Conductivity

The conductivities of HMMCs have been measured in two ways. Polycrystalline pellets ®®'®® or single crystals (four-point analysis) have been analysed.®^ Platinum Iinear-chain complexes are semiconductors, typical of Class II mixed-valence species. Conductivity along the chain, a//, is generally two to three orders of magnitude greater than that perpendicular to it, aj_. a// follows similar trends to the IVCT edge energy regarding change of halogen or metal.®® Application of pressures in excess of 100 kbar has been shown to increase chain conductivity by as much as 10®, but ct// tends to a limit so that the chains do not reach a metallic state.®® Doping halogen molecules into crystals can enhance conductivity along the chain, but it has far more effect on g± , s o much so that it becomes greater than ct//.®^ The drift mobilities for hole

and electron polarons have shown that the halogen acts as an inter-chain bridge for holes alone.®^

1.5.3 Luminescence

The self-trapped exciton (STX) described in section 1.4 is luminescent. A polarised single-crystal study on the emission band it produces (L band) was carried out first on Wolff ram's Red.^ Luminescence is z-polarised and is known to result from a localised interaction because the shape of the observed peak is Gaussian. The energy of the L band is only about 50 % of that of the IVCT edge, but it has similar metal and halogen dependencies.®® The intensity of the L band falls away with increase in pressure,®® because the energy required for the exciton to separate, and thereby relax by some other route, is reduced. The energy of the exciting radiation is known to alter both peak position and intensity.^® ®® As the excitation wavelength is shortened, the L band gains intensity and moves to higher energy, tending to a limit in both cases. Time-resolved studies have shown the lifetime of the luminescent state to be of the order of 100 ps for PtCI complexes.®^ ®^ Because this is significantly shorter than that calculated for a free (untrapped) exciton, non-radiative processes must determine the rate of decay of the excited state. There is a separate, broad unstructured emission (the B band, unrelated to the B absorption) which occurs at higher energy than the L band, and has an associated lifetime of less than 7 ps.®^ This band is produced as a result of the recombination of electron and hole as the excited free state relaxes to the STX.

1.5.4 Resonance Raman spectroscopy

In luminescence experiments there are discrete emission peaks in addition to the L and B bands that occur at energies approaching that of the exciting radiation.®^ When the energy of this radiation is greater than E c t . these peaks form a progression of evenly spaced

signals whose intensity increases towards the exciting line. The lifetime of the excited state that causes them is less than 7 ps, i.e. much shorter than that for the STX.®^ Resonance Raman (rR) theory indicates that they arise from decay of the excited state to various vibrational levels of the electronic ground state.®® Strong enhancement is predicted for totally symmetric modes within the threshold of the rR condition. The high energy emission lines are z-polarised, so it is generally accepted that the periodic signals seen in HMMC complexes are due to the fundamental breathing mode (vi) and its overtones.^'*

HMMCs have been studied by resonance Raman techniques In their own right for several years/^ As for other analytical methods, the single crystal has become the preferred state for the analysed sample Instead of a polycrystalline substrate. The improvement In resolution that came with single-crystal analysis (and the use of chilled samples) enabled the true structure of the vi(RCI) mode to be revealed (see section 1.5.5).^ vi Is dependent on the exciting line, v q, when in resonance, both in Its structure and in its wavenumber.®®-®® The dispersion of vi over a given range of excitation energy Increases in the order Cl < Br < I. Excitation profiles have been measured for disc samples by determining the intensity of the vi mode against that of an intemal standard,®®"^ but attempts to correlate them with electronic spectra of powders have failed because of the erroneous assignment of the IVCT band In the latter.®®'®^ Overtone progressions In the spectra of pellets extend further and are more Intense than in those of single crystals. Therefore even comparisons of vi values between complexes containing the same metal and halogen are not particularly useful. The effect of changing the bridging halogen or the metal is even more complicated, because the dynamics of the chain will be affected primarily by the alteration of masses and related force constants (see section 1.6.5). Characteristic frequency ranges can be determined for particular metal-halogen combinations, but no relation to delocalisation can be d e d u c e d . T h e value of vi is affected by temperature, but not pressure, is shifted to larger wavenumber by increasing the temperature,^®® while a hydrostatic pressure study has found little change in vi up to 3 GPa.^® Where a phase boundary is crossed, there is often a discontinuity in the v-i value.®^"®®

The electronic spectra of HMMCs contain several peaks besides the P absorption edge. The bands A-D are distinct from this edge and provide a separate range of excitation frequencies through which llnear-chains show resonant behaviour. For example, there are three peaks In [Pt(en)2][Pt(en)2Cl2](CI0 4 ) 4 which are enhanced by exciting lines In this range

(see Figure 1.5.2).^®^'^®® They have been assigned provisionally to the localised vibrations of three polaron defects: electron polarons (e‘ or p ) at 263 cm'^, electron bipolarons (e^* or p^") at 272 cm'^ and hole polarons (h* or p^ at 287 cm'^. Similar defects have been observed in [Pt(en)2][Pt(en)2Br2](CI0 4)4, although only electron polarons and bipolarons have been

1 8 2 cm'^ h a v e b een su g gested fo r th e hole polaron m o d e . R e s o n a n c e R a m a n s tu d ie s h a v e show n th a t d e fe c ts are m o re co m m o n in o rth orho m b ic crystals o f [P t(e n )2][P t(e n )2B r2]( C I0 4 ) 4

th an th e y a re in th e m o n o c lin ic ones.^°^ A b so rp tion 2 6 3 cm'^ 2 7 2 cm'^ 2 8 7 cm*"' B " M ' 'y ' " ' / r \_{vAV V ' V ~} , XAvt- V A ^ E n e rg y (e V ) 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3

Figure 1.5.2 Representation of the enhancement ranges o f the three defect modes compared with bands observed in the absorption spectra.101-103

P h oto lysis g e n e ra lly in c re a s e s th e intensity of d e fe c t m o d es, alth ou g h if th e re is a high c o n ce n tra tio n o f electro n polarons th ey m a y co m b in e to fo rm b ip o laro ns. P h o to ly s e d d efe c ts are not p e rm a n e n t and can be re m o ve d by a n n e a l i n g . T h e m o d e ls o utlined in s e ctio n 1 .6 h a v e b een used to p red ict th e positions of the polaron sig nals fo r H M M C s p e c ie s , w ith v a ria b le su ccess. T h e re a re o th e r w e a k m o d es th at o ccu r natu rally in H M M C re s o n a n c e R a m a n sp ectra b esid es polaron sig nals, and assig nm en t o f th e m has raised co n ce rn s o v e r th e purity o f H M M C sampl es. ^®’" ' T h i s is b e c a u s e th e positions o f th e w e a k sig nals m a tc h s o m e o f th e m a n y b an ds found in th e re s o n a n c e R a m a n spectra o f m ix e d -h a lid e c o m p o u n d s .'^ In te res t in th e m ix e d -h a lid e c o m p le x e s has grown from th e b e lie f th a t th ey exist a s block co p o ly m e rs , with fe w units o f [B rP t'^ C I] in th e c h a i n . P h o t o a b s o r p t i o n is e x p e c te d to result in ch arg e s e p a ra tio n across th e p h a s e boundary b etw ee n h alid e s e g m en ts.^ ^ ’^® E lec tro n p olaro n s are pred icted to be lo c ate d p refe re n tially in th e s e g m e n t co n tainin g th e h a lid e w ith g re a te r e le c tro n e g a tiv ity , with h ole polarons in the less e le c tro n e g a tiv e section.^-^®-^®

1.5.5 Vibrational spectroscopy

B e c a u s e th e re a re so m a n y p eaks in s o m e H M M C e le c tro n ic s p e c tra , it is a q u e stio n of s e m a n tic s w h e th e r R a m a n spectra are co n sidered to be re s o n a n c e o r not. F o r a d e fe c t fre e ch ain , e x citatio n e n e rg y s m a lle r than Ec t will result in n orm al R a m a n sc atte rin g fo r th e vi

mode. The nature of the splitting in vi has been investigated both by Raman and infrared 1 0 9,1 1 1 ,1 1 2 spectroscopies, and is considered in more detail in section 1.6.5, Ligand

modes exhibit normal scattering, and have been probed at low temperature. [Pt(en)2][R(en)2Cl2](CI0 4 ) 4 has been analysed around its phase transition tem perature,^with

a large change in the v ( C - H ) modes observed between orthorhombic and monoclinic phases; little alteration is seen in v ( M - N ) or ô ( N - C - C - N ) . This implies that phase transition mainly involves a modification of the methylene-perchiorate interactions. The hydrogen-bonding of the H - ( N ) atoms in certain HMMCs has been examined by infrared spect r oscopy. ^The separation of v ( N - H ) - P t “ and v ( N - H ) - P t ' ^ is found to correlate well with p . The 0 band can be observed in the near infrared range at around 3500 Reflectivity experiments in the far infrared have revealed that in addition to the stretching modes vi, V2 (the asymmetric

halogen stretch) and V3 (the anti-phase motion of M'^ % 2 units against the m" centres), HMMCs

exhibit chain-bending modes.®^-^'^^^'^^®