Attila Demeter

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.

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.

Dual Fluorescence and Ultrafast Intramolecular Charge Transfer with 6-N,N-Dialkylaminopurines. A Two-State Model

6-N,N-Dimethyl-9-methyladenine (DMPURM) and 6-N,N-dimethyladenine (DMPURH) show dual fluorescence from a locally excited (LE) and an intramolecular charge transfer (ICT) state in solvents of different polarity over extended temperature ranges. The fluorescence quantum yields are very small, in particular those of LE. For DMPURM in acetonitrile (MeCN) at 25 °C, for example, Φ(ICT) = 3.2 × 10(-3) and Φ(LE) = 1.6 × 10(-4). The large value of Φ(ICT)/Φ(LE) indicates that the forward LE → ICT reaction is much faster than the back reaction. The data obtained for the intersystem crossing yield Φ(ISC) show that internal conversion (IC) is the dominant deactivation channel from LE directly to the ground state S(0). For DMPURM in MeCN with Φ(ISC) = 0.22, Φ(IC) = 1 - Φ(ISC) - Φ(ICT) - Φ(LE) = 0.78, whereas in cyclohexane an even larger Φ(IC) of 0.97 is found. The dipole moment gradually increases upon excitation, from 2.5 D (S(0)), via 6 D (LE) to 9 D (ICT) for DMPURM and from 2.3 D (S(0)), via 7 D (LE) to 8 D (ICT) for DMPURH. From the temperature dependence of Φ(ICT)/Φ(LE), a reaction enthalpy -ΔH of 11 kJ/mol is obtained for DMPURM in n-hexane (ε(25) = 1.88), increasing to 17 kJ/mol in the more polar solvent di-n-butyl ether (ε(25) = 3.05). With DMPURM in diethyl ether, an activation energy of 8.3 kJ/mol is determined for the LE → ICT reaction (k(a)). The femtosecond excited state absorption spectra at 22 °C undergo an ultrafast decay: 1.0 ps in CHX and 0.63 ps in MeCN for DMPURM, still shorter (0.46 ps) for DMPURH in MeCN. With DMPURM in n-hexane, the LE fluorescence decay time τ(2) increases upon cooling from 2.6 ps at -45 °C to 6.9 ps at -95 °C. The decay involves ICT and IC as the two main pathways: 1/τ(2) ≅ k(a) + k(IC). As a model compound (no ICT) is not available, its lifetime τ(0)(LE) ∼ 1/k(IC) is not known, which prevents a separate determination of k(a). The excited state reactions of DMPURM and DMPURH are treated with a two-state model: S(0) → LE ⇄ ICT. With 6-N-methyl-9-methyladenine (MPURM) and 9-methyladenine (PURM), the fluorescence quantum yield is very low (<5 × 10(-5)) and dominated by impurities, due to enhanced IC from LE to S(0).

Internal conversion in 4-substituted 1-naphthylamines. Influence of the electron donor/acceptor substituent character

The thermally activated internal conversion (IC) taking place in 4-substituted 1-(dimethylamino)naphthalenes (14DMX) and 1-aminonaphthalenes (14ANX) with X=CN, Cl, H, CH3 and OCH3 was investigated in three solvents n spanning the polarity scale, hexane, diethyl ether and acetonitrile. In both series 14DMX and 14ANX, the n efficiency of the IC reaction decreases substantially when X changes from CN to OCH3, the order in which the electron donor character of the 4-substituent increases. Considerably larger IC reaction rate constants are obtained n for the first group of compounds. This difference is connected with the ground state structure of the amino group, which is more strongly twisted for 14DMX (ca. 60°) than for 14ANX (ca. 20°), whereas both sets of 1-naphthylamines are planarised in the S1 n excited state. The IC process slows down with increasing solvent polarity for each of the 14DMX and 14ANX molecules. The substituent X and the solvent polarity mainly affect the IC activation energy EIC. With 14DMX in hexane, EIC n increases from 10 kJ mol−1 for X=CN to 34 kJ mol−1 n for X=OCH3, whereas with, e.g., 14DMCL a solvent polarity dependent increase of EIC from 16 kJ mol−1 n in hexane n to 28 kJ mol−1 in acetonitrile is observed. The height of the barrier EIC is governed by the energy gap ΔE(S1,S2) between the two lowest excited singlet states. The influence of ΔE(S1,S2) on EIC is attributed to vibronic coupling caused by the proximity of the S1 and S2 states, which flattens the S1 potential energy surface n and thereby lowers the IC barrier when ΔE(S1,S2) becomes smaller. It is assumed that the IC reaction of the 1-naphthylamines n passes through a conical intersection, which exists as a consequence of the relative displacement of the S1 and S0 n surfaces caused by the different amino twist angles in the two states.

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).

Intramolecular Charge Transfer with the Planarized 4-Cyanofluorazene and Its Flexible Counterpart 4-Cyano-N-phenylpyrrole. Picosecond Fluorescence Decays and Femtosecond Excited-State Absorption

The fluorescence spectrum of the rigidified 4-cyanofluorazene (FPP4C) in n-hexane consists of a dual emission from a locally excited (LE) and an intramolecular charge-transfer (ICT) state, with an ICT/LE fluorescence quantum yield ratio of Phi(ICT)/Phi(LE) = 3.3 at 25 degrees C. With the flexible 4-cyano- N-phenylpyrrole (PP4C) in n-hexane, such an ICT reaction also takes place, with Phi(ICT)/Phi(LE) = 1.5, indicating that for this reaction, a perpendicular twist of the pyrrole and benzonitrile moieties is not required. The ICT emission band of FPP4C and PP4C in n-hexane has vibrational structure, but a structureless band is observed in all other solvents more polar than the alkanes. The enthalpy difference Delta H of the LE --> ICT reaction in n-hexane, -11 kJ/mol for FPP4C and -7 kJ/mol for PP4C, is determined by analyzing the temperature dependence of Phi(ICT)/Phi(LE). Using these data, the energy E(FC,ICT) of the Franck-Condon ground state populated by the ICT emission is calculated, 41 (FPP4C) and 40 kJ/mol (PP4C). These large values for E(FC,ICT) lead to the conclusion that with FPP4C and PP4C, direct ICT excitation, bypassing LE, does not take place. FPP4C has an ICT dipole moment of 15 D, similar to that of PP4C (16 D). Picosecond fluorescence decays allow the determination of the ICT lifetime, from which the radiative rate constant k(f)(ICT) is derived, with comparable values for FPP4C and PP4C. This shows that an argument for a twisted ICT state of PP4C cannot come from k(f)(ICT). After correction for the solvent refractive index and the energy of the emission maximum nu(max)(ICT), it appears that k(f)(ICT) is solvent-polarity-independent. Femtosecond transient absorption with FPP4C and PP4C in n-hexane reveals that the ICT state is already nearly fully present at 100 fs after excitation, in rapid equilibrium with LE. In MeCN, the ICT state of FPP4C and PP4C is likewise largely developed at this delay time, and the reaction is limited by dielectric solvent relaxation, which shows that the ICT reaction is ultrafast, at the experimental time limit of 50 fs. PP4C and FPP4C have a similar planar ICT structure, without an appreciable twist of the pyrrole and benzonitrile subgroups. Their crystal structure is compared with calculations for the S0 ground state.

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 ncrossing 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 nP4CF3 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.

The influence of aryl substitution on the photophysics of 1-aryl-fluorenones

Abstract 1-aryl-fluorenone derivatives were prepared from fluorenone via 9-fluorenone-1-boronic acid by coupling with the corresponding aryl-halides. The photophysical properties of these derivatives were found to be strongly influenced by a new CT transition, resulting in some cases in dual luminescence.

Fast internal conversion, dual fluorescence and intramolecular charge transfer in 9-cyano-10-(dimethylamino)anthracene.

The deactivation of the singlet excited state of 9-cyano-10-(dimethylamino)anthracene (CDA) in a variety of solvents predominantly takes place by way of internal conversion (IC), with a yield of ~0.97 in n-propylcyanide, whereas intersystem crossing and intramolecular charge transfer (ICT) only play a minor role. The ICT reaction leads to dual fluorescence, from a locally excited (LE) and a charge transfer (CT) state. The CT/LE fluorescence quantum yield ratio Φ(CT)/Φ(LE) becomes larger with increasing solvent polarity, from ~0.1 in cyclopentane to 3.7 in n-propylcyanide at -92 °C. The LE and CT fluorescence decays, with shortest decay times of several tens of picoseconds, are measured as a function of temperature and a growing-in of the CT fluorescence is observed at -110 °C in n-propylcyanide. The occurrence of IC is connected with the amino twist angle relative to the anthracene plane of CDA in the ground state. The S 2 fluorescence reported for CDA in the literature is attributed to the presence of a small amount of a highly fluorescing impurity.