Direct Observation of a Hydrogen‐Bonded Charge‐Transfer State of 4‐Dimethylaminobenzonitrile in Methanol by Time‐Resolved IR Spectroscopy

Abstract

Charge-transfer excited states have frequently been studied by using 4-dimethylaminobenzonitrile (DMABN) as a model. In nonpolar solvents, a single fluorescence band is observed from a locally excited (LE) state. In polar solvents, the initially populated LE state reacts further to produce a stable intramolecular charge-transfer (ICT) state, which gives rise to a second fluorescence band that overlaps with, but is abnormally red-shifted from, the LE emission.[1] Results of experiments using aprotic solvents are well described by models in which polarity is the only solvent property that affects the charge transfer reaction activation energy and the relative stabilization of the ICT and LE states.[2] Whilst much work continues to concentrate on determining the structures of the LE and ICT states, [3][4][5][6][7] the precise nature of the difference between the properties of the excited state in protic and aprotic solvents is little understood. For example, the fluorescence quantum yield of DMABN in protic solvents is lower and the fluorescence spectrum is further red-shifted and broadened, relative to measurements in aprotic solvents of the same polarity, [8,9] and the fluorescence decay kinetics are difficult to interpret.[2] Hydrogen bonding in protic solvents can lead to complicated interactions [10] but although specific solute-solvent and solute-solute interactions have been discussed, [8,[11][12][13][14] there is no generally accepted explanation. There are similar problems in other cases of dual fluorescence.[15]The time-resolved infrared (TRIR) absorption spectra presented here demonstrate and monitor the formation of a hydrogen-bonded charge-transfer state of photoexcited DMABN in the protic solvent methanol (MeOH), through the development of the CN IR absorption band from an initial singlet into a doublet. The initial single band is interpreted as belonging to an ICT state like that created in aprotic acetonitrile (MeCN), where only one absorption band is observed at all delay times. The second component is interpreted as being due to the hydrogen-bonded chargetransfer state; the kinetics show the populations of the free and hydrogen-bonded species coming to dynamic equilibrium. We designate the hydrogen-bonded state as HICT. This is the first direct observation of hydrogen bonding in an excited state. Since the populations in the LE state and the two charge-transfer states coexist, the fluorescence will be triple, not dual in character. Neglect of this major factor is considered to account for much of the difficulty in interpreting the fluorescence results. [2,8,[11][12][13] A mechanism of this kind has not to our knowledge been proposed before. We believe this interpretation is applicable to other molecules with solvent-dependent dual fluorescence. Figure 1 shows TRIR spectra of DMABN in MeCN (a) and MeOH (b) recorded with sub-picosecond time resolution at pump-probe delays from 2 to 3000 ps after excitation; Figure 2 gives the time-dependence of the absorption band areas. Kinetics parameters were dete...

Spin–Orbit Ab Initio Investigation of the Photolysis of Bromoiodomethane

“…The results are consistent with the structures being at the minima, and agree with the previous B3LYP results [14] and experiments [10,14,46,47] with respect to both the magnitude and assigned vibrational modes (see Table 2). …”

“…The spin-free MS-CASPT2 vertical excitation energies (T v ) and oscillator strengths (f) of the five lowest-lying spin-free states of CH 2 BrI, iso-CH 2 Br-I, and iso-CH 2 I-Br. [8,9,47] have shown that photoexcitation of CH 2 BrI within the A-band absorption results in only CÀI bond cleavage.…”

“…In a femtosecond pump-probe experiment in acetonitrile at room temperature, the lifetime of iso-CH 2 Br-I was measured to be 2.5 ns while it was established that iso-CH 2 I-Br has a longer lifetime. [11] TRRS experiments in cyclohexane at room temperature [14,47] showed mostly iso-CH 2 I-Br, whereas studies in low-temperature (12 K) argon matrices [46] only found the CH 2 Br-I isomer. While the computed energetics of the isomerization process supports the observations in solvents, the solid-state results are not explained.…”

Ultrafast Intramolecular Charge Transfer and Internal Conversion with Tetrafluoro‐aminobenzonitriles

“…[3,6,9,10,[56][57][58] The still faster ICT reaction found here for AZABN4F in n-hexane (7.7 10 12 s À1 or t 2 = 0.13 ps) as compared with DEABN4F, however, is contrary to the slowing down of the ICT reaction observed for AZABN relative to DMABN and DEABN. [5a, 10] With the XABN4Fs in MeCN, the shortest decay time t 2 has a similar value (around 0.09 ps) for all five molecules.…”

Do fluorescence and transient absorption probe the same intramolecular charge transfer state of 4-(dimethylamino)benzonitrile?

“…It is possible that the slower picosecond rise time of the ICT states in ethanol, as compared to acetonitrile, is related to H-bonding in ethanol. 23 To summarize, our data suggest the existence of two charge-transfer states of DMABN in polar solvents, namely, the dark TICT state formed form the ‫ء‬ state and the fluorescent ICT state formed from the LE state. Differing time scales of the two ICT states indicate the lack of communication ͑i.e., rapid population equilibration͒ between them.…”