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STUDXES OF- EXCITON INTERACTIONS IN MOI,ECUx,AR AGGREGATES LEON PAUL GIANNESCHI, B Sc (Hons ) Department of Physical and Inorganic Chemistry, UniversÍty of Adelde Thesis presented for the degree of Doctor of'Phllosophy January, 1977 To El-izabeth and Ben Surnrnary Exciton effects were studÍed in three types of molecular aggregate These studíes involved a predominantly theoretical analysis of the exciÈon migration phenomenon in molecular dirners and experimental investigations of exciton transfer effects in míxed molecular crystals and dye-polymer complexes The adÍabatic approximation was applied Èo Èhe problem of vibronic exciton interactions in dimers It was determined that a weak inter- action, arísing from the nuclear kínetic energy, exists and excited st.ates within each molecule ín the between the ground climer and that, índirectly, this interaction introduces an additÍonal signífícant coupling excited states of the dimer inducedtt 'effects model wa's of dimers between the The problem whích íncludes such I'kinetically formulated and solved for the usual tight-binding exciton The results of this analysís were used to calculate theoretical absorption spectra which srere crude adíabatic exciton theory, compared with those from a previous An experimenËal verifÍcat.ion of the theory developed in this work was carrÍed out by the study of the dimeric species spectra of three xanthene dyes; Pyronine Y, Pyronine B Rhodamine B and In particular, the Pyronine Y sysÈem was the subject of the applícation of an experÍmental planning technique knorrn as prediction analysis which r¿as aimed at optímizing the experåmental conditions in order to reduce the errors in determining the dineric specÈra species of the various dimeric species were fitted to, theoretical spectra calculated from the adiabatíe theoríes The existence of the weak spectra The and compared with, and crude adiabatic kinetically induced interactíon predicted by the adíabatic theory was confirmed and was lnterpreted as an excitorì-nuclear momentum interaction The study of mixed molecular crystals r,¡as aímed at ínvestigating the extenÈ to r,rhich the propertie" of an" guest could be interpreted in terms of variatíons in the orientation of the guesÈ transítíon moment The systems 1- and 2.- amínoanthracene as guests ín ànthracene as the host provide excellent examples as each of these appear to produce orientations of the guest crysÈal1ographically distinct tt'ro This üras con- firrned by the measurements of the polarized guest absorption as a functlon of the angle of incidence at very low temperaÈures in these mÍxed rnolecular crystals transition of the By the application of classícal optics, effective moment oríentations for the guests r¡rere calculated, measurements system tetracene i-n anthracene providing a test of the method used Classical local field correctíons ürere carried ouÈ and appeared to be only slightly anisotropic The results of computer símulations crystal packing were used to assign specífic measured transiÈÍon moment the transition províded directions moments were guesË of the orientatíons to the In this way, the dírections of fixed with respect to the molecular axes and additional, ímportant verificaEions of the crystallographic in- equlvalence of the observed guest transitions It was concluded thaË the differences in excitation energy observed for the dífferent orientations of the same guest species could be accounted for purely in terms of the guest- host resonance ínteractíon, although Èhe effects of dÍfferences ln dispersion forces may need to be lncluded as well Preliminary investigatÍons were effeeted in a study aÍmed at examining the presence to similar studies of exciton band structure in polymers, In r,riÈh shallow analogy traps in mixed molecular ciystals, the absorption spectra of dye-polymer complexes lrere measured at low temperatures fihns of the phenothíazine dyes, Methyrene Blue and rhÍonine, in poly (vinyl alcohol) (PvA) and atactic sodíum poly (styrenesulphonate) Mixed (NaPSS) were ature studied at líquid nitrogen temperâtures and at room temper- The resulEs of this work indícated that spectral shifÈs caused by solvenÈ effects would tend to out-weigh expected excíton resonance shífts, As well, the spectral region of interest ín the proximity of the host absorption band was not resolved at low temperatures so that deconvolutíon techníque was employed, Differenáes were observed between dyes bound a PVA ín absorption intensiÈy the free dye Èransítions and transítions for the to NaPSS Thís effect matríx" a However, Èhe than trould be expecËed rdas not observed for dyes held r¿ithin intensity differences observed are larger for exciton ínteractions in an atactíc polymer as NaPSS, Although inconclusive, the such results of this study have índicated the linits faced Ín attemptíng an investigatíon of this type for polymeric aggregates, To the best of my knowledge and no material- prevíously published belíef, this thesis contains or wriËten by another person, nor any materíal previousl-y submitted for a degree or diploma at any universíty, except were due reference ís made ín the text L;P, Giẳneschi Aaknowledgements I wish to express my appreciation to Dr T Kurucsev for his encouragement and asÈute guidance throughout the duraÈion of this work Èty thanks also are extended to Dr N.J Brídge who helped make my stay aÈ the University of Kent at Canterbury, where the crystal studies v/ere cai'ried out, both fruitful and enjoyable, I wish to thank the various members of the technical staff who helped in the construction of the apparatus required as well as my fellow students for their contínuing support and fríendship I also thank !lrs J Schillíng for the typing of this thesis and the Advisory CenÈre for University Education for the photo-reduction of some of the diagrams and tables A Commonwealth Postgraduate Research Award ís also acknowledged L.P, Gianneschi IÞpartment of Physical and Inorganic Chemistry, Ttre UnÍversíty of Adelaide, South Australía January, 1977 Contenüs CHAPTER T INTRODUCTION THE EXCITON o TOPICS IN THE STI]DY OF EXCITON 2.L ExcÍton Migration Effects 2.1.L The Tnterpretation 2,2 Excíton Transfer Effects 2.2.L The Guest-Host Interaction in MoLecuLar CrgstaTs 2.2.2 2.3 of the Spectra of CHAPTER TT Dimers The Guest-Host Interactíon as an ExcÍton Probe in PoTgmers Structure of t.he Thesis 10 11 13 REFERENCES EFFECTS GENERAL EXPTRII"IEIITAL DETAILS CRYOSTATS 18 3.1 Liqd Helium Cryostat 18 3.2 Liquid NÍtrogen Cryostat 2r 26 SPECTROPHOTOMETRY 4.L Hilger D460 26 4.2 Zíess PlfqII 27 4,3 Suuuuary of Experimental Errors 28 30 MATERIAIS 5.1 Puriflcation of Materíals 30 5.1.1 I'Iaterials for Crgstal Studies 30 5,L.2 t¡lateriaTs 5.1.3 traterials fot for Dimer Studjes 33 PoTgmer Studjes 36 5.2 Preparatíon of Samples 38 5.2.L Mixed CrgsÈals 38 5.2.2 Dge SoTutions 40, 5.2.3 Dge-PoTgmer and Polgmer Fil-ms 40 42 REFERENCES CHAPTER TTT THE DiMERIC AGGREGATE AN ADIAB'ATIC TREATMENT OF EXCITON INTERACTIONS IN DIMERS 6.1 Treatment for: Each Moiety 6.1.1 The Molecular HamíTtonian in CoupJed Form 47, 6.I.2 The MoTecular Hamil-tonian ìn Decoupled Form 50 6.2 TreatmenÈ 6.2.I Exciton CoupTing between Dimeric States 46 for the DÍmer 6.2.2 KineticaTTg Inducd s4 CoupTing between Dimeric States 6.2.3 SoTution of the CoupTed EigenvaTue Equatiof for the Dimer 44 for Absorptíon 6.3 The TransiÈion Moments 6.3.1 The MoLecuLar 6.3.2 6.4 The Dimeric Transìtion Moments TransitÍon Inloments Summary EXPERIMENTAL STUDY OF THE DIMERIC SPECTRI]M 7.I The ApplÍcation of Prediction Analysis 55 58, 59 64 65 69 7L 73 76 7.1.1 Optimization of Experimental ConditÍons 78 7.L.2 Ðetermination of Species Spectra 81 7.2 Computation 7.2.L The l[ononer Spectrum 86 7.2.2 The Dimer specËrum 88 of Species Spectra 86 COMPARISONS 89 OF THEORY AND EXPERIMENT 8.1 Theoretical Comparisons 89 8.2 Experimental Comparisons 9L 94 CONCLUSIONS 9.L Precliction Analysís 94 9.2 Exciton Interactíons in Dímers 94, 96 REFERENCES CHAPTER 10 TV THE CRYSTALLINE AGGREGATE THE CLASSICAL THEORY OF TRANSMISSION AND ABSORPTION LIGHT IN 99, of Anisotropíc 10.1 The Optics IO.2 Doped CrysÈal Absorption IO.2.l optical Data for LO,2.2 11 T2, 13 OF CRYSTALS The LocaL Media Anthracene Efectric FieTd 100 103 114 115 Ð(DERIMENTAI STUDY L20, 11.1 Materials 120 LI,2 Spectrophotometry t24 11,3 Calculatíon of Ancíllary Data 126 11.3.L opticaT Data L26 11,3.2 L29 ooped Crgstal Parameters RESULTS AND DISCUSSION OF TRANSITION MOI4ENI ORIENTATIONS L32 L2.L Theoretlcal Calculations 136 L2.2 Experimental Calculations and Results 137 CONCLT]SIONS 146 REFERENCES 148,