Near-Infrared Intense

Nov 2, 2017 - Department of Chemistry, Faculty of Science, Sultan Qaboos University, P.O. Box 36, Postal Code 123, Muscat, Sultanate of Oman. ‡ Cent...
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Letter Cite This: J. Phys. Chem. Lett. 2017, 8, 5603-5608

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New Insight into the Origin of the Red/Near-Infrared Intense Fluorescence of a Crystalline 2‑Hydroxychalcone Derivative: A Comprehensive Picture from the Excited-State Femtosecond Dynamics N. Idayu Zahid,†,‡ Mohamad Syafie Mahmood,§ Balamurugan Subramanian,§ Suhana Mohd Said,§ and Osama K. Abou-Zied*,† †

Department of Chemistry, Faculty of Science, Sultan Qaboos University, P.O. Box 36, Postal Code 123, Muscat, Sultanate of Oman Centre for Fundamental and Frontier Sciences in Nanostructure Self-Assembly, Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia § Department of Electrical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia ‡

S Supporting Information *

ABSTRACT: Fluorescence upconversion and transient absorption techniques are used to explain the source of the intense red/near-infrared emission of crystalline 4dimethylamino-2′-hydroxychalcone. We found that the initially excited enol form undergoes tautomerization in 3 ps to form the keto tautomer. The latter is stable in the ground state as a consequence of J-type aggregation in the crystal packing and is manifested in an absorption peak at 550 nm that spectrally overlaps with the short-lived enol emission, leading to self-reabsorption and adding a factor to the complete depletion of the enol emission. Relaxation of the keto tautomer takes place in the form of intense fluorescence (600−750 nm) with 1.7 ns lifetime. The different spectroscopy in solution is due to vibrational cooling (300 fs), followed by solvation dynamics (5 ps in methanol) and twisting of the hydroxyphenyl ring (16 ps), before relaxation of the enol tautomer in the form of weak green fluorescence with 350 ps lifetime. fluorescence intensity through intramolecular twisting and hydrogen-bond disruption. A comprehensive theoretical investigation of the gas-phase DHC and similar derivatives identified two major intramolecular rotation pathways in the enol and keto excited states.16 These twisting motions lead to a conical intersection that drives the molecule back to the ground-state nonradiatively. In contrast, the source of the intense emission in the crystalline form was attributed to the πconjugated framework, a strong donor−acceptor skeleton (donor: N(Me)2−acceptor: CO), and an ESIPT reaction (enol-keto tautomerization). Fluorescence quenching due to molecular twisting and π−π stacking was efficiently eliminated in the crystalline form of DHC due to its rigid and slip (edgeto-face) packing. To fully understand the excited-state behavior of DHC, it is necessary to resolve the excited-state dynamics in real time. We studied the ultrafast spectroscopy of DHC in crystalline form and in solution using fluorescence upconversion17 and transient absorption18 techniques. We show that the first several hundred femtoseconds, following a femtosecond pump laser excitation, involve the key dynamics that are behind the different spectroscopic behavior of DHC in solution versus that of the

he development of novel fluorescent molecules that emit in the physiologically relevant optical window 600−1000 nm is of increasing interest due to the large penetration of nearinfrared (NIR) radiation in most biological media, offering imaging at significant depths in living tissues.1,2 Chalcones, which comprise an aromatic ketone and an enone system, are a core structure found in many naturally occurring compounds and have been widely applied in medicinal chemistry, such as detection and treatment of viral disorders, cardiovascular diseases, and cancer.3−7 Fluorescent chalcones were obtained by attaching electron-pushing and electron-pulling functional groups in the molecular backbone as well as an OH group in the phenyl ring to induce excited-state intramolecular proton transfer (ESIPT).8−11 Some of these derivatives have been utilized as fluorescent probes in enzymatic reactions and in detecting fingerprints.12,13 A series of highly efficient deep red to NIR emissive organic crystals based on the structurally simple 2′-hydroxychalcone derivatives have recently been synthesized.12,14,15 Among these derivatives, 4-dimethylamino2′-hydroxychalcone (DHC) is shown to produce amplified spontaneous emission with a quantum yield of ϕem = 0.32 and λem (max) = 650−710 nm that is dependent on the crystal size14 (see Scheme 1 for the structure of DHC). In solution, DHC was found to have no or weak green fluorescence that is solvent-dependent. The different spectroscopy in solution was attributed to active nonradiative channels that deplete the

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© XXXX American Chemical Society

Received: October 2, 2017 Accepted: November 2, 2017 Published: November 2, 2017 5603

DOI: 10.1021/acs.jpclett.7b02601 J. Phys. Chem. Lett. 2017, 8, 5603−5608

Letter

The Journal of Physical Chemistry Letters Scheme 1. Ground- and Excited-State Structures of DHC in Crystalline and Solution Phasesa

a

Different mechanisms are derived from the steady state and ultrafast dynamic results. TD-DFT twist energies are shown in the top right.

crystalline form. A comprehensive picture of the excited-state deactivation was produced based on the observed femtosecond−nanosecond dynamics. The steady-state absorption and fluorescence (excitation at 450 nm) spectra of DHC in crystalline form and dissolved in methanol are shown in Figure 1. In methanol, an absorption peak at 425 nm and a weak fluorescence peak at 575 nm are assigned to the enol-tautomer.14,16 We measured the fluorescence quantum yield (ϕF) in MeOH to be 80 nm (i.e., λem ≈ 600 nm) compared with DHC. Their results clearly demonstrate that the donor−acceptor structure in DHC is crucial for the observed intense fluorescence. In this regard, MeOH solvation of the lone pair of electrons on the nitrogen atom of the amino group (see Scheme 1) is a major factor in weakening the donor−acceptor mechanism in which the lone pair of electrons is not free to participate in conjugation. A similar effect was also reported when the hydroxyl group was replaced with a methoxy group, indicating the equal importance of the ESIPT process for the observed intense fluorescence of crystalline DHC.14 In summary, we studied the excited-state spectroscopy of DHC in solution and in crystalline form by measuring the ultrafast dynamics using femtosecond fluorescence upconversion and transient absorption techniques. The results show that the source of the intense red fluorescence in the crystalline form is due to an ESIPT process that leads to a keto tautomer. This process takes place during the initial 3 ps after excitation and is manifested in a decay component when probing the enol tautomer (λem = 580 nm) and a rise time component when probing the keto tautomer (λem = 700 nm). The ultrafast ESIPT process is absent in solution due mainly to solvation of the polar sites of DHC (lifetime 5 ps in MeOH) and a twisting motion within the molecular backbone (lifetime 17 ps). We also show that the stability of the keto tautomer in the ground state of the crystalline form provides an additional channel for accessing the keto excited state and depleting the enol emission via self-reabsorption. The complex dynamics in MeOH were

Figure 5. Femtosecond transient absorption spectra of DHC dissolved in MeOH. The dynamics were recorded during the first 400 fs immediately after time zero between the pump (photoexcitation at 450 nm) and the probe pulses. The insets are the temporal profiles monitored at different probe wavelength, as indicated, along with the best multiexponential fits (solid lines) for 8 ps time window. The instrument response function is shown by the dashed curve in the lower right panel.

change recorded over the initial period of 400 fs after time zero between the pump (λex = 450 nm) and the probe. The results show the evolution of an excited-state absorption (ESA) band in the region 490−550 nm, with a maximum at 510 nm, and a negative band that develops within the same time frame in the region 550−750 nm. The latter band matches the steady-state fluorescence region and is then due to stimulated emission (SE) and not ground-state recovery. Both ESA and SE bands show a clear dynamic red shift. The red shift is much more pronounced in the SE band, which gets much broader with time. The red shift is an indication of excited state cooling via vibrational relaxation, in addition to solute−solvent interaction that takes place over a longer period of time (∼5 ps). The spectral broadening indicates the existence of internal degrees of freedom such as twisting about one or more single bonds during the relaxation of the excited state. The insets in Figure 5 show the TA temporal profiles of the ESA and the SE bands for 5607

DOI: 10.1021/acs.jpclett.7b02601 J. Phys. Chem. Lett. 2017, 8, 5603−5608

Letter

The Journal of Physical Chemistry Letters

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reproduced by femtosecond TA spectroscopy. The results explain the weak, green fluorescence of DHC in solution, where solvation of the polar sites of the molecule weakens the ICT process and the internal twisting motion breaks the intramolecular hydrogen bond. Both mechanisms hinder the ESIPT process and act as nonradiative channels that lower the fluorescence quantum yield of the enol species. In contrast, the slip-packing and the near-planarity of the DHC molecules in the crystal structure lead to the formation of J-aggregation, leading to the observed aggregation-induced emission. The findings from this work provide a comprehensive picture of the underlying mechanism for the intense red/NIR fluorescence of DHC in the crystalline state using excitedstate femtosecond dynamics. This will be a valuable pathway for molecular design of chalcone derivatives that are able to provide intense fluorescence in the solid state for applications such as optoelectronics and biomedical imaging.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b02601. Detailed information on the experimental methods, crystal data and structure refinement, and theoretical calculations of the energy barriers. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: (+968) 2414-1468. Fax: (+968) 2414-1469. E-mail: [email protected]. ORCID

Osama K. Abou-Zied: 0000-0003-0497-8412 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank The Research Council of Oman (RC/SCI/CHEM/ 14/01) and Sultan Qaboos University (IG/SCI/CHEM/16/ 01) for supporting this work.

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REFERENCES

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DOI: 10.1021/acs.jpclett.7b02601 J. Phys. Chem. Lett. 2017, 8, 5603−5608