Double intramolecular hydrogen transfer assisted dual emission in a carbazole-embedded porphyrin-like macrocycle

Article abstract of DOI:10.1039/d1cc00868d

The introduction of a pyrrole ring at one of themesopositions of carbazole-based porphyrins lowers the structural symmetry and results in dual emission, which strongly depends on the excitation wavelength and temperature. The origin of dual emission induced by NH-tautomerism is confirmedviaphotophysical and DFT calculations.

Full text of DOI:10.1039/d1cc00868d

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Double intramolecular hydrogen transfer assisted
dual emission in a carbazole-embedded
porphyrin-like macrocycle†
Cite this: Chem. Commun., 2021,
57, 4420
Received 15th February 2021,
Accepted 23rd March 2021
Arumugan Kalaiselvan, Aswini Spergen, Isukapalli Sai Vamsi Krishna,
Vennapusa Sivaranjana Reddy
and Sabapathi Gokulnath
*
DOI: 10.1039/d1cc00868d
rsc.li/chemcomm
The introduction of a pyrrole ring at one of the meso positions of vibrationally excited level without populating its relatively high
carbazole-based porphyrins lowers the structural symmetry and energy cis-intermediate.13,14 In addition, porphycenes featuring an
results in dual emission, which strongly depends on the excitation NHÁÁÁN distance of 2.53 Å, which is relatively small as compared to
wavelength and temperature. The origin of dual emission induced porphyrin, and thereby exhibit dual-fluorescence. Interestingly,
by NH-tautomerism is confirmed via photophysical and DFT dual emission was also observed in corroles due to the coexistence
calculations.
of both tautomers (T1 and T2) even at a lower temperature (77 K),
which suggests that the energy difference between these tautomers
NH tautomerism plays a vital role in developing mole- is very small (2.45 kcal molÀ1).15,16 Very recently, a meso-modified
cular switches1,2 and information storage.3 In porphyrinoids, heteroporphyrin with the replacement of one methine carbon with
NH-tautomerism can be distinguishable and exhibit two dif-
ferent optical properties when porphyrin possesses irregular
substitution leading to unsymmetrical structures on the cores
(Fig. 1a).4 Such NH tautomerism in porphyrins occurs stepwise
through a cis-intermediate supported by tunnelling mechanism
in solution (Fig. 1a).5,6 The unequal ground and excited state
energies in metal-free porphyrins result in a dual emission
involving NH-tautomerism. Dual emitters (DEs) have gained
immense attention recently due to their potential applications
in various fields, such as sensors,7 multicolour displays,8 white
organic light emitting diodes (WOLEDs),9 data encryption10
and bio-imaging.11,12 Due to the close relation between
NH-tautomerism and dual emission in various macrocyclic
compounds, we devised a meso-modification strategy on the
carbazole embedded porphyrin with a meso-pyrrole substituent.
Such substitution provides nonsymmetric structures that can
lead to two distinguishable pathways for the tautomeric pro-
cess. Herein, efforts have been made to study whether the
tautomerism can take place either via a step-wise or a simulta-
neous double intramolecular hydrogen transfer (DIHT). Speci-
fically, NH-tautomerism in porphycenes (trans
" trans)
takes place simultaneously via concerted tunnelling from a
School of Chemistry, Indian Institute of Science Education and Research,
Thiruvananthapuram-695551, Maruthamala P.O., Vithura, Kerala, India.
E-mail: gokul@iisertvm.ac.in
† Electronic supplementary information (ESI) available: Experimental details and
compound data. CCDC 2062803, 2059574 and 2059575. For ESI and crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/d1cc00868d
Fig. 1 (a) Selected examples of dual emitters. (b) A plausible pathway for
NH-tautomerization between 3T1 and 3T2; the relative energy for each
tautomer is given with the respective chemical structure (units: kcal molÀ1).
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oxygen exhibited dual fluorescence through its unique NH- but in a slightly lower yield of 14% (route D in ESI†). The
tautomerism.17 1H NMR spectra of 4 and 5 in CDCl3 showed a simple spectral
The syntheses of carbazole incorporated meso-pyrrole 3, pattern attributed to their C2 symmetric structures. The b-CHs
meso-unsubstituted 4 and meso-pentafluorophenyl substituted of the pyrrole (ring B) in 4 and 5 resonate as singlets at 7.45 and
5 porphyrin-like macrocycles are shown in Scheme 1. At first, 6.95 ppm, respectively. The two pyrrole rings (ring A) connected
we performed a ‘‘3+1’’ acid catalysed condensation reaction of at the 1,8-positions of the carbazole subunit displayed two well-
118 with 1H-pyrrole-2-carbaldehyde 2a in the presence of resolved doublets (7.64 ppm and 7.97 ppm for 4; 7.48 ppm and
CF3COOH followed by DDQ oxidation to provide macrocycles 8.26 ppm for 5) that are similar to what is seen in the case of 3
3 in 12% and 4 in 4%. The HRMS of 3 showed a distinct peak (Fig. S4.11 and S4.13, ESI†). The downfield shifts of the pyrrole
at m/z = 562.2961 ([M + H]+, calcd for C38H36N5:562.2971) (Fig. S3.5, and carbazole NH resonances in 4 and 5 are consistent with the
ESI†) and 4 showed a molecular peak at m/z = 497.2709([M + H]+, existence of strong hydrogen–bonding interactions within the
calcd for C34H33N4:497.2705) (Fig. S3.6, ESI†). Considering the less shielded environment of N4 cores.
mass difference of one pyrrole moiety between 3 and 4, diluted
Further insights into the molecular structure of 3 came from
CF3COOH added to 3 did not produce 4, thereby indicating that 4 single crystal X-ray structural analyses of crystals obtained by
was not derived from 3 during the reaction. All our attempts to the slow diffusion of methanol into a toluene solution of 3 at
improve the yield of 4 were unsuccessful, and hence we condensed room temperature which crystallized in a triclinic crystal lattice
%
1 with 1H-pyrrole-2,5-dicarbaldehyde 2b under similar conditions, with a P1 space group (Table S6.1, ESI†). The structure is a
which provided 4 in a surprisingly high yield of 33%. The 1H NMR nearly planar macrocycle with a tilted meso-pyrrole ring (ring D)
spectrum of 3 in CDCl3 showed (Fig. S4.7, ESI†) a Cs symmetric by 53.91 and two adjacent pyrrole rings (rings B and C) were slightly
pattern with relatively upfield shifted signals for ring D correlating inclined with respect to the mean plane (C5–C15–C25–C30) (Fig. 2a
1
with each other in H–1H COSY spectral analysis (Fig. S4.8, ESI†). and b). Four intramolecular N–HÁÁÁN hydrogen bonding interac-
Three singlets between 8.63 and 8.79 ppm are assigned to pyrrole tions were observed inside the macrocycle (N1–HÁÁÁN2:2.306 Å;
and carbazole NHs, as confirmed by D2O experiments (Fig. S4.9, N2–HÁÁÁN3:2.360 Å; N3–HÁÁÁN4:2.554 Å and N4–HÁÁÁN1:2.458 Å),
ESI†). Two closely associated multiplets at 7.27 and 7.31 ppm for typical for regular porphyrins. Additionally, two strong intermole-
ring B and three well separated multiplets between 7.64 and cular N–HÁÁÁN hydrogen bonding interactions were also found with
8.08 pm integrating to four protons are due to the two pyrrole distances of 2.290 Å and 2.306 Å to form the self-assembled dimer
rings (ring A and C).
(Fig. 2c). Similarly, the slow diffusion of methanol into a chloro-
In order to compare the optical and redox properties of 3 form solution of 4 and 5 provided X-ray quality crystals of 4 and 5.
and 4, a bis-meso-pentafluorophenyl substituted analogue 5 was Both crystallized in a monoclinic crystal lattice with P21/n and
also prepared by condensation of equimolar amounts of 1 with P21/c space groups, respectively. The structures of both 4 and 5
(perfluorophenyl)(1H-pyrrol-2-yl)methanol 6a19 in the presence revealed a more planar conformation and showed intramolecular
of a catalytic amount of para-toluenesulfonic acid (p-TsOH) N–HÁÁÁN hydrogen bonding interactions typical for regular por-
under an inert atmosphere for 2 h, followed by oxidation with phyrins (Fig. 3a and b).
1.0 equiv. of DDQ (Route C and D, ESI†) in 19% yield. The
The absorption and fluorescence spectra of 3, 4 and 5 in
initial characterization of 5 was performed using HRMS analy- CH2Cl2 are shown in Fig. 4. The UV/Vis absorption spectrum of
sis which shows a molecular ion peak at m/z = 829.2374 3 shows a slightly broad Soret-like band at lmax = 422 nm with
([M + H]+, calcd for C46H31F10N4:829.2374) and thus proved two Q-like bands at 580 and 626 nm, respectively. Whereas 4
the exact composition of 5 (Fig. S3.7, ESI†). Alternatively, and 5 showed similar absorption profiles and displayed blue
condensation of 7a20 with 1 also provided the macrocycle 5, shifted absorption peaks at lmax = 375 and 385 nm along with
distinct Q-like bands at 572, 621 nm and 582, 631 nm. Upon
protonation, both 3 and 4 exhibited a red-shift of the Soret-like
Fig. 2 Single crystal X-ray structures of macrocycle 3: (a) a top view
showing N–HÁ Á ÁN distances, (b) a side view showing the tilting of ring D,
and (c) a self-assembled dimer with two intermolecular N–HÁ Á ÁN interac-
Scheme 1 Synthetic routes to the preparation of meso-pyrrolyl substi-
tuted carbazole embedded porphyrin 3.
tions. Solvent molecules and tert-butyl groups have been omitted in (b and
c). Ellipsoids are shown at a 50% probability level.
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Fig. 5 (a) Emission spectra of 3 recorded at various excitation wave-
lengths in CH2Cl2. Spectra have been normalized at 651 nm. (b) Energy
level diagrams of the tautomerism between 3T1 and 3T2 in the ground and
excited states, calculated at the B3LYP/6-31g(d) level of theory (units:
kcal molÀ1). The solvent effect was accounted for using PCM. Excitation
and emission wavelengths (units: nm) are also shown.
Fig. 3 Single crystal X-ray structures of macrocycles (a) 4 and (b) 5.
Solvent molecules and meso-pentafluorophenyl groups have been
omitted for clarity in the respective side views. Ellipsoids are shown at a
50% probability level.
spectra (Fig. S5.4, ESI†). We found a red shifted emission for 3
upon increasing the solvent polarity (Fig. S5.7, ESI†). To con-
firm the NH tautomeric behaviour in 3, we examined the
1
variable-temperature H NMR spectra, which showed an indis-
tinguishable proton signal as expected for the existence of a fast
interconversion between both tautomeric forms (3T1 and 3T2)
(Fig. S4.15, ESI†). Even at 213 K in CDCl3, we were unable to
detect two separate pairs of signals corresponding to the
tautomers 3T1 and 3T2, thus revealing the coexistence of these
two NH tautomers. Identical excitation and absorption spectra
of macrocycles 4 and 5 confirm the presence of a single species.
We found a bi-exponential feature for the fluorescence decay
of 3, probed at 650 nm, with an average lifetime of 0.26 ns. This
bi-exponential feature clearly indicates the presence of two
different NH tautomeric forms in the excited state. In contrast,
we observed single exponential fluorescence lifetimes of 5.25
and 5.51 ns for 4 and 5, respectively (Fig. S10.1, ESI†). The
photophysical properties and kinetic parameters (kr and knr,
Fig. 4 A comparison of the absorption and fluorescence spectra (blue
lines) of macrocycles (a) 3, (b) 4, and (c) 5 recorded in CH2Cl2 at 298 K. All
the samples were excited at lex = 440 nm.
and Q-like bands with a distinct broadening of the Q-like bands respectively) of 3, 4 and 5 are summarized in (Table S10.1,
due to the altered HOMO–LUMO levels (Fig. S5.6, ESI†). Curiously, ESI†). Relative fluorescence quantum yields (FF) were deter-
5 did not show a spectral change upon protonation, indicating the mined using a previously reported compound as a reference in
weak basicity of the imino-pyrrolic nitrogen due to the strong toluene.18,21 The derived FF for 4 and 5 were found to be ca.
electron-withdrawing nature of meso-C6F5 groups. The fluorescence 0.309 and 0.261, respectively, which may be attributed to the
spectra of 3, 4 and 5 exhibit a broad emission profile that is notably low rates of intersystem crossing (ISC). Interestingly, the quan-
shifted with respect to the lowest-energy transition (Fig. 4).
tum yield varies from ca. 0.072 to 0.033 upon changing the
We observed two different emission spectra for 3 which are excitation wavelength from 440 to 600 nm for 3. The observed
attributed to two different NH tautomers 3T1 and 3T2 gener- lower FF for 3, compared to those for 4 and 5, could be due to
ated through a double intramolecular hydrogen transfer (DIHT) the strong intermolecular hydrogen bonding between the meso-
(Fig. 1b). The tautomer 3T1 shows two emission maxima at 651 pyrrole NH and the imino-nitrogen of ring C.22 This is in good
and 709 nm upon excitation in the range of 400–550 nm. We agreement with the theoretically calculated oscillator strengths
also observed two emission maxima at 773 and 803 nm for the originating from two different tautomers, 3T1 and 3T2
3T2 tautomer in addition to the 3T1 emission features (Fig. 5a). (Fig. S7.4, ESI†).
Notably, the intensities of the 3T2 tautomer increase upon
The HOMO–LUMO energy gaps for all these macrocycles
excitation at a longer wavelength, whereas the 3T1 intensities were estimated via cyclic voltammetry (CV) and differential
remain unchanged. Further, to confirm the existence of tauto- pulse voltammetry (DPV). Estimated electrochemical HOMO–
mers 3T1 and 3T2 (trans-to-trans), we recorded the excitation LUMO (H–L) gaps of 1.82 V for 3, 1.53 V for 4 and 1.79 V for 5
spectra of 5 in CH2Cl2 at two emission maxima (lem = 651 and (Fig. S9.1, ESI†) were obtained. All three macrocycles showed
773 nm). These spectra indicate the presence of two different irreversible oxidations, but exhibited two consecutive reversible
absorbing species, as evident from two dissimilar excitation to quasi-reversible reduction waves as judged by both CV and
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DPV analysis (Fig. S9.1, ESI†). Compared with macrocycles 3
This work was supported by IISER Thiruvananthapuram and
(Eox1 = 1.32 V) and 4 (Eox1 = 1.04 V), the first oxidation of 5 SERB Core Research Grant No. CRG/2019/006303. A. K. thanks
(Ered1 = 1.51 V) occurs at a significantly more positive potential. CSIR and A. S. thanks IISER TVM for their fellowships. We
Such an effect is inherent due to the meso-pentafluoro sub- thank Alex P. Andrews for solving the X-ray structures of 3, 4,
stituents of 5 being harder to oxidize and easier to reduce and 5.
(Ered1 = À0.28 V) than those of the meso-unsubstituted homo-
logue 4. No significant change in the reduction potential was
Conflicts of interest
observed between 3 (Ered1 = À0.50 V) and 4 (Ered1 = À0.49 V),
signifying a minimal redox change by the introduction of
pyrrole at one of the meso-position of 4.
There are no conflicts to declare.
The analysis of frontier molecular orbitals (FMOs) reveals
that the highest occupied molecular orbitals (HOMOs) and
lowest unoccupied molecular orbitals (LUMOs) of macrocycles
4 and 5 are identical in shape with a similar energy gap
(Fig. S8.6, ESI†). For all the compounds, the LUMOs are readily
isolated from other unoccupied orbitals. The HOMO–LUMO
gap for tautomer 3T1 is B2.34 eV and decreases to B2.01 eV for
the tautomer 3T2 (Fig. S8.5, ESI†). The HOMO and LUMO of 5
are relatively stabilized as compared with those of 3 and 4 due
to the electron withdrawing meso-C6F5 substituents. The broad
absorbances in 3–5 are both composed of transitions that are a
mixture of HOMO - LUMO and HOMOÀ1 - LUMO transi-
tions. To further evaluate the aromatic character, we calculated
NICS values at the global centre of macrocycles 3, 4 and 5,
estimated to be À2.84, À3.29 and À3.58 ppm, respectively,
thereby suggesting the weak aromatic nature of these macro-
cycles (Fig. S8.1–3, ESI†). Further, to evaluate the aromaticity,
we conducted an anisotropy of the induced current density
(ACID) plot visualizing their distinct clockwise ring current and
corroborating its aromatic character (Fig. S10.2, ESI†). DIHT
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planar conformations of macrocycles 4 and 5, but slight dis-
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molecular N–HÁ Á ÁN hydrogen bonding interactions was
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