Fraser University (1992). [ 21 Y.L. Chow, S.P. Wu and X.X. Ouyang, J. Org. Chem. 59 (1994) 381; S.P. Wu, M.Sc. Thesis, Simon Fraser University (1994). [ 31 Y.L. Chow, X.E. Cheng and C.I. Johansson, J. Photochem. Photobiol. A 57 ( 199 I ) 247. [4] Y.L. Chow, S.S. Wang, Z.L. Liu, V. Wintgens, P. Valat and J. Kossanyi, New J. Chem. 18 (1994) 923; C.I. Johansson, Ph.D. Thesis, Simon Fraser University, November (1994). [ 5 ] Y.L. Chow and X.E. Cheng, Can. J. Chem. 69 ( 1991) 1575; Y.L. Chow, S.S. Wang and X.E. Cheng, Can. J. Chem. 71 (1993) 846. [6] Y.L. Chow and C.I. Johansson, J. Photochem. Photobiol. A 74 (1993) 171. [7] W.H. Melhuish, J. Phys. Chem. 65 (1961) 229. [8] J.N. Demas, in: Optical radiation measurements, Vol. 3. Measurements of photoluminescence, ed. K.D. Mielenz (Academic Press, New York, 1982) p. 195.

[ 91 V.E. Karasev and O.A. Korotkikh, Russian J. Inorg. Chem. (Engl. Transl.) 3 I ( 1986) 493. [lo] S.L. Murov, I. Carmichael and G.L. Hug, Handbook of photochemistry, 2nd Ed. (Marcel Dekker, New York, 1993) Table 1. [ 111 C.J. Schlesener, C. Amatore and J.K. Kochi, J. Phys. Chem. 90 (1986) 3747; I.R. Gould, D. Ege, J.E. Moser and S. Farid, J. Am. Chem. Sot. 112 ( 1990) 4290. [ 121 Y.L. Chow and C.I. Johansson, Res. Chem. Interm. 19 (1993) 191. [ 131 Y.L. Chow and C.I. Johansson, J. Chin. Chem. Sot. 40 (1993) 531; J.L. Charlton, D.E. Townsend, B.D. Watson, P. Shannon, J. Kowalewska and J. Saltiel, J. Am. Chem. Sot. 99 ( 1983) 5992; R.A. Caldwell, D. Creed, L.A. Melton, H. Ohta and P.H. Wine, J. Am. Chem. Sot. 102 ( 1981) 2369. [ 141 P.G.T. Fogg and W. Gerrard, Solubility of gases in liquids (Wiley, New York, 1991) pp. 24,294. [ 151 H. Beens, H. Knibbe and A. Weller, J. Chem. Phys. 47 (1967) 1183. [ 161 N.J. Turro, Modem molecular photochemistry (Benjamin/ Cummings, Menlo Park, 1978) p. 90, Eq. (5.23). [ 171 D.D. Eads, B.G. Dismer and G.R. Fleming, J. Chem. Phys. 93 (1990) 1136. [ 18 ] J.B. Birks, Prog. React. Kinetics 5 ( 1970) 18 1. [ 191 T.O. Harju, J. Erostyak, Y.L. Chow and J.E.I. KorppiTommola, Chem. Phys. 18 1 ( 1994) 259.