Primary Photophysical Processes 5 toreduce absorption by minimum.Transmission curves for the e mo important vents n in [2, o The UV transmission of solvents depends critically upon the solvent purity. For tha reason,some manufacturers supply solvents specially purified for UV spec- troscopy [5,6]. 2.2 Primary Photophysical Processes On the basis of Eq.(3),the energy states of a molecule are summarized in an energy-level diagram. A gene eral energy diagram of electronic states used to explain the primary photophysical processes is shown in Fig.2.without taking the vibrational states into acc ount The indiv ridual levels co espond to the different energies of electrons in singlet and triplet sta ates.Of all the transitions sho Flu standard abs ves only t ition orescence and phosphorescence an respectively Transitions XI,XII and XIII are non-ra iative tr ansitions which are known as internal conversions.Transitions XIV and XVII are intersystem crossings.Transition II (T-So)represents singlet-triplet absorption and being an intercombination transition is spin-forbidden.Thus,it occurs with a very low intensity and special methods of measurement are required to ob- serve this transition [7].Transitions III and IV are two-photon transitions Radiative transitions Fig.2.General energy-level diagram for electronic excitation
Primary Photophysical Processes 5 to reduce absorption by atmospheric oxygen to a minimum. Transmission curves for the most important solvents are shown in [2], volume 5. The UV transmission of solvents depends critically upon the solvent purity. For that reason, some manufacturers supply solvents specially purified for UV spectroscopy [5, 6]. On the basis of Eq. (3), the energy states of a molecule are summarized in an energy-level diagram. A general energy diagram of electronic states used to explain the primary photophysical processes is shown in Fig. 2, without taking the vibrational states into account. The individual levels correspond to the different energies of electrons in singlet and triplet states. Of all the transitions shown, standard absorption spectroscopy involves only transition I. Fluorescence and phosphorescence arise from transitions V and VI respectively. Transitions XI, XII and XIII are non-radiative transitions which are known as internal conversions. Transitions XIV and XVII are intersystem crossings. Transition II (T 1 ~So) represents singlet-triplet absorption and being an intercombination transition is spin-forbidden. Thus, it occurs with a very low intensity and special methods of measurement are required to observe this transition [7]. Transitions III and IV are two-photon transitions i i I I I 5, it! I I I I I I I I I I I VXlllW i I I I I ~ ~ I I -. : XII TIIlX i m I ~:- l I \ \XN I , I ., I \ I , I " I JJW? i ; ; ; ; I ! ~ ~ T, S = Singlet states T = Triplet states Radiative transitions } Non-radiative transitions { Internal conversion intersystem crossing Fig. 2. General energy-level diagram for electronic excitation
6 Principles lonization lim cm 50L50 4 2 70 loge Fig.3.Singlet-singlet transitions and their assignment to the absorption spectrum where T or S,must be excited primarily by the first photon.The process of resonance fluorescence is represented by VII and in practice it can only be observed in gases under reduced pressure.Birks has given an account of the primary photophysical processes [8]. Singlet-singlet transitions have been assigned to the measured absorption sctruminFigtourate that absorption rodtoqte specific ener states,i.e.excitation energies.Furthermore,this figure ora fact that,tothe n maximum,the extinction coefficient,is also very significant when interpreting spectra 2.3 Vibrational Structure of Electronic Spectra The enerey level diagrams in figs.2 and 3 do not take into consideration bra ional states are superimposed on the elec. tronic states.Int ase of molecules having the dir nensions with which we are concerned tes car cause the surrounding solve nt molecule strongly hindero Consequently,the observed structure is causec by a superposi ition of vibrational states only.Figure 4 shows an energy-level diagram which for
6 Principles a Lu 57 56 55 54 5J 125 51 Ionization limit ~- - I +-t-+-c.- V4 I +-t-+-I-r-~ -"-j I I ++-+-rr.- ~V2~+iT:r: - III I III I 111 I II I +-t-+-f-++-,- I I I I I II I III I III I III I III I III I III I £51 E4 I I EJI E2 III I III I I" , - lOgE 50 '10\m-l 45 40 35 'i> 30 25 Fig.3. Singlet-singlet transitions and their assignment to the absorption spectrum where T 1 or SI must be excited primarily by the first photon. The process of resonance fluorescence is represented by VII and in practice it can only be observed in gases under reduced pressure. Birks has given an account of the primary photophysical processes [8]. Singlet-singlet transitions have been assigned to the measured absorption spectrum in Fig. 3 to illustrate that absorption maxima correspond to quite specific energy states, i.e. excitation energies. Furthermore, this figure demonstrates the important fact that, in addition to the position of the absorption maximum, the extinction coefficient, e, is also very significant when interpreting spectra. :' ". ., < '; •• ' - ., - t· . '. : .'. . . -, - . \ I ~:- ';', . :-: I. :'4,·' -. ._ . ". \.~ . , ' . I~~_.~_: .:-. :'~'~.'" '. .~_.~ ~,' . r . 't~-~ . ; . ~.,.~·· . lr.!.~::~.·:;~;.~~,~~.:.:!';I~ ;., The energy level diagrams in Figs. 2 and 3 do not take into consideration the fact that vibrational and rotational states are superimposed on the electronic states. In the case of molecules having the dimensions with which we are concerned here, rotational states can no longer be resolved because the surrounding solvent molecules strongly hinder rotation in solution. Consequently, the observed structure is caused by a superposition of vibrational states only. Figure 4 shows an energy-level diagram which for
Vibrational Structure of Electronic Spectra 7 +一节incm Fig.4.Energy-level diagram including the superposition of one vibrational progression simplification illustrates only the superposition of one vibration,i.e.vibra tional progression,in each of the ground and excited states.By reference to the absorption,fluorescence and phosphorescence spectra shown in this energy-level diagram,the characteristics of such spectra can be illustrated: a)The vibrational quanta of the excited state can be observed in the absorp- beanppbanaepeh tion spectrum and,in contrast,those of the ground state in the fluores- 。之onsa of the triplet state T,the phosphores- cence spectrum ngly toward the red so that the fluorescence and phosphorescence spectra are normally clearly separated (for examples see 9)). which have the same geometry in the ground and only rarely,and the maxima in absorption and emission ar e y i placed toward higher vibrational transitions,i.e.the0-0 transition is no lon ger the most intense.The Franck-Condon principle explains this behaviour [10,11,12)
Vibrational Structure of Electronic Spectra 7 y' 5 4 ~ 1 S, 0 So -vincm-' Fig. 4. Energy-level diagram including the superposition of one vibrational progression T, v" 5 4 ~ d simplification illustrates only the superposition of one vibration, i.e. vibrational progression, in each of the ground and excited states. By reference to the absorption, fluorescence and phosphorescence spectra shown in this energy-level diagram, the characteristics of such spectra can be illustrated: a) The vibrational quanta of the excited state can be observed in the absorption spectrum and, in contrast, those of the ground state in the fluorescence and phosphorescence spectra. b) Frequently, the fluorescence spectrum is approximately a mirror image of the absorption spectrum (for examples see [9]). c) On account of the low energy of the triplet state, Th the phosphorescence spectrum is displaced strongly toward the red so that the fluorescence and phosphorescence spectra are normally clearly separated (for examples see [9]). The spectra shown schematically in Fig. 4 illustrate the case of molecules which have the same geometry in the ground and excited states. This occurs only rarely, and the maxima in absorption and emission are usually displaced toward higher vibrational transitions, i.e. the 0-0 transition is no longer the most intense. The Franck-Condon principle explains this behaviour [10, 11, 12]
8 Principles 2.4 Electronic Spectra and Molecular Structure The discrete molecular states assigned theoretically to the electronic states are shown in the energv-level diagram.the elect ronic states der nd ver critically upon the number of electrons in a molecule as well as on the and the of that stru ture or ge mm 、1a y,c ra are able aid to ectro de igenfur ed round states he ion rules and thus the intensities of elec ons.The correlation between theory and experiment may be expressed by the oscillator strength"f"which may be calculated theoretical- ly and can also be established experimentally from 8=f(v)using Eq.(5) (see particularly Chapter 8). p-2303mc2 Nn e()dy (5) Here m=mass of an electron;c=velocity of light;e=electronic charge; N=Loschmidt number;n=refractive index of the solvent. The integral represents the "integrated intensity"which can be simply determined experi mentally.It can alsobe approximated by dei向d=6andn ⑥ fk-8amc±G1M产 3.he2 () is the statistical weight which equals 1 for a pure electronic transition, Mik is the transition dipole moment which can be calculated theoretically. The transition dipole moment determines the intensity of transition This mor and is compos d of three compon n the Carte sian coord system.Consequently,for many plar ar molecules,the com- iospresent hih is of great interet in m o the mol lane is missing and an anisotropy of elec- summary,it can be said that electronic spectra supply the following infor- mation:
8 Principles The discrete molecular states assigned theoretically to the electronic states are shown in the energy-level diagram. The electronic states depend very critically upon the number of electrons in a molecule as well as on the structure or geometry and the symmetry of that molecule. Consequently, electronic spectra are an extremely valuable aid to structure determination. The molecular eigenfunctions of the ground state and the different excited states also determine the selection rules and thus the intensities of electronic transitions. The correlation between theory and experiment may be expressed by the oscillator strength "f" which may be calculated theoretically and can also be established experimentally from e = f(v) using Eq. (5) (see particularly Chapter 8). f = 2303'm'c2 J (-)d- exp eV v. ;rr'e'NLn Band (5) Here m = mass of an electron; c = velocity of light; e = electronic charge; NL = Loschmidt number; n = refractive index of the solvent. The integral represents the "integrated intensity" which can be simply determined experimentally. It can also be approximated by the expression J e(v)d v '"" emaxLl V1l2 Band LlV1l2 is the width of the band at half its maximum intensity (fwhh). Equation (7) gives the theoretical expression for fl,k: vl,k is the wavenumber of the 0-0 transition (1-+ k), (6) (7) G is the statistical weight which equals 1 for a pure electronic transition, MI, k is the transition dipole moment which can be calculated theoretically. The transition dipole moment determines the intensity of a transition. This moment is a vector and is composed of three components in the Cartesian coordinate system. Consequently, for many planar molecules, the component vertical to the molecular plane is missing and an anisotropy of electronic excitation is present which is of great interest in molecular theory. In summary, it can be said that electronic spectra supply the following information:
References 9 1.Absorption maximax which correspond to the discrete molecular states which are strongly dependent on the molecular structure,geometry and symmetry. 2.Extinction coefficients emax,or the integral absorption over an absorp tion band,which give the magnitude of the transition dipole moment and are also dependent on geometry and symmetry. 3.The structure within an absorption band or within the fluorescence or phosphorescence spectrum supplies information about normal vibra- 4.The a y of light absorption or emission gives information about orie of the electr nic transitions and is very susceptible to changes of geometry and symmetry. Electronic excitati pet and visible reionscn ply extremely val about lar structure (see Sect.4.3). References 1.Ko um G(1962)Kolorimetrie,Photometrie und Spektrometrie,Kap1.5,4.Aufl.Springer )()DMS-UVAas,Vol 3.Kortuim G(1936)Z Physik Chemie (B)33:243 4.Helima-Koverten./Ba den,Kata opie Merck,Darmstadt 6.Baker Analyzed Reagenz fur die UV-Spektroskopie,Katalog 780,Baker-Chemikalien, 7.Gro6. Ger (1954)I Chem Phys 22-255:MeGlynn SP.Azumi T n Rev 58:1113;Evans DF: Chem Soc 1957:1351;195 :2753;Robinson GW 8. Molecular Pho Wiley.Lo ndon Nev ap 10.Ja Spectroscopy.Wiley 11.Murrell JN(1963)The Theory of the Electronic Spectra of Organic Molecules.Methuen, 12 Rs(1969)T
References 9 1. Absorption maxima vmax which correspond to the discrete molecular states which are strongly dependent on the molecular structure, geometry and symmetry. 2. Extinction coefficients Bmax, or the integral absorption over an absorption band, which give the magnitude of the transition dipole moment and are also dependent on geometry and symmetry. 3. The structure within an absorption band or within the fluorescence or phosphorescence spectrum supplies information about normal vibrations coupled to the electronic excitation. 4. The anisotropy of light absorption or emission gives information about the orientation of the electronic transitions and is very susceptible to changes of molecular geometry and symmetry. Electronic excitation spectra in the UV and visible regions can also supply extremely valuable information about molecular structure (see Sect. 4.3). References 1. Kortum G (1962) Kolorimetrie, Photometrie und Spektrometrie, Kap 1.5, 4. Aufl. Springer Berlin GOttingen Heidelberg, S 21 ff 2. Perkampus H-H, Sandemann I, Timmons CJ (Hrsg) (1966-1971) DMS-UV-Atlas, Vol I-V. Verlag Chemie, Butterworth, London Weinheim 3. Kortum G (1936) Z Physik Chemie (B) 33:243 4. Hellma-Kuvetten, Mfilheim/Baden, Katalog 67/32 u. 76/34 5. Uvasole, LOsungsmittel und Substanzen fUr die Spektroskopie. Merck, Darmstadt 6. Baker Analyzed Reagenz fUr die UV-Spektroskopie, Katalog 780, Baker-Chemikalien, GroB-Gerau 7. McClure DS, Blake NW, Hanst PL (1954) J Chern Phys 22:255; McGlynn SP, Azumi T, Hasha M (1964) J Chern Phys 40:507; McGlynn SP (1958) Chern Rev 58:1113; Evans DF: J Chern Soc 1957:1351; 1959:2753; Robinson GW (1961) J Mol Spectrosc 6:58 8. Birks JB (1973) In: Organic Molecular Photophysics, Chap 1. Vol 1. Wiley, London New York Sidney Thronto, p 1 ff 9. Perkampus H-H, Vollbrecht HR (1971) Spectrochim Acta Part A 27a:2173 10. Jaffe HH, Orchin M (1962) Theory and Applications of Ultraviolet Spectroscopy. Wiley, New York London 11. Murrell IN (1963) The Theory of the Electronic Spectra of Organic Molecules. Methuen, London 12. Becker RS (1969) Theory and Interpretation of Fluorescence and Phosphorescence. Wiley, New York London Sidney Toronto