Optics and Spectroscopy, Vol. 90, No. 1, 2001, pp. 67–77. Translated from Optika i Spektroskopiya, Vol. 90, No. 1, 2001, pp. 76–87. Original Russian Text Copyright © 2001 by Knyukshto, Sagun, Shul’ga, Bachilo, Starukhin, Zen’kevich. MOLECULAR SPECTROSCOPYManifestation of Nonplanarity Effects and Charge-Transfer Interactions in Spectral and Kinetic Properties of Triplet States of Sterically Strained Octaethylporphyrins V. N. Knyukshto, E. I. Sagun, A. M. Shul’ga, S. M. Bachilo, D. A. Starukhin, and É. I. Zen’kevich Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, Minsk, 220072 Belarus Received March 30, 2000 Abstract—Properties of the triplet states of octaethylporphyrins with the steric hindrance (free bases and Pd complexes) are studied by the methods of stationary and kinetic spectroscopy in the temperature range from 77 to 293 K. The mono-mesophenyl substitution results in a decrease in the quantum yield and shortening of the phosphorescence lifetime of Pd complexes by 250–3500 times in degassed toluene at 293 K. The phospho- rescence quenching is caused by nonplanar dynamic conformations of the porphyrin macrocycle in the T1 state, which also lead to the appearance of new bands at l ~ 1000 nm in the T—T absorption spectra. As the number of meso-phenyls (Pd-octaetyltetraphenylporphyrin) increases, the quantum yield of phosphorescence further decreases (<10—5) at 293 K, the lifetime of the T1 state shortens (<50 ns), and the efficiency of the singlet oxygen generation abruptly decreases (<0.01). The intense bathochromic emission of this compound at 705 nm with a lifetime of 1 ms at 77 K is assigned to the phosphorescence of a nonplanar conformation. Upon meso-ortho- nitrophenyl substitution, the quenching of phosphorescence of Pd complexes (by more than 104 times at 293 K) is caused by direct nonadiabatic photoinduced electron transfer from the T1 state to the nearest charge-transfer state with the probability = (1.5—4.0) · 106 s—1. The induced absorption of ortho-nitro derivatives in the region between 110 and 1400 nm is caused by mixing of pure pp * states with charge-transfer states. © 2001 MAIK “Nauka/Interperiodica”. ket TINTRODUCTION Recent spectral, kinetic, and physicochemical stud- ies [1–4] showed that a number of “hybrid” porphyrins with the steric hindrance (free bases and complexes with metals Ni, Cu, and Fe) have the nonplanar porphy- rin macrocycle in the initial S0 state, which is mani- fested in a considerable change in the absorption spec- trum (strong bathochromic shift and broadening of the bands) and in redox properties compared to planar tet- rapyrrole macrocycles. Upon excitation of such mole- cules, the dynamic conformational mobility of the tet- rapyrrole macrocycle in the S1 state is manifested in strong deformations of the fluorescence spectra (batho- chromic shift and broadening of the bands, an increase in the Stokes shift) and in a strong decrease in the quan- tum yield j F and shortening of the fluorescence life- time t S. However, at present there is no detailed infor- mation on spectral and kinetic parameters of the T1 states of hybrid porphyrins under the conditions of dynamic conformational lability of the porphyrin mac- rocycle in liquid solutions at 293 K. The data available are scarce and are often indirect. Thus, in experiments on bleaching of the absorption bands of porphyrins with the steric hindrance (saddle and ruffle conforma- tions), the quantum yields of the S1 T1 intersystem 0030-400X/01/9001- $21.00 © 20067crossing were estimated to be g = 0.2–0.7, the lowest values being typical for the ruffle structures [5–9]. In some cases, differential spectra of the induced absorp- tion have been obtained in the region from 600 to 900 nm upon picosecond excitation [5, 8, 9]. Direct experimental data on the decay of the triplet states for two types of nonplanar tetra-substituted porphyrins are presented in the only paper [9] known to us, where it was shown that, along with a decrease in the quantum yield of intersystem crossing, the lifetime t T of the T1 state greatly shortened at room temperature in deoxy- genated solutions and the quantum yield g D of the sin- glet oxygen generation decreased. It was found that, for these compounds, g D < g , whereas, for usual porphyrins, g D @ g [10, 11]. Recently we found [12–14] that the introduction of even one phenyl ring into the meso position of b -alkyl- substituted porphyrins [octaethylporphyrins (OEPs), ethioporphyrin II, and their Zn complexes] results in a drastic shortening of t T at room temperature in liquid solutions and virtually does not affect the spectral and kinetic parameters of the S1 state decay. We showed in these papers that this effect is related to torsion vibra- tions (librations) of the phenyl ring around a single C–C bond in porphyrins with the steric hindrance. As a result, the nonplanar dynamic conformations of the tet-001 MAIK “Nauka/Interperiodica” 68 KNYUKSHTO et al . rapyrrole macrocycle appear in the excited T1 state in which nonradiative intersystem crossing is activated (an increase in the T1 level energy, a decrease in the overlap integrals for the n orbitals of nitrogen and p orbitals of the macrocycle upon the displacement of nitrogen atoms from the macrocycle plane, and activa- tion of new types of accepting modes). Note in this connection that the most convenient objects for a detailed study of spectral manifestations of the conformational dynamics of porphyrins with the N N N N Pd EtEt Me Me Me Me 4: N N N N Pd Et Et Et Et Et Et EtEt 5: N N N N Pd REt Et Et Et Et Et EtEt 1: R = H 2: R = 3: R = –CH3 6: R = NO2 Fig. 1. Structural formulas of molecules studied and their abbreviations. (1) 2,3,7,8,12,13,17,18-octaethylporhyri- nato-Pd(II), PdOEP; (2) 5-phenyl-2,3,7,8,12,13,17,18- octaethylporhyrinato-Pd(II), PdOEP-Ph; (3) 5-methyl- 2,3,7,8,12,13,17,18- octaethylporhyrinato-Pd(II), PdOEP- CH3; (4) 5-phenyl-2,8,12,18-tetramethyl-13,17-diethylpor- phyrinato-Pd(II), PdTMDEP-Ph; (5) 2,3,7,8,12,13,17,18- octaetyl-5,10,15,20-tetraphenylporphyrinato-Pd(II), PdO- ETPP; (6) 5-(ortho-nitrophenyl)-2,3,7,8,12,13,17,18-octa- ethylporphyrinato-Pd(II), PdOEP-Ph(o-NO2)).steric hindrance in the T1 state are the Pd complexes. They were chosen for the study for a number of rea- sons. Palladium complexes of porphyrins have high quantum yields of the S1 T1 intersystem crossing (g » 1 [15]) and the T1 S0 phosphorescence (j P » 0.1—0.5 [16–18]) both at room and low temperature. Therefore, a simultaneous analysis of the data on spec- tral and kinetic properties of the induced T–T absorp- tion and phosphorescence at 293 K allows one to obtain detailed information on the mechanisms of dynamic relaxation of the T1 states of porphyrins with the steric hindrance. In addition, because of the high probability of the S1 T1 intersystem crossing (r = 8.3 · 1010 s—1 [19]), Pd porphyrins are attractive for the study of photoin- duced electron transfer (PET) involving triplet states. Such an approach was used for studying PET from the T1 state in covalently bonded complexes of Pd-tet- raphenylporphyrin-quinone (PdTPP-Q) in toluene at 293 K [19]. We have shown that efficient PET (the probability = 9.5 · 109 s—1 in toluene at 293 K) occurs from the excited S1 states of steric hindered meso-ortho-nitrophenyloctaetylporphyrins and their chemical dimers to the NO2 group, which can be described by the Marcus theory for nonadiabatic elec- tron transfer [20–22]. The study of Pd complexes of these compounds is of interest from the point of view of the elucidation of mechanisms of PET involving triplet states. In this work, we performed a comparative experi- mental study of spectral and kinetic parameters of mol- ecules of PdOEP, PdOEP-CH3, and PdTMDEP-Ph, which are not subject to the conformational rearrange- ment in the T1 state or their steric hindered meso-phe- nyl-substituted analogs PdOEP-Ph and PdOETPP, as well as meso-nitrophenyloctaethylporphyrins PdOEP- Ph(p-NO2) and PdOEP-Ph(o-NO2) in liquid solutions at 293 K. The aim of this study was to obtain detailed information on the effect of the conformational dynam- ics of the tetrapyrrole macrocycle and charge-transfer interactions on the properties of triplet states. A com- prehensive analysis of the structure and dynamic prop- erties of such objects and of their photophysical param- eters is not only of interest in itself but is also required for the understanding of the role of labile conforma- tions of metalloporphyrins in their interaction with membrane components in vivo [23], as well as for the study of artificial supramolecular systems based on tet- rapyrrole macrocycles simulating a variety of biologi- cal processes [24]. EXPERIMENTAL Structural formulas of molecules studied in this paper are presented in Fig. 1, where the corresponding abbreviations are also given. The method of synthesis of initial metal-free OEPs containing meso-substitu- ket SOPTICS AND SPECTROSCOPY Vol. 90 No. 1 2001 MANIFESTATION OF NONPLANARITY EFFECTS 69 ents of different type is described in detail in [25]. OETPP was synthesized by the method developed in [26]. The corresponding Pd complexes were synthe- sized from free base porphyrins of different structure by the method described in [27]. As solvents, toluene, acetone, and dimethylformamide (spectroscopic grade) were used at 293 K and transparent glassy methylcyclo- hexane–diethyl ester (1 : 1) and methylcyclohexane- toluene (6 : 1) mixtures, at 77 K. Concentrations of por- phyrin solutions were, as a rule, ~10—6—10—5 M to decrease the influence of annihilation effects. The solu- tions were degassed to a residual pressure of 5 · 10- 5 Torr. Experiments were performed, as a rule, within 1–2 h after the sample preparation. Spectral and luminescent studies and measurements on kinetic parameters of phosphorescence were per- formed using a high-sensitivity automated setup, which was described in detail in [28] where basic procedures, measurement errors, and the references used are also given. The decay kinetics of the excited T1 states, the induced absorption spectra (which were measured for the first time in the near-IR region away from the S0—Sn absorption bands), as well as the singlet oxygen gener- ation were measured with the universal measuring unit described in [29]. Samples were excited by the 532-nm second-harmonic pulses from an actively Q-switched LTI-401 Nd3+:YAG laser (the pulse energy was 1–5 mJ with an accuracy of – 5% and the pulse duration was D t1/2 = 15 ns). The detection system that included a dif- fraction monochromator, detectors (a FEU-84 PMT and FD-10GA germanium photodiodes), an S9-8 digi- tal oscillograph, and a computer processing unit per- mitted the measurement of nonstationary absorption with an optical density as low as D D ~ 10—4 and an time resolution as low as 50 ns over the entire visible spec- tral range and in the near-IR region from 850 to 1660 nm. The relative error of measurements of the optical density of the induced nonstationary absorption did not exceed – 5%. RESULTS AND DISCUSSION Absorption and Luminescence Spectra In analyzing the entire spectral information obtained, it is expedient to separate the influence of a few factors that determine the spectral properties of compounds under study. Consider the role of confor- mational effects caused by meso-aryl substitution. One can see from Fig. 2 and the table that the transition from PdOEP to PdOEP-Ph at 293 K is accompanied by a small bathochromic shift (Dl ~ 4 nm) of the long-wave- length Q absorption bands, whereas the intensity and shape of the bands do not change appreciably. These molecules emit weak fluorescence at room tempera- ture, the shape of the fluorescence bands being virtually independent of the meso-aryl substitution. A compari- son of the phosphorescence spectra of PdOEP, PdTM- DEP-Ph, and PdOEP-Ph at 293 K shows that the phenylOPTICS AND SPECTROSCOPY Vol. 90 No. 1 2001substitution is also manifested in a small bathochromic shift of the bands (see the table), which is accompanied by a weak increase in the relative intensity of the vibronic band (see curves 4 in Figs. 2a and 2b). The phosphorescence of PdOEP-Ph is strongly quenched compared to that of PdTMDEP-Ph (by a factor of ~350). According to the results of our studies [13, 14], this quenching is related to the nonstationary nonpla- narity of the porphyrin macrocycle in the excited T1 state. The search for new bands in the spectral region up to 1100 nm at 293 K performed in this paper did result in the detection of the phosphorescence bands belong- ing to the nonplanar conformation of PdOEP-Ph mole- cules. We will show below that nonplanar conforma- tions of PdOEP-Ph in the T1 state are reliably detected in the T—T absorption spectra. One can see from Fig. 3 and the table that the tran- sition from mono- (PdOEP-Ph) to tetra-meso-phenyl- substitution (PdOETPP) is accompanied by a strong bathochromic shift (Dl ~ 30—40 nm) and broadening of the absorption and fluorescence bands at 293 K. Under these conditions, fluorescence is weakly quenched and phosphorescence is not observed. As the temperature was decreased to 77 K, the hypsochromic shift (Dl ~ 6— 8 nm) was observed in rigid glassy matrices and the absorption and fluorescence bands were abruptly nar- rowed. Note that PdOETPP exhibits an intense phos- phorescence band (j P = 0.25) at 705 nm at 77 K that is red-shifted relative to the phosphorescence band of usual porphyrins. Taking into account that the molecule 500 550 600 650 700 750 0 0.2 0.4 0.6 0.8 1.0 0.2 0.4 0.6 0.8 1.0 0.2 0.4 0.6 0.8 1.0 1 2 3 4 1 2 3 4 1 2 3 D/Dmax l , nm 1.0 0.8 0 0.2 0.4 0.6 0.8 1.0 0.2 0.4 0.6 0.8 1.0 0.2 0.4 0.6 I/Imax (à) (b) (c) Fig. 2. (1) Absorption, (2) fluorescence, and (3, 4) phospho- rescence spectra of (a) PdOEP, (b) PdOEP-Ph, and (c) PdOEP-Ph(o-NO2). (1, 2, 4) 293 K; (3) 77 K. 70 KNYUKSHTO et al.Spectral and photophysical parameters of Pd porphyrins Compound , nm, 293 K , nm, 293 K , nm, 293 K , nm, 77 K j F · 104, 293 K t T , m s, 293 K , m s, 293 K t T , ms, 77 K , 293 K j P , 77 K kT · 10–9, M–1 s–1 g D PdOEP 546 551 663 654 3.1 0.25 650 1.8 0.15 0.35 2.3 1* PdOEP-Ph 550 555 668 658 3.2 0.15 0.2 1.7 0.0006 0.30 0.9 0.57 PdOEP-CH3 555 559 668 670 3.4 0.27 350 1.2 0.08 – 2.1 1 PdTMDEP-Ph 548 554 668 658 3.8 0.26 530 1.6 0.22 0.30 2.2 1 PdOETPP 581 594 – 705 2 – <0.05 1.0 <10–5 0.25 – <0.01 PdOEP-Ph(o-NO2) 554 558 – 665 1.4 0.62 0.65 1.6 ~10–5 0.45 0.04 ~0.01 Note: , , and are the maxima of the absorption (the long-wavelength transition), fluorescence, and phosphorescence bands, respectively. j P and g D are the quantum yields of phosphorescence and singlet oxygen generation, respectively. The bimolecular rate constants kT of quenching of the T1 states by molecular oxygen were calculated by the Stern–Volmer equation / t T = 1 + kTC , where C = 1.8 · 10–3 M is the concentration of O2 in toluene at 293 K. The superscript “0” refers to degassed solutions. The PET probability for PdOEP-Ph(o-NO2) was calculated by the formula = 1/ . The error of measurement of quantum yields of emission was 5–7% for j P , j F ‡ 0.1; for lower values of j P and j F, the relative error increased to 15–20%. The lower limit of the measured values of j P and j F was 10–5. The relative errors in time decays t T and of the triplet states measured upon nanosecond laser excitation in the accumulation mode from the induced T–T absorption at 293 K and in phosphorescence studied did not exceed, as a rule, 5%. The relative error of measurements of the quantum yields of singlet oxygen generation was 10%; the minimum measured value of g D was estimated as ~0.01. *PdOEP was used as a reference (g D = 1.0 [38]). l 00 A l 00 F l 00 P l 00 P t T 0 j P 0 l 00 A l 00 F l 00 P t T 0 t T 0 ket T t T 0 t T 0of tetra-mesophenyl-substituted free base OETPP has a nonplanar conformation in the ground S0 state [5], the low-temperature emission of PdOETPP should be treated as phosphorescence of the nonplanar conforma- tion. Note that phosphorescence of nonplanar confor- mations of steric hindered porphyrins at 77 K was not reported earlier in the literature. In the case of mono-mesophenyl-substituted PdOEP-Ph, a passage to rigid matrices at 77 K elimi- 500 550 600 650 700 750 800 850 0 0.2 0.4 0.6 0.8 1.0 0 0.2 0.4 0.6 0.8 1.0 1 2 3 4 5 D/Dmax I/Imax l , nm Fig. 3. (1, 2) Absorption, (3, 4) fluorescence, and (5) phos- phorescence spectra of PdOETPP. (1, 3) 293 K; (2, 4, 5) 77 K.nates the conformation dynamics of the p -conjugated macrocycle in the T1 state caused by the steric interac- tion of volume substituents [13, 14]. As a result, the phosphorescence spectra detected under these condi- tions coincide with those of PdOEP and PdTMDEP-Ph (see Fig. 2 and table). By analyzing conformational effects, note the fol- lowing experimental fact that is typical for all Pd por- phyrins. The phosphorescence and phosphorescence excitation spectra of these porphyrins in glassy matri- ces at 77 K (low-concentration solutions in the 6 : 1 methylcyclohexane-toluene mixture) suggest the pres- ence of two forms having different spectra. In the case of PdOEP, the presence of these forms is manifested both in the absorption and phosphorescence spectra even at room temperature. Earlier, based on the temper- ature dependence of phosphorescence spectra of Pd- porphin in nonane studied in the range from 161 K to 79 K, it was assumed that it exists in two forms related to the displacement of the central Pd ion from the por- phyrin macrocycle plane in the excited state [16]. Our studies of optical spectra, polarized phosphorescence, and the phosphorescence decay for different forms showed that Pd-porphyrins in solutions have two con- formations already in the ground state. This issue will be discussed in detail elsewhere. One can see from Fig. 2 and the table that the ortho- nitro-substitution (a passage from PdOEP-Ph to PdOEP-Ph(o-NO2)) is weakly manifested in theOPTICS AND SPECTROSCOPY Vol. 90 No. 1 2001 MANIFESTATION OF NONPLANARITY EFFECTS 71absorption and fluorescence spectra of this compound. PdOEP-Ph(o-NO2) exhibit a small bathochromic shift and broadening of the purely electronic transition band by a factor of ~ 1.5, the Stokes shift being invariable. Similarly to the situation known for meso-tetra-(ortho- nitrophenyl)porphyrin [30], the spectral changes observed for PdOEP-Ph(o-NO2) can be explained by admixing of the charge-transfer states to the pp * states of the porphyrin macrocycle. Induced Absorption Spectra Our earlier measurements [13, 14, 20 22] showed that free bases of steric hindered mono- and dimeso- phenyl-substituted OEP molecules do not emit notice- able phosphorescence in liquid solutions at 293 K (j P < 10—5). This is also the case for most free bases of usual porphyrins in liquid solutions at room temperature. In this case, the analysis of induced absorption spectra can give information on the spectral properties of triplet states of steric hindered porphyrins in the presence of conformation dynamics. One can see from Fig. 4 that the intensity of the T- T absorption spectrum of the OEP molecule monotoni- cally decreases in the near-IR region up to 1660 nm. In passing to the steric hindered OEP-Ph molecule, the T- T absorption spectra in this region exhibit two dis- tinct maxima at 740 and 1000 nm, the half-width of the long-wavelength band being considerably greater. The T–T absorption spectrum of the OEP-Ph(o-CH3) mole- cule does not exhibit a band at 1000 nm, whereas its short-wavelength band is less pronounced and is shifted to the blue (720 nm) compared to the corresponding band in the OEP-Ph spectrum. These spectra can be interpreted as follows. We showed recently [12–14] that the introduction of volume substituents into the ortho position of the phe- nyl ring of b -alkyl-substituted porphyrins (a passage from OEP-Ph to OEP-Ph(o-CH3)) substantially increases steric interactions and hinders torsion vibra- tions of mesophenyl around a singular C–C bond. In this case, unlike OEP-Ph, the OEP-Ph(o-CH3) mole- cule remains planar in the excited T1 state and has a long lifetime in degassed solutions at 293 K. Taking into account these facts, the presence of the long-wave- length band at 1000 nm in the T—T absorption spectrum of the OEP-Ph molecule should be related to the dynamic nonplanarity of the porphyrin macrocycle. Because, at present, a rigorous theoretical model that would describe the T—T absorption spectra of porphy- rins is absent, we can only qualitatively explain the spectral features observed. According to [26], nonpla- nar porphyrins in the initial S0 state are characterized by the destabilization of the p -conjugated system of the macrocycle, which results in a substantial increase in the energy of the highest-energy occupied molecular orbitals (HOMOs) and a small change in the energy of t T 0OPTICS AND SPECTROSCOPY Vol. 90 No. 1 2001the lowest-energy unoccupied molecular orbitals (LOMOs). As a result, a decrease in the energy gap between the LOMO and HOMO is accompanied by the red shift of the absorption bands. Because steric inter- actions in the OEP-Ph molecule lead to the dynamical distortion of the tetrapyrrole macrocycle only in the triplet state, the presence of the long-wavelength band at 1000 nm in the T - T absorption spectrum can be attributed to a change in the HOMO energy in the non- planar conformation. In addition, the distortion of the porphyrin macrocycle can result in the additional mix- ing of triplet states of different types (pp * and np * tran- sitions) caused by the displacement of nitrogen atoms from the macrocycle plane [13, 14] and by the corre- sponding increase in the contributions from one-center overlap integrals for n orbitals of nitrogen and p orbit- als of the macrocycle [31]. Because the dynamic distor- tion of the porphyrin macrocycle of the OEP-Ph(o- CH3) molecule in the triplet state is eliminated, the long-wavelength band at 1000 nm is absent. The presence of the 740-nm band in the T—T absorp- tion spectrum of OEP-Ph and its absence in the induced absorption spectrum of the OEP molecule can be attrib- uted to a change in the nature of the lowest triplet state upon meso-aryl substitution. The lowest T1 level of the OEP-Ph molecule, as an analogous level of the tet- 600 800 1000 1200 1400 1600 740 720 1000 770 760 1180 1 2 3 1 2 0 20 40 4 8 12 0 (à) (b) D D · 103 l , nm Fig. 4. Induced T—T absorption spectra of (a) free bases and (b) Pd complexes of octaethylporphyrins and their meso- phenyl-substituted derivatives in degassed toluene at 293 K. (a) (1) OEP, (2) OEP-Ph, (3) OEP-Ph(o-CH3); (b) (1) PdOEP, (2) PdOEP-Ph. 72 KNYUKSHTO et al.raphenylporphyrin (TPP) related to the 740-nm band in the T—T absorption spectrum [32] corresponds to the 3(3a2u4eg) configuration, whereas the lowest level of the OEP molecule is the T1 level with the 3(1a1u4eg) configuration. In passing to the OEP-Ph(o-CH3) mole- cule, the band under consideration becomes weaker and experiences the hypsochromic shift to 720 nm. These can be explained by the fact that, unlike OEP-Ph, the influence of mesophenyl in the OEP-Ph(o-CH3) molecule is minimal because the phenyl ring is virtu- ally perpendicular to the porphyrin macrocycle plane due to steric interactions [20, 22]. The nonplanarity effects are also observed in the T—T absorption spectra of the Pd complexes of molecules under study (Fig. 4b). The short-wavelength at 760– 770 nm observed in the spectra of both PdOEP and PdOEP-Ph can be attributed, in our opinion, to nonpla- nar deformations of the tetrapyrrole macrocycle caused by the displacement of the central Pd ion from the por- phyrin ligand plane. In this case, due to a large ion radius of Pd2+ (0.86 Å [33]), the Pd complexes of OEP have the dome conformation in the ground state, whose symmetry is lower that the initial symmetry D4h. The out-of-plane vibrations of a metal in this conformation 600 800 1000 1200 1400 1600 0 5 10 15 20 30 0 10 1000 1200 1400 10 20 1110 1290 1 · 10 l , nm 810 740 720 980 870 880 1380 1400 l , nm D D · 103 1 23 1 2 (à) (b) D D · 103 0 Fig. 5. Induced absorption spectra of (a) free bases and (b) Pd complexes of meso-nitrophenyl-substituted octaeth- ylporphyrins in degassed solutions at 293 K. (a) (1) OEP- Ph(o-NO2) in toluene, (2) OEP-Ph(p-NO2) in toluene, (3) OEP-Ph(p-NO2) in acetone; (b) (1) PdOEP-Ph(o-NO2) in toluene, (2) PdOEP-Ph(o-NO2) in acetone.are strongly anharmonic [34]. Note that such a band was observed at ~760 nm in the T—T absorption spec- trum of PdTPP in pyridine [35]. However, the origin of this band was not discussed, although the formation of mono-pyridinate complexes should result in the distor- tion of the planarity of the porphyrin macrocycle due to the displacement of the Pd ion from the macrocycle plane. The T—T absorption spectrum of PdOEP-Ph exhibits a broad structureless band at 1180 nm (Fig. 4b). This band and its large width can be caused by a combina- tion of at least two effects: the dynamic distortion of the tetrapyrrole macrocycle in the triplet state (as in the case of OEP-Ph) and a change in the nature of the low- est triplet state upon meso-aryl substitution (the pres- ence of the 845-nm band in the T—T absorption spec- trum of PdTPP and ZnTPP [35]). Consider the influence of the meso-nitrophenyl sub- stitution on the induced absorption spectra of mole- cules under study upon variation of the position of the NO2 group in the phenyl ring (ortho- and para-substitu- tion). One can see from Fig. 5a that the induced absorp- tion spectrum of OEP-Ph(p-NO2) contains a long- wavelength band at 1000 nm and in fact coincides with the T—T absorption spectrum of OEP-Ph (Fig. 4a). This result is explained by the fact that the introduction of the bulk substitute into the para-position does not affect torsion librations of the phenyl ring and, thus, does not eliminate the dynamic nonplanarity of the porphyrin macrocycle in the T1 state [12–14]. In addition, in the presence of the electron-acceptor group in the para- position that is the farthest removed from porphyrin the charge-transfer interactions is inefficient [20–22], so that their influence on the induced absorption spectra is negligible. Thus, the induced absorption spectrum of OEP-Ph(p-NO2) is in fact the T—T absorption spectrum. The decrease in the intensity of the 1000-nm band of OEP-Ph(p-NO2) in acetone (Fig. 5a) can be explained by the enhancement of charge-transfer interactions in polar media resulting in the shortening of the triplet- state lifetime [20] and, hence, in a decrease in the effi- ciency of formation of nonplanar conformations related to this band. The induced absorption spectrum of OEP-Ph(o- NO2) (Fig. 5a) substantially differs from the two spectra considered above. Note that the 1000-nm band related to nonplanar conformations in the T1 state completely disappears. Along with the residual absorption at 740 nm (caused by the mesophenyl substitution), an intense narrow band appears at 810 nm and two low- intensity bands appear at 1110 and 1290 nm. We found earlier [20] that the energy of the charge-transfer (CT) state for OEP-Ph(o-NO2) in toluene exceeds the energy of the locally excited T1 state by 0.24 eV. Therefore, from energy considerations, the new bands observed cannot be assigned to the T1 CT absorption transi- tions. A manifestation of the intrinsic absorption in a system of the states of the ion–radical pair is alsoOPTICS AND SPECTROSCOPY Vol. 90 No. 1 2001 MANIFESTATION OF NONPLANARITY EFFECTS 73unlikely because of a fast transition from the 3[P+É ] CT state to the lower-lying T1 state (ECT > ET1 [20]). One can assume that the induced absorption spectrum of OEP-Ph(o-NO2) is caused by the mixing of pure pp * states with the CT states due to the overlap of the orbitals of the porphyrin macrocycle and nitro group. Assumptions of this type were used for the explanation of the transformation of absorption spectra of porphyrin–quinone complexes [30]. One can see from Fig. 5b that the induced absorp- tion spectra of PdOEP-Ph(o-NO2) are similar to the analogous spectrum of OEP-Ph(o-NO2), except as regards the bathochromic shift of the bands. The low- intensity at 760–770 nm in the spectrum of PdOEP- Ph(o-NO2) observed both for PdOEP and PdOEP-Ph is caused by the displacement of the central Pd ion from the porphyrin ligand plane and is not related to the pres- ence of the nitro group. The bathochromic shift of the bands can be explained by several causes : a change in the symmetry of the OEP-Ph(o-NO2) molecule caused by the introduction of the Pd ion (which also results in the transformation of two long-wavelength bands in a region of 1100–1400 to one band), thermal mixing of the transitions in a system of locally excited triplet lev- els and the CT states in an ion–radical pair, and an inversion of the energies of the T1 and CT states in pass- ing to the Pd complex (D E = ET1 — ECT » 2kT in toluene; this question is considered below in detail in analysis of the PET properties). Note finally that the induced absorption spectrum of PdOEP-Ph(o-NO2) virtually does not change in passing to highly polar media (Fig. 5b). This can be caused by a weak dependence of the CT state energy in this system on the medium polar- ity due to the screening effect and specific solvation (see below analysis of the PET properties). Deactivation of Excited States Analysis of the experimental data presented in the table revealed the following features. The mesophenyl substitution in PdOEP-Ph, as for free bases of OEP molecules [12–14], does not affect the quantum yield of fluorescence j F in degassed solutions at 293 K; how- ever, it results in a drastic decrease in the quantum yield of phosphorescence and an abrupt shortening of the triplet-state lifetime from 650 m s to 200 ns. At the same time, such effects was not observed upon meso- alkyl substitution (PdOEP-CH3) and in the absence of bulk substituents in the b -positions of pyrrole rings adjacent to meso-phenyl (PdTMDEP-Ph). A small shortening of for PdOEP-CH3 and PdTMDEP-Ph compared to PdOEP was attributed to a decrease in the symmetry of the molecules and a decrease in the energy of the T1 level upon mono-meso-substitution, resulting in the increase in the probability of intersystem cross- ing [36]. The quenching of the T1 state of PdOEP-Ph is NO2 – j P 0 t T 0 t T 0OPTICS AND SPECTROSCOPY Vol. 90 No. 1 2001completely absent in frozen solutions at 77 K. In this case, the values of and virtually coincide with those for PdOEP. Our data on quenching of the T1 states of the mole- cules under study by molecular oxygen showed that the mono-mesosubstitution does not affect the quenching rate constant (kT » 2 · 109 Œ—1 s—1) for PdOEP-CH3 and PdTMDEP-Ph, which is typical for usual metallopor- phyrins [37]. The rate constant of quenching of the T1 state for PdOEP-Ph in toluene decreases to kT = 9 · 108 Œ—1 s—1 and the efficiency of singlet oxygen gener- ation decreases correspondingly. The properties of triplet states of PdOEP-Ph observed in liquid solutions at 293 K reflect all the fea- tures of the deactivation of the T1 states of free bases of mono- and di-mesophenyl-substituted steric hindered octaetylporphyrins studied earlier by us [12–14]. Note that, as the number of meso-phenyls was increased (i.e., in passing to PdOETPP), the quantum yield of phos- phorescence of PdOEP-Ph in toluene at 293 K further decreased ( < 10—5), the lifetime of the T1 state short- ened ( < 50 ns), and the efficiency of the singlet oxy- gen generation drastically decreased (g D < 0.01). According to [5], tetra-mesophenyl-substituted octa- ethylporhyrins have the nonplanar p -conjugated mac- rocycle in the ground S0 state, which results in a consid- erable increase in the probability of nonradiative S1 T1 and S1 S0 transitions. Our data showed that the probability of the nonradiative T1 S0 decay of the T1 state in such steric hindered porphyrins also substantially increases. One can see from the table that the quantum yield of fluorescence of the PdOETPP molecule virtually coincides with the values of j F mea- sured for other Pd complexes. This means that the non- radiative decay of the S1 state in PdOETPP is mainly determined by the internal heavy atom effect and the exchange d—p * interactions [18], whereas the dynamic distortion of the porphyrin macrocycle is inefficient. As was mentioned above, the introduction of bulk substituents (CH3, OCH3, F) into the ortho position of phenyl in molecules of the type OEP-Ph prevents the dynamic nonplanarity of the porphyrin macrocycle in the T1 state in liquid phase at 293 K and is accompanied by a considerable lengthening of the lifetime . At the same time, the value of does not increase after the introduction of the bulk NO2 group into the ortho posi- tion of the phenyl ring (in passing from OEP-PH to OEP-Ph(o-NO2)) but even shortens, which is caused, as was shown earlier by us [13, 20], by mixing of the locally excited T1 state of porphyrin with the higher- lying CT state of the ion–radical pair and the thermally activated T1 CT transition. j P 0 t T 0 j P 0 t T 0 t T 0 t T 0 74 KNYUKSHTO et al.One can see from the table that the quantum yield of phosphorescence of PdOEP-Ph(o-NO2) decreases to ~ 10—5 and the lifetime of the induced absorption of this molecule shortens to = 650 ns. The decrease in the rate of quenching of the excited T1 states of PdOEP- Ph(o-NO2) by molecular oxygen (kT » 4 · 107 Œ—1 s—1) can be caused by a decrease in the contribution of the CT states of oxygen in the collision complex [3PdOEP- Ph(o-NO2)É3O2] due to the presence of the electron- acceptor nitro group bonded with the macrocycle [38]. In addition, a drastic decrease in the efficiency of the singlet oxygen generation (g D ~ 0.01) along with rela- tively long lifetimes of the induced absorption ( = 650 ns) suggests that the detected absorption is mainly related to the ion–radical pair found in the thermody- namical equilibrium with the T1 state. Note, however, that the quenching mechanism of the T1 state in the PdOEP-Ph(o-NO2) molecule substan- tially differs from that for its metal-free analog OEP- Ph(o-NO2), which is explained by different mutual arrangement of the locally excited T1 level of the por- phyrin macrocycle and the CT state in these molecules. The energy of the CT state for PdOEP-Ph(o-NO2) in a condensed phase can be estimated from the known expression [39] (1) where = 0.82 V [40] is the one-electron oxidation potential of the donor PdOEP and = –1.08 V [41] is the reduction potential of the electron acceptor (nitrobenzene) determined with respect to a calomel electrode in dimethylformamide (DMF, e = 36.7). The Coulomb stabilization energy W = e2/4 p e 0 e rDA in DMF for the distance rDA = 5.7 Å between the centers of por- phyrin and the NO2 group for the optimized OEP-Ph(o- NO2) structure [20] is 0.07 eV. Because the reactant radii rD = 5 Å and rA = 3.5 Å [20] for OEP-Ph(o-NO2) and PdOEP-Ph(o-NO2) are comparable to the inter- center distance rDA and the condition r rDA > rD + rA [42] is not fulfilled, the above value of W is only an approximate estimate. The experimental value of the CT state energy can be determined from the value of the energy gap D E = ECT — ET obtained for OEP-Ph(o- NO2) in [20] from the Boltzmann temperature depen- dence of the quenching rate of the T1 state in solvents of different polarity. The values of the CT state energy for OEP-Ph(o-NO2) obtained in this way were 1.75 eV in acetone and 1.78 eV in toluene. Taking into account the closeness of the oxidation potentials of OEP (0.81 V [40]) and PdOEP (0.82 V [40]) and the close- ness of the optimized structures of OEP-Ph(o-NO2) and PdOEP-Ph(o-NO2), we can assume that the experimen- tal values of the CT state energy for PdOEP-Ph(o-NO2) j P 0 t T 0 t T 0 ECT e E1/2 ox E1/2 red –( ) W ,–= E1/2 ox E1/2 redin DMF is 1.76 eV and 1.79 eV in toluene. Thus, we will use the experimental values of the CT state in solu- tions of different polarity in our quantitative analysis of the CT interactions in PdOEP-Ph(o-NO2). The measurements of phosphorescence of PdOEP- Ph(o-NO2) at 77 K, taking into account the bathochro- mic shift of the phosphorescence bands with increasing temperature to 293 K (see Fig. 2 and the table), gave the T1 state energy of this molecule in DMF at room tem- perature equal to 1.84 eV. Therefore, unlike the OEP- Ph(o-NO2) molecule, where the T1 state energy (ET = 1.56 eV in DMF) is lower than the CT state energy, for the PdOEP-Ph(o-NO2) molecule, ET > ECT both in polar and nonpolar solvents at 293 K. This means that the quenching of the T1 state of PdOEP-Ph(o-NO2) is caused by the photoinduced electron transfer to the lower-lying CT state of the ion–radical pair 3[porphy- rin+É ]. It follows from the data presented in the table that the rate constant of PET involving the triplet states of PdOEP-Ph(o-NO2) is = 1.5 · 106 s—1 in toluene at 293 K, and it increases to = 4.0 · 106 s—1 in DMF. Such a tendency is typical for the exothermic nonadia- batic PET [39]. In this case, the quantum yield of for- mation of the ion–radical pair 3[porphyrin+É ] for PdOEP-Ph(o-NO2) in the solutions used is (2) where p + q = 1.5 · 103 s—1 is the total probability of the T1 S0 phosphorescence and the nonradiative T1 S0 decay of the PdOEP molecule at 293 K. Note that, for the porphyrin–quinone pair at rDA = 14 Å in polar media, ~ 104—105 s—1 [43]. Within the framework of the semiclassical Marcus theory [39, 42], the probability of the nonadiabatic PET in the “normal” region has the form (3) where kB is the Boltzmann constant; V is the matrix element of the electronic interaction in the DA pair; l = l in + l solv is the reorganization energy; and D G* is the value of the activation PET barrier. The reorganization energy of a solvate is calculated from the expression (4) where e op = n2 and e are optical and static dielectric con- stants of a solvent, respectively (n = 1.49693 and e = NO2 – ket T ket T NO2 – j et T ket T p q ket T + +( ) ----------------------------- 0.99,= = ket T ket T ket T 2 p " ----- V2 4 p l kBT( )1/2 ----------------------------- D G* kBT -----------– Ł ł æ ö ,exp= l solv e 2 4 p e 0 ----------- 1 2rD -------- 1 2rA -------- 1 rDA -------–+ 1 e op ------ 1 e --– ,=OPTICS AND SPECTROSCOPY Vol. 90 No. 1 2001 MANIFESTATION OF NONPLANARITY EFFECTS 75S1 k13 kR kA q CT r 3[IP] f q pkR * OEP-Ph(o-NO2) 2.2 2.0 1.9 1.8 1.7 1.6 1.5 0.1 0 2.1 E, eV ~ ~ T1 S0 kR* k31 1[IP] ket S PdOEP-Ph(o-NO2) f ket S r S1 T1 ‘’ S0 3[IP]1[IP] k13, k31 kA ket T 0.2 Fig. 6. Energy level diagrams and decay channels of locally excited S1 and T1 states of the porphyrin macrocycle and the CT states in the ion-radical pair for OEP-Ph(o-NO2) and PdOEP-Ph(o-NO2) molecules in polar solvents at 293 K. f, r, p, and q are the prob- abilities of the S1 S0 fluorescence, the S1 T1 intersystem crossing, the T1 S0 phosphorescence, and the T1 S0 nonradiative decay, respectively; and are the rate constant of PET involving the excited S1 and T1 states, respectively; kÄ is the probability of thermally activated population of the higher-lying states; k31 and k13 are the probabilities of spin dephasing of the ion–radical pair; and kR are the probabilities of recombination of the singlet and triplet ion–radical pair, respectively; and 1[IP] and 3[IP] are the singlet and triplet states of the ion–radical pair. ket S ket T kR*2.38 for toluene; and n = 1.43047 and e = 36.7 for DMF [41]). Therefore, the reorganization energy of the sol- vent is, according to our estimates, l solv = 0.03 eV for toluene and 0.45 eV for DMF. The internal reorganiza- tion energy l in, which is determined by a change in the geometrical parameters of the equilibrium conforma- tions of the DA pair, is estimated as ~0.2 eV for por- phyrin macrocycles [43]. Therefore, the total reorgani- zation energy for PdOEP-Ph(o-NO2) in toluene and DMF is l = 0.23 and 0.65 eV, respectively. The free Gibbs energy D G0 of PdOEP-Ph(o-NO2) calculated from the expression (5) is –0.05 eV in toluene and –0.08 eV in DMF. The PET activation energies D G* determined from the expression (6) are estimated as 0.035 eV in toluene and 0.125 eV in DMF. Because, in both solvents, |—D G0 | < l , PET in PdOEP-Ph(o-NO2) involving the triplet state corre- sponds to the “normal” region in the Marcus parabolic dependence, = f(—D G0). D G0 e E1/2 ox E1/2 red –( ) W– ET ,–= D G* D G 0 l+( )2 4 l-------------------------- ,= ketlogOPTICS AND SPECTROSCOPY Vol. 90 No. 1 2001Using the PET parameters estimated in this way, experimental values of the rate constants ket, and expression (3), we calculated the matrix elements of the electronic interaction for the PdOEP-Ph(o-NO2) mole- cule in polar and nonpolar media V = 0.05 cm—1 in tol- uene and V = 1.2 cm—1 in DMF. An increase in V in pass- ing from weakly polar solvents to polar solvents has also been observed for the porphyrin-quinine complex and has been explained by a change in the geometry and intercenter distances in a hydrophobic D–A pair in the polar environment [44]. It is possible that, in the case of PdOEP-Ph(o-NO2), the equilibrium conforma- tion of the D–A pair changes upon variation of the polarity of the solvate environment. This results in the change in the overlap of molecular orbitals of the p - conjugated macrocycle and the NO2 group, which determines the value of V in PET according to the through-space mechanism [39, 42]. The values of V = 0.05–1.2 cm—1 obtained by us for different solvents sat- isfy the known Landau–Zener criterion for nonadia- batic PET reactions [45] (7) where w » 100 cm—1 at 300 K are averaged frequencies of the molecular vibrations of reagents and of the ori- entation motion of the solvent. Therefore, quenching of the triplet state of PdOEP-Ph(o-NO2) both in polar and 4 p 2V 2/h w 2 l kBT( )1/2 1,< 76 KNYUKSHTO et al.nonpolar solvents at room temperature corresponds to the nonadiabatic PET. Thus, using the PET parameters and the data on the decay of the excited states of PdOEP-Ph(o-NO2), we can suggest the following scheme of relaxation pro- cesses taking place upon photoexcitation of this mole- cule (Fig. 6): where r is the probability of the S1 T1 intersystem crossing, is the PET probability, kÄ is the probability of the thermally activated population of the T1 state of porphyrin from the lower-lying CT state of the ion-rad- ical pair, k31 and k13 are the probabilities of the spin dephasing of the ion-radical pair, and is the proba- bility of recombination of the singlet ion-radical pair to the ground state. It follows from the energy level dia- gram that, in principle, the direct PET from the S1 state of PdOEP-Ph(o-NO2) is also possible. Our calculations showed that, in this case, the free Gibbs energy in tolu- ene D G0 = –0.43 eV. Therefore, |—D G0 | > l = 0.23 eV and PET corresponds in this case to the “inverted” region of the Marcus dependence, = f(—D G0). For this reason, an abrupt increase in for PdOEP- Ph(o-NO2) compared to the value = 9.5 · 109 s—1 for OEP-Ph(o-NO2) in toluene [20] in unlikely. As a result, upon photoexcitation, almost all PdOEP-Ph(o-NO2) molecules transfer to the locally excited T1 state because the probability of the S1 T1 conversion r = 8.3 · 1010 s—1 considerably exceeds the estimated rate constant of PET involving the S1 states. One can see from the energy level diagram that the CT state of PdOEP-Ph(o-NO2) in toluene is located lower by 0.05 eV (~2kBT) than the T1 state. Therefore, the lower- lying triplet state of the ion-radical pair is populated with the probability = 1.5 · 106 s—1 due to the direct PET. Because the hyperfine interaction energy in a spa- tially separated ion-radical pair is small (i.e., the exchange integrals are small), the singlet–triplet split- ting in this pair is also small. For this reason, the 3[por- phyrin+É ] 1[porphyrin+É ] transitions between the triplet and singlet states of the ion-radical pair are highly efficient (k31 » k13 » 5 · 107 s—1 [43]). The singlet state of the ion-radical pair 1[porphy- rin+É ] experiences the nonradiative decay during recombination to the S0 ground state. It is this barrier recombination process that determines the lifetime of the CT state. S1( ) P1 …NO2[ ] T1( ) P3 …NO2[ ] P+…NO2 –[ ]3 P+…NO2–[ ] 1 P…NO2[ ] S0( ), r ket T kÄ k31 k13 kR* ket T kR* ketlog ket S ket S ket T NO2 – NO2 – NO2 –Note that the lifetime of the induced absorption of PdOEP-Ph(o-NO2) measured in solvents under study is determined by a complex character of the deactivation of the closely spaced locally excited T1 state of the por- phyrin and the CT state of the ion-radical pair. For this reason, the measured values of represent the effec- tive values, which in fact reflect the fast deactivation of the T1 state due to the direct PET to the lower-lying CT state under the conditions of the Boltzmann thermal population of these states. To elucidate the features of the direct deactivation of the locally excited T1 state of PdOEP-Ph(o-NO2) in more detail, one should perform direct picosecond kinetic measurements. Note also that the value of weakly depends on the solvent polarity. The absence of a distinct stabilization effect of the ion pair upon increasing the solvent polarity [39, 42] can be explained in our case by the fact that a small distance between the donor (porphyrin macrocycle) and the acceptor (nitro group) prevents the formation of indi- vidual solvates. ACKNOWLEDGMENTS This work was supported by the Belarussian Foun- dation for Basic Research, project nos. F99-104 and F98-243. REFERENCES 1. S. Gentemann, N. Y. Nelson, L. Jaguinod, et al., J. Phys. 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