Indian Journal of ChemistryVol. 20A, September 1981, pp. 861-863

Intermolecular Potentials In Dimer & Excimers of PhenanthreneE. JOy PADMA MALAR

Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012

Received 5 December 1980; accepted 2 February 1981

Interactions in ground dimer, singlet and triplet excimers of phenanthrene have been studied theoretically. Theresults show that phenantbrene can form weak dimer and excimers. The binding energies and the conformations ofthe ground dimer and the triplet excimer are nearly identical. The singlet excimer has relatively large bindingenergy because the exciton resonance stabilisation in the singlet excimer is quite large, The exciton resonance termis also responsible for the conformational difference in the singlet and the triplet excimers.

SINGLET excimer formation is a widespreadphenomenon in a large number of aromatichydrocarbons and many of their derivativesunder a variety of conditions'. However, the existenceof triplet excimers has been established only in a fewcases such as halobenzenes=", naphthalene and someof its derivativesv". Phenanthrene is a typical examp-le of an aromatic hydrocarbon for which contradic-ting views exist regarding the formation of both thesinglet and the triplet excimersv+'.

The aim of the .r.resent work is to establish theoreti-cally the possibility of formation of singlet andtriplet excimers of phenanthrene, their probableconformation and binding energies. The binding andthe conformation of the ground dimer of phenanth-renes have also been studied.

Computational MethodSince the ground dimer and the excimers of phe-

nanthrene are weak dimeric species (in the groundstate and excited state respectively), the interaction'energies have been calculated using the exchange per-turbation approach 15,16. For this the a and n; elec-tron interactions have been treated separately. Thevarious a-electron interaction terms are calculatedusing the standard forrnulae-". The important a-electron terms are electrostatic energy, non-bondedrepulsion energy and a-a-dispersion energy'". Theseterms are common for the ground dimer as well asthe excimers since it is assumed that the a-cores ofthe excimers are the same as those of the grounddimer. Since both the singlet and triplet exci-mer states are assumed to originate from n:-n:*tran-sition-", it is obvious that all the 7t-electron terms ofthe excimers differ from those of the ground dimer.The expressions for the -e-electron interaction termsin the ground dimer are well-known", Quite recentlywe have formulated the interaction terms in both thesinglet and the triplet excimers using the exchangeperturbation theory15·16. The important n-electronterms are n:-n:overlap repulsion, charge-transfer in-teraction, n:-n:dispersion and exciton resonance in-teraction (in excimers). These-terrnshavebeencal-culated using the expressions given in earlierpapers17,lS, In the calculation of the n;-electron terms,

the parameters K = -1 eV and ~ = 1.273have beenused=. In the non-bonded repulsion term, in additionto the C .... Hand H .... H repulsions, the C .... Crepulsions have been incorporated using the expo-nential function B exp (_CR)lMO,

The following geometrical arrangements of thedimers and excimers have been analysed : symmetricsandwich, rotated sandwich, tilted and translatedstructures. These are illustrated in Fig. 1. 0 and 0'denote the centre of mass of the two molecules. Inthe symmetric sandwich structure IX = 8 = 0°, Rota-ted sandwich and tilted structures are obtained byvarying 8 and IX respectively, keeping one moleculefixed. Translated structures are obtained by dis-placing one molecule along its long axis (y-axis) bya distance 6.S. D is defined as the shortest distancebetween the two molecular planes.

Results and DiscussionThe variation of the different energy terms with

intermolecular separation(D) in the ground dimerand the excimers of phenanthrene of symmetricsandwich geometry are illustrated in Figs 2 and 3

z,z~

xFig. 1 - Relative orientation of two phenanthrene moleculesin the different conformations of the dimer and excimers.

D, II and 0 are the structural parameters used.

861

INDIAN J. CHEM., VOL. 20A, SEPTEMBER 1981

10r-~.----'r----r-------------'

\\

\b\ -,

"- toa:UJzUJ

zo;::u..•a:UJI-z

-6'--_-- U(S)

o 20 406(degrees)

Fig. 4 - Variation with 6 of (a) electrostatic, (b) re-te over-lap repulsion, (c) exciton-resonance, (d) non bonded repulsion,(e) charge-transfer, (f) 7t-l't dispersion, (g) a-a dispersionand (U) total interaction energy terms of the singlet(S) andtripletfT) excimers of ophenanthrene for the rotated sandwichstructure at D = 4.0A. U(G) is the total potential curve ofthe ground dimer of phenanthrene for the same structure.

60 80

PADMA MALAR: DIMER & EXCIMERS OF PHENANTHRENE

.."0E

~ -2...~~ .T,D=4.4A

>C>a:•••Z•••z -4o;::u«a:•••...~ -6-'s~

20 40' 600( (degrees)

eo

Fig. 5 - Variation with IX of the total interaction energiesin the ground dimer(G), the singlet(S) and the triplet(T)excimers of phenanthrene at the indicated values of D.

TABLE 1 - EQUIUBRlUM CONFORMATIONSAND THE BINDINGENERGmS OF THE GROUND DrMER AND THE EXCIMERSOF

PHENANfHRENE

System Equilibrium structure Binding energy(kcal/mol)

1.9Ground dimer Rotated sandwichD = 4.3A, 6 80°

IX = 0°

Singletexcimer

Tilted °D = 4.0A, IX = 5°

II = 0°

8.59

Tripletexcimer

2.55Rotated sandwichD = 4.2A, 6 80°

IX = 0°

The results for the tilted structure show that themagnitudes of the repulsive and the attractive termsdecrease gradually with IX, except the exciton re-sonance term of the singlet excimer which experiencesa rapid decrease with IX. The total potentials of thedimer and excimers are presented in Fig. 5. It isnoted that at small values of D, minima appear inthe potential curves. However, at large values of D,the potential energies increase with IX.

The equilibrium conformations and the bindingenergies of the ground dimer, the singlet excimer,and the triplet excimer of phenanthrene are presen-ted in Table 1. The results indicate that phenan-threne can form weak dimer and excimers. It isobserved that both the ground dimer and tripletexcimer have nearly identical structures and binding

energies. We have reached similar conclusion for thedimer and the triplet excimer of naphthalene-". Thishas been experimentally proved very recently in thecase of dinaphthylalkanes by Lim21• This agreementindicates that the present calculations though semi-empirical in nature, are reliable. Our results predictthat the singlet excimer of phenanthrene has tiltedstructure with IX,= 5". This structure is very closeto the symmetric sandwich structure. The structuraldifference between the singlet and triplet excimersarises mainly from the exciton resonance term.Bous-Laurent et al.14 observed that the photodimersof phenanthrene derivatives have structures whichare obtained from the symmetric sandwich geometryof the dimer pair and the singlet excimer is the inter-mediate in the process. The present results suggestthat since the singlet excimer has nearly sandwichstructure, singlet excimer is a precursor for the photo-dimerisation process. The involvement of the tripletexcimer as intermediate is ruled out since it doesnot have the requisite geometry.

AcknowledgementThe author thanks Prof. A. K. Chandra for his

keen interest in this work and encouragement and the,Computer Centre, Indian Institute of Science,Bangalore for providing facilities of DEC 1090Computer.

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