Photoreduction of triplet benzophenone by amines: role of their structure

CHEMICAL

12 April 1996

PHYSICS LETTERS ELSEVIER

Chemical Physics Letters 252 (1996) 263-266

I

Photoreduction of triplet benzophenone by amines: role of their structure Markus von Raumer, Paul Suppan, Edwin Haselbach Institute of Physical Chemistry of the University of Fribourg, P~rolles, CH-1700Fribourg, Switzerland Received 13 December 1995; in final form 5 February 1996

Abstract The reaction of triplet benzophenone with the tertiary amines triisopropylamine and diisopropyl-3-pentylami~e does not lead to photoreduction. An explanation of this unusual behaviour is given based on the particular structure of these amines. The initial charge transfer events are compared with those exhibited by 1,4-diazabicyclo[2.2.2]octaneand triethylamine.

1. Introduction

explained [6] by the orthogonal orientation of the radical p-lobe at C~ and the adjacent N-lone pair which would result from proton loss of th~ intermediate radical cation 1 +. Hence, no stabilising 2centre-3-electron interaction is possible s~ch as encountered in the case of amines with conformationally flexible substituents. We report here studies with other tertiary amines which behave in a similar manner: triisop!opylamine (TiPA, 2) and diisopropyl-3-pentylamine (DiPPA, 3).

The photoreduction of triplet benzophenone (3 BP) by amines is a well known reaction which has been extensively investigated [1,2]. Most tertiary amines with hydrogens at the C,~-atom react with 3Bp by forming the neutral ketyl radical BPH. Very few exceptions are known, e.g. 1,4-diazabicyclo[2.2.2]octane (DABCO, 1) which quenches 3Bp by electron transfer (ET) but no subsequent proton transfer is observed in polar solvents [1,3-5]. This outcome is

/

L

C3

DABCO

TIPA

DiPPA

TEA

Manxlne

I

2

3

4

5

0009-2614/96/$12.00 © 1996 Elsevier Science B.V. All rights reserved PII S 0 0 0 9 - 2 6 1 4(96)001 38-8

264

M. yon Raumer et a l . / Chemical Physics Letters 252 (1996) 263-266

2. Experimental The transient absorption (TA) and transient photocurrent (TP) apparatus have been described previously [7]. TP measurements were made in single shot mode to prevent degradation of the solutions. The energy of the laser pulses was monitored with an integrated photodiode which allowed an assessment of the pulse variations and the normalization of the peak currents. Amine concentrations were chosen so that at least 99% of the triplets were quenched. TA measurements were made with the triplets quenched at 90%. For the measurement of the quenching rates, phosphorescence (Aobs = 450 nm) lifetime measurements were made. The excitation source (Aexc = 337 nm) was a PRA ps nitrogen laser (model LN100) operated at 10 Hz repetition rate (300 ps pulse length, 40 /~J energy per pulse). The emitted light was observed through a monochromator by a Hamamatsu R928 PM tube. The signal was displayed through an anodic load of 50 ohms and stored on a 100 MHz digital oscilloscope (Gould 4074) linked to an Olivetti M240 computer. In order to apply pseudo first order kinetics the triplet concentrations must lie well below the quencher concentrations which were in the range 1 0 - 5 - 1 0 -3 M. By actinometry (Aberchrome 540/toluene system) [8,9] the concentration of 3Bp was found to be around 10 - 7 M. Fitting the lifetime of 3BP to pseudo first-order kinetics at different amine concentrations allowed us to obtain SternVolmer plots consisting of at least 5 points. No significant deviations from linearity were observed. The BP solutions (5 raM) had an absorbance (1 cm pathway) of 0.5 at 355 nm and 0.78 at 337 nm. Deoxygenation of the samples was done by bubbling Ar for at least 20 rain through the solutions. BP (Aldrich, Gold Label) was sublimed, DABCO (Fluka, purum) was twice sublimed, DiPPA (Fluka, purum) was distilled, TiPA was synthesized and purified according to Ref. [I0]. MeCN (Fluka, UV grade) and triethylamine (TEA, 4) (Fluka, puriss) were used without further purification.

maximum of the B P H absorption at 540 nm is near the 3Bp absorption maximum at 525 nm. One can see clearly that with 1, 2 and 3 only a shortening of the 3Bp lifetime takes place and no long-lived B P H is formed as with 4. The absorption of the radical anion B P - in MeCN was observed at 710 nm. It is clear that it is formed from 3BP with 1, 2 and 3 (Fig. 2). On the other hand, this ionic transient could not be observed in the presence of 4.

0.08

a) b)

0.04

"--" • ..... - . . . . . . . . . . . .

0.00 0

10

20

30

""

40

0.09

2

0.06

~

b)

0.03 0.00

2O

3O

4O

0

I 0

t

I

i

I

0

10

20

30

40

0,06 0.04 0.02 0.00

0.06 b) 0.04 0.02

3. Results and discussion

0.00 10

Fig. 1 shows the TA decay kinetics at 540 nm of 3Bp in MeCN, and in the presence of 1-4. The

20

30

t [l&s]

Fig. I. TA decay kinetics at 540 nm. 3Bp (a) in MeCN, and (b) in the presence of 1, 2, 3 and 4.

M. yon Raumer et a l . / Chemical Physics Letters 252 (1996) 263-266 w

A

600 400 200 0

0.08 = ~ 0.04 0,00

i

-

~ -

10

i

..........

i "pv ,

20 30

-~

(~ion

determined by steady state irradiation. Table 1 summarizes quantum yields and quenching rateiconstants for the four amines. The adiabatic ionisatibn potentials reflect the amine's propensity to act Ks electron donor. Consider 4 as an ordinary tertiary amine with flexible substituents. For such a case it i~ currently assumed that the process as depicted in Scheme 1 is operational [14,15]: after ET proton transfer (PT) occurs and leads to a radical which is istabilized through a 2-center-3-electron bond (Fig. 5a). The situation is different for 2. Due t o its bulky substituents this amine possesses three essentially coplanar N-C bonds [10] (Fig. 4), a surprising struc-

kq

IPa

for TP

109 M - t s - l

2.9 1.7 2.0 8.8

0.02 0.02 0.02 0.09

7.8 7.8 7.2 1.7

10 - 4 10 - 4 10 _4 10 - 4

10 t [IX~]

of (b) 4.

for TA X × × ×

5

and (~ketyl for the quenching of 3BP with amines 1-4

[Donor]/M

1 2 3 4

0

Fig. 3. TP kinetics of BP in MeCN in the presence of ia) 1-3, and

in MeCN in the

Fig. 3 shows examples of TP kinetics. With this technique the presence of mobile charge carriers can be confirmed. Assuming that the ion yield in the system B P / 1 is unity [11,12] then the ion yield for BP with 2 and 3 can be calculated by comparing the peak photocurrents (Table 1) [7]. BP in MeCN without added amine nevertheless yields a small photocurrent due to triplet-triplet annihilation [13]. In the case of 4 another spurious little photocurrent persists which must arise from the processes which follow the formation of BPH. By comparison with the BP/benzhydrol system [14], the quantum yield of B P H formation (~ketyl) Can be obtained. ~ketyl does not correspond to the overall quantum yield as

Table 1 Rate constants, peak photocurrents,

= = ~

t [.tts]

Fig. 2. TA decay kinetics at 710 nm. B P presence of (a) 1-3, and of (b) 4.

Donor

265

eVa 7.24 c 6.95 d 6.85 7.5

Peak current

(~ion

(i~ketyl

/~A b 768 712 679 e

i q

1.00 0.92 0.88

0 ~

e

1 f

0 0

Note, that the concentrations are different for TA and TP measurements, see Experimental part. a ipa is taken as the band onset which was determined by the tangential extrapolation technique [20]. b Values corrected for variable laser intensity, see text. c Ref.[16]. d Ref. [10]. e No peak photocurrent can be obtained within the same timescale, as employed for 1-3. f Ref. [ 14].

~c---o

.

R2~--c.

R,

R'

~

,-..~ pxodaets

--o"

R' Scheme 1.

266

M. yon Raumer et al. / Chemical Physics Letters 252 (1996) 263-266

~ ~/~._"

similar geometrical arrangement of the C ~ - H bond with respect to the axes of the N-lone pair.

...c~

Acknowledgements Fig. 4. Structure of 2 from Ref. [10]. The CNC angle fl = 119.2° indicates essential coplanarityof the three N-C bonds. The outof-plane angle 8 for the C,,-H bond with respect to the N(Ctt) 3plane is 5°.

tural feature encountered before only in bicyclic manxine 5. The present results add to the spectroscopic ramifications [10,16,17] of such special amine structures: there is neither a favourable approach of 3BP to the hydrogen at C a of 2, nor could a stabilizing interaction analogous to the one of flexible 4 subsequently develop as the involved orbitals would then be essentially orthogonal (Fig. 5b). Consequently, 2 engages in electron transfer, but ensuing 2 +. is reluctant to cleave the C ~ - H bond. As pointed out in the introduction, the rigid bicyclic structure of 1 ÷" would equally prevent the 2-centre-3-electron bonding after proton loss (Fig. 5c). On the other hand, this ion enjoys special stabilization from the 'through b o n d ' charge resonance interaction between the two N-atoms [18]. Our data reveal efficient electron transfer from 1 to 3Bp and the formation of B P - , but no subsequent proton transfer from 1 ÷" to B P - which would result in BPH ". (The absorption of the intermediate cation 1 + with E(Amax, 470 rim) = 650 M - l c m - 1 cannot be detected under our experimental conditions [19]). No structural analysis of 3 is available, but the similarity of the results to those of 2 suggests a

0 a)

h)

O

Fig. 5. Possibilitiesof 2-center-3-electronbonding for (a) 4, (b) 2 and (c) 1 after formal H-abstractionat C,,.

This work was supported by the 'Fonds National Suisse de la Recherche Scientifique' through Project No. 2028-040398.94/1

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