Sergey I. Druzhinin

Dual fluorescence and fast intramolecular charge transfer with 4-(diisopropylamino)benzonitrile in alkane solvents

Abstract Dual fluorescence and fast intramolecular charge transfer (ICT) is observed with 4-(diisopropylamino)benzonitrile (DIABN) in alkane solvents. The rate constant ka for the reaction from the locally excited (LE) to the ICT state has a value of 3.4×1011 s−1 in n-hexane at 25°C, with an activation energy Ea of 6 kJ mol−1. Efficient intersystem crossing with a yield of 0.94 takes place from the ICT state. With 4-(dimethylamino)benzonitrile, in contrast, dual fluorescence is not observed in alkanes. The charge transfer reaction of DIABN is mainly favoured by its small energy gap ΔE(S1,S2), in accordance with the PICT model for ICT in aminobenzonitriles.

Ultrafast Intramolecular Charge Transfer with N-(4-Cyanophenyl)carbazole. Evidence for a LE Precursor and Dual LE + ICT Fluorescence

The photophysics of N-(4-cyanophenyl)carbazole (NP4CN) was investigated by using absorption and fluorescence spectra, picosecond fluorescence decays, and femtosecond transient absorption. In the nonpolar n-hexane as well as in the polar solvent acetonitrile (MeCN), a locally excited (LE) state is detected, as a precursor for the intramolecular charge transfer (ICT) state. A LE → ICT reaction time τ(2) at 22 °C of 0.95 ps in ethyl cyanide (EtCN) and 0.32 ps in MeCN is determined from the decay of the LE excited state absorption (ESA) maximum around 620 nm. In the ESA spectrum of NP4CN in n-hexane at a pump-probe delay time of 100 ps, an important contribution of the LE band remains alongside the ICT band, in contrast to what is observed in EtCN and MeCN. This shows that a LE ⇄ ICT equilibrium is established in this solvent and the ICT reaction time of 0.5 ps is equal to the reciprocal of the sum of the forward and backward ICT rate constants 1/(k(a) + k(d)). In the photostationary S(0) → S(n) absorption spectrum of NP4CN in n-hexane and MeCN, an additional CT absorption band appears, absent in the sum of the spectra of its electron donor (D) and acceptor (A) subgroups carbazole and benzonitrile. This CT band is located at an energy of ∼4000 cm(-1) lower than for N-phenylcarbazole (NPC), due to the larger electron affinity of the benzonitrile moiety of NP4CN than the phenyl subunit of NPC. The fluorescence spectrum of NP4CN in n-hexane at 25 °C mainly consists of a structured LE emission, with a small ICT admixture, indicating that a LE → ICT reaction just starts to occur under these conditions. In di-n-pentyl ether (DPeE) and di-n-butyl ether (DBE), a LE emission is found upon cooling at the high-energy edge of the ICT fluorescence band, caused by the onset of dielectric solvent relaxation. This is not the case in more polar solvents, such as diethyl ether (DEE) and MeCN, in which a structureless ICT emission band fully overlaps the strongly quenched LE fluorescence. For the series of D/A molecules NPC, N-(4-fluorophenyl)carbazole (NP4F), N-[4-(trifluoromethyl)phenyl]carbazole (NP4CF), and NP4CN, with increasing electron affinity of their phenyl subgroup, an ICT emission in n-hexane 25 °C only is present for NP4CN, whereas in MeCN an ICT fluorescence is observed with NP4CF and NP4CN. The ICT fluorescence appears when for the energies E(ICT) of the ICT state and E(S(1)) of the lowest excited singlet state the condition E(ICT) ≤ E(S(1)) holds. E(ICT) is calculated from the difference E(D/D(+)) - E(A(-)/A) of the redox potentials of the D and A subgroups of the N-phenylcarbazoles. From solvatochromic measurements with NP4CN an ICT dipole moment μ(e)(ICT) = 19 D is obtained, somewhat larger than the literature values of 10-16 D, because of a different Onsager radius ρ. The carbazole/phenyl twist angle θ = 45° of NP4CN in the S(0) ground state, determined from X-ray crystal analysis, has become smaller for its ICT state, in analogy with similar conclusions for related N-phenylcarbazoles and other D/A molecules in the literature.

Singlet excited state dipole moments of dual fluorescent N-phenylpyrroles and 4-(dimethylamino)benzonitrile from solvatochromic and thermochromic spectral shifts

The excited state dipole moments mue(ICT) and mue(LE) of the dual fluorescent molecules N-phenylpyrrole (PP), N-(4-cyanophenyl)pyrrole (PP4C) and N-(3-cyanophenyl)pyrrole (PP3C) are determined from solvatochromic and thermochromic measurements. It is shown that the best results are obtained when the solvatochromic as well as the thermochromic analysis of the spectral shifts is made relative to 4-(dimethylamino)benzonitrile (DMABN) as the model compound. Direct thermochromic experiments with PP4C, PP3C and DMABN in diethyl ether lead to reasonable results, but unrealistically large dipole moments mue(ICT) are found for PP, PP4C, PP3C and DMABN in acetonitrile, ethyl cyanide and n-propyl cyanide. The mue(ICT) values obtained for the N-phenylpyrroles from the thermochromic analysis in these solvents relative to DMABN (17 D) do not depend on solvent polarity: 13 D for PP, 15 D for PP4C and PP3C. The spectral shifts for the LE emission of the N-phenylpyrroles and aminobenzonitriles are much smaller than those for the ICT fluorescence, resulting in relatively small values for mue(LE). With PP and N-(4-methylphenyl)pyrrole (PP4M) the problem arises that one of the two values calculated by solving the quadratic equation for mue(LE) in the solvatochromic and thermochromic analysis cannot be discarded on photophysical or molecular grounds, as is the case for the other molecules. The experimental data for mue(ICT) of PP and PP4C are compared with theoretical values calculated for coplanar (PICT) and perpendicular (TICT) conformations of the pyrrole and phenyl or cyanophenyl groups. The experimental ICT dipole moment of PP4C has a value in between the theoretical results for mue(PICT) and mue(TICT), whereas the data for PP tend to favour the TICT configuration. It appears that in the LE state of PP and PP4M a negative charge remains on the pyrrole moiety, whereas a charge reversal takes place for the LE state of PP3C and the ICT state of PP, PP4C and PP3C.

Intramolecular charge transfer with 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6) and other aminobenzonitriles. A comparison of experimental vapor phase spectra and crystal structures with calculations.

Calculations of molecular structures in the electronic ground state S(0) and of excited state and fluorescence energies generally refer to the gas phase. This complicates a comparison with experimental data, which often are only available for molecules in solution. Therefore, experimental absorption and fluorescence spectra in the vapor phase are presented for 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6), 1-methyl-6-cyano-1,2,3,4-tetrahydroquinoline (NMC6), 4-(dimethylamino)benzonitrile (DMABN), and 4-(diisopropylamino)benzonitrile (DIABN). NTC6 and DIABN show a dual fluorescence in the gas phase, with emissions from an intramolecular charge transfer (ICT) and a locally excited (LE) state, whereas with NMC6 and DMABN only LE emission is observed. For a comparison of the experimental molecular structure in S(0) with the results of recent computations, X-ray crystal structures of NTC6, NMC6, and several analogues are presented. For DMABN, NMC6, and NTC6, LE/ICT energy diagrams are constructed, in which the experimental energies of the Franck-Condon singlet excited states S(1) and S(2), and the LE and ICT states together with their emissions, are compared with the calculations. The LE and ICT dipole moments are also discussed. This comparison reveals substantial differences, in particular for the ICT energies, but even for the structure of the S(0) ground states. It is concluded that the computed ICT states of NTC6 and DMABN, in which the full conjugation of the phenyl ring is interrupted, is different from the ICT states measured in the experiments.

Intramolecular charge transfer of 4-(dimethylamino)benzonitrile probed by time-resolved fluorescence and transient absorption: No evidence for two ICT states and a pisigma( *) reaction intermediate.

For the double exponential fluorescence decays of the locally excited (LE) and intramolecular charge transfer (ICT) states of 4-(dimethylamino)benzonitrile (DMABN) in acetonitrile (MeCN) the same times tau(1) and tau(2) are observed. This means that the reversible LE<==>ICT reaction, starting from the initially excited LE state, can be adequately described by a two state mechanism. The most important factor responsible for the sometimes experimentally observed differences in the nanosecond decay time, with tau(1)(LE)<tau(1)(ICT), is photoproduct formation. By employing a global analysis of the LE and ICT fluorescence response functions with a time resolution of 0.5 ps/channel in 1200 channels reliable kinetic and thermodynamic data can be obtained. The arguments presented in the literature in favor of a pisigma( *) state with a bent CN group as an intermediate in the ICT reaction of DMABN are discussed. From the appearance of an excited state absorption (ESA) band in the spectral region between 700 and 800 nm in MeCN for N,N-dimethylanilines with CN, Br, F, CF(3), and C(=O)OC(2)H(2) p-substituents, it is concluded that this ESA band cannot be attributed to a pisigma( *) state, as only the C-C[Triple Bond]N group can undergo the required 120 degrees bending.

Fluorescence excitation spectra of jet-cooled 4-(diisopropylamino)benzonitrile and related compounds

Abstract Fluorescence excitation spectra of 4-(diisopropylamino)benzonitrile (DIABN) and 4-(dimethylamino)benzonitrile (DMABN) in thermal vapour and seeded jet expansions are compared. The spectrum of jet-cooled DIABN shows an intense 0–0-transition at 31751.8 cm −1 . The spectrum collapses at excess excitation energies above 800 cm −1 , indicating the presence of an efficient non-radiative decay channel. In the gas phase, fluorescence emission of DIABN occurs from the intramolecular charge transfer (ICT) state. The non-radiative decay channel, therefore, is attributed to rapid ICT in the isolated molecule. The related compounds 4-(methylamino)-3,5-dimethylbenzonitrile (MHD), 4-(azetidinyl)-3,5-dimethylbenzonitrile (M4D), and 4-(dimethylamino)-3,5-dimethylbenzonitrile (MMD) in the jet show extremely weak and structureless emission.

Dual fluorescence and intramolecular charge transfer with crystalline 4-(diisopropylamino)benzonitrile

Abstract Dual fluorescence from a locally excited (LE) and an intramolecular charge transfer (ICT) state is observed with 4-(diisopropylamino)benzonitrile (DIABN) crystals, from 80 down to −110 °C. With crystalline 4-(methylamino)benzonitrile (MABN) only LE emission is observed over this temperature range. Crystals of 4-(dimethylamino)benzonitrile (DMABN) at room temperature mainly show LE fluorescence, whereas the red-shifted structured phosphorescence increases in importance upon cooling. Its spectral shape resembles that of DMABN phosphorescence in low-temperature glassy media, red-shifted by around 2800 cm −1 . The ICT fluorescence of DIABN crystals at −110 °C has a risetime of 55 ps, which becomes shorter with increasing temperature.

Intramolecular charge transfer with 4-fluorofluorazene and the flexible 4-fluoro-N-phenylpyrrole

With 4-fluorofluorazene (FPP4F) and its flexible counterpart 4-fluoro-N-phenylpyrrole (PP4F) an intramolecular charge transfer (ICT) reaction occurs in the singlet excited state in sufficiently polar solvents. The ICT reaction begins to appear in tetrahydrofuran (epsilon = 7.4) for FPP4F and in the more polar 1,2-dichloroethane (epsilon = 10.4) with PP4F, showing its presence by dual fluorescence from a locally excited (LE) and an ICT state. Only LE fluorescence is observed in less polar solvents such as n-hexane. The ICT reaction is more pronounced with FPP4F than for PP4F, due to the smaller energy gap DeltaE(S1,S2) of the former molecule, in accordance with the PICT model. The occurrence of an ICT reaction is confirmed by the ICT dipole moments mu(e)(ICT) of 12 D (FPP4F) and 10 D (PP4F), clearly larger than mu(e)(LE) of approximately 4 D for FPP4F and PP4F. Isoemissive points are found in the fluorescence spectra of FPP4F and PP4F in acetonitrile (MeCN), ethyl cyanide (EtCN), and n-propyl cyanide (PrCN) as a function of temperature, confirming the two-state (LE and ICT) reaction mechanism. From plots of the logarithm of the ICT/LE fluorescence quantum yield ratio versus the reciprocal absolute temperature in these solvents, the ICT reaction enthalpies DeltaH are determined, with larger -DeltaH values for FPP4F than for PP4F: 19.2 as compared with 14.9 kJ/mol in MeCN, as an example. The picosecond fluorescence decay of PP4F at -45 degrees C becomes slower with decreasing solvent polarity, 5.1 ps (MeCN), 14 ps (EtCN), and 35 ps (PrCN), from which the LE --> ICT reaction rate constant is calculated, decreasing from 19 x 10(10) to 2.1 x 10(10) s(-1) between MeCN and PrCN. The femtosecond LE excited-state absorption spectra of FPP4F and PP4F do not undergo any time development in n-hexane (no ICT reaction), but show a fast ICT reaction in MeCN at 22 degrees C, with decay times of 1.1 ps (FPP4F) and 3.3 ps (PP4F). It is concluded that FPP4F and PP4F have a planar ICT state (PICT model), indicating that a perpendicular twist of the donor and acceptor subgroups in a donor/acceptor molecule is not a requirement for fast and efficient ICT. The molecular structures of FPP4F and PP4F obtained from X-ray crystal analysis reveal that the pyrrole group of PP4F is twisted over an angle theta = 25 degrees relative to the fluorophenyl moiety in the ground state, whereas as expected FPP4F is practically planar (theta = 2 degrees). The pyrrole-phenyl bond length of FPP4F (140.7 pm) is shorter than that for PP4F (141.8 pm).

Counterintuitive absence of an excited-state intramolecular charge transfer reaction with 2,4,6-tricyanoanilines. Experimental and computational results.

The fluorescence spectra of 2,4,6-tricyano-N,N-dimethylaniline (TCDMA), 2,4,6-tricyano-N-methylaniline (TCMA), and 2,4,6-tricyanoaniline (TCA) consist of a single emission band, even in the polar solvent acetonitrile (MeCN). This indicates that an intramolecular charge transfer (ICT) reaction from the initially prepared locally excited (LE) state does not take place with these molecules, in contrast to 4-(dimethylamino)benzonitrile (DMABN), although the electron accepting capability of the tricyanobenzene moiety in TCDMA, TCMA, and TCA is substantially larger than that of the benzonitrile group in DMABN. In support of this conclusion, the picosecond fluorescence decays of the tricyanoanilines are single-exponential. Only with TCDMA in MeCN at the highest time resolution, double-exponential decays are observed. On the basis of a similar temporal evolution of around 2 ps in the femtosecond excited-state absorption (ESA) spectra of TCDMA in this solvent, the time development is attributed to the presence of two rapidly interconverting S(1) conformers. The same conclusion is reached from CASPT2/CASSCF computations on TCDMA, in which two S(1) minima are identified. The ESA spectra of TCDMA, TCMA, and TCA resemble that of the LE state of DMABN, but are different from its ICT ESA spectrum, likewise showing that an ICT reaction does not occur with the tricyanoanilines. From the luminescence spectrum of TCDMA in n-propyl cyanide at -160 degrees C, it follows that intersystem crossing and not internal conversion is the main S(1) deactivation channel. The radiative rate constant of TCDMA in MeCN is smaller than that of TCMA and TCA, which indicates that the S(1) state of TCDMA has a larger ICT contribution than in the case of TCMA and TCA, in accordance with the results of the calculations, which show that the S(1) state displays ICT valence bond character. Extrapolated gas-phase data for TCDMA and TCA are compared with the results of the computations, revealing a good agreement. The calculations on TCDMA and TCA also lead to the conclusion that the lowest excited singlet state S(1) determines its photophysical behavior, without the occurrence of an LE --> ICT reaction, in the sense that the initially excited LE state has already a strong ICT character and there is no equilibrium between two electronic states with strongly different electronic structures (i.e., LE and ICT with very different dipole moments) leading to dual (LE + ICT) fluorescence.

Internal conversion with 4-(azetidinyl)benzonitriles in alkane solvents. Influence of fluoro substitution

The introduction of a fluoro-substituent in the phenyl ring of 4-(1-azetidinyl)benzonitrile (P4C) leads to smaller fluorescence quantum yields Φf and shorter decay times τ in alkane solvents (cyclopentane, n-hexadecane, n-hexane and 2-methylpentane). In cyclopentane at 25°C, Φf and τ equal 0.02 and 0.14 ns for 2-fluoro-4-(1-azetidinyl)benzonitrile (P4CF2) and 0.11 and 0.85 ns for 3-fluoro-4-(1-azetidinyl)benzonitrile (P4CF3), as compared with 0.27 and 4.55 ns for P4C. The fluorescence originates from a locally excited (LE) state and dual fluorescence due to intramolecular charge transfer is not observed for the three aminobenzonitriles at any temperature in the alkane solvents. By measuring the yields of intersystem crossing ΦISC, it follows that this enhancement of the radiationless deactivation of the first excited singlet state S1 is due to thermally activated internal conversion (IC). The IC yield ΦIC in cyclopentane at 25°C, as an example, is considerably larger for P4CF2 (0.93) than for P4CF3 (0.35) and of minor importance for P4C (0.03). The IC activation energies EIC of P4CF2 (12.6 kJ mol−1), P4CF3 (19.3 kJ mol−1) and P4C (38.1 kJ mol−1) in cyclopentane were determined by fitting τ measured as a function of temperature, together with data for Φf and ΦISC. Similar EIC values were obtained in n-hexane and n-hexadecane. These data show that the increase in IC efficiency from P4C via P4CF3 to P4CF2 is caused by a decrease in EIC. The radiative rate constants kf in cyclopentane of P4CF2 and P4CF3 are about twice that of P4C, probably due to the mixing of the S2(1La,CT) and S1(1Lb) states of P4C caused by the molecular asymmetry introduced by the F-substituents. It is assumed that the lowering of the IC barriers in P4CF2 and P4CF3 is governed by an F-substituent-dependent difference in the energies of the molecular configuration of the azetidinylbenzonitriles that can be reached in S1 as compared with those in S0.