Part I

PHYSICAL ASPECTS OF PHOTOCHEMISTRY

1

Photophysical Processes in Condensed Phases

BY R. B. CUNDALL AND M. W. JONES

1 Introduction

The pattern of research in the area under review in this Chapter is much the same as in recent years, although there is an emphasis on detailed fluorescence data and their interpretation. The increase in exploitation of fluorescence spectroscopy in biochemical studies is striking and we have not perhaps treated this aspect as comprehensively as the quality of the work merits.

Some of the basic problems have still not been adequately clarified and effort is still being directed towards these.

2 Excited Singlet-state Processes

The dependence of the electronic transition probabilities on the refractive index of the medium has been reviewed and verified experimentally by a comparison of the gas phase and solution phase fluorescence from 9-methylanthracene. Where the absorption is essentially independent of the medium, the radiative rate constant does depend on n2. The spectrum of the secondary radiation emitted from excitonic molecules under resonant two-photon excitation has been investigated theoretically. Both polarization and angular dependence of emission are derived, reflecting directly the relaxation stage of the molecules. The photochemical consequences of exciting molecules containing two or more functional groups have been considered. Observations are reviewed for three interesting bichromophore systems namely, aryl-olefins, keto- and benzoyl-olefins, and aryl-esters. An absorptivity related constant and a quantum efficiency related constant have been derived by a simple graphical method from data obtained in a standard fluorimeter.' Absolute values of absorptivity and quantum efficiency can be obtained by comparison with a fluorophore of known absorptivity and quantum efficiency. The method relies on the approximately hyperbolic relationship between fluorescence and fluorophore concentration. Synchronous fluorescence spectroscopy has been applied to the examination of petroleum products and also to forensic science. The paper reviews the experimental variables present in such an analysis and attempts to provide criteria for satisfactory and reproducible collection of data. A quantum chemical investigation of the electric dipole moments in the ground and excited states and also transition moments of 2,2'-p-phenylenebis (5-phenyloxazole) and 2,2'-p-phenylenebis(4-methyl-5-phenyloxaole) have been carried out. Perhaps surprisingly, it is shown that the directions of these quantities do not necessarily coincide.

A review on the role of molecular photochemistry in polymer-growing processes and in phototransformation of polymers has been given. The correlation with photophysical properties is especially noteworthy. The influences of neighbouring groups and of high local concentrations are discussed in relation to polymer transformation and excimer formation.

Fluorescence has been observed from 20 β,γ-unsaturated ketones in cyclohexane solution at room temperature.

Ab initio and CNDO/S calculations support a mechanism of geometrical relaxation through molecular twisting for the appearance of the long-wavelength fluorescence (Fa) observed in some p-substituted dimethylanilines.

A luminescence study of 96 toxic and hazardous materials has recently been presented. The survey contains a brief literature review, estimated detection limits, and a spectral code for preliminary identification of these materials. Furthermore, the high sensitivity of luminescence plus the specificity afforded by excitation and emission spectra provides a simple and inexpensive approach to the identification of hazardous materials.

Direct fluorimetric analysis of the insecticide carbaryl (1-naphthyl-N -methylcarbamate) and its hydrolysis product 1-naphthol in the range 0-l µml-1 have been made. Spectra, quantum efficiencies, and relative sensitivities are reported for this insecticide. The fluorescence and phosphorescence properties of another pesticide 2-isopropoxy phenyl-N-methyl carbamate (1) have been reported in polar and non-polar solvents at room termperature and 77 K.

For highly purified acetophenone, no evidence for any prompt, short-lived fluorescence emission, which can be distinguished from room-temperature phos-phorescence and the solvent blank, is seen at the 10-6 quantum-yield level. The solvent dependence of the lowest triplet state supports the n-π* triplet assignment in polar solvents and a π [right arrow] π* assignment in non-polar solvents. The interaction of ground- and excited-state m-aminoacetophenone (MAAP) with several aliphatic alcohols in non-polar viscous solvents has been investigated by steady-state and nanosecond fluorescence spectroscopy. Exciplex formation of MAAP with several alcohols is demonstrated.

The photophysical primary processes in solutions of p-substituted dialkylanilines has been investigated by measurements of fluorescence and fluorescence-polarization. Variation of the acceptor substituent in these molecules leads to a reversal of the 1Lb, and 1La type excited states. (See Table 1.)

The fluorescence quantum yields and lifetime of styrene at 298 and 77 K for dilute solutions of styrene and three deuteriated isomers have been measured and are presented in Table 2.

The photochemistry of pentahelicene, acetophenone, and several ethoxycarbonyl- and ethoxycarbonyl-methoxy-substituted cycloheptatrienes has also been investigated with some emphasis on the photophysical aspects.

The mechanisms of hydrogen abstraction of aralkyl thiones to give cyclopentyl thiols has been investigated by fluorescence techniques. The results show that hydrogen abstraction occurs through a π,π* state in a manner very similar to that in the alkanone-type II singlet process.

2-Arylnaphtho(1,2-d)thiazole has been identified as the fluorescent product formed when benzaldehyde or p-hydroxybenzaldehyde reacts in dilute sulphuric acid with 2,2'-dithiobis (1-aminonaphthalene) in the presence of tri-n-butylphosphine, phosphate, and sulphate ions at ambient temperature.

Absorption, fluorescence, and luminescence excitation spectra of 2,2'-bipyridyl (bipy), bipy H+, and bipy H2++ and of the complex ion Zn(bipy) have been measured in rigid and fluid media. The authors conclude that the weak fluorescence previously ascribed to bipy or bipy H+ appear to be due to the strongly fluorescent species Zn(bipy)++, which results from the contamination of polar solvents with traces of Zn++.

Proton transfer has been demonstrated for p-aminosalicyclic acid in fluid solutions and in the solid state. Activation parameters for the deprotonation occurring in the excited state has been obtained by following the fluorescence emission as a function of time and temperature.

Emission from S1 [right arrow] So, T1 [right arrow] So and laser single-site singlet excitation (S1 [right arrow] So) spectra for the various insertion sites of coronene in n-heptane at 1.5 K have been presented. The results indicate strongly that the distortion of coronene in n-heptane is primarily crystal-field induced and is not greatly changed upon excitation of the molecule to its lowest excited states.

Double fluorescence of 1,4-diazatriphenylene (1,4-DAT) and 1,4-diazaphen-anthrene (1,4-DAP) has been observed in alcoholic solutions at temperatures 200–100K. Two bands with different lifetimes have been assigned, one being the relaxed strongly hydrogen-bonded structure Y and the other to a 'frozen' structure X.

Delayed fluorescence has been observed for [2.2]paracyclophane both in crystals and in rigid glass solutions at 4.2 K. A mechanism based on dissociation–recombination reactions in the singlet manifold is proposed as a possible mechanism for the delayed fluorescence. Delayed fluorescence (P-type) has also been observed from viscous solutions of 1-naphthyl-N-methylcarbamate and attributed to triplet–triplet annihilation processes.

The photochemistry of diazocompounds and azides in argon and 3-ferrocenyl-1-p-methoxyphenylprop-1-ene have been examined. The fluorescence of 1,3,4-oxadiazole derivatives in n-paraffins has been studied in detail and also the photophysical properties of dimethyl 2,6-naphthalenedicarboxylate have been reported on.

King discusses the use of Hammett σ constants as a more general measure of the excess electron density in benzene and substituted benzenes. In substituted benzenes, fluorescence can be shown to be largely determined by the electron density in the aromatic nucleus.

Fluorescence decay-time measurements of solid benzene at very low temperature have been made and compared with the high-pressure value of benzene vapour. A value of 83.8 ns was obtained and compares favourably with that of the benzene vapour.

Further work has been carried out by Hochstrasser and co-workers on the two-photon excitation spectra of benzene crystals at 4.2 K in the wavelength range 400–530 nm. Evaluation of the two-photon tensors for the eight ungerade vibrational fundamentals v11, v14-v19 in the crystal axis system have been made.

Exciplex formation and diabatic and adiabatic isomerizations of hexamethyl(Dewar benzene) (HMDB) to hexamethylbenzene (HMB) have been studied.

Lifetime measurements of 3,4,9,10-dibenzopyrene, 20-methylcholanthrene, and 2- and 1-aminoanthracene, some carcinogenic compounds, have been obtained as a function of temperature and singlet [right arrow] singlet radiationless-transition rate parameters evaluated.

Absorption and emission spectral properties of several benzyl aromatic esters have been studied in polar and non-polar solvents.

Fluorescence and phosphorescence spectra of some p-substituted benzylidenyl-aniline (BA) molecules and their planar analogues have been obtained in EPA solvent. The large Stokes shifts seen at room temperature are attributed to a rapid relaxation of the Franck–Condon (FC) states in these molecules. In glassy matrices however, the stokes shifts are much reduced; a hypsochromic (blue) shift of the fluorescence is seen and is explained in terms of a decrease in relaxation of the FC state.

Gas-phase and solution spectra of methoxy nitroanilines have been reported. Excited-state dipole moments associated with the first intense electronic transition were evaluated.

Several papers have appeared on various aspects of stilbene photochemistry. Fluorescence quantum yields and the rates for initial photoisomerization for trans-stilbene in several deoxygenated solvents have been obtained as a function of solvent viscosity. (See Table 3.) These results indicate that in high-viscosity solvents such as parah oil and glycerol, Φf approaches a limiting value associated with complete inhibition of molecular twist. Significant isomerization in viscous solvents via the triplet state is also postulated. Further observations of the effect of viscosity on the lifetimes of trans-stilbene have been carried out using a frequency-tunable u.v. picosecond laser source and streak-camera system. Over the viscosity range 1.2–230cp, the lifetime increased from 57 to 152ps. A much longer and weaker contribution was also observed. The effect of temperature on the lifetime of trans-stilbene has also been measured in the range -10 to 30 °C. Two components of emission were detected, one varying between 64 and 125 ps and a much weaker component varying from 690 to 1450ps. The authors postulate that the longer-lived component may indicate a return to the trans-geometry following initial photoisomerization. Of further interest is a study of the orientational-relaxation times of excited-state stilbene using sub-picosecond laser pulses generated by a cavity-dumped passively-mode-locked CW dye laser.

Intersystem crossing in several trans-stilbene singlet exciplexes has been attributed to transfer via oxygen-independent and oxygen-catalysed pathways. Rate constants for oxygen-independent intersystem crossing are in the range 1–5 x 107 s-1. An investigation of the interaction between excited stilbene and several electron-poor alkenes has been carried out.* Quenching by dimethyl fumarate (DF) and fumaronitrile is accompanied by appearance of exciplex fluorescence in non-polar solvents. Quenching of stilbene excimer by DF results in the formation of an oxetane cycloadduct and not the cyclobutane formed from singlet stilbene with DF. Several papers have appeared on the photoisomerization of stilbene. Picosecond time-resolved measurements have been made on the photoisomerization of both trans and cis stilbene. In the first, Greene et al. reports on a thermalized distribution of excited states, which is produced in about 25 ps. These S1 states then twist with a rate constant of 1.1 x 1010 s-1. In the second, Sumitani et al. report on the rate constants for cis–trans photoisomerization of stilbene. The main reaction is shown to occur in the singlet manifold via a perpendicular singlet state with a lifetime shorter than a few picoseconds. Triplet-state involvement in the main isomerization process was ruled out by flash photolysis experiments. Analysis of the intense and sharp Sn [left arrow] S1 absorption of cis- and trans-stilbene indicates that the transition is charge resonant in character and arises in the ethylenic and two phenyl moieties in stilbene.

Time-resolved resonance Raman (TR3) spectroscopy has been used to monitor the photolysis of the dianion of trans-stilbene (T2-) induced by pulsed-laser excitation at 532nm. A short-lived intermediate was identified as the trans-radical anion of stilbene (T-).

The effect of substitution on the S1-T1 energy difference in trans-stilbenes has been investigated and the anisotropic fluorescence of 4'-isocyanato-4'-dimethylamino-trans-stilbene (CDS) in various solvents. A value for r = 0.35 (P = 0.45) was obtained for CDS in cyclohexane and cyclohexanol.

The conformational equilibria in 1,2-diarylethylenes has also been reported on by examination of the absorption and emission spectra and fluorescence lifetimes.

Numerous papers have appeared on various aspects of the photochemistry of anthracene and related compounds.

Time-resolved fluorescence spectroscopy has been applied to the study of non-crystalline layers of anthracene, and this shows the presence of three distinct excimer bands at 21 900 cm-1, 21 500 cm-1, and 19 600 cm-1 the corresponding lifetimes being 10, 35, and 170ns, respectively. Intramolecular anthracene 'dimerization' in 1,3-dianthrylpropane has also been reported. The photodissociation of the 9-methylanthracene photodimer has also been investigated in non-polar solvents between 300 and 77 K by means of steady-state and flash experiments. A detailed reaction scheme is proposed for the photodissociation process at temperatures above and below 200 K. The effect of long u.v. irradiations on the fluorescence emission of anthracene crystals has been studied by examining microphotographs of the crystal surface, and has been correlated with changes in fluorescence-emission profile. Sub-nanosecond time-resolved transient photo-conductivity studies in anthracene have been reported and are associated with photogeneration, polaron formation, and hot-carrier motion that take place within 800ps of the light illumination.