Fluorescence quenching of tertiary amines by halocarbons

Physics 76 (1983) 103-109 North-Holland PublishingCompany

Chemical

FLUORESCENCE QUENCHING OF TERTIARY AMINES BY HALOCARBONS Peter C- ALFORD.

Chfford C_ CURETON

*, R A. LAMPERT and David PHILLIPS

Dmy-Farada_v Laboratory. i%e Royal Insrirution. 21 Albemarle Sweet. London WlX 4BS

Received 17 hlay 1982,in

fiial form 13 December

*

UK

1982

The quenchmg of fluorescence of a variety of tertiary amines by halocarbons has been imestigated in both vapour and solutron phases In the vapour phase, quenching cross sections do not correlate v.el.l with a parameter based upon x\elI-depth for colhsion complexes, but reasonable correlatron betueen rate constant and free energy change for charge-transfer complex formation is obtained. The quenching by halocarbons is very efficient_ In solution phase, evidence of static quenching is presented.

tronic

1_ Introduction

and steric

factors

can affect

the magnitude

of

Keq

Amines such as diphenylamine are of interest in that they are photosensitive materials [l] and both aromatic amines and ahphatic amines in solutions contaming halogenated alkanes undergo facile photochemical reactions [2-5]_ 3 has been proposed that the absorbing species in these cases are 1 I 1 complexes of amines and halocarbon [X,3] which have been observed through UV absorption spectra [6,7]. Thus

the sequence of reactions occurrmg in the case of the reaction between for example n-butylamine and Ccl, is given as [3] RNHz - Ccl, + hu + (RNH2)+CCl,-. (RNH?)+(CCJ,)-

--, RNHf Cl- i- - CC13.

(1) (3

For the dlphenylamine-Ccl4 reaction, the diphenylamine radical cation Ph,Np has been observed as an mtermedlate by flash photolysis [5]. The studies above were carried out at high concentrations of halocarbon, since the equilibrium constants for the ground-state complexes are extremely small. of the order of 0.05-0.1 [3,6,7]. Both eiec* Work carried out in Department of Chemistry, University of Southampton. l Present address: Department of Chemistry, The Universrty, Heslington. York. UK.

0301-0104/83/0000-0000/S

03.00 0 1983 North-Holland

[S]. The very small magnitudes of Keq measured indicate that ar least for ahphatlc amines and Ccl, as acceptor the charge-transfer interaction 1s so loose as to approach the nature of contact pairs in the hlulliken sense [9] _Nevertheless, at high concentrations. absorption by ground-slate donor-acceptor pairs dominates, and thus obscures excited-state interactions_ Since many smmple amines such as trimethylamine and its derivatives are volatile and lughly fluorescent [lo121 m the vapour phase, a study has been carried out on the fluorescence quenching of such amines by halocarbons in this phase in the hope that excitedstate interactions could be quantified_ This study amplifies the perhaps surprising observation that the fluorescence of triethylamine is quenched efficiently by fully fluorinated alkanes [l I] _Vapour-phase results are compared with some obtained m dilute cyclohexane

solution.

2. Experimental Solution-phase results were obtained using standard techniques. III the vapour phase relative fluorescence quantum yields QF were measured on a standard optical arrangement comprising a 6 cm long, 2 cm diameter

cylindrical quartz gas cell with a 2 cm diameter T-window. a medium-pressure mercury arc or xenon arc

104

P C. Alford et al ki’uorescence quenciung of tertiwy amines by halocarbons

Table 1

Qucnchmg parametersfor simple ammes by halocarbons in cycloheune solution ~--

--

Amine

_-__

---

a)

Quencher

Stem-Volmer slope q-0 (dm3 ,‘llOl-‘) -~ 224

“Q

perfluoromethylcyclohexane perfluoromethylcyclohexane CHaCla

tnethylamine triethylamine N,N’dunethylaniline N,N’-dimethylaniline N,N’drphenylamine N,N’diphenylamine

Decay time TFO

(IIS)

Quenching rate constant (dm3 mol-’ s-l ) (1.3 4 0.2) x 10’0

17.2

(7 2.65 43 3 70 b)

22 22 2.9 b)

f 3 )X 109

1.2x 1.8~ 2.4 x 97x

109 1O’O 1O’O 109

3) Amine concentration10-4 mol/P. b, from decay time measurements, thus euted-state quenchmg only_ c, From ref. [5], wheune

solution.

lamp for excitation, a 0.25 m Bausch and Lamb grating monochromator with resolution 16 A/mm for excitation wavelength selection, and a RCA 935 photodiode for momtoring transmitted hght, and a lP28 photo-

multipher for monitoring emitted light Gas mixtures were made up on a grease-free vacuum system, and all materials were thoroughly out-gassed by repeated freeze-pump-thaw cycles before use.

Table 2 Quenching of sunple amines by halocarbons m the vapour phase Amine

Quencher

triethyhimine a) triethylamiie a) triethylamine a) triethylamimea)

triethylamine a) tnethylamine a) triethylamine a1 triethylamine a) tr~ethylammea) txiethylamme a) tnethylamme a)

trietlrylamine a) titrp;$e

a)

anujne b: c) anihne b, c) N,N’diiethylanihne b* d, N,N’-dimethylanilhteby d) N,N’+hmethylanihne by d)

---1 cc14 2 CHC13 3 perfluoro-but-2-ene 4 perfluoro cycle-butane 5 1,3-his-trifluoromethylbenzene 6 7 CFClaCFaCI trifhroromethylbenzene 9 CH$& 10 CHaClCH$Zl 11 CF4 1202 13 NO 14 cc14 15 CHC13 16 CHzClz 17 cCl4 18 CHC13 19 CHaCla

a) Excitatton wavelength240 run, amine decay time 58 ns, 0.5 Torr. b, ‘1 Excitation wavelength265 nrn, 0.5 Ton_ Aniline decay time 3 3 ns, 0.5 Torr. d, Dimethylamline decay time 1.8 ns, 0.5 Ton.

Apparent quenching rate constant (dm3 mol-’ s-l) ~-- -6.5 x 10” 4.2x 10” 4.0x 10” 3.3 x 10” 2.9 x 10” 2 2 x 10” 1.9 x 10” 1.6 x 10” 6.7 x 10” 2.9 x 10’0 <3.5 x 10s 4.0 x 10” 1.1 x 10” 1.9 x 10’2 5 5 x 10” 2 0 x 10” 3 0 x 10’2 5.3 x 10” 2.7 x 10”

P-C Alford er al fl?uorescence quenching of rerh-aryammes by halocarbom

Fluorescence decay times rF were measured by the timecorrelated single photon-counting technique usmg an apparatus which has been described earlier

I

A&A*+

Quenching parameters denved from Stem-Volmer plots of either &-&jF against [Q]or Trl agaiIISt [Q] usmg the following equations

I

)

A*Q

quenchlng

“R A + hvF where

(3) +kQ[Q],

AQ

hu

P31.

-I=+; Tt-

e

A+Q

105

k=

kR+kNR

Scheme 1.

(4)

of where Gro and rro refer to zero concentration quencher, are given in table 1_ Results for the quenching of simple annnes by halocarbons in the vapour phase using continuous rllumination are collated in table 2, where apparent kQ values are quoted.

plex whrch can also absorb incident hght to form the exciplex A*Q. Smce single-component emission 1s observed throughout. the reverse dissocration of the e\crplex. step k_. can be ignored. The rate constant k is the sum of all the unimolecular rate constants for decay of A”. The steady-state treatment applied to +xcited amine A* is d[A”] /dr - I,(A)

3. Discussion It rs evident from the results above that the excited smglet states of tertiary amines are quenched very effectively by a wide variety of halogenated compounds. For molecules with lower energy levels than the arnmes, quenching through electronic energy transfer could occur. as has been seen recently m benzeneTEA mixtures in solution [ 14,15]_ in the case of all quencher molecules studied here however with the exception of benzene derivatives, ener,y transfer would be an endothermic process. Since the rate constants observed in many cases are extremely large, energy transfer seems an unlikely explanation. We may further discount enhancement of non-radiative decay from the singlet state to the tnplet since spin-orbit coupling constants for the fluorine-containing and chlonnecontaining compounds used as quenchers are small, and there is no evidence at all for triplet-state formatton in simple amines. The most likely contender for the observed quenching is complex formation either between excited-state or ground-state amine with the additrve, as has been proposed previously for amine fluorescence quenching by oxygen and NO [12], and amine quenching by carbon dioxide [16] _Both ground-state and excited-state quenching processes are accommodated in scheme 1, in which the quenching of amine A by a molecule Q is considered. In scheme 1 AQ is a ground-state com-

- k[A”]

- k+]A”]

[Q] = 0.

where IA(A) IS the rate of hghr lbsorprron plexed A, and so [A*] =1*(A)/@

b> uncom-

+ k+]Ql).

Now the measured 1s given by

quantum

(5)

(6) yield of fluorrscencc.

QF = kRIA”]/IA.

or-_

(7)

where I, is the (oral light absorbed IA( therefore

by A_ I__,(X) +

when Q = 0. +- = kRfk_ and so +. -=

k

+kJQl k

Or-

I,(AQ) ( l-l- iA )_

At the low concentrations present. the approtimatron can safely be used:

of anune and comple\ to the Beer-Lunbert

I, = I& @FO -=

QF

k +x-,[Ql k

=k +k,[Ql k

1 + EAQ

tAQ1

EA I-4

(9)

law

106

P_C Alfkd

et al /Fluorescence

where Kfc is the equlbbrrum constant of ground-state complex, i.e., $c

=

quenching

for formation

MQl/[Al tQ1

(11)

anmes

by halocarbons

well-tried treatment of Weller et al. [ 181, one expects a correlation between quenching rate constant and electron affinity EA accordmg to the equation In kQ 0: (IP - EA -Eoo),

and (12)

K;c = @AC@A)Kfc EA, EAQ

are ITIOh decadlc absorptron coeffclents for A and AQ respectively. The first term on the right-hand side m eq (10) can be written I +

of ternary

x-+[Ql/k= 1 + k+~,-~tQ1 = T~-~/T~,

(13)

where we recogmze that l/k= 7Fo,the fluorescence lifetime m the absence of quencher. Thus, substituting eqs. (12) and (13) into eq (10) we have

(@FOh-As = 1 +K;,[Q]. TI-O/TI-

(14)

The subscript ss denotes that the quenclung ratio 1s that measured under steady-state condrtions whereas the ratio of the hfetlmes is that measured under kinetic conditions. For normal Stern-Volmer quenching, these terms would be identical and the ratio would be unity.

4. Vapour-phase

results

For TEA/Ccl4 mixtures, there seems to be no systematic variation with [Q] as would be anticipated from (14). The data are somewhat imprecise, and we merely conclude that ground-state complexes in the vapour phase between TEA and Ccl, may be formed, aithough absorption spectra give no evidence of this. Furthermore, static and dynamic quenching parameters for TEA/PFMCH mixtures are identical, mdleating httle or no vapour-phase ground-state complex formation. Attempts to correlate the observed quenching efficiencies in the present work with a recent model for complex formation [ 171 which assumes attractive forces predominate fail completely with the present data. Smce we anticrpate that the dominant force in stabilization of an encounter complex between an amme and a halogeno compound would be electron or charge transfer, this is not surprising. Using the

(I9

where IP is the ionization potential of the donor, EA the electron affinity of the acceptor, and Eoo the excltatron energy of the amme. This equation was tested for quenching by the chloromethanes and for quenching of TEA by 0, and NO. The absence of electron affinity values for the ff uorinated molecules precluded the posslbrhty of applying the model to those also. Often, electrode reduction potentrals have been used for want of an appropriate electron affinrty value. but these also are lacking for the fluorocarbons. Plots of In kQ versus IP - EA - Eoo are shown m fig 1 using the data from table 3 In each case, good correlatrons are observed for a particular amme for quenching by chloromethanes and the straight lines observed have closely similar slopes However, the data points do not lie on one common straight lme as wouid be expected if the model were followed rigorously. Furthermore, the points for O2 and NO quenching TEA are displaced from the line for chlorocarbons quenching TEA. Clearly the assumption of complete electron transfer 1s not good in the vapour phase even though rt is a hkely mechanism in solution, especially m polar solvents. The real situation in a charge-transfer complex

I

2 -LKi/fZV

Fig 1. Plots of logarithm of quenching rate constant against estimated free energy change for excitedstate complex formation for excited singlet TEA, DMA and anihne, with compounds identied III table 3. hex TMA, 240 run. DMA 265 nm, aniline. 265 nm. Pressure of amine 0.5 Torr.

RC. Alfordetal /Fluorescence qrtetrchingof terfmry omines by hdocarbom Table 3 Tree energy parameters for vapour-phase amine quenchm9 - ----- ------ - --Amine

EA(eV) a)

hi kQ

fK(e\q

4.6

cc14 CHC13 CHzClz 02 NO

4.2

cc14

2.06 1.7 1.31 0440 0.0240 2 06 1.7 1.31 2 06 1.7 1.31

27 2 26.8 24.9 26.7 25.4 28 3 27.0 26.0 28.7 27.0 26.3

OX4 1.2 1.59 2.46 2.88 1.44 1.8 2.19 0 94 1.30 1.69

Eoouun)

E00Cev)

TEA

7.5 a)

270 =)

aniline

7 7b)

294d)

DhlA

7.1 b,

303 d)

--Quencher

WeV)

--__--

CHCla CHaC1, cc4 CHC13 CH*Cl2 __

41

--a) Ref 1261.

b, Ref. 1271.

involves contnbutrons

‘1 This work

‘1 Ref. 1281

from a number of interactions

which may be summarized

107

e, Ref 1291

D Ref. 1301

5. Solution-phase

results

by writing the following

structures for the exciplex: (AQ)* = A*Q * AQ* ++ A+Q- +r A-Q+.

(16)

The experimentally observed behaviour will then depend on the extent to which one or more of these is

dommant It will be noted that the above discussion does not take steric considerations into account; these have proved to be important in at least one previous study [I91 _ Fig 1 strongly suggests that for the molecules included, charge transfer complex formation is the hkely quenching mechanism. Since cross sections for quenchmg of the amines in the vapour phase by fully fluonnated compounds, with the exception of CF4, show srmilar trends, one may extend the inodel

to include these compounds. The high cross sections for quenchmg observed in ah cases are consistent with the proposal that the emrtting state of the amine is Rydberg in character, the electron involved in charge transfer thus being in an orbital far removed from the nitrogen nucleus in the amine [ 12,131. The resulting exciplex may subsequently initiate chemical reaction. Thus irradiatron of a TEA/WI, mrxture in the gas phase using a more powerful laser source has been observed to produce an aerosol of product whereas TEA/perfluorobut-2-ene drd not [30] _ In view of the strength of the C-F bond with respect to the C-Cl bond, this is reasonable.

There are no marked deviations from linearity in solution-phase Stern-Volmer plots. but it is very clear from table 1 that apparent quenchmg rate constants derived from intensity measurements under condrtions of continuous illumination differ markedly from those obtained from decay time measurements, as is the case for the N,N’-drphenylamine!CC14

system [5] _

This 1s to be expected both on the grounds of diffusion-controlled quenching, that is, a time-dependent value of kQ [2 1 _X!] and when sratrc quenching due to ground-state interactions 1s of importance. as discussed above. Smce in the present case all decay times couid reasonably be fitted to a smgle exponential decay. diffusional kinetics cannot be of importance. and the main

source of this discrepancy must be static quenching_ Static quenching results from ground-state complex formation, thus m this case scheme 1 suffices From (14) we expect plots of (~p~/Q~)/(rp~/~p) to be linear with [Q] _Appropnate data from our work here, and of DPA in methanol from the literature are given in table 3 and results are shown in fig. 2. In all cases a lmear correlation is absent. wirh the fraction approaching an asymptotic value at high quencher concentrations_ Evidently an alternative model is required_ Deviations from lineanty have been observed prevrously [22], but these have been in the positrve sense. and are explicable in terms of quenching of the complex by the quencher, or “exterplex” fomration. In

RC. Alford et al /Fluorescence querrchng of tertiary amhes by halocarbom

108

A TEA/

PFMCH

in cyclohcxanc

111 R.H.

D

DPA/CCl,

!n hexone

l

DPA/CCl4

in methanol

5

3

[o]/icFx

mole

dK3

Fig. 2. Plots of (@F(J/@F)/(TF&f-) PTMCH in cyclohexane

and methanol

against [Q] for TEA/ (thrs work) and DPA/CCI4 m hexane

[5] .

the present case the deviations from linearity are negative, and the present results are supported by those from the literature. The functron (j) plotted in fig. 2 must therefore have the functional form

f = 0 +A [QIYU +WQl), where A

(17)

The so-called

leadmg subsequent of Analysis this yields for of 56,0.64 032 TEA cyclohexane. in and in which reasonable. equihbrium for complex defrom treatment are much than obtained absorption [3, and work necessary verify premechanism statrc in systems.

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[18]

[19] 1201 [21]

The of Suter C. thrrd project in some the is knowledged. from Science EngineerResearch and Royal for ment mamtenance also acknowledged.

[22] [ 231

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109

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